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ABIM Infectious Disease study materials
ABIM Infectious Disease study resources

ABIM Infectious Disease practice questions

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Question 1Bacterial Infections and Antimicrobial Therapy

A 30-year-old man presents with a painless penile ulcer with a clean base and firm, nontender inguinal lymphadenopathy. Darkfield microscopy is positive. What is the treatment of choice?

  1. Azithromycin 1 g PO once
  2. Benzathine penicillin G 2.4 million units IM once Correct answer.
  3. Doxycycline 100 mg PO twice daily for 14 days
  4. Ceftriaxone 250 mg IM once

Explanation

Primary syphilis in a patient without penicillin allergy is treated with a single intramuscular dose of benzathine penicillin G 2.4 million units, which achieves treponemicidal levels for the required duration. Alternative regimens are reserved for penicillin-allergic patients, and azithromycin resistance is now widespread.

Question 2Bacterial Infections and Antimicrobial Therapy

Which organism is the most common cause of community-acquired pneumonia requiring hospitalization in adults?

  1. Streptococcus pneumoniae Correct answer.
  2. Mycoplasma pneumoniae
  3. Haemophilus influenzae
  4. Legionella pneumophila

Explanation

Streptococcus pneumoniae remains the single most frequently identified cause of community-acquired pneumonia across all severity levels, including patients requiring admission. Atypical organisms and Haemophilus are important but less common overall.

Question 3Bacterial Infections and Antimicrobial Therapy

A patient with a prosthetic heart valve develops fever and blood cultures growing methicillin-susceptible Staphylococcus aureus. Which agent is preferred for definitive therapy?

  1. Vancomycin
  2. Linezolid
  3. Cefazolin Correct answer.
  4. Daptomycin

Explanation

For methicillin-susceptible S. aureus bacteremia and endocarditis, an antistaphylococcal beta-lactam such as cefazolin or nafcillin is superior to vancomycin in efficacy and is preferred. Vancomycin and other agents are reserved for MRSA or beta-lactam intolerance.

Question 4Bacterial Infections and Antimicrobial Therapy

Which finding best distinguishes Clostridioides difficile infection from other causes of hospital-acquired diarrhea?

  1. Presence of fecal leukocytes
  2. Recent use of a proton pump inhibitor
  3. Positive stool glutamate dehydrogenase with positive toxin assay Correct answer.
  4. Abdominal cramping with watery stools

Explanation

CDI is diagnosed by detecting toxigenic C. difficile, typically with a two-step algorithm combining glutamate dehydrogenase antigen and toxin testing or nucleic acid amplification. Clinical features such as diarrhea and cramping are nonspecific.

Question 5Bacterial Infections and Antimicrobial Therapy

A 24-year-old college student has a stiff neck, fever, and headache. CSF shows elevated neutrophils, low glucose, and high protein with gram-negative diplococci. What is the empiric therapy of choice?

  1. Meropenem plus vancomycin
  2. Ceftriaxone plus vancomycin plus ampicillin
  3. Ceftriaxone alone Correct answer.
  4. Vancomycin plus ceftriaxone

Explanation

In adults with suspected pneumococcal or meningococcal meningitis, empiric therapy is vancomycin plus a third-generation cephalosporin to cover resistant pneumococcus. In a young adult with gram-negative diplococci consistent with Neisseria meningitidis, ceftriaxone alone is adequate and vancomycin adds no benefit.

Question 6Bacterial Infections and Antimicrobial Therapy

A patient with a wound infection has a gram stain showing gram-positive cocci in clusters that grow on mannitol salt agar and are coagulase positive. Which organism is responsible?

  1. Staphylococcus aureus Correct answer.
  2. Streptococcus pyogenes
  3. Staphylococcus epidermidis
  4. Enterococcus faecalis

Explanation

Coagulase-positive, mannitol-fermenting gram-positive cocci in clusters identify Staphylococcus aureus. Coagulase-negative staphylococci such as S. epidermidis do not ferment mannitol and are coagulase negative.

