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Egyptian Board Infectious Diseases - independent subject foundations study materials
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Egyptian Board Infectious Diseases - independent subject foundations practice questions

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Question 1Microbiology, resistance and antimicrobial pharmacology

Which molecular target distinguishes the bactericidal mechanism of glycopeptides such as vancomycin from that of beta-lactam antibiotics during bacterial cell wall synthesis?

  1. Terminal D-alanyl-D-alanine pentapeptide precursor side chains Correct answer.
  2. Penicillin-binding protein transpeptidase active catalytic sites
  3. Undecaprenyl pyrophosphate lipid carrier dephosphorylation cycles
  4. N-acetylglucosamine-N-acetylmuramic acid disaccharide backbone polymerases

Explanation

Vancomycin exerts its bactericidal effect by binding with high affinity to the terminal D-alanyl-D-alanine moieties of cell wall peptidoglycan precursors. This steric hindrance prevents both transglycosylation and transpeptidation reactions. In contrast, beta-lactams bind directly to the catalytic active site of penicillin-binding proteins (transpeptidases) rather than binding the peptide substrate.

Question 2Microbiology, resistance and antimicrobial pharmacology

A clinical isolate of Klebsiella pneumoniae demonstrates high-level resistance to amikacin, gentamicin, and tobramycin. Genetic sequencing confirms the presence of an armA gene. What is the precise biochemical mechanism of this resistance?

  1. Active proton-dependent efflux through tripartite MexAB-OprM membrane pumps
  2. 16S ribosomal RNA methylation preventing drug binding to the 30S decoding A-site Correct answer.
  3. Enzymatic O-adenylylation of the 2-deoxystreptamine aminoglycoside scaffold
  4. Mutational alteration of the 50S L4/L22 ribosomal proteins disrupting translocation

Explanation

The armA gene encodes a 16S rRNA methyltransferase that methylates the G1405 nucleotide within the 30S ribosomal decoding region (A-site). This specific modification confers pan-aminoglycoside resistance across 4,6-disubstituted 2-deoxystreptamines, including amikacin, gentamicin, and tobramycin. Aminoglycoside-modifying enzymes typically confer narrower subclass-specific phenotypes.

Question 3Microbiology, resistance and antimicrobial pharmacology

An isolate of Escherichia coli recovered from a bacteremic patient is resistant to ceftriaxone, ceftazidime, and aztreonam, but tests susceptible to piperacillin-tazobactam, cefepime, and ertapenem. Addition of clavulanic acid produces a >=5 mm increase in the zone diameter of ceftriaxone. Which resistance enzyme is most consistent with this phenotypic profile?

  1. Chromosomal inducible class C beta-lactamase (AmpC)
  2. Carbapenem-hydrolyzing class D oxacillinase (OXA-48)
  3. Plasmid-mediated class A extended-spectrum beta-lactamase (CTX-M) Correct answer.
  4. Acquired Ambler class B metallo-beta-lactamase (NDM-1)

Explanation

Extended-spectrum beta-lactamases (ESBLs, predominantly CTX-M variants) are Ambler class A serine enzymes that hydrolyze penicillins, oxyimino-cephalosporins (ceftriaxone, ceftazidime), and aztreonam. They are characteristically inhibited by beta-lactamase inhibitors like clavulanate and tazobactam, demonstrating synergistic zone expansion, and spare carbapenems. AmpC enzymes and metallo-beta-lactamases are not inhibited by clavulanic acid.

Question 4Microbiology, resistance and antimicrobial pharmacology

According to the Ambler classification system for beta-lactamases, which enzyme class utilizes active-site divalent zinc ions rather than a catalytic serine residue to hydrolyze beta-lactam rings?

  1. Class B metallo-beta-lactamases (e.g., NDM, VIM, IMP) Correct answer.
  2. Class A carbapenemases (e.g., KPC, GES, SME)
  3. Class C cephalosporinases (e.g., AmpC, CMY, ACT)
  4. Class D oxacillin-hydrolyzing carbapenemases (e.g., OXA-48)

Explanation

Ambler class B enzymes are metallo-beta-lactamases (MBLs) that utilize divalent zinc cations (Zn2+) in their active site to coordinate and activate a water molecule that cleaves the beta-lactam ring. In contrast, classes A, C, and D are serine beta-lactamases that form a transient covalent acyl-enzyme intermediate through an active-site serine residue. MBLs are inhibited by metal chelators such as EDTA but not by serine beta-lactamase inhibitors.

Question 5Microbiology, resistance and antimicrobial pharmacology

A blood culture from a liver transplant recipient flags positive for Klebsiella pneumoniae harboring both blaNDM-1 and blaCTX-M-15. Which antimicrobial regimen provides targeted microbiological activity against this dual-carbapenemase/ESBL-producing pathogen?

  1. Meropenem-vaborbactam monotherapy
  2. Aztreonam combined with ceftazidime-avibactam Correct answer.
  3. Imipenem-cilastatin-relebactam monotherapy
  4. High-dose extended-infusion cefepime monotherapy

Explanation

Aztreonam is stable to NDM hydrolysis, while avibactam can protect it from co-produced serine beta-lactamases such as CTX-M. The listed combination can therefore have activity; it is not a claim that CTX-M is a carbapenemase or that this is the only preferred 2026 treatment.

Question 6Microbiology, resistance and antimicrobial pharmacology

What is the primary genetic and biochemical basis of methicillin resistance in Staphylococcus aureus (MRSA)?

  1. Hyperproduction of BlaZ beta-lactamase hydrolyzing all penicillin derivatives
  2. Methylation of the 23S rRNA subunit preventing beta-lactam binding
  3. Acquisition of mecA encoding penicillin-binding protein 2a (PBP2a) Correct answer.
  4. Active drug extrusion mediated by the plasmid-borne norA efflux pump

Explanation

Methicillin resistance in S. aureus is mediated by the acquisition of the staphylococcal cassette chromosome mec (SCCmec), which carries the mecA or mecC gene. These genes encode an altered penicillin-binding protein, PBP2a (or PBP2c), which exhibits markedly low binding affinity for virtually all standard beta-lactam antibiotics, allowing cell wall synthesis to proceed despite therapeutic beta-lactam concentrations. Ceftaroline is the only FDA-approved beta-lactam active against PBP2a.

Question 7Microbiology, resistance and antimicrobial pharmacology

An isolate of Enterococcus faecium recovered from a bone marrow aspirate exhibits a vancomycin MIC of >256 mcg/mL and a teicoplanin MIC of 64 mcg/mL. PCR testing confirms the VanA operon. What specific biochemical modification to peptidoglycan precursors accounts for this high-level glycopeptide cross-resistance?

  1. Replacement of terminal D-Ala-D-Ala with D-alanyl-D-serine causing a 10-fold drop in binding
  2. Enzymatic phosphorylation of the D-alanyl side chain by VanY carboxypeptidase
  3. Mutational truncation of the stem pentapeptide to a tripeptide lacking D-alanine
  4. Substitution of terminal D-alanyl-D-alanine with D-alanyl-D-lactate reducing affinity by 1,000-fold Correct answer.

