FREE AP-MICRO Mycology and Parasitology 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?

Accepted Answer

Histoplasma capsulatum

Answer

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?

Accepted Answer

Plasmodium falciparum

Answer

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?

Accepted Answer

Zygomycetes

Answer

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?

Accepted Answer

Entamoeba histolytica

Answer

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?

Accepted Answer

Phenol

Answer

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]

AP Practice Test

AP Practice Test

FREE AP-MICRO Mycology and Parasitology Questions and Answers