I85PREP
⌘ K
ABMM study materials
ABMM study resources

ABMM practice questions

One question at a time. Build your ABMM understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Bacteriology

Which bacterial structure is the primary determinant of the Gram stain reaction?

  1. The peptidoglycan layer of the cell wall Correct answer.
  2. The teichoic acids embedded in the wall
  3. The polysaccharide capsule surrounding the cell
  4. The lipopolysaccharide of the outer membrane

Explanation

Gram-positive cells retain crystal violet-iodine because their thick peptidoglycan layer resists decolorization, while the thin peptidoglycan of Gram-negative cells does not. The stain therefore reflects cell wall architecture.

Question 2Bacteriology

A colony on blood agar shows a clear zone of complete red cell lysis around it. This pattern is described as:

  1. Gamma-hemolysis
  2. Delta-hemolysis
  3. Beta-hemolysis Correct answer.
  4. Alpha-hemolysis

Explanation

Beta-hemolysis is complete lysis of erythrocytes, producing a clear transparent zone. Alpha-hemolysis is partial lysis with greenish discoloration, and gamma-hemolysis means no hemolysis at all.

Question 3Bacteriology

Which organism is the most common cause of community-acquired urinary tract infection in otherwise healthy women?

  1. Enterococcus faecalis
  2. Staphylococcus saprophyticus
  3. Escherichia coli Correct answer.
  4. Proteus mirabilis

Explanation

Uropathogenic E. coli accounts for the majority of uncomplicated community-acquired UTIs. Other uropathogens occur but at much lower frequency in this population.

Question 4Bacteriology

Catalase converts hydrogen peroxide into which products?

  1. Superoxide and oxygen
  2. Water and hydrogen gas
  3. Water and oxygen Correct answer.
  4. Hydroxyl radicals and water

Explanation

Catalase detoxifies hydrogen peroxide by splitting it into water and molecular oxygen, which appears as bubbling in the slide test. The enzyme is present in staphylococci but absent in streptococci.

Question 5Bacteriology

Which genus is composed of obligate intracellular bacteria that cannot be cultured on standard cell-free media?

  1. Legionella
  2. Chlamydia Correct answer.
  3. Bordetella
  4. Mycoplasma

Explanation

Chlamydia species lack the ability to synthesize sufficient ATP and depend on host cells, so they are obligate intracellular parasites. Mycoplasma, Legionella, and Bordetella can all be grown on cell-free media with appropriate supplements.

Question 6Bacteriology

Coagulase production is used in the laboratory primarily to distinguish which pair?

  1. Enterococcus from Streptococcus
  2. Streptococcus pyogenes from Streptococcus agalactiae
  3. Staphylococcus from Micrococcus
  4. Staphylococcus aureus from coagulase-negative staphylococci Correct answer.

Explanation

Coagulase converts fibrinogen to fibrin and is a key marker separating S. aureus from other staphylococci. Coagulase-negative species such as S. epidermidis are clinically important but test negative.

Question 7Bacteriology

A spore-forming, anaerobic, Gram-positive bacillus is isolated from a wound with a black eschar. The most likely organism is:

  1. Clostridium perfringens Correct answer.
  2. Listeria monocytogenes
  3. Bacillus anthracis
  4. Clostridioides difficile

Explanation

C. perfringens is an anaerobic, spore-forming Gram-positive rod associated with gas gangrene and wound infections. B. anthracis is aerobic, C. difficile causes colitis, and Listeria is a non-spore-forming rod.

Question 8Bacteriology

Which specimen transport condition is most appropriate for a suspected anaerobe?

  1. A blood culture bottle incubated at 4 degrees Celsius
  2. An aerobic swab left in ambient air for several hours
  3. A dry tube without transport medium
  4. A sealed anaerobic transport vial kept at room temperature Correct answer.

Explanation

Anaerobic organisms lose viability when exposed to oxygen, so specimens must be transported in an oxygen-free container and processed promptly. Room-temperature anaerobic vials preserve viability without refrigeration.

Question 9Bacteriology

A patient with a duodenal ulcer undergoes biopsy, and a urease-positive, microaerophilic, curved Gram-negative rod is seen. The organism is most likely:

  1. Vibrio cholerae
  2. Salmonella enterica
  3. Helicobacter pylori Correct answer.
  4. Campylobacter jejuni

Explanation

H. pylori is a microaerophilic, urease-producing curved rod that colonizes gastric mucosa and causes peptic ulcer disease. Its strong urease activity is the basis of the rapid urease biopsy test.