Question 7Bacterial Infections and Antimicrobial Therapy

A 45-year-old woman has a productive cough with rusty sputum and a chest radiograph showing lobar consolidation. Which organism is most likely?

  1. Streptococcus pneumoniae Correct answer.
  2. Mycoplasma pneumoniae
  3. Chlamydia psittaci
  4. Klebsiella pneumoniae

Explanation

Rusty sputum and lobar consolidation are classic for Streptococcus pneumoniae pneumonia. Klebsiella may produce currant-jelly sputum but is more typical in patients with alcohol use disorder or aspiration.

Question 8Bacterial Infections and Antimicrobial Therapy

A patient with a prosthetic joint infection grows methicillin-resistant Staphylococcus aureus. Which antibiotic is generally preferred for oral step-down therapy after initial intravenous treatment?

  1. Cephalexin
  2. Oxacillin
  3. Trimethoprim-sulfamethoxazole
  4. Linezolid Correct answer.

Explanation

Linezolid has excellent oral bioavailability and activity against MRSA, making it a useful oral step-down option. Beta-lactams such as cephalexin and oxacillin lack reliable MRSA activity, and TMP-SMX is less reliable for deep staphylococcal infection.

Question 9Bacterial Infections and Antimicrobial Therapy

Which statement about the mechanism of action of vancomycin is correct?

  1. It inhibits DNA gyrase
  2. It inhibits bacterial protein synthesis at the 30S ribosomal subunit
  3. It binds the D-Ala-D-Ala terminus of peptidoglycan precursors Correct answer.
  4. It disrupts the bacterial cell membrane

Explanation

Vancomycin binds the D-Ala-D-Ala terminus of cell wall precursors, blocking peptidoglycan cross-linking. This mechanism explains its activity against gram-positive organisms and the resistance seen with vanA-mediated D-Ala-D-Lac alteration.

Question 10Bacterial Infections and Antimicrobial Therapy

A patient with suspected bacterial meningitis is being started on empiric therapy. Which factor most strongly determines the need to add ampicillin for Listeria coverage?

  1. Serum creatinine level
  2. Patient age and immune status Correct answer.
  3. Recent travel history
  4. Presence of a rash

Explanation

Listeria monocytogenes meningitis occurs predominantly in neonates, adults over 50, pregnant women, and immunocompromised patients, so age and immune status determine whether ampicillin is added. Other clinical features do not reliably predict Listeria.

Question 11Bacterial Infections and Antimicrobial Therapy

A 60-year-old man with cirrhosis develops fever and abdominal pain, and ascitic fluid shows 600 polymorphonuclear cells per microliter. Which organism is the most common cause of spontaneous bacterial peritonitis?

  1. Escherichia coli Correct answer.
  2. Enterococcus faecalis
  3. Clostridioides difficile
  4. Bacteroides fragilis

Explanation

Escherichia coli is the most frequent cause of spontaneous bacterial peritonitis, followed by Klebsiella species. Anaerobes and enterococci are less common in this setting.

Question 12Bacterial Infections and Antimicrobial Therapy

A patient with a urinary tract infection grows an organism that is ESBL-producing Escherichia coli. Which agent is the most appropriate choice for a serious infection requiring parenteral therapy?

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

Explanation

ESBLs hydrolyze extended-spectrum cephalosporins and aztreonam, and carbapenems such as meropenem remain the most reliable agents for serious ESBL infections. Fluoroquinolone resistance is frequently co-transmitted, and piperacillin-tazobactam is not dependable for ESBL organisms.

Question 13Bacterial Infections and Antimicrobial Therapy

Which antibiotic requires monitoring for the adverse effect of peripheral neuropathy and tendinopathy?

  1. Azithromycin
  2. Doxycycline
  3. Amoxicillin
  4. Levofloxacin Correct answer.

Explanation

Fluoroquinolones such as levofloxacin are associated with tendinopathy, tendon rupture, and peripheral neuropathy. These risks are the basis for boxed warnings and restricted indications in less severe infections.

Question 14Bacterial Infections and Antimicrobial Therapy

A patient with endocarditis has blood cultures growing Enterococcus faecalis that is susceptible to ampicillin and gentamicin. What is the preferred regimen?