Explanation

The VanA phenotype confers high-level inducible resistance to both vancomycin and teicoplanin. The VanA ligase synthesizes depsipeptide cell wall precursors terminating in D-alanyl-D-lactate (D-Ala-D-Lac) instead of D-alanyl-D-alanine. This ester linkage eliminates a critical hydrogen bond, reducing glycopeptide binding affinity by approximately 1,000-fold. In contrast, the VanC phenotype produces D-Ala-D-Ser precursors, conferring low-level intrinsic vancomycin resistance without teicoplanin resistance.

Question 8Microbiology, resistance and antimicrobial pharmacology

Which pharmacokinetic/pharmacodynamic (PK/PD) parameter best predicts the clinical bactericidal efficacy of once-daily high-dose aminoglycoside therapy, and which parameter correlates most closely with nephrotoxicity?

  1. Peak concentration-to-MIC ratio (Cmax/MIC) for efficacy; trough concentration (Cmin) for nephrotoxicity Correct answer.
  2. Time above MIC (%T > MIC) for efficacy; peak concentration (Cmax) for nephrotoxicity
  3. Area under the curve-to-MIC ratio (AUC/MIC) for efficacy; total clearance for nephrotoxicity
  4. Post-antibiotic effect duration for efficacy; free unbound drug fraction for nephrotoxicity

Explanation

Aminoglycosides exhibit concentration-dependent bactericidal killing; their clinical and microbiological efficacy is maximized when the peak concentration exceeds the organism's MIC by at least 8- to 10-fold (Cmax/MIC >= 8-10). Nephrotoxicity results from accumulation of drug in renal proximal tubular cells, which is driven by sustained trough levels (Cmin) rather than transient high peaks. Thus, once-daily extended-interval dosing achieves optimal Cmax while allowing prolonged drug clearance to undetectable troughs.

Question 9Microbiology, resistance and antimicrobial pharmacology

A clinical isolate of Salmonella enterica serovar Typhi shows resistance to ciprofloxacin with an MIC of 4 mcg/mL. Which chromosomal mutations in the quinolone resistance-determining regions (QRDR) are most commonly responsible for high-level fluoroquinolone resistance in Enterobacterales?

  1. Point mutations in the rpoB gene affecting RNA polymerase elongation
  2. Point mutations in gyrA (DNA gyrase) and parC (topoisomerase IV) Correct answer.
  3. Frameshift deletions in the ftsI gene disrupting transpeptidation
  4. Tandem duplications in the folP gene increasing dihydropteroate synthase activity

Explanation

Fluoroquinolones inhibit bacterial DNA replication by trapping complexes of DNA gyrase (encoded by gyrA and gyrB) and topoisomerase IV (encoded by parC and parE). High-level fluoroquinolone resistance in Gram-negative bacilli results stepwise from point mutations within the quinolone resistance-determining regions (QRDR), typically involving Ser83 or Asp87 in gyrA and Ser80 in parC, which decrease drug binding affinity.

Question 10Microbiology, resistance and antimicrobial pharmacology

A critically ill ICU patient with ventilator-associated pneumonia caused by extensively drug-resistant Acinetobacter baumannii is prescribed intravenous colistin (colistimethate sodium, CMS). Which physiological and pharmacological consideration is essential regarding colistin administration?

  1. CMS is an active drug that is rapidly inactivated by hepatic glucuronidation
  2. Colistin is eliminated solely via biliary excretion and requires no renal dose titration
  3. Neurotoxicity is mediated by selective uncoupling of mitochondrial oxidative phosphorylation
  4. CMS is an inactive prodrug that requires a loading dose and carries high risk of acute tubular necrosis Correct answer.

Explanation

Colistimethate sodium (CMS) is an inactive prodrug that must undergo slow, non-enzymatic spontaneous hydrolysis in vivo to release active colistin (polymyxin E). Because conversion is slow and renal excretion of unconverted CMS is substantial, an initial loading dose (e.g., 9 million IU or 300 mg colistin base activity) is required to achieve therapeutic plasma levels rapidly. Colistin binds anionic proximal tubular membranes, causing acute tubular necrosis and dose-dependent nephrotoxicity.

Question 11Microbiology, resistance and antimicrobial pharmacology

Which statement accurately describes the mechanism of action, antimicrobial spectrum, and activity profile of echinocandin antifungals (e.g., caspofungin, micafungin, anidulafungin)?

  1. Non-competitive inhibition of 1,3-beta-D-glucan synthase; fungicidal against Candida and fungistatic against Aspergillus Correct answer.
  2. Competitive inhibition of fungal lanosterol 14-alpha-demethylase; fungicidal against Aspergillus and Cryptococcus
  3. Binding to ergosterol causing membrane pore formation; fungicidal against Zygomycetes and Fusarium
  4. Inhibition of fungal DNA and RNA synthesis via toxic fluorouracil conversion; fungicidal against all molds

Explanation

Echinocandins inhibit 1,3-beta-D-glucan synthase (encoded by FKS1 and FKS2), disrupting fungal cell wall integrity. They exhibit rapid concentration-dependent fungicidal activity against most Candida species, but are fungistatic against Aspergillus species (targeting hyphal tips and branch points). They possess no meaningful activity against Cryptococcus neoformans, Mucorales, or Fusarium species due to low cell wall 1,3-beta-D-glucan content.

Question 12Microbiology, resistance and antimicrobial pharmacology

When comparing liposomal amphotericin B (L-AmB) to conventional amphotericin B deoxycholate (d-AmB), what is the primary pharmacokinetic and safety advantage demonstrated by the liposomal formulation?

  1. Complete absence of hepatic reticuloendothelial system uptake, resulting in 100% renal elimination
  2. Lower free drug delivery to the pulmonary alveoli, reducing acute transfusion-related lung injury
  3. Reduced nephrotoxicity and infusion reactions due to stable lipid bilayer encapsulation and targeted fungal delivery Correct answer.
  4. Enhanced oral bioavailability, enabling conversion to once-weekly enteral outpatient maintenance therapy

Explanation

Liposomal amphotericin B (L-AmB) incorporates amphotericin B into small, true unilamellar liposomes (diameter 60-80 nm). This formulation limits exposure of free drug to mammalian cholesterol membranes in renal proximal tubules, significantly reducing acute tubular toxicity, hypokalemia, and renal vasoconstriction compared with conventional amphotericin B deoxycholate. It also substantially attenuates systemic cytokine release, minimizing infusion-related fever, chills, and rigors.

Question 13Microbiology, resistance and antimicrobial pharmacology

A multidrug-resistant Candida auris isolate recovered from a central line bloodstream infection demonstrates elevated minimum inhibitory concentrations to fluconazole (>64 mcg/mL) and amphotericin B (2 mcg/mL), but remains susceptible to caspofungin (0.25 mcg/mL). Which molecular mechanism is most commonly responsible for echinocandin resistance when treatment failure subsequently develops during caspofungin monotherapy?

  1. Upregulation of the CDR1 ABC transporter family promoting echinocandin efflux
  2. Amplification of the ERG11 gene resulting in sterol 14-alpha-demethylase overexpression
  3. Loss-of-function nonsense mutations in the FUR1 cytosine deaminase gene
  4. Point mutations in hot spot 1 or 2 of the FKS1 gene encoding 1,3-beta-D-glucan synthase Correct answer.