Question 10Bacteriology

A sputum culture grows an organism that is oxidase-positive, non-fermenting, and produces a blue-green pigment. This organism is:

  1. Klebsiella pneumoniae
  2. Escherichia coli
  3. Acinetobacter baumannii
  4. Pseudomonas aeruginosa Correct answer.

Explanation

P. aeruginosa is oxidase-positive, non-lactose-fermenting, and produces pyocyanin, a blue-green pigment. The combination of oxidase positivity and pigment is characteristic.

Question 11Bacteriology

A stool culture from a child with bloody diarrhea grows a non-lactose-fermenting, oxidase-negative rod that is motile and produces colorless colonies on MacConkey agar. The likely genus is:

  1. Salmonella Correct answer.
  2. Yersinia
  3. Vibrio
  4. Shigella

Explanation

Salmonella is motile, oxidase-negative, and non-lactose-fermenting, producing colorless colonies. Shigella is also non-lactose-fermenting but is non-motile, which differentiates the two.

Question 12Bacteriology

Which test result combination best supports identification of Streptococcus agalactiae?

  1. Bacitracin sensitive and bile-esculin positive
  2. Optochin sensitive and bile soluble
  3. Bacitracin resistant and hippurate hydrolysis positive Correct answer.
  4. Pyrrolidonyl arylamidase negative and salt tolerant

Explanation

S. agalactiae (group B) is bacitracin resistant and hydrolyzes hippurate, distinguishing it from group A streptococci. Group A is bacitracin sensitive and hippurate negative.

Question 13Bacteriology

A newborn develops meningitis, and a Gram-positive, catalase-positive coccus is isolated. Which organism should be strongly suspected?

  1. Staphylococcus aureus Correct answer.
  2. Streptococcus agalactiae
  3. Neisseria meningitidis
  4. Listeria monocytogenes

Explanation

S. aureus is a catalase-positive Gram-positive coccus that can cause neonatal meningitis and is distinguished from streptococci by catalase production. Group B streptococcus and Listeria are important neonatal pathogens but are catalase-negative.

Question 14Bacteriology

The capsule of Cryptococcus neoformans is best demonstrated using which stain?

  1. Ziehl-Neelsen
  2. Giemsa stain
  3. India ink Correct answer.
  4. Gram stain

Explanation

India ink is a negative stain in which the background is darkened and the capsule appears as a clear halo. It is classically used for Cryptococcus, though antigen testing is more sensitive.

Question 15Bacteriology

Which characteristic best differentiates Mycobacterium tuberculosis from other mycobacteria in a clinical laboratory?

  1. Positive niacin accumulation and nitrate reduction Correct answer.
  2. Presence of mycolic acids in the cell wall
  3. Ability to grow on Lowenstein-Jensen medium
  4. Susceptibility to standard Gram staining

Explanation

M. tuberculosis is niacin-positive and reduces nitrate, features used in biochemical identification. Mycolic acids and growth on Lowenstein-Jensen are shared by many mycobacteria, and mycobacteria are not reliably Gram-stained.

Question 16Bacteriology

A patient with a dog bite develops a wound infection. A fastidious Gram-negative rod that grows best in carbon dioxide-enriched conditions is isolated. The likely organism is:

  1. Bartonella henselae
  2. Pasteurella multocida Correct answer.
  3. Brucella melitensis
  4. Francisella tularensis

Explanation

P. multocida is a small Gram-negative coccobacillus that is part of the normal oral flora of cats and dogs and causes rapidly developing bite wound infections. It grows on blood agar, often enhanced by CO2.

Question 17Bacteriology

A patient with neutropenia develops a bloodstream infection, and blood culture grows Gram-positive cocci in clusters that are catalase-positive and coagulase-negative. Which organism is most likely?

  1. Streptococcus pneumoniae
  2. Staphylococcus aureus
  3. Enterococcus faecium
  4. Staphylococcus epidermidis Correct answer.

Explanation

Coagulase-negative staphylococci such as S. epidermidis are common causes of bacteremia in neutropenic and catheterized patients. Their catalase positivity and cluster arrangement separate them from streptococci and enterococci.

Question 18Bacteriology

Which organism is the most frequent cause of acute bacterial meningitis in adults beyond the neonatal period?