  1. Daptomycin alone
  2. Vancomycin alone
  3. Ampicillin plus gentamicin Correct answer.
  4. Ampicillin plus ceftriaxone

Explanation

For ampicillin-susceptible Enterococcus faecalis endocarditis, combination therapy with ampicillin plus gentamicin provides synergistic bactericidal activity. Ampicillin plus ceftriaxone is an alternative when aminoglycoside toxicity is a concern.

Question 15Bacterial Infections and Antimicrobial Therapy

Which patient is at highest risk for infection with Pseudomonas aeruginosa?

  1. A young adult with acute otitis media
  2. A patient with cystic fibrosis and bronchiectasis Correct answer.
  3. A patient with uncomplicated cystitis
  4. A healthy college student with a sprained ankle

Explanation

Pseudomonas aeruginosa commonly infects patients with structural lung disease such as cystic fibrosis or bronchiectasis, as well as neutropenic and hospitalized patients. Community-acquired infections in healthy patients are usually due to other organisms.

Question 16Bacterial Infections and Antimicrobial Therapy

A patient with a severe penicillin allergy requires treatment for a serious infection caused by a gram-negative rod. Which approach is most appropriate before using a cephalosporin?

  1. Perform allergy evaluation or use a non-beta-lactam alternative Correct answer.
  2. Avoid all beta-lactams permanently
  3. Administer the cephalosporin without further evaluation
  4. Give a test dose of penicillin followed by the cephalosporin

Explanation

Cross-reactivity between penicillins and cephalosporins depends on shared side chains rather than the beta-lactam core, so allergy evaluation or non-beta-lactam alternatives are preferred. A history of severe reaction warrants formal assessment rather than empiric avoidance or unmonitored challenge.

Question 17Bacterial Infections and Antimicrobial Therapy

Which organism is the leading cause of bacterial meningitis in neonates?

  1. Haemophilus influenzae type b
  2. Streptococcus agalactiae Correct answer.
  3. Neisseria meningitidis
  4. Streptococcus pneumoniae

Explanation

Group B Streptococcus (Streptococcus agalactiae) is the most common cause of neonatal meningitis, along with E. coli and Listeria. Meningococcus, pneumococcus, and Hib predominate in older children and adults.

Question 18Bacterial Infections and Antimicrobial Therapy

A patient with a deep neck abscess grows a mixture of aerobic and anaerobic organisms. Which antibiotic provides appropriate anaerobic coverage?

  1. Ceftriaxone
  2. Aztreonam
  3. Azithromycin
  4. Metronidazole Correct answer.

Explanation

Metronidazole has excellent activity against anaerobic bacteria such as Bacteroides and is commonly combined with a cephalosporin for polymicrobial head and neck infections. Ceftriaxone and aztreonam lack reliable anaerobic coverage.

Question 19Bacterial Infections and Antimicrobial Therapy

Which of the following best describes the mechanism by which aminoglycosides exert their bactericidal effect?

  1. Inhibition of folate synthesis
  2. Irreversible binding to the 30S ribosomal subunit causing misreading of mRNA Correct answer.
  3. Inhibition of cell wall synthesis
  4. Inhibition of RNA polymerase

Explanation

Aminoglycosides bind irreversibly to the 30S ribosomal subunit, causing misreading of mRNA and production of abnormal proteins, which contributes to their concentration-dependent bactericidal activity.

Question 20Bacterial Infections and Antimicrobial Therapy

A patient with suspected sepsis is started on empiric antibiotics. Which factor is the most important determinant of the initial antibiotic regimen?

  1. The color of the patient's sputum
  2. The likely source of infection and local resistance patterns Correct answer.
  3. The patient's body mass index
  4. The patient's preferred oral intake

Explanation

Empiric therapy should be guided by the suspected source, likely pathogens, and local susceptibility patterns to ensure adequate coverage. Patient preferences and nonspecific findings should not drive initial selection.

Question 21Bacterial Infections and Antimicrobial Therapy

A patient with a history of Clostridioides difficile infection has a recurrence after completing a first course of oral vancomycin. What is the most appropriate next step?

