Dimorphic Fungi Practice Questions

Correct answer: Mold at 25°C and Yeast at 37°C

Dimorphic fungi exist as saprophytic molds in the environment (25°C) and transition to a parasitic yeast or specialized tissue form at body temperature (37°C). This morphological switch is a key virulence factor for surviving within the host.

Correct answer: Paracoccidioides brasiliensis

Paracoccidioides brasiliensis produces large, round yeast cells with multiple narrow-necked buds attached all around the periphery. This distinctive 'mariner's wheel' appearance is diagnostic for paracoccidioidomycosis.

Correct answer: Histoplasma capsulatum

Histoplasma capsulatum thrives in soil enriched with bird or bat guano, particularly in the Ohio and Mississippi River Valleys. Inhalation of microconidia leads to histoplasmosis, which often mimics tuberculosis.

Correct answer: Coccidioides immitis

Coccidioides immitis is a unique dimorphic fungus because its parasitic phase is a thick-walled spherule. These spherules undergo internal cleavage to produce hundreds of endospores, each capable of forming a new spherule.

Correct answer: Large, thick-walled yeasts with a broad-based bud

Blastomyces dermatitidis yeast cells are large (8–15 µm) and possess a highly characteristic broad-based bud. The cell wall is notably thick and refractile, appearing 'doubly contoured' under the microscope.

Correct answer: Traumatic inoculation via thorns or splinters

Sporotrichosis is typically an occupational hazard for gardeners and farmers. The fungus is introduced into the subcutaneous tissue through minor trauma, often following a linear lymphatic spread (lymphocutaneous sporotrichosis).

Correct answer: Talaromyces marneffei

Talaromyces marneffei is the only thermally dimorphic species in its genus. It produces a soluble red pigment that diffuses into the medium and presents as small yeast cells that divide by fission (not budding) in tissue.

Correct answer: Arthroconidia

Coccidioides produces highly infectious, barrel-shaped arthroconidia in the environment. These spores are easily aerosolized when soil is disturbed and are considered a significant laboratory hazard due to their low infectious dose.

Correct answer: Coccidioides immitis

Coccidioides immitis and C. posadasii are found in the alkaline, arid soils of the 'Lower Sonoran Life Zone'. This includes regions like the San Joaquin Valley in California and southern Arizona.

Correct answer: Sabouraud Dextrose Agar (SDA)

Sabouraud Dextrose Agar is the standard medium for fungal isolation and the cultivation of the saprophytic mold phase. BHI or Blood agar at 37°C is usually required to convert the isolate to its yeast or parasitic phase.

Correct answer: Tuberculate macroconidia

The mold phase of H. capsulatum produces large, thick-walled, spherical macroconidia with finger-like projections known as 'tubercles'. These structures are highly characteristic and used for definitive identification.

Correct answer: Histoplasma capsulatum

Histoplasma capsulatum is an intracellular parasite of the reticuloendothelial system. The yeast cells are very small and can be seen packed within the cytoplasm of monocytes and macrophages in bone marrow or lymph node smears.

Correct answer: They are pleomorphic, often described as 'cigar-shaped'

Sporothrix yeast cells are typically oval to elongated, often resembling small cigars. This morphology is observed when the fungus is grown at 37°C or seen in tissue, although they are often sparse in clinical specimens.

Correct answer: Valley Fever

Valley Fever refers to the primary pulmonary infection caused by Coccidioides in the San Joaquin Valley. 'Cave Disease' and 'Darling's Disease' both refer to histoplasmosis, and 'Chicago Disease' is an old name for blastomycosis.

Correct answer: Blastomyces dermatitidis

Blastomycosis occurs in the Great Lakes, St. Lawrence River, and Southeastern US. While it begins in the lungs, it frequently disseminates to the skin (verrucous lesions) and bones, often in otherwise healthy individuals.

Correct answer: Blastomyces dermatitidis

The mold form of Blastomyces produces single, round to oval conidia at the ends of short conidiophores or directly on the hyphae, resembling a lollipop. This is distinct from the tuberculate macroconidia of Histoplasma.

Correct answer: They enable rapid identification without yeast conversion

Traditional conversion of a mold to its yeast phase can take weeks and is often difficult. Molecular probes and exoantigen tests can confirm the identity of the mold culture much faster and safer for laboratory personnel.

Correct answer: Adult male agricultural workers in Latin America

Paracoccidioidomycosis is endemic to Central and South America. There is a strong male predilection (up to 15:1) because estrogen is thought to inhibit the conversion of the mold to the parasitic yeast form.

Correct answer: Sporothrix schenckii

While Sporothrix is thermally dimorphic, it is usually categorized as a subcutaneous mycosis rather than a primary systemic (endemic) mycosis like Histoplasma, Blastomyces, Coccidioides, and Paracoccidioides.

Correct answer: Chitin and Alpha-glucan

The thick cell wall of Blastomyces consists of an outer layer of alpha-1,3-glucan and chitin. This structure provides significant mechanical strength and results in the characteristic thick-walled appearance under light microscopy.