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

Explanation

S. pneumoniae is the leading cause of community-acquired bacterial meningitis in adults. Meningococcal disease occurs but is less frequent overall, and Hib disease has declined markedly with vaccination.

Question 19Bacteriology

A patient with a history of rheumatic fever is at risk for endocarditis. Which organism is classically associated with subacute endocarditis on previously damaged valves?

  1. Bacteroides fragilis
  2. Viridans group streptococci Correct answer.
  3. Staphylococcus aureus
  4. Pseudomonas aeruginosa

Explanation

Viridans streptococci are normal oral flora that adhere to damaged heart valves, causing subacute endocarditis. S. aureus more often causes acute endocarditis, including on normal valves.

Question 20Bacteriology

A patient with cystic fibrosis has repeated respiratory infections. Which organism is most characteristically associated with this condition?

  1. Streptococcus pyogenes
  2. Haemophilus ducreyi
  3. Mycoplasma hominis
  4. Burkholderia cepacia complex Correct answer.

Explanation

Burkholderia cepacia complex is a recognized respiratory pathogen in cystic fibrosis and is associated with accelerated decline. P. aeruginosa is also common, but B. cepacia is specifically linked to CF and transmissibility between patients.

Question 21Bacteriology

Which mechanism best explains why Mycoplasma species are intrinsically resistant to beta-lactam antibiotics?

  1. They produce beta-lactamase enzymes
  2. They have a thickened outer membrane
  3. They lack a peptidoglycan cell wall Correct answer.
  4. They possess an efflux pump for beta-lactams

Explanation

Mycoplasma species lack a cell wall and therefore have no peptidoglycan target for beta-lactam antibiotics. This also explains their resistance to other cell-wall-active agents such as vancomycin.

Question 22Bacteriology

A patient with a prosthetic hip joint develops infection. A Gram-positive coccus that grows in clusters, is catalase-positive, and is coagulase-negative is isolated. Which feature most strongly suggests the isolate is a true pathogen rather than a contaminant?

  1. Presence of a biofilm on the prosthesis Correct answer.
  2. Growth in broth culture
  3. Ability to grow at 37 degrees Celsius
  4. Fermentation of glucose

Explanation

Biofilm formation on prosthetic material is a key virulence mechanism for coagulase-negative staphylococci and supports a pathogenic role in device-related infection. Common biochemical traits such as glucose fermentation and growth at 37 degrees Celsius do not distinguish pathogen from contaminant.

Question 23Bacteriology

A patient with diarrhea after antibiotic therapy is tested for toxin production. Which organism and toxin pair is correctly matched?

  1. Vibrio cholerae and cholera toxin
  2. Clostridioides difficile and toxin A/B Correct answer.
  3. Clostridium perfringens and Shiga toxin
  4. Bacillus cereus and botulinum toxin

Explanation

C. difficile produces toxin A (enterotoxin) and toxin B (cytotoxin), which mediate antibiotic-associated colitis. These toxins are detected by immunoassay or nucleic acid amplification.

Question 24Bacteriology

Which laboratory finding most strongly supports the diagnosis of typhoid fever caused by Salmonella Typhi rather than nontyphoidal Salmonella gastroenteritis?

  1. Isolation of the organism from blood during the first week of illness Correct answer.
  2. Growth of the organism from stool on MacConkey agar
  3. Presence of lactose fermentation
  4. Positive oxidase reaction

Explanation

S. Typhi causes a bacteremic illness with organisms recoverable from blood early in the disease, whereas nontyphoidal salmonellae typically cause gastroenteritis with stool positivity. Blood culture is the key diagnostic specimen in the first week.

Question 25Bacteriology

A patient with a chronic skin ulcer has a Gram-negative rod that is oxidase-positive and grows on MacConkey agar at 42 degrees Celsius. Which organism is most consistent with these findings?

  1. Pseudomonas aeruginosa Correct answer.
  2. Proteus vulgaris
  3. Escherichia coli
  4. Klebsiella pneumoniae

Explanation

P. aeruginosa is oxidase-positive and can grow at 42 degrees Celsius, a feature used in identification. The other enteric organisms listed are oxidase-negative and do not share this temperature tolerance.

Question 26Bacteriology

Which bacterial structure is primarily responsible for the ability of Neisseria gonorrhoeae to adhere to and invade mucosal surfaces?