  1. Switch to oral metronidazole
  2. Repeat the same standard course of oral vancomycin
  3. Start intravenous vancomycin
  4. Extend the course of oral vancomycin with a tapered or pulsed regimen Correct answer.

Explanation

For a first recurrence, guidelines recommend either a standard course of fidaxomicin or a vancomycin taper or pulse regimen, which reduces recurrence by eradicating spores more effectively. Intravenous vancomycin does not reach the colonic lumen.

Question 22Bacterial Infections and Antimicrobial Therapy

A patient with a known MRSA infection is being treated with vancomycin. Which monitoring parameter best reflects adequate exposure?

  1. Peak serum concentration
  2. Serum creatinine alone
  3. White blood cell count
  4. Trough concentration or AUC Correct answer.

Explanation

Vancomycin exposure is best assessed with an AUC-based approach or trough concentration to ensure efficacy while minimizing nephrotoxicity. Peak levels are not routinely used for vancomycin monitoring.

Question 23Bacterial Infections and Antimicrobial Therapy

Which organism is the most common cause of infective endocarditis in patients with native valves in high-income countries?

  1. Coagulase-negative staphylococci
  2. Staphylococcus aureus Correct answer.
  3. Viridans group streptococci
  4. Enterococcus faecalis

Explanation

Staphylococcus aureus is now the most common cause of native valve endocarditis in many high-income settings, reflecting healthcare exposure and invasive procedures. Viridans streptococci remain important but are less frequent overall.

Question 24Bacterial Infections and Antimicrobial Therapy

A patient with suspected Legionnaires' disease has severe pneumonia with hyponatremia and gastrointestinal symptoms. Which antibiotic is the preferred treatment?

  1. Ceftriaxone
  2. Clindamycin
  3. Amoxicillin
  4. Levofloxacin Correct answer.

Explanation

Legionella pneumophila is an intracellular organism, and a respiratory fluoroquinolone such as levofloxacin or a macrolide is preferred. Beta-lactams are ineffective because Legionella does not have a typical peptidoglycan cell wall target.

Question 25Bacterial Infections and Antimicrobial Therapy

Which of the following is the most appropriate initial empiric regimen for a patient with a diabetic foot infection and no recent antibiotic exposure?

  1. Cephalexin alone
  2. Metronidazole alone
  3. Azithromycin
  4. Vancomycin plus piperacillin-tazobactam Correct answer.

Explanation

Diabetic foot infections are typically polymicrobial, requiring coverage of gram-positive cocci, gram-negative rods, and anaerobes, so broad-spectrum therapy such as vancomycin plus piperacillin-tazobactam is appropriate for severe infection. Narrow agents may be used for mild infections after culture.

Question 26Bacterial Infections and Antimicrobial Therapy

A 28-year-old woman presents with fever, chills, and flank pain. Urinalysis shows pyuria and bacteriuria, and a urine culture grows Escherichia coli susceptible to most antibiotics. She has no allergies and is not pregnant. Which oral antibiotic is most appropriate for definitive therapy?

  1. Nitrofurantoin
  2. Ciprofloxacin
  3. Trimethoprim-sulfamethoxazole Correct answer.
  4. Cefazolin

Explanation

For acute pyelonephritis caused by susceptible E. coli, oral trimethoprim-sulfamethoxazole is an appropriate definitive agent when the organism is susceptible. Fluoroquinolones are also effective but are generally reserved for more severe cases or when other options are not suitable. Nitrofurantoin does not achieve adequate renal tissue concentrations and should not be used for pyelonephritis.

Question 27Bacterial Infections and Antimicrobial Therapy

A 35-year-old man presents with a painless ulcer on his penis and enlarged, rubbery inguinal lymph nodes. Darkfield microscopy of the ulcer exudate is positive for spirochetes. Which antibiotic is the treatment of choice for this condition?