Explanation

Echinocandin resistance in Candida species, including Candida auris, is mediated almost exclusively by amino acid substitutions in conserved 'hot spot' regions (HS1 and HS2) of the FKS1 gene (and FKS2 in C. glabrata). These mutations alter the catalytic subunit of 1,3-beta-D-glucan synthase, dramatically decreasing echinocandin binding affinity. Efflux pumps like CDR1 do not transport echinocandins.

Question 14Microbiology, resistance and antimicrobial pharmacology

Which pharmacokinetic/pharmacodynamic (PK/PD) index best correlates with the clinical bactericidal efficacy of beta-lactam antibiotics (e.g., penicillins, cephalosporins, carbapenems), and what administration strategy is used to optimize this parameter?

  1. Time above MIC (%T > MIC); optimized by prolonged or continuous intravenous infusions Correct answer.
  2. Peak concentration-to-MIC ratio (Cmax/MIC); optimized by rapid bolus infusions once daily
  3. Area under the curve-to-MIC ratio (AUC/MIC); optimized by single-dose ultra-high administration
  4. Post-antibiotic leukocyte enhancement index; optimized by alternating-day dosing schedules

Explanation

Beta-lactam antibiotics display time-dependent bactericidal killing. Their clinical and microbiological efficacy correlates with the percentage of time that free (unbound) drug concentrations remain above the pathogen's minimum inhibitory concentration (%fT > MIC) during the dosing interval. For cephalosporins, penicillins, and carbapenems, optimal targets are typically 50-70%, 50%, and 40% fT > MIC, respectively. Extended (3-4 hour) or continuous infusions maximize this index in critically ill patients.

Question 15Microbiology, resistance and antimicrobial pharmacology

A clinical pharmacist reviews therapeutic dosing strategies for a bacteremic patient infected with Pseudomonas aeruginosa. Which antibacterial agent relies predominantly on the peak-to-MIC ratio (Cmax/MIC) to drive bactericidal action and mitigate adaptive resistance?

  1. Cefepime administered via 4-hour extended intravenous infusion
  2. Amikacin administered as a single high-dose daily infusion Correct answer.
  3. Vancomycin administered via continuous intravenous infusion
  4. Linezolid administered every 12 hours via enteral suspension

Explanation

Amikacin, like other aminoglycosides, exhibits concentration-dependent bactericidal killing. Its pharmacodynamic efficacy is maximized by optimizing the Cmax/MIC ratio (target >= 8-10). Delivering the total daily dose in a single extended-interval infusion achieves a high initial peak concentration that drives rapid bactericidal clearance and overcomes adaptive resistance, while permitting prolonged trough clearance that prevents accumulation and reduces nephrotoxicity.

Question 16Microbiology, resistance and antimicrobial pharmacology

According to the CDC and IDSA core elements of hospital antimicrobial stewardship programs, which two proactive interventions represent the foundational active core strategies recommended to optimize antimicrobial use?

  1. Prospective audit and feedback AND formulary preauthorization Correct answer.
  2. Passive education campaigns AND antibiotic cycling policies
  3. Automatic 72-hour pharmacy stop orders AND post-discharge surveillance
  4. Mandatory infectious disease consultation for all IV prescriptions AND restricted access

Explanation

The CDC and IDSA guidelines identify prospective audit and feedback (PAF) and formulary restriction with preauthorization as the two primary, evidence-based proactive stewardship strategies. PAF engages prescribers post-prescription to optimize selection, dosing, and duration without impeding initial therapy, while preauthorization regulates access to restricted broad-spectrum agents prior to dispensing. Passive education alone or antibiotic cycling lacks sustained efficacy.

Question 17Microbiology, resistance and antimicrobial pharmacology

A patient with refractory osteomyelitis is receiving long-term linezolid therapy (600 mg PO BID). At week 4, the patient develops progressive paresthesias in a stocking-glove distribution, blurred vision, and a platelet count drop to 45,000/mcL. What is the fundamental cellular mechanism underlying these cumulative toxicities?

  1. Inhibition of mammalian cytoplasmic 80S ribosomes disrupting ubiquitin turnover
  2. Autoimmune destruction of megakaryocytes mediated by drug-dependent IgG antibodies
  3. Direct alkylation of mitochondrial DNA disrupting nuclear membrane synthesis
  4. Inhibition of human mitochondrial 55S ribosomes due to structural homology with bacterial 70S ribosomes Correct answer.

Explanation

Linezolid binds to the domain V region of the 23S rRNA within the bacterial 50S ribosomal subunit. Human mitochondrial ribosomes (55S) share substantial evolutionary sequence and structural homology with bacterial 70S/50S ribosomes. Prolonged linezolid therapy (>2-4 weeks) inhibits mitochondrial protein synthesis, leading to mitochondrial respiratory chain impairment that manifests as myelosuppression (thrombocytopenia, anemia), peripheral neuropathy, optic neuropathy, and lactic acidosis.

Question 18Microbiology, resistance and antimicrobial pharmacology

Which antimicrobial agent lacks reliable clinical activity against Bacteroides fragilis and therefore requires the addition of metronidazole when treating intra-abdominal polymicrobial infections?

  1. Ceftriaxone Correct answer.
  2. Piperacillin-tazobactam
  3. Meropenem
  4. Ertapenem

Explanation

Third-generation cephalosporins such as ceftriaxone and ceftazidime possess excellent activity against Gram-negative Enterobacterales and streptococci, but exhibit poor and unreliable activity against intra-abdominal obligate anaerobes, specifically Bacteroides fragilis group organisms. When ceftriaxone is used for community-acquired intra-abdominal infections, it must always be combined with an antianaerobic agent such as metronidazole. Piperacillin-tazobactam and carbapenems possess inherent potent anaerobic coverage.

Question 19Healthcare infection, prevention and diagnostic stewardship

According to IDSA/ATS guidelines, which clinical parameter defines hospital-acquired pneumonia (HAP) and distinguishes it from community-acquired pneumonia?

  1. Development of pulmonary infiltrates within 12 hours of admission to an acute care facility
  2. Pneumonia developing 48 hours or more after hospital admission, not incubating at admission Correct answer.
  3. Pneumonia occurring in any patient who has visited an outpatient clinic in the preceding 30 days
  4. New respiratory symptoms developing within 24 hours following endotracheal extubation

Explanation

Hospital-acquired pneumonia (HAP) is formally defined as pneumonia that occurs 48 hours or more after admission and was not incubating at the time of admission. Ventilator-associated pneumonia (VAP) is defined as pneumonia occurring more than 48 hours after endotracheal intubation. The previous category of 'healthcare-associated pneumonia' (HCAP) was retired in the 2016 IDSA/ATS guidelines.

Question 20Healthcare infection, prevention and diagnostic stewardship

An 80-year-old bedbound female with a chronic Foley catheter for neurogenic bladder has a routine urinalysis showing 50 WBC/HPF and a urine culture growing >10^5 CFU/mL Klebsiella pneumoniae. She is completely afebrile, hemodynamically stable, has no suprapubic tenderness, and eats normally. What is the recommended management?