  1. Pili Correct answer.
  2. Lipopolysaccharide
  3. Peptidoglycan
  4. Capsule

Explanation

Pili (fimbriae) mediate initial attachment to host epithelial cells and are a major virulence factor for gonococci. Capsule and LOS also contribute to pathogenesis but are not the primary adhesins.

Question 27Bacteriology

A Gram-positive rod that is catalase-positive, motile at 25°C but non-motile at 37°C, and exhibits tumbling motility is most likely which organism?

  1. Listeria monocytogenes Correct answer.
  2. Corynebacterium diphtheriae
  3. Clostridium perfringens
  4. Bacillus cereus

Explanation

Listeria monocytogenes is a Gram-positive rod with characteristic tumbling motility in broth and temperature-dependent motility, being motile at room temperature but not at 37°C. Corynebacteria are non-motile, Bacillus species are motile at both temperatures, and Clostridium perfringens is non-motile.

Question 28Bacteriology

Which laboratory test is most specific for identifying Streptococcus pyogenes (group A strep) from a throat swab?

  1. Optochin sensitivity
  2. Bile esculin hydrolysis
  3. Growth in 6.5% NaCl
  4. Bacitracin sensitivity Correct answer.

Explanation

Streptococcus pyogenes is bacitracin-sensitive, a key presumptive test for group A streptococci. Optochin sensitivity is used for pneumococci, bile esculin for group D enterococci, and 6.5% NaCl for enterococci.

Question 29Bacteriology

A patient presents with severe pneumonia and a Gram stain of sputum shows Gram-negative rods. The organism grows on buffered charcoal yeast extract (BCYE) agar but not on standard blood agar. Which organism is most likely?

  1. Klebsiella pneumoniae
  2. Pseudomonas aeruginosa
  3. Haemophilus influenzae
  4. Legionella pneumophila Correct answer.

Explanation

Legionella pneumophila requires BCYE agar with L-cysteine for growth and does not grow on routine blood agar. Klebsiella, Pseudomonas, and Haemophilus can grow on standard media or require other supplements.

Question 30Bacteriology

Which of the following best explains why Mycoplasma pneumoniae is resistant to cell wall-active antibiotics such as penicillin?

  1. It has an outer membrane that excludes drugs.
  2. It grows intracellularly.
  3. It lacks a peptidoglycan cell wall. Correct answer.
  4. It produces beta-lactamase.

Explanation

Mycoplasma species lack a peptidoglycan cell wall, making beta-lactam antibiotics ineffective. Beta-lactamase production, outer membrane permeability, and intracellular location are not the primary reasons.

Question 31Bacteriology

A blood culture from a patient with endocarditis grows a Gram-positive coccus in chains that is bile esculin positive and grows in 6.5% NaCl. Which organism is most likely?

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

Explanation

Enterococcus faecalis is a Gram-positive coccus in chains, bile esculin positive, and able to grow in 6.5% NaCl. Pneumococci are optochin sensitive, S. pyogenes is bacitracin sensitive, and S. aureus forms clusters and is catalase positive.

Question 32Bacteriology

Which virulence factor of Corynebacterium diphtheriae is responsible for the formation of pseudomembranes and systemic toxicity?

  1. Toxic shock syndrome toxin-1
  2. Diphtheria toxin Correct answer.
  3. Pyrogenic exotoxin
  4. Panton-Valentine leukocidin

Explanation

Diphtheria toxin, encoded by a lysogenic bacteriophage, inhibits protein synthesis and causes local tissue damage and systemic effects. The other toxins are produced by different organisms.

Question 33Bacteriology

A patient with a history of recurrent urinary tract infections has a urine culture growing a Gram-negative rod that is oxidase-negative, lactose-positive, and indole-positive. Which organism is most likely?

  1. Proteus mirabilis
  2. Pseudomonas aeruginosa
  3. Klebsiella pneumoniae
  4. Escherichia coli Correct answer.

Explanation

Escherichia coli is the most common cause of UTIs and is oxidase-negative, lactose-positive, and indole-positive. Klebsiella is indole-negative, Proteus is lactose-negative and indole-variable, and Pseudomonas is oxidase-positive.

Question 34Bacteriology

Which test is most useful for differentiating Staphylococcus aureus from other coagulase-negative staphylococci?

  1. Bile esculin test
  2. Optochin sensitivity
  3. Bacitracin sensitivity
  4. Coagulase test Correct answer.

Explanation

Staphylococcus aureus is coagulase-positive, while other staphylococci are coagulase-negative. Optochin and bacitracin tests are used for streptococci, and bile esculin for enterococci.