  1. Benzathine penicillin G Correct answer.
  2. Ceftriaxone
  3. Doxycycline
  4. Azithromycin

Explanation

The clinical presentation and darkfield findings are consistent with primary syphilis, caused by Treponema pallidum. The treatment of choice for primary, secondary, or early latent syphilis is a single intramuscular dose of benzathine penicillin G. Doxycycline is an alternative for patients with penicillin allergy, but penicillin remains the preferred agent.

Question 28Bacterial Infections and Antimicrobial Therapy

A 72-year-old man with chronic obstructive pulmonary disease develops a productive cough, fever, and dyspnea. Chest radiograph shows a new infiltrate. Sputum Gram stain shows gram-negative rods, and culture grows Haemophilus influenzae susceptible to ampicillin. Which antibiotic is most appropriate for this patient?

  1. Ampicillin Correct answer.
  2. Azithromycin
  3. Levofloxacin
  4. Amoxicillin-clavulanate

Explanation

When H. influenzae is confirmed susceptible to ampicillin, ampicillin is an appropriate and narrow-spectrum choice for respiratory infection. Amoxicillin-clavulanate is broader than necessary and adds no benefit for a beta-lactamase-negative, susceptible strain. Fluoroquinolones and azithromycin are alternatives but are not preferred when a narrower agent is active.

Question 29Bacterial Infections and Antimicrobial Therapy

A 24-year-old college student presents with fever, headache, neck stiffness, and a petechial rash. Cerebrospinal fluid shows gram-negative diplococci. Which antibiotic is the treatment of choice for this infection?

  1. Vancomycin
  2. Doxycycline
  3. Ampicillin
  4. Ceftriaxone Correct answer.

Explanation

The presentation is consistent with meningococcal meningitis caused by Neisseria meningitidis. A third-generation cephalosporin such as ceftriaxone is the treatment of choice for meningococcal meningitis. Vancomycin and ampicillin do not provide reliable coverage against N. meningitidis.

Question 30Bacterial Infections and Antimicrobial Therapy

A 58-year-old woman is admitted with fever, hypotension, and a urinary source of infection. She has a history of anaphylaxis to penicillin. Which antibiotic is most appropriate for empiric gram-negative coverage?

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

Explanation

Aztreonam is a monobactam with activity against gram-negative organisms and minimal cross-reactivity with penicillin, making it a suitable choice in patients with severe penicillin allergy. Ceftriaxone, meropenem, and piperacillin-tazobactam all contain beta-lactam structures that may cross-react in patients with severe penicillin allergy.

Question 31Bacterial Infections and Antimicrobial Therapy

A 40-year-old man presents with a 3-day history of bloody diarrhea and abdominal cramps after a course of clindamycin. Stool testing confirms Clostridioides difficile infection. Which antibiotic is the preferred first-line treatment for this initial episode?

  1. Metronidazole
  2. Fidaxomicin Correct answer.
  3. Rifaximin
  4. Vancomycin

Explanation

Fidaxomicin is recommended as a preferred first-line agent for initial episodes of C. difficile infection because it is associated with lower rates of recurrence compared with vancomycin. Metronidazole is no longer preferred for initial therapy when fidaxomicin or vancomycin is available. Rifaximin is used for recurrent cases, not initial treatment.

Question 32Bacterial Infections and Antimicrobial Therapy

A 65-year-old man with a history of prostate cancer and recent urinary instrumentation develops fever and chills. Blood cultures grow Pseudomonas aeruginosa susceptible to multiple agents. Which antibiotic is most appropriate for definitive therapy?

  1. Meropenem
  2. Ceftazidime-avibactam
  3. Cefepime Correct answer.
  4. Ciprofloxacin

Explanation

For susceptible P. aeruginosa, cefepime is an appropriate antipseudomonal cephalosporin for definitive therapy. Ceftazidime-avibactam and meropenem are broader agents typically reserved for resistant organisms. Ciprofloxacin is active against Pseudomonas but is not preferred when a beta-lactam is active, due to resistance concerns.

Question 33Bacterial Infections and Antimicrobial Therapy

A 30-year-old woman presents with fever, polyarthralgia, and a new heart murmur. Blood cultures grow Streptococcus gallolyticus (formerly bovis). Which additional evaluation is most important to perform?