  1. Prescribe oral ciprofloxacin for 7 days to clear the colonization
  2. Withhold antimicrobial therapy; neither treatment nor routine screening is indicated Correct answer.
  3. Administer intravesical gentamicin bladder irrigation for 5 consecutive days
  4. Replace the catheter and initiate 14 days of intravenous ertapenem

Explanation

This patient has asymptomatic bacteriuria (ASB), which occurs in virtually 100% of chronically catheterized patients within 30 days. High-quality evidence and IDSA guidelines strongly advise against screening for or treating ASB in catheterized patients, as antimicrobial therapy does not prevent symptomatic UTI, does not improve survival, and directly promotes multidrug-resistant organisms and Clostridioides difficile infection. The only clear indications to treat ASB are pregnancy and prior to invasive urological procedures with mucosal bleeding.

Question 21Healthcare infection, prevention and diagnostic stewardship

A patient with a non-tunneled internal jugular central venous catheter develops rigors and hypotension. Blood cultures drawn from both the line and periphery grow Staphylococcus aureus. What is the mandatory immediate management regarding the central line?

  1. Prompt catheter removal combined with targeted intravenous antistaphylococcal therapy Correct answer.
  2. Retention of the catheter with instillation of high-dose vancomycin lock therapy
  3. Exchange of the catheter over a guidewire under sterile barrier precautions
  4. Preservation of the line while administering oral linezolid for 7 days

Explanation

According to IDSA guidelines, central catheter removal is mandatory and must be performed promptly for catheter-related bloodstream infections caused by Staphylococcus aureus, Pseudomonas aeruginosa, Candida species, or multidrug-resistant organisms, as well as in patients with severe sepsis, hemodynamic instability, suppurative thrombophlebitis, or persistent bacteremia >72 hours. S. aureus lines cannot be salvaged due to high rates of metastatic seeding, endocarditis, and treatment failure.

Question 22Healthcare infection, prevention and diagnostic stewardship

In which clinical scenario is antibiotic lock therapy (ALT) combined with systemic antimicrobial therapy considered acceptable to salvage a long-term central venous catheter?

  1. A patient with Candida albicans candidemia and a permanent cuffed hemodialysis catheter
  2. An uncomplicated CRBSI due to coagulase-negative staphylococci in a patient with limited venous access Correct answer.
  3. A bacteremic patient with Pseudomonas aeruginosa and severe septic shock requiring vasopressors
  4. A patient with a persistent tunnel tract abscess and S. aureus bacteremia

Explanation

Catheter salvage using antibiotic lock therapy (ALT) instillation along with systemic antibiotics for 10-14 days is acceptable only for uncomplicated CRBSI involving long-term tunneled catheters or implanted ports when the pathogen is coagulase-negative staphylococci (CoNS) or enterococci, in patients with critical or exhausted venous access. It is strictly contraindicated in infections due to S. aureus, Candida spp., Pseudomonas, tunnel/pocket infections, endocarditis, or septic shock.

Question 23Healthcare infection, prevention and diagnostic stewardship

A 68-year-old male develops profuse watery diarrhea and abdominal cramps 5 days after completing a course of clindamycin. Stool nucleic acid amplification testing is positive for Clostridioides difficile. His WBC count is 11,500/mcL and serum creatinine is 1.0 mg/dL (baseline 0.9 mg/dL). According to updated IDSA/SHEA guidelines, which initial therapeutic regimen is preferred?

  1. Oral fidaxomicin 200 mg twice daily for 10 days (or oral vancomycin 125 mg four times daily for 10 days) Correct answer.
  2. Oral metronidazole 500 mg three times daily for 14 days as first-line monotherapy
  3. Intravenous vancomycin 1000 mg twice daily combined with intravenous metronidazole
  4. Immediate fecal microbiota transplantation delivered via colonoscopy

Explanation

The updated IDSA/SHEA guidelines recommend oral fidaxomicin (200 mg BID for 10 days) as the preferred first-line agent for an initial episode of non-severe C. difficile infection, with oral vancomycin (125 mg QID for 10 days) as an acceptable alternative. Fidaxomicin provides comparable cure rates with significantly lower recurrence rates due to minimal disruption of normal colonic microbiota. Oral metronidazole is no longer recommended as first-line therapy.

Question 24Healthcare infection, prevention and diagnostic stewardship

A 74-year-old inpatient with C. difficile infection develops marked abdominal distension, severe tenderness, a WBC count of 38,000/mcL, blood pressure of 80/50 mmHg requiring norepinephrine, and a cecal diameter of 8.5 cm on CT imaging. What is the recommended antimicrobial regimen and urgent specialty consultation for this fulminant presentation?

  1. Oral fidaxomicin monotherapy 200 mg twice daily and discharge home on hospice care
  2. High-dose oral vancomycin (500 mg QID) plus intravenous metronidazole (500 mg TID) +/- rectal vancomycin enemas, and emergency surgical consult Correct answer.
  3. Intravenous daptomycin 10 mg/kg daily plus oral rifaximin 550 mg TID, with observation in a general ward
  4. Single dose of intravenous bezlotoxumab with discontinuation of all enteral antimicrobial therapy

Explanation

Fulminant C. difficile infection is characterized by hypotension, shock, ileus, or toxic megacolon (cecal diameter >6 cm). Treatment requires high-dose oral vancomycin (500 mg four times daily via nasogastric tube) combined with intravenous metronidazole (500 mg every 8 hours) to ensure drug delivery despite impaired intestinal transit. If ileus is present, rectal vancomycin retention enemas (500 mg in 100 mL saline every 6 hours) should be added. Immediate surgical consultation is mandatory for possible subtotal colectomy or diverting loop ileostomy.

Question 25Healthcare infection, prevention and diagnostic stewardship

A college student is admitted to the emergency department with fever, petechial rash, and nuchal rigidity. Lumbar puncture reveals Gram-negative diplococci. Which infection control transmission-based precaution must be immediately implemented?

  1. Contact Precautions with gown and gloves for all room entries
  2. Droplet Precautions with a surgical mask worn when within 1 to 2 meters of the patient Correct answer.
  3. Airborne Precautions requiring a negative-pressure isolation room and PAPR unit
  4. Protective Environment isolation with positive-pressure HEPA filtration

Explanation

Neisseria meningitidis is transmitted via large respiratory droplets generated during coughing, sneezing, or close contact. Droplet Precautions require healthcare personnel to wear a surgical mask upon entry or when working within 1-2 meters (3-6 feet) of the patient. Droplet Precautions must be maintained until the patient has completed 24 hours of effective targeted antimicrobial therapy (e.g., ceftriaxone).

Question 26Healthcare infection, prevention and diagnostic stewardship

Which patient room requires the routine use of Contact Precautions (dedicated gown and gloves donned upon entry and doffed upon exit) to prevent horizontal transmission within the hospital?

  1. A patient with uncomplicated community-acquired Streptococcus pneumoniae lobar pneumonia
  2. A patient with acute hepatitis C viral infection without active bleeding
  3. A patient colonized or infected with Carbapenem-Resistant Enterobacterales (CRE) Correct answer.
  4. A patient presenting with acute Epstein-Barr virus infectious mononucleosis

Explanation

Contact Precautions are indicated to prevent horizontal nosocomial transmission of epidemiologically important multidrug-resistant organisms (MDROs), such as Carbapenem-Resistant Enterobacterales (CRE), vancomycin-resistant enterococci (VRE), and extensively drug-resistant Acinetobacter or Pseudomonas. Healthcare personnel must don a clean gown and gloves upon entering the patient's room and remove them prior to exiting, followed by immediate hand hygiene.

