Sporothrix schenckii causes lymphocutaneous sporotrichosis. What is the temperature-dependent dimorphism of this organism and which medium/characteristic is used for diagnosis at the laboratory bench?

• A 37°C: hyphae; 25°C: spherules — reverse dimorphism
• B Both temperatures produce yeast; dimorphism based on CO2 concentration
• C 37°C: yeast; 25°C: thick-walled chlamydoconidia only
• D 37°C (host): yeast (cigar-shaped/asteroid bodies); 25°C: hyphae with 'daisy' conidia on SDA — temperature-induced dimorphism ✓

Explanation

Sporothrix schenckii is a classic thermally dimorphic fungus. At 37°C (body temperature), it grows as cigar-shaped budding yeast cells (asteroid bodies seen in tissue). At 25°C on Sabouraud's dextrose agar (room temperature), it grows as a dark brownish-black mold with delicate hyphae bearing characteristic rosette-like clusters of pyriform (pear-shaped) conidia arranged in 'daisy-head' or 'floret' pattern on the tips of conidiophores. The conversion from mold to yeast at 37°C (and reversion at 25°C) is pathognomonic dimorphism. Diagnosis is confirmed by culture.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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