Question 35Bacteriology

A patient with neutropenia develops a bloodstream infection. Blood culture grows a Gram-negative rod that is oxidase-positive, non-fermenting, and grows at 42°C. Which organism is most likely?

  1. Klebsiella pneumoniae
  2. Pseudomonas aeruginosa Correct answer.
  3. Salmonella Typhi
  4. Escherichia coli

Explanation

Pseudomonas aeruginosa is an oxidase-positive, non-fermenting Gram-negative rod that grows at 42°C and is a common cause of infections in neutropenic patients. The other organisms are oxidase-negative fermenters.

Question 36Bacteriology

Which of the following is the primary mechanism by which Clostridioides difficile causes antibiotic-associated colitis?

  1. Invasion of colonic epithelial cells
  2. Production of enterotoxins A and B Correct answer.
  3. Formation of pseudomembranes by direct cytotoxicity
  4. Production of a single cytotoxin

Explanation

C. difficile produces two major toxins, enterotoxin A and cytotoxin B, which damage the colonic mucosa and cause pseudomembranous colitis. It does not invade epithelial cells.

Question 37Bacteriology

A patient with a wound infection after a human bite grows a Gram-negative rod that is oxidase-positive, catalase-positive, and produces a brownish pigment on blood agar. Which organism is most likely?

  1. Bacteroides fragilis
  2. Eikenella corrodens Correct answer.
  3. Pasteurella multocida
  4. Fusobacterium nucleatum

Explanation

Eikenella corrodens is a Gram-negative rod that is oxidase-positive and catalase-positive, and is associated with human bite infections. Pasteurella is oxidase-positive but catalase-positive and associated with animal bites; Bacteroides and Fusobacterium are anaerobes.

Question 38Bacteriology

Which component of the Gram-negative cell wall is responsible for endotoxic shock?

  1. Lipid A Correct answer.
  2. O antigen
  3. Core polysaccharide
  4. Peptidoglycan

Explanation

Lipid A is the toxic portion of lipopolysaccharide (endotoxin) that triggers the inflammatory cascade leading to shock. O antigen and core polysaccharide are antigenic but not the primary toxic component.

Question 39Bacteriology

A patient with a prosthetic joint infection has a Gram-positive coccus in clusters that is catalase-positive, coagulase-negative, and novobiocin-resistant. Which organism is most likely?

  1. Staphylococcus aureus
  2. Staphylococcus saprophyticus Correct answer.
  3. Staphylococcus epidermidis
  4. Staphylococcus lugdunensis

Explanation

Staphylococcus saprophyticus is coagulase-negative and novobiocin-resistant, and can cause UTIs and occasionally prosthetic joint infections. S. epidermidis is novobiocin-sensitive, S. aureus is coagulase-positive, and S. lugdunensis is novobiocin-sensitive.

Question 40Bacteriology

Which of the following best describes the mechanism of action of streptolysin O produced by Streptococcus pyogenes?

  1. It is an ADP-ribosyltransferase that inhibits protein synthesis.
  2. It is a superantigen that activates T cells.
  3. It is a cholesterol-dependent cytolysin that lyses cells. Correct answer.
  4. It is an IgG protease that cleaves antibodies.

Explanation

Streptolysin O is a cholesterol-dependent pore-forming cytolysin that is oxygen-labile and causes beta-hemolysis. Superantigen activity is associated with pyrogenic exotoxins, and other activities do not apply.

Question 41Bacteriology

A patient with a history of rheumatic fever develops endocarditis. Which organism is most likely to be isolated?

  1. Staphylococcus aureus
  2. Streptococcus pneumoniae
  3. Viridans group streptococci Correct answer.
  4. Enterococcus faecalis

Explanation

Viridans group streptococci are the most common cause of subacute endocarditis, especially in patients with pre-existing valvular damage such as from rheumatic fever. S. aureus is more acute, and enterococci are less common.

Question 42Bacteriology

Which laboratory finding best supports the diagnosis of typhoid fever caused by Salmonella Typhi rather than nontyphoidal Salmonella?

  1. Growth of lactose-fermenting colonies on MacConkey agar
  2. Isolation of the organism from blood culture in the first week of illness Correct answer.
  3. Positive oxidase test
  4. Production of urease

Explanation

Salmonella Typhi often causes bacteremia early in infection, so blood cultures are positive in the first week. Nontyphoidal Salmonella typically cause gastroenteritis and are less commonly isolated from blood. Salmonella are lactose-negative, oxidase-negative, and urease-negative.