Question 27Healthcare infection, prevention and diagnostic stewardship

What is the foundational first step an infection prevention and control team must execute when initiating a formal hospital outbreak investigation?

  1. Immediately close the implicated surgical intensive care unit to all new admissions
  2. Confirm the existence of an outbreak by verifying that the observed case count exceeds the baseline rate Correct answer.
  3. Perform whole-genome sequencing on all environmental surfaces across the hospital
  4. Administer mass prophylactic antibiotics to all healthcare personnel in the hospital

Explanation

The established CDC/WHO 10-step outbreak investigation framework dictates that investigators must first confirm the diagnosis and verify the existence of an outbreak (i.e., establish that the observed number of cases statistically exceeds the expected baseline occurrence for that population and time frame). Premature unit closures or mass antimicrobial prophylaxis before establishing baseline verification and a case definition are contraindicated.

Question 28Community syndromes, endocarditis and sepsis

A 70-year-old male presents with cough, fever, and purulent sputum. On examination, he is alert and oriented (score 0). Vital signs show BP 115/75 mmHg, pulse 92 bpm, respiratory rate 32 breaths/min, and temperature 38.6°C. Laboratory testing reveals blood urea nitrogen 28 mg/dL (10 mmol/L). What is his CURB-65 score, and what is the recommended management setting?

  1. CURB-65 score 1; safe for outpatient oral therapy with home follow-up
  2. CURB-65 score 4; requires emergency tracheal intubation and intensive care unit transfer
  3. CURB-65 score 3; requires hospital admission and consideration of intensive care monitoring Correct answer.
  4. CURB-65 score 0; discharge home with 3 days of oral amoxicillin

Explanation

CURB-65 awards 1 point each for: Confusion (0), Urea >7 mmol/L (1 point, value 10), Respiratory rate >=30 (1 point, value 32), Blood pressure SBP <90 or DBP <=60 (0 points), and Age >=65 (1 point, age 70). This patient has a score of 3. CURB-65 score 3 is associated with a 30-day mortality of 14-17% and mandates urgent inpatient hospitalization with evaluation for intensive care unit placement.

Question 29Community syndromes, endocarditis and sepsis

A 58-year-old previously healthy woman is admitted to the general medical ward with non-severe community-acquired pneumonia (CURB-65 score 2). She has no risk factors for MRSA or Pseudomonas aeruginosa. According to IDSA/ATS guidelines, which antimicrobial regimen represents standard first-line empirical coverage?

  1. Ciprofloxacin monotherapy 400 mg IV every 12 hours
  2. Vancomycin plus cefepime plus tobramycin triple therapy
  3. Oral azithromycin monotherapy 500 mg daily
  4. Intravenous ceftriaxone combined with oral azithromycin (or respiratory fluoroquinolone monotherapy) Correct answer.

Explanation

For hospitalized adults with non-severe community-acquired pneumonia without risk factors for MRSA or P. aeruginosa, IDSA/ATS guidelines recommend either: (1) combination therapy with a beta-lactam (ampicillin-sulbactam 1.5-3 g q6h, ceftriaxone 1-2 g q24h, or cefotaxime 1-2 g q8h) plus a macrolide (azithromycin 500 mg daily), OR (2) monotherapy with a respiratory fluoroquinolone (levofloxacin 750 mg daily or moxifloxacin 400 mg daily).

Question 30Community syndromes, endocarditis and sepsis

A 52-year-old hotel maintenance engineer presents with severe community-acquired pneumonia, high-grade fever, watery diarrhea, confusion, and right lower lobe consolidation. Laboratory testing reveals serum sodium of 124 mEq/L and AST 142 U/L. What is the most rapid diagnostic test and drug of choice for this suspected infection?

  1. Serum cold agglutinin titer; intravenous penicillin G
  2. Nasopharyngeal viral swab; oral oseltamivir
  3. Sputum silver methenamine stain; oral trimethoprim-sulfamethoxazole
  4. Urinary antigen test; intravenous levofloxacin or azithromycin Correct answer.

Explanation

The Legionella urinary antigen test commonly detects Legionella pneumophila serogroup 1. A positive result in the stated pneumonia supports directed therapy; a negative result does not exclude other Legionella species or serogroups.

Question 31Community syndromes, endocarditis and sepsis

A 19-year-old university student presents with a 2-week history of dry nagging cough, low-grade fever, headache, and fatigue. Chest X-ray reveals bilateral reticulonodular infiltrates out of proportion to physical exam findings. Laboratory studies reveal cold agglutinin titers of 1:128 and Coombs-positive autoimmune hemolytic anemia. Why are all beta-lactam antibiotics inherently ineffective against this causative organism?

  1. Mycoplasma pneumoniae completely lacks a peptidoglycan cell wall Correct answer.
  2. The pathogen secretes constitutive chromosomal Metallo-beta-lactamases
  3. Bacterial porin closure completely prevents beta-lactam periplasmic diffusion
  4. The organism actively metabolizes penicillins into an essential carbon energy source

Explanation

Mycoplasma pneumoniae is a wall-less bacterium bounded only by a single cholesterol-containing cell membrane. Because it lacks a peptidoglycan cell wall and penicillin-binding proteins (PBPs), it is intrinsically resistant to all beta-lactam antibiotics (penicillins, cephalosporins, carbapenems) and glycopeptides. It is treated with ribosomal inhibitors (macrolides, doxycycline) or fluoroquinolones.

Question 32Community syndromes, endocarditis and sepsis

A 45-year-old male with native aortic valve endocarditis caused by methicillin-susceptible Staphylococcus aureus (MSSA) has been receiving IV cefazolin for 6 days. Today, he develops acute pulmonary edema, a new loud diastolic murmur, and refractory cardiogenic shock requiring inotropic support. Transesophageal echocardiography reveals severe aortic regurgitation with a 14 mm flail vegetation and early mitral valve closure. What is the immediate definitive management?

  1. Add intravenous rifampin and gentamicin to cefazolin and observe for 48 hours
  2. Switch cefazolin to high-dose daptomycin combined with ceftaroline
  3. Perform emergency percutaneous balloon aortic valvuloplasty
  4. Emergent surgical aortic valve replacement/repair within 24 hours Correct answer.

Explanation

According to AHA/ACC and ESC guidelines, emergency surgery (within 24 hours) is indicated for infective endocarditis complicated by acute severe aortic or mitral regurgitation causing refractory pulmonary edema, cardiogenic shock, or severe hemodynamic compromise. Early mitral valve closure on echocardiography signals critically elevated left ventricular end-diastolic pressure that will rapidly culminate in cardiac arrest without emergency surgical intervention.

Question 33Community syndromes, endocarditis and sepsis

A 28-year-old previously healthy male presents with severe headache, fever of 39.5°C, vomiting, and photophobia. On examination, Kernig and Brudzinski signs are positive. Lumbar puncture reveals cloudy cerebrospinal fluid. Which CSF profile is characteristic of acute community-acquired bacterial meningitis?