  1. Dental radiographs
  2. HIV testing
  3. Colonoscopy Correct answer.
  4. Bone marrow biopsy

Explanation

Streptococcus gallolyticus (S. bovis) bacteremia is strongly associated with colorectal neoplasia, and colonoscopy is recommended to evaluate for underlying colorectal cancer or polyps. Dental evaluation is relevant for viridans streptococci but is not the specific association for S. gallolyticus. HIV testing and bone marrow biopsy are not indicated based on this organism.

Question 34Bacterial Infections and Antimicrobial Therapy

A 50-year-old man with a history of alcohol use disorder presents with fever, productive cough, and a lung abscess on imaging. Sputum culture grows Klebsiella pneumoniae. Which antibiotic is most appropriate for this patient?

  1. Azithromycin
  2. Vancomycin
  3. Ceftriaxone Correct answer.
  4. Ampicillin

Explanation

Klebsiella pneumoniae is a gram-negative rod that can cause severe pneumonia and lung abscess, particularly in patients with alcohol use disorder. A third-generation cephalosporin such as ceftriaxone is an appropriate treatment for susceptible strains. Ampicillin, vancomycin, and azithromycin do not provide reliable coverage against Klebsiella.

Question 35Bacterial Infections and Antimicrobial Therapy

A 45-year-old man is diagnosed with infective endocarditis of a native mitral valve caused by viridans group streptococci. He has no allergies and normal renal function. Which antibiotic regimen is most appropriate?

  1. Vancomycin plus gentamicin
  2. Penicillin G plus gentamicin Correct answer.
  3. Ceftriaxone alone
  4. Ampicillin plus gentamicin

Explanation

For endocarditis caused by penicillin-susceptible viridans group streptococci, the standard regimen is penicillin G plus an aminoglycoside (gentamicin) for synergistic killing. Ampicillin plus gentamicin is an alternative but penicillin G is the preferred penicillin. Vancomycin is reserved for penicillin allergy or resistant organisms, and ceftriaxone alone is not standard for this indication.

Question 36Bacterial Infections and Antimicrobial Therapy

A 55-year-old man with a history of kidney transplant on chronic immunosuppression develops fever and pulmonary nodules. Blood cultures grow Nocardia species. Which antibiotic is the preferred treatment?

  1. Trimethoprim-sulfamethoxazole Correct answer.
  2. Vancomycin
  3. Ciprofloxacin
  4. Doxycycline

Explanation

Nocardia infections are typically treated with trimethoprim-sulfamethoxazole, which is the preferred agent for most susceptible strains. Vancomycin, ciprofloxacin, and doxycycline are not first-line therapies for Nocardia. Treatment often requires prolonged courses, especially in immunosuppressed patients.

Question 37Bacterial Infections and Antimicrobial Therapy

A 60-year-old woman is admitted with fever, jaundice, and right upper quadrant pain. Blood cultures grow Bacteroides fragilis. Which antibiotic is most appropriate for this anaerobic infection?

  1. Azithromycin
  2. Metronidazole Correct answer.
  3. Vancomycin
  4. Ceftriaxone

Explanation

Bacteroides fragilis is an anaerobic gram-negative rod that is effectively treated with metronidazole, which has excellent activity against most anaerobes. Vancomycin, azithromycin, and ceftriaxone are not reliably active against B. fragilis. Metronidazole is often used in combination with other agents for mixed aerobic-anaerobic infections.

Question 38Bacterial Infections and Antimicrobial Therapy

A 70-year-old man with a prosthetic hip joint develops pain and fever 6 months after implantation. Joint fluid culture grows Staphylococcus epidermidis that is methicillin-resistant. He is scheduled for debridement and implant retention. Which antibiotic is most appropriate for definitive therapy?

  1. Cefazolin
  2. Vancomycin Correct answer.
  3. Nafcillin
  4. Clindamycin

Explanation

Methicillin-resistant coagulase-negative staphylococci are best treated with vancomycin, especially in prosthetic joint infections where biofilm formation is a concern. Cefazolin and nafcillin are ineffective against methicillin-resistant strains. Clindamycin is not preferred due to potential resistance and inferior efficacy.

