ABPath - American Board of Pathology Medical Microbiology Questions and Answers 1 — Questions and Answers
Question 1: A lung biopsy from a severely immunocompromised patient with a pulmonary nodule is examined. Microscopic evaluation reveals fungal elements consisting of septate hyphae that branch at acute, 45-degree angles. This morphological finding is most characteristic of which of the following organisms?
- Mucor species
- Aspergillus fumigatus (Correct answer)
- Cryptococcus neoformans
- Candida albicans
Correct answer: Aspergillus fumigatus
The presence of septate hyphae with acute-angle (approximately 45-degree) branching is a classic histopathologic feature of Aspergillus species. In contrast, Mucor species (Zygomycetes) are characterized by broad, pauciseptate or non-septate hyphae with right-angle branching. Cryptococcus neoformans is an encapsulated yeast, and Candida albicans typically appears as budding yeasts and pseudohyphae.
Question 2: A 48-year-old renal transplant recipient presents with fever, malaise, and leukopenia. A gastrointestinal biopsy performed to evaluate diarrhea reveals enlarged cells containing prominent basophilic intranuclear inclusions surrounded by a clear halo, creating an "owl's eye" appearance. Which of the following is the most likely causative agent?
- Herpes Simplex Virus (HSV)
- Epstein-Barr Virus (EBV)
- Adenovirus
- Cytomegalovirus (CMV) (Correct answer)
Correct answer: Cytomegalovirus (CMV)
The clinical scenario of an organ transplant recipient with systemic symptoms and the pathognomonic finding of large cells with "owl's eye" intranuclear inclusions is classic for Cytomegalovirus (CMV) infection. While HSV, EBV, and Adenovirus can also cause disease in this patient population, the specific cytopathic effect described is the hallmark of CMV.
Question 3: A peripheral blood smear from a patient who recently returned from a trip to sub-Saharan Africa shows multiple small, ring-form trophozoites within some erythrocytes. Additionally, several distinct crescent-shaped gametocytes are observed. These findings are diagnostic for an infection with which of the following species?
- Plasmodium falciparum (Correct answer)
- Plasmodium vivax
- Babesia microti
- Plasmodium ovale
Correct answer: Plasmodium falciparum
The presence of crescent-shaped (or banana-shaped) gametocytes is a pathognomonic feature of Plasmodium falciparum. The observation of multiple ring forms within a single red blood cell and high levels of parasitemia are also characteristic of this species. Plasmodium vivax and ovale have amoeboid trophozoites and round gametocytes, while Babesia microti appears as ring forms but does not produce gametocytes in human blood and may form tetrads ('Maltese cross').
Question 4: A blood culture from a patient with a catheter-related bloodstream infection grows Gram-positive cocci in clusters. The organism is catalase-positive. Which of the following biochemical tests is most critical for differentiating Staphylococcus aureus from coagulase-negative staphylococci (CoNS)?
- Bacitracin susceptibility
- Novobiocin susceptibility
- Coagulase test (Correct answer)
- PYR test
Correct answer: Coagulase test
The coagulase test is the principal laboratory test used to differentiate Staphylococcus aureus (positive) from coagulase-negative staphylococci (CoNS), such as S. epidermidis. The production of coagulase is a key virulence factor for S. aureus. Novobiocin susceptibility is used to differentiate S. saprophyticus (resistant) from other CoNS. Bacitracin is used for differentiating Streptococcus pyogenes, and PYR is used for identifying Enterococcus and S. pyogenes.
Question 5: A 75-year-old patient develops profuse watery diarrhea after a 10-day course of antibiotics for pneumonia. A stool sample is tested using a multi-step algorithm. The initial glutamate dehydrogenase (GDH) antigen test is positive, but the toxin A/B enzyme immunoassay (EIA) is negative. According to current guidelines, what is the most appropriate next step to resolve this discrepancy?
- Report the result as negative for C. difficile infection.
- Repeat the toxin A/B EIA test.
- Perform a nucleic acid amplification test (NAAT) for toxin genes. (Correct answer)
- Culture the stool for C. difficile and perform toxigenic culture.
Correct answer: Perform a nucleic acid amplification test (NAAT) for toxin genes.
When a sensitive screening test like GDH is positive but a less sensitive, specific test like the toxin EIA is negative, a third, highly sensitive and specific test is needed for resolution. A nucleic acid amplification test (NAAT) for the C. difficile toxin genes (e.g., tcdB) is the recommended reflex test in this scenario. GDH is a sensitive but non-specific marker, as it detects both toxigenic and non-toxigenic strains. The EIA for toxin is specific but can lack sensitivity. NAAT provides a definitive answer on whether a toxigenic strain is present.
Question 6: A urine culture from a patient with a complicated urinary tract infection grows a gram-negative rod identified as Klebsiella pneumoniae. Antimicrobial susceptibility testing reveals it is resistant to ceftriaxone and cefepime but susceptible to imipenem. A synergy test using a ceftazidime disk and a clavulanate-containing disk shows an enhanced zone of inhibition ('keyhole' effect). This phenotype is characteristic of what resistance mechanism?
- AmpC beta-lactamase production
- Carbapenemase (KPC) production
- Efflux pump overexpression
- Extended-Spectrum Beta-Lactamase (ESBL) production (Correct answer)
Correct answer: Extended-Spectrum Beta-Lactamase (ESBL) production
This pattern is classic for an Extended-Spectrum Beta-Lactamase (ESBL) producer. ESBLs are enzymes that hydrolyze extended-spectrum cephalosporins (like ceftriaxone and cefepime) but are inhibited by beta-lactamase inhibitors like clavulanate. The positive synergy test confirms this. Carbapenemase production would confer resistance to imipenem. AmpC beta-lactamases are typically not inhibited by clavulanate.
ABPath - American Board of Pathology Medical Microbiology Questions and Answers 1
A lung biopsy from a severely immunocompromised patient with a pulmonary nodule is examined.
Microscopic evaluation reveals fungal elements consisting of septate hyphae that branch at acute, 45-degree angles.
This morphological finding is most characteristic of which of the following organisms?