  1. Opening pressure 120 mmH2O; WBC 25/uL (95% lymphocytes); protein 35 mg/dL; CSF:plasma glucose 0.7
  2. Opening pressure 150 mmH2O; WBC 150/uL (80% monocytes); protein 80 mg/dL; CSF:plasma glucose 0.55
  3. Opening pressure 320 mmH2O; WBC 3,500/uL (92% neutrophils); protein 280 mg/dL; CSF:plasma glucose 0.18 Correct answer.
  4. Opening pressure 80 mmH2O; WBC 2/uL; protein 25 mg/dL; CSF:plasma glucose 0.65

Explanation

Acute community-acquired bacterial meningitis characteristically produces marked CSF abnormalities: elevated opening pressure (>200-300 mmH2O), marked neutrophilic pleocytosis (typically 1,000 to >5,000 WBC/uL with >80% polymorphonuclear cells), substantially elevated protein (>100-500 mg/dL), and severely depressed CSF glucose with a CSF-to-serum glucose ratio <0.4 (often <0.2 or absolute CSF glucose <40 mg/dL).

Question 34Community syndromes, endocarditis and sepsis

A 68-year-old woman with type 2 diabetes mellitus is admitted with acute bacterial meningitis. What constitutes the evidence-based empirical antimicrobial and adjunctive regimen for this patient, and why is ampicillin specifically included?

  1. Vancomycin plus piperacillin-tazobactam; ampicillin is added to prevent candidal superinfection
  2. Ceftriaxone plus ciprofloxacin; ampicillin is added to provide synergy against Streptococcus pneumoniae
  3. Meropenem plus tobramycin plus flucytosine; ampicillin is added to eradicate Pseudomonas aeruginosa
  4. Ceftriaxone plus vancomycin plus ampicillin plus dexamethasone; ampicillin is required to cover Listeria monocytogenes Correct answer.

Explanation

In adults aged >=50 years, neonates, and immunocompromised hosts, Listeria monocytogenes is an important cause of bacterial meningitis. Cephalosporins (including ceftriaxone and cefotaxime) have zero activity against Listeria due to penicillin-binding protein non-affinity. Therefore, IV ampicillin (2 g q4h) must be added to vancomycin and ceftriaxone. Dexamethasone (10 mg IV) is administered prior to or with the first antimicrobial dose to reduce neurological morbidity and mortality from pneumococcal meningitis.

Question 35Community syndromes, endocarditis and sepsis

A 36-year-old previously healthy accountant is brought to the emergency department with acute onset of fever, behavioral changes, olfactory hallucinations, and expressive aphasia. Brain MRI demonstrates asymmetric hyperintensity localized to the left temporal and orbitofrontal lobes. What is the diagnostic test of choice and the immediate therapeutic intervention?

  1. Serum anti-NMDA receptor antibody titer; high-dose methylprednisolone
  2. Cerebrospinal fluid HSV PCR; immediate initiation of intravenous acyclovir (10 mg/kg every 8 hours) Correct answer.
  3. Cryptococcal antigen latex agglutination; intravenous liposomal amphotericin B
  4. Brain biopsy of the temporal lobe; observation without empiric antiviral therapy

Explanation

The clinical presentation of fever, frontotemporal focal neurological deficits (aphasia, personality change, olfactory hallucinations), and asymmetric temporal lobe abnormalities on MRI is classic for Herpes Simplex Virus type 1 (HSV-1) encephalitis. CSF HSV-1/2 PCR is the diagnostic gold standard (>95% sensitivity and >98% specificity). Empiric IV acyclovir (10 mg/kg q8h) must be initiated immediately upon suspicion, as mortality rises steeply with every hour of delay.

Question 36Community syndromes, endocarditis and sepsis

According to IDSA skin and soft tissue infection guidelines, how should clinicians distinguish between non-purulent and purulent skin infections, and what is the primary management difference?

  1. Purulent infections are caused by Pseudomonas and require colistin; non-purulent require oral ciprofloxacin
  2. Purulent infections only occur in burn patients; non-purulent infections require emergent hyperbaric oxygen therapy
  3. Non-purulent infections (cellulitis/erysipelas) are driven by beta-hemolytic streptococci and managed with beta-lactams; purulent infections (abscesses) are driven by S. aureus and require drainage Correct answer.
  4. Both forms require immediate wide surgical fasciotomy and lifelong oral suppressive doxycycline

Explanation

A nonpurulent cellulitis pattern commonly supports streptococcal coverage, whereas an abscess requires assessment for drainage and staphylococcal infection. The need for additional antibiotics depends on severity, host factors and associated findings.

Question 37Community syndromes, endocarditis and sepsis

According to the Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3), what is the definitive modern definition of sepsis?

  1. Life-threatening organ dysfunction caused by a dysregulated host response to infection, represented by an acute increase in SOFA score >=2 Correct answer.
  2. The presence of systemic inflammatory response syndrome (SIRS) criteria combined with a confirmed positive blood culture
  3. A confirmed bacteremia accompanied by transient fever >38.5°C and leukocytosis without end-organ dysfunction
  4. Severe refractory hypotension requiring epinephrine infusion regardless of underlying infectious etiology

Explanation

The Sepsis-3 consensus defines sepsis as life-threatening organ dysfunction caused by a dysregulated host response to infection. Clinically, organ dysfunction is operationalized as an acute increase in the total Sequential Organ Failure Assessment (SOFA) score of >=2 points attributable to the infection. Sepsis-3 explicitly abandoned the older SIRS criteria due to poor specificity and lack of independent correlation with mortality.

Question 38Community syndromes, endocarditis and sepsis

Under the Sepsis-3 consensus framework, which combination of clinical and laboratory criteria defines septic shock?

  1. Systolic blood pressure <90 mmHg responding promptly to 500 mL of normal saline
  2. Persisting hypotension requiring vasopressors to maintain MAP >=65 mmHg AND serum lactate >2 mmol/L despite adequate fluid resuscitation Correct answer.
  3. Elevated serum procalcitonin >5 ng/mL combined with positive urine cultures in an afebrile patient
  4. Oliguria <0.5 mL/kg/h for 2 hours with normal blood pressure and serum lactate 1.2 mmol/L

Explanation

Under Sepsis-3, septic shock is defined as a subset of sepsis in which underlying circulatory and cellular/metabolic abnormalities are profound enough to substantially increase mortality (>40%). It is clinically identified by persisting hypotension requiring vasopressors to maintain mean arterial pressure (MAP) >=65 mmHg AND a serum lactate level >2 mmol/L (18 mg/dL) despite adequate volume resuscitation.

Question 39Community syndromes, endocarditis and sepsis

What is the precise immunological mechanism by which bacterial superantigens (e.g., Toxic Shock Syndrome Toxin-1 [TSST-1] of Staphylococcus aureus and SpeA of Streptococcus pyogenes) induce life-threatening toxic shock syndrome?