Question 39Bacterial Infections and Antimicrobial Therapy

A 25-year-old man presents with dysuria and urethral discharge. Nucleic acid amplification testing is positive for Chlamydia trachomatis. Which antibiotic is the recommended treatment?

  1. Doxycycline Correct answer.
  2. Ceftriaxone
  3. Metronidazole
  4. Azithromycin

Explanation

The recommended treatment for uncomplicated chlamydial urethritis is doxycycline 100 mg orally twice daily for 7 days. Azithromycin is an alternative but is no longer preferred due to concerns about efficacy and resistance. Ceftriaxone is used for gonorrhea, and metronidazole is for trichomoniasis.

Question 40Bacterial Infections and Antimicrobial Therapy

A 68-year-old woman with a history of breast cancer and lymphedema develops a rapidly spreading, painful erythematous rash on her arm with fever and chills. Blood cultures grow Streptococcus pyogenes. She has no allergies. Which antibiotic is most appropriate?

  1. Ceftazidime
  2. Vancomycin
  3. Clindamycin
  4. Penicillin G Correct answer.

Explanation

Streptococcus pyogenes (group A streptococcus) remains universally susceptible to penicillin, making penicillin G the treatment of choice for streptococcal cellulitis. Vancomycin is reserved for resistant gram-positive organisms, ceftazidime is for gram-negative coverage, and clindamycin is an alternative for penicillin-allergic patients or for toxin suppression.

Question 41Bacterial Infections and Antimicrobial Therapy

A 30-year-old woman presents with fever, abdominal pain, and a history of an intrauterine device. Pelvic examination reveals cervical motion tenderness, and laparoscopy confirms pelvic inflammatory disease. Which empiric antibiotic regimen is recommended?

  1. Ceftriaxone plus doxycycline plus metronidazole Correct answer.
  2. Amoxicillin alone
  3. Ciprofloxacin alone
  4. Azithromycin alone

Explanation

Empiric treatment for pelvic inflammatory disease should cover Neisseria gonorrhoeae, Chlamydia trachomatis, and anaerobic bacteria. The recommended regimen includes ceftriaxone, doxycycline, and metronidazole. Single-agent regimens like azithromycin, ciprofloxacin, or amoxicillin do not provide adequate broad-spectrum coverage.

Question 42Bacterial Infections and Antimicrobial Therapy

A 75-year-old man with a history of chronic bronchitis develops a severe exacerbation with increased sputum purulence and dyspnea. He has received multiple courses of antibiotics in the past year. Sputum culture grows beta-lactamase-producing Haemophilus influenzae. Which antibiotic is most appropriate?

  1. Ampicillin
  2. Amoxicillin
  3. Amoxicillin-clavulanate Correct answer.
  4. Cefazolin

Explanation

Beta-lactamase-producing H. influenzae is resistant to amoxicillin and ampicillin. Amoxicillin-clavulanate overcomes this resistance by inhibiting the beta-lactamase enzyme. Cefazolin is an intravenous first-generation cephalosporin with poor activity against H. influenzae.

Question 43Bacterial Infections and Antimicrobial Therapy

A 40-year-old man with a history of intravenous drug use presents with fever and a new tricuspid murmur. Blood cultures grow methicillin-susceptible Staphylococcus aureus. He has no allergies. Which antibiotic is the treatment of choice for this infection?

  1. Nafcillin Correct answer.
  2. Vancomycin
  3. Cefazolin
  4. Linezolid

Explanation

For methicillin-susceptible S. aureus endocarditis, a penicillinase-resistant penicillin such as nafcillin is the treatment of choice and is superior to vancomycin. Cefazolin is an alternative but nafcillin is preferred for native valve endocarditis. Vancomycin and linezolid are reserved for resistant organisms or specific situations.

Question 44Bacterial Infections and Antimicrobial Therapy

A 22-year-old woman presents with fever, headache, and a rash that started on her wrists and ankles and spread centrally. She recently returned from a camping trip in North Carolina. Which antibiotic is the treatment of choice for this condition?

