**Question:** A patient made a self-diagnosis of athlete's foot (tinea pedis) and began using a product advertised on television. The condition improved but did not clear and then the patient showed himself to a Dermatologist. A skin scraping was sent to the laboratory for culture, including culture for fungi. The fungal culture yielded a slow growing colony, which produced a few small microconidia. This is consistent with isolation of a dermatophyte of the genera

A. Trichophyton
B. Epidermophyton
C. Microsporum
D. Nannizzia

**Correct Answer:** C. Microsporum

**Core Concept:** Dermatophytes are a group of fungi that cause superficial infections in humans, particularly affecting keratinized tissues such as the skin, hair, and nails. Dermatophytes are classified into three groups based on their growth characteristics: Mucormycetes, Chrysosporium, and Dermatophytes. Athlete's foot, also known as tinea pedis, is primarily caused by dermatophytes of the genera Trichophyton, Epidermophyton, Microsporum, and Nannizzia.

**Why the Correct Answer is C:**

The correct answer is Microsporum because the patient presented with a slow-growing colony and few small microconidia in the fungal culture. Microconidia are a distinctive feature of the genus Microsporum, which is known for causing athlete's foot (tinea pedis). In contrast, the other options (Trichophyton, Epidermophyton, and Nannizzia) are associated with faster-growing colonies and larger macroconidia, which are not present in this case.

**Why the Wrong Options are Incorrect:**

1. **Option A (Trichophyton):** Trichophyton spp. are known for their faster growth and larger macroconidia production, not microconidia. They are more commonly associated with ringworm infections in other parts of the body.

2. **Option B (Epidermophyton):** Similar to Trichophyton, Epidermophyton spp. are associated with faster growth and larger macroconidia production, not microconidia. They primarily cause ringworm infections on the skin, hair, and nails.

3. **Option D (Nannizzia):** Nannizzia spp. are part of the Microsporum group and share similar characteristics, including the presence of microconidia. They are not typically associated with athlete's foot but with onychomycosis (fungal infection of the nail).

4. **Option E (Microsporum):** Microsporum spp. are a subgroup of Microsporum, again, sharing similar characteristics, including microconidia production. They are not specifically associated with athlete's foot but rather with ringworm infections on