  1. Direct enzymatic cleavage of circulating complement factor C3 into active anaphylatoxins C3a and C3b
  2. Unprocessed cross-linking of MHC class II molecules to specific Vbeta regions of T-cell receptors, activating up to 20% of all circulating T cells Correct answer.
  3. Intracellular phosphorylation of nuclear factor kappa B causing pure B-cell apoptosis without T-cell engagement
  4. Selective destruction of natural killer cell perforin granules resulting in acute hemophagocytosis

Explanation

Conventional peptide antigens are processed by antigen-presenting cells (APCs) and presented within the peptide-binding groove of MHC class II molecules, stimulating ~0.01% of naive T cells. In contrast, superantigens bypass conventional intracellular processing and directly cross-link the invariant outer aspect of MHC class II molecules to variable beta (Vbeta) domains of T-cell receptors. This non-specific bridge activates up to 20-30% of all circulating T lymphocytes simultaneously, triggering a massive cytokine storm (IL-1, IL-2, IL-6, TNF-alpha, IFN-gamma) causing capillary leak, shock, and multiorgan failure.

Question 40HIV, opportunistic and transplant infection

According to current IAS-USA and DHHS antiretroviral guidelines, what constitutes the preferred initial antiretroviral therapy (ART) regimen for most treatment-naive adults living with HIV?

  1. Two non-nucleoside reverse transcriptase inhibitors (NNRTIs) plus one boosted protease inhibitor
  2. Zidovudine plus lamivudine plus nevirapine
  3. A CCR5 antagonist combined with an entry inhibitor and an unboosted protease inhibitor
  4. A second-generation integrase strand transfer inhibitor (e.g., bictegravir or dolutegravir) plus two NRTIs Correct answer.

Explanation

Current guidelines recommend a second-generation integrase strand transfer inhibitor (INSTI)-specifically bictegravir (BIC) or dolutegravir (DTG)-combined with a two-nucleoside reverse transcriptase inhibitor (NRTI) backbone (tenofovir alafenamide [TAF] or tenofovir disoproxil fumarate [TDF] plus emtricitabine [FTC], or abacavir [ABC] plus lamivudine [3TC] if HLA-B*5701 negative). Second-generation INSTIs offer rapid viral suppression, minimal drug interactions, and an exceptionally high genetic barrier to resistance.

Question 41HIV, opportunistic and transplant infection

What is the primary pharmacological and safety distinction between tenofovir alafenamide (TAF) and tenofovir disoproxil fumarate (TDF) in HIV clinical practice?

  1. TAF concentrates intracellularly in PBMCs with 90% lower plasma levels, reducing renal tubulopathy and bone loss compared to TDF Correct answer.
  2. TDF is an active triphosphate drug requiring no intracellular processing, whereas TAF is entirely metabolized in saliva
  3. TAF is associated with severe acute pancreatitis and optic neuropathy, whereas TDF has no documented toxicities
  4. TDF cannot be co-formulated with emtricitabine, whereas TAF can only be administered as monotherapy

Explanation

Both TDF and TAF are prodrugs of tenofovir. However, TAF is more stable in plasma and is selectively hydrolyzed to tenofovir inside lymphatic cells and peripheral blood mononuclear cells (PBMCs) by cathepsin A. Consequently, TAF delivers higher antiviral active intracellular metabolite concentrations at one-tenth of the systemic dose, resulting in >90% lower circulating plasma tenofovir levels. This significantly diminishes tenofovir exposure to renal proximal tubules and bone, markedly lowering rates of Fanconi syndrome, acute kidney injury, and bone mineral density loss.

Question 42HIV, opportunistic and transplant infection

A 32-year-old male with newly diagnosed HIV presents with progressive exertional dyspnea, dry cough, and low-grade fever for 3 weeks. His CD4 count is 85 cells/uL. Chest radiography shows diffuse bilateral symmetric perihilar interstitial infiltrates, and serum lactate dehydrogenase (LDH) is 480 U/L. What is the definitive first-line therapeutic agent and primary prophylactic regimen for this condition?

  1. Oral fluconazole 400 mg daily for 14 days
  2. Trimethoprim-sulfamethoxazole (TMP-SMX) orally or intravenously Correct answer.
  3. Inhaled pentamidine 300 mg once every 6 months
  4. Intravenous ganciclovir combined with oral foscarnet

Explanation

The clinical syndrome of subacute exertional dyspnea, dry cough, hypoxemia, elevated serum LDH, and bilateral perihilar interstitial infiltrates in an HIV patient with CD4 <200 cells/uL is pathognomonic for Pneumocystis jirovecii pneumonia (PCP). High-dose trimethoprim-sulfamethoxazole (TMP-SMX, 15-20 mg/kg/day of TMP component divided q6-8h for 21 days) is the undisputed first-line treatment. TMP-SMX (one double-strength tablet daily or three times weekly) is also the drug of choice for primary PCP prophylaxis.

Question 43HIV, opportunistic and transplant infection

A 40-year-old HIV-infected patient is admitted with severe Pneumocystis jirovecii pneumonia. Room air arterial blood gas reveals pH 7.44, PaCO2 32 mmHg, and PaO2 58 mmHg. Which adjunctive medication must be initiated prior to or concurrently with antimicrobial therapy to decrease mortality?

  1. Inhaled hypertonic saline three times daily
  2. Intravenous recombinant human granulocyte colony-stimulating factor (G-CSF)
  3. Systemic corticosteroids (intravenous methylprednisolone or oral prednisone) Correct answer.
  4. High-dose intravenous vitamin C and N-acetylcysteine infusions

Explanation

In moderate-to-severe PCP, adjunctive systemic corticosteroids significantly reduce the incidence of respiratory failure, need for mechanical ventilation, and mortality. Corticosteroids are indicated when the room air arterial PaO2 is <70 mmHg or the alveolar-arterial oxygen gradient (A-a DO2) is >=35 mmHg. Steroids (prednisone 40 mg PO BID tapered over 21 days, or IV methylprednisolone) must be started before or at the time of initiating antimicrobials to attenuate the inflammatory lysis of fungal organisms.

Question 44HIV, opportunistic and transplant infection

A 29-year-old patient with AIDS (CD4 count 28 cells/uL) presents with severe headache, nausea, and photophobia. Lumbar puncture demonstrates an opening pressure of 340 mmH2O. CSF India ink preparation shows encapsulated budding yeast cells, and CSF cryptococcal antigen (CrAg) titer is >1:1024. What is the evidence-based induction antifungal regimen and essential procedural management to control intracranial pressure?

  1. Oral fluconazole 200 mg daily; placement of an immediate permanent ventriculoperitoneal shunt
  2. Intravenous micafungin 100 mg daily; administration of intravenous mannitol and furosemide
  3. Intravenous voriconazole 6 mg/kg BID; immediate lumbar drain placement with prophylactic antibiotics
  4. Liposomal amphotericin B plus flucytosine (or AMBITION-cm single-dose liposomal amphotericin B with flucytosine/fluconazole); therapeutic serial lumbar punctures for opening pressure >200 mmH2O Correct answer.

Explanation

For cryptococcal meningitis in HIV-infected patients, WHO and IDSA guidelines recommend induction therapy with liposomal amphotericin B (3-4 mg/kg/day) plus oral flucytosine (100 mg/kg/day divided q6h) for 2 weeks, or the AMBITION-cm regimen (a single high dose of liposomal amphotericin B 10 mg/kg on day 1 combined with 14 days of flucytosine and fluconazole). Elevated intracranial pressure (opening pressure >200 mmH2O) must be aggressively managed with daily therapeutic lumbar punctures removing 20-30 mL until pressure normalizes; failure to relieve ICP is a major cause of death.