  1. Doxycycline Correct answer.
  2. Ceftriaxone
  3. Amoxicillin
  4. Azithromycin

Explanation

The presentation is consistent with Rocky Mountain spotted fever, caused by Rickettsia rickettsii and transmitted by ticks. Doxycycline is the treatment of choice for all ages, including children, and should be started promptly. Amoxicillin, azithromycin, and ceftriaxone are not effective against Rickettsia.

Question 45Viral Infections and Antiviral Therapy

A 28-year-old healthy adult has acute infectious mononucleosis with fever, exudative pharyngitis, and cervical adenopathy. Which antiviral agent is indicated for routine treatment?

  1. Valacyclovir
  2. No antiviral therapy is indicated Correct answer.
  3. Acyclovir
  4. Ganciclovir

Explanation

Epstein-Barr virus mononucleosis in immunocompetent patients is managed with supportive care; antivirals do not shorten the illness meaningfully and are not routinely indicated. Acyclovir has been studied but is not recommended for routine treatment. Ganciclovir is reserved for severe EBV disease in immunocompromised hosts.

Question 46Viral Infections and Antiviral Therapy

Which medication is the preferred first-line treatment for chronic hepatitis B virus infection when a high genetic barrier to resistance is desired?

  1. Entecavir Correct answer.
  2. Lamivudine
  3. Adefovir
  4. Telbivudine

Explanation

Entecavir and tenofovir are the preferred first-line nucleos(t)ide analogues for chronic hepatitis B because of potent activity and a high genetic barrier to resistance. Lamivudine, adefovir, and telbivudine have lower barriers and higher rates of resistance emergence.

Question 47Viral Infections and Antiviral Therapy

A 34-year-old receives a needlestick from a patient with known HIV. Which regimen represents appropriate HIV postexposure prophylaxis?

  1. Two-drug therapy started only after a positive HIV test at 6 weeks
  2. A three-drug regimen started as soon as possible for 28 days Correct answer.
  3. Zidovudine monotherapy for 4 weeks
  4. Single-dose nevirapine at exposure

Explanation

HIV postexposure prophylaxis should be started as soon as possible after exposure and continued for 28 days, using a three-drug regimen. Monotherapy and delayed initiation are inadequate. Nevirapine is not used for PEP because of hepatotoxicity risk.

Question 48Viral Infections and Antiviral Therapy

A previously healthy adult has laboratory-confirmed influenza and symptoms for 36 hours. Which antiviral is recommended for early treatment?

  1. Oseltamivir Correct answer.
  2. Famciclovir
  3. Cidofovir
  4. Ribavirin

Explanation

Oseltamivir, a neuraminidase inhibitor, is recommended for treatment of influenza when started within about 48 hours of symptom onset. Ribavirin, famciclovir, and cidofovir are not influenza therapies.

Question 49Viral Infections and Antiviral Therapy

Which drug is the treatment of choice for acyclovir-resistant mucocutaneous herpes simplex virus infection in an immunocompromised patient?

  1. Amantadine
  2. Ganciclovir
  3. Valacyclovir
  4. Foscarnet Correct answer.

Explanation

Foscarnet is the preferred agent for acyclovir-resistant HSV because it does not require thymidine kinase activation, the step lost in resistant strains. Valacyclovir is a prodrug of acyclovir and would also fail. Ganciclovir is not active against HSV.

Question 50Viral Infections and Antiviral Therapy

A pregnant woman in the third trimester has recurrent genital herpes. Which strategy reduces the risk of neonatal HSV transmission?

  1. Cidofovir at 36 weeks
  2. No intervention; cesarean delivery is always required
  3. Intravenous foscarnet until delivery
  4. Daily suppressive acyclovir or valacyclovir near term Correct answer.

Explanation

Suppressive antiviral therapy with acyclovir or valacyclovir beginning around 36 weeks reduces recurrences at delivery and the need for cesarean section. Cesarean delivery is indicated for active lesions at the time of labor, not universally.