Free AP-MICRO Mycology and Parasitology Questions and Answers 1 — Questions and Answers
Question 1: A patient with a history of cave exploration in the Ohio River Valley presents with respiratory symptoms. A fungal culture of a lung biopsy grows a mold at 25°C and a yeast at 37°C. Microscopic examination of the mold phase reveals large, thick-walled, tuberculate macroconidia. Which organism is most likely the cause of the infection?
- Coccidioides immitis
- Blastomyces dermatitidis
- Histoplasma capsulatum (Correct answer)
- Sporothrix schenckii
Correct answer: Histoplasma capsulatum
Histoplasma capsulatum is a dimorphic fungus endemic to the Ohio and Mississippi River Valleys, often associated with soil contaminated with bird or bat guano, such as in caves. [3, 11, 17] Its characteristic mold phase feature is the presence of large (8–15 µm), thick-walled, spherical macroconidia with distinctive tuberculate or finger-like projections. [19]
Question 2: A Giemsa-stained peripheral blood smear reveals multiple ring-form trophozoites in some red blood cells (RBCs), including appliqué forms (rings on the periphery of the cell). Several crescent-shaped gametocytes are also observed. The infected RBCs are normal in size. Which species of Plasmodium is most likely present?
- Plasmodium vivax
- Plasmodium falciparum (Correct answer)
- Plasmodium malariae
- Plasmodium ovale
Correct answer: Plasmodium falciparum
Plasmodium falciparum is characterized by several key features: high parasitemia with multiple rings per RBC, appliqué (or accole) forms, and pathognomonic crescent- or banana-shaped gametocytes. [2, 7, 25] Unlike P. vivax and P. ovale, it does not typically cause the infected RBCs to become enlarged. [2]
Question 3: A fungal culture from a sinus biopsy of an immunocompromised patient grows a rapidly spreading, woolly, gray-to-brown mold. Microscopic examination reveals broad, ribbon-like, pauciseptate (aseptate) hyphae with right-angle branching. What class of fungi is described?
- Ascomycota
- Dermatophytes
- Basidiomycota
- Zygomycetes (Correct answer)
Correct answer: Zygomycetes
The class Zygomycetes (now often classified within Mucoromycota) includes organisms like Rhizopus and Mucor. [10, 23] They are characterized by primitive, broad (6-25 µm), ribbon-like hyphae that are pauciseptate (having few or no septa) and exhibit irregular, wide-angle (often 90-degree) branching. [6, 13, 21] These fungi are known for causing aggressive opportunistic infections (mucormycosis). [13]
Question 4: A trichrome-stained stool smear from a patient with dysentery reveals amoebic trophozoites measuring approximately 20 µm. The cytoplasm is finely granular and contains ingested red blood cells. The single nucleus has even, peripheral chromatin and a small, central karyosome. Based on these observations, what is the most likely identification?
- Endolimax nana
- Entamoeba coli
- Entamoeba histolytica (Correct answer)
- Iodamoeba bütschlii
Correct answer: Entamoeba histolytica
The presence of ingested red blood cells (erythrophagocytosis) is a key diagnostic feature of Entamoeba histolytica, distinguishing it from the morphologically identical but non-pathogenic E. dispar. [9, 14, 20] The characteristic nuclear morphology—a small, centrally located karyosome and fine, uniform peripheral chromatin—further supports this identification. [34, 35]
Question 5: In the Lactophenol Cotton Blue (LPCB) stain used for microscopic examination of fungi, which component acts as a killing agent, deactivating lytic enzymes and preserving the organism's morphology?
- Lactic acid
- Glycerol
- Cotton blue
- Phenol (Correct answer)
Correct answer: Phenol
Lactophenol Cotton Blue (LPCB) is a combination stain and mounting medium. The phenol component acts as a fungicide, killing any live organisms and inactivating their lytic enzymes to prevent autolysis. [4, 12, 15] Lactic acid acts as a clearing agent, cotton blue stains the chitin in the fungal cell walls, and glycerol prevents the slide from drying out. [28, 31]
Question 6: Which of the following is the specimen of choice for the laboratory diagnosis of an Enterobius vermicularis infection?
- Stool specimen in preservative
- Cellulose tape paddle preparation (Correct answer)
- Peripheral blood smear
- Duodenal aspirate
Correct answer: Cellulose tape paddle preparation
The gravid female Enterobius vermicularis (pinworm) migrates to the perianal region, typically at night, to deposit her eggs. [18] Consequently, eggs are rarely found in stool specimens. The most effective diagnostic method is the cellulose tape paddle (or "Scotch tape") preparation, which collects eggs directly from the perianal folds, ideally first thing in the morning before bathing or defecation. [5, 8, 22]
Free AP-MICRO Mycology and Parasitology Questions and Answers 1
A patient with a history of cave exploration in the Ohio River Valley presents with respiratory symptoms.
A fungal culture of a lung biopsy grows a mold at 25°C and a yeast at 37°C.
Microscopic examination of the mold phase reveals large, thick-walled, tuberculate macroconidia.
Which organism is most likely the cause of the infection?