Question 45HIV, opportunistic and transplant infection

A 44-year-old male with untreated HIV and a CD4 count of 22 cells/uL presents with drenching night sweats, persistent fever, 10 kg weight loss, chronic diarrhea, and severe abdominal pain. Laboratory testing reveals an alkaline phosphatase of 450 U/L and severe anemia. Blood mycobacterial cultures grow acid-fast bacilli identified as Mycobacterium avium complex (MAC). What is the preferred initial antimicrobial regimen for disseminated MAC disease?

  1. Clarithromycin (or azithromycin) plus ethambutol (with or without rifabutin) Correct answer.
  2. Standard four-drug antituberculous therapy (isoniazid, rifampin, pyrazinamide, ethambutol)
  3. Intravenous vancomycin plus intravenous piperacillin-tazobactam for 6 weeks
  4. Oral trimethoprim-sulfamethoxazole monotherapy 1 double-strength tablet daily

Explanation

Disseminated Mycobacterium avium complex (MAC) occurs primarily in patients with CD4 counts <50 cells/uL. Preferred treatment requires a combination of at least two antimycobacterial drugs to prevent the emergence of macrolide resistance: a macrolide (clarithromycin 500 mg BID preferred, or azithromycin 500-600 mg daily) PLUS ethambutol (15 mg/kg daily). A third agent, such as rifabutin (300 mg daily), is added in patients with advanced immunosuppression (CD4 <50) or high mycobacterial burden.

Question 46HIV, opportunistic and transplant infection

A 48-year-old patient with AIDS (CD4 count 18 cells/uL) complains of blurred vision, painless floaters, and progressive loss of peripheral visual fields in the right eye. Dilated funduscopic examination reveals extensive perivascular yellow-white retinal opacifications with full-thickness retinal necrosis and brushfire hemorrhages ('pizza pie' retinopathy). What is the treatment of choice?

  1. Intravenous amphotericin B plus oral flucytosine
  2. Oral valganciclovir (or intravenous ganciclovir) Correct answer.
  3. Topical trifluridine eye drops combined with oral acyclovir
  4. Intravitreal pegaptanib sodium injection

Explanation

The clinical presentation of painless visual loss, floaters, and classic 'pizza pie' or 'ketchup and cottage cheese' funduscopic findings (retinal hemorrhages adjacent to yellow-white necrotic exudates along retinal vessels) in an AIDS patient with CD4 <50 cells/uL is diagnostic of Cytomegalovirus (CMV) retinitis. Oral valganciclovir (900 mg BID for 21 days for induction, followed by 900 mg daily maintenance) or IV ganciclovir (5 mg/kg q12h) is the treatment of choice, with intravitreal ganciclovir/foscarnet injections added for sight-threatening lesions near the fovea or optic nerve.

Question 47HIV, opportunistic and transplant infection

According to IDSA guidelines, what clinical temperature threshold and absolute neutrophil count (ANC) parameters formally establish the diagnosis of febrile neutropenia in a patient undergoing cancer chemotherapy?

  1. Any temperature >=37.5°C sustained over 24 hours with an ANC <1,500 cells/uL
  2. Axillary temperature >=38.0°C with an ANC <1,000 cells/uL persisting for >48 hours
  3. Rectal temperature >=38.5°C regardless of ANC in an inpatient oncology ward
  4. A single oral temperature >=38.3°C (or >=38.0°C sustained over 1 hour) with an ANC <500 cells/uL (or expected to fall <500 cells/uL within 48 hours) Correct answer.

Explanation

The IDSA and ASCO guidelines define fever in neutropenic patients as a single oral temperature measurement >=38.3°C (101°F) or a sustained temperature >=38.0°C (100.4°F) over a 1-hour period. Neutropenia is defined as an absolute neutrophil count (ANC) <500 cells/uL, or an ANC <1,000 cells/uL with a predicted nadir <=500 cells/uL within the subsequent 48 hours. Rectal temperatures are strictly contraindicated in neutropenic patients due to the risk of mucosal trauma and bacteremic seeding.

Question 48HIV, opportunistic and transplant infection

A 52-year-old male with acute myeloid leukemia undergoing induction chemotherapy develops high-risk febrile neutropenia (ANC 80 cells/uL, temperature 39.0°C). He is hemodynamically stable without a central catheter-site infection. According to IDSA guidelines, what constitutes appropriate initial empirical antibacterial therapy?

  1. Vancomycin plus gentamicin plus metronidazole triple therapy
  2. Monotherapy with an intravenous antipseudomonal beta-lactam (cefepime, piperacillin-tazobactam, or meropenem) Correct answer.
  3. Oral ciprofloxacin plus oral amoxicillin-clavulanate administered in the outpatient clinic
  4. Intravenous colistin combined with aerosolized amikacin

Explanation

IDSA guidelines strongly recommend urgent monotherapy with an antipseudomonal beta-lactam (cefepime 2 g q8h, piperacillin-tazobactam 4.5 g q6h, or meropenem 1 g q8h) as standard empiric therapy for high-risk febrile neutropenia. The routine addition of vancomycin or an aminoglycoside is explicitly NOT recommended unless specific indications are present (hemodynamic instability, clinically suspected catheter-related infection, skin/soft tissue infection, pneumonia, or known MRSA colonization).

Question 49HIV, opportunistic and transplant infection

According to the classic timeline of post-transplant infections described by Fishman and Rubin, which group of pathogens characteristically manifests during the intermediate period (1 to 6 months post-transplantation) under standard maintenance immunosuppression?

  1. Surgical site wound dehiscence and line-related bacterial thrombophlebitis
  2. Community-acquired encapsulated respiratory pathogens such as Streptococcus pneumoniae
  3. Primary cutaneous dermatophyte tinea pedis infections without systemic dissemination
  4. Opportunistic and latent viral reactivations (CMV, EBV, BK polyomavirus, Pneumocystis jirovecii, and Listeria monocytogenes) Correct answer.

Explanation

The first one to six months is a classic period of opportunistic infection risk after transplantation. Prophylaxis, rejection treatment, net immunosuppression and exposure can substantially change that timeline, so it is not a fixed diagnostic clock.

Question 50HIV, opportunistic and transplant infection

Which solid organ transplant donor-recipient cytomegalovirus (CMV) serostatus pairing carries the highest risk for developing severe, invasive primary CMV disease post-transplantation?

  1. Donor seropositive / Recipient seronegative (D+/R-) Correct answer.
  2. Donor seronegative / Recipient seropositive (D-/R+)
  3. Donor seropositive / Recipient seropositive (D+/R+)
  4. Donor seronegative / Recipient seronegative (D-/R-)

Explanation

The CMV seronegative recipient receiving an allograft from a CMV seropositive donor (D+/R-) represents the highest risk category for CMV disease post-transplant. Because the recipient possesses no pre-existing CMV-specific memory T-cell or humoral immunity, primary CMV transmission via the donor organ leads to unchecked viral replication, severe clinical disease (tissue-invasive pneumonitis, hepatitis, colitis), and high rates of graft rejection. Extended antiviral prophylaxis with oral valganciclovir (typically for 6 months) is standard of care.