Question 43Bacteriology

A patient with a chronic skin ulcer has a Gram-negative rod that is oxidase-positive and grows on MacConkey agar at 42°C. Which organism is most likely?

  1. Escherichia coli
  2. Serratia marcescens
  3. Pseudomonas aeruginosa Correct answer.
  4. Proteus mirabilis

Explanation

Pseudomonas aeruginosa is oxidase-positive and grows at 42°C, and is a common cause of chronic wound infections. E. coli and Proteus are oxidase-negative, and Serratia is oxidase-negative and does not grow at 42°C.

Question 44Bacteriology

Which of the following is the most appropriate specimen for diagnosing pertussis caused by Bordetella pertussis?

  1. Blood culture
  2. Sputum
  3. Throat swab
  4. Nasopharyngeal swab Correct answer.

Explanation

Bordetella pertussis colonizes the ciliated respiratory epithelium, so nasopharyngeal specimens are optimal. Throat swabs and sputum have lower yield, and blood cultures are rarely positive.

Question 45Bacteriology

A patient with a hospital-acquired pneumonia has a sputum culture growing a Gram-negative rod that is oxidase-negative, non-lactose-fermenting, and resistant to multiple antibiotics. Which organism is most likely?

  1. Escherichia coli
  2. Acinetobacter baumannii Correct answer.
  3. Klebsiella pneumoniae
  4. Haemophilus influenzae

Explanation

Acinetobacter baumannii is an oxidase-negative, non-lactose-fermenting Gram-negative rod that is often multidrug-resistant and causes hospital-acquired pneumonia. Klebsiella is lactose-fermenting, E. coli is lactose-fermenting, and Haemophilus is oxidase-positive and fastidious.

Question 46Bacteriology

Which bacterial structure is the target of lysozyme?

  1. Lipopolysaccharide
  2. Outer membrane
  3. Teichoic acid
  4. Peptidoglycan Correct answer.

Explanation

Lysozyme cleaves the beta-1,4 glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine in peptidoglycan, weakening the cell wall. LPS, teichoic acid, and the outer membrane are not substrates for lysozyme.

Question 47Bacteriology

A patient with a wound infection caused by Clostridium perfringens is treated with penicillin. Which toxin is primarily responsible for the tissue necrosis and gas production seen in this infection?

  1. Epsilon toxin
  2. Alpha toxin Correct answer.
  3. Iota toxin
  4. Beta toxin

Explanation

Alpha toxin (phospholipase C) is the major lethal toxin of C. perfringens and causes tissue necrosis and gas gangrene. Beta, epsilon, and iota toxins are produced by other types of C. perfringens and have different roles.

Question 48Bacteriology

Which of the following is the most appropriate method to detect Helicobacter pylori infection non-invasively?

  1. Serology for IgM antibodies
  2. Urea breath test Correct answer.
  3. Culture of gastric biopsy
  4. Gram stain of gastric biopsy

Explanation

The urea breath test is a non-invasive method that detects H. pylori urease activity. Culture and Gram stain require invasive biopsy, and IgM serology is not reliable for active infection.

Question 49Bacteriology

A patient with a respiratory infection has a Gram-negative coccobacillus that grows on chocolate agar but not on blood agar. It requires both X factor (hemin) and V factor (NAD). Which organism is most likely?

  1. Haemophilus parainfluenzae
  2. Haemophilus influenzae Correct answer.
  3. Bordetella pertussis
  4. Legionella pneumophila

Explanation

Haemophilus influenzae requires both X and V factors for growth, which are available in chocolate agar but not blood agar. H. parainfluenzae requires only V factor, and Bordetella and Legionella have different growth requirements.

Question 50Bacteriology

Which of the following best explains the mechanism of action of the toxin produced by Bacillus anthracis that leads to edema?

  1. The capsule inhibits phagocytosis.
  2. Edema factor is an adenylate cyclase that increases cAMP. Correct answer.
  3. Lethal factor cleaves MAP kinase kinase.
  4. Protective antigen binds to cell receptors.

Explanation

Edema factor is a calmodulin-dependent adenylate cyclase that raises intracellular cAMP, causing edema. Lethal factor is a protease that cleaves MAPKK, and protective antigen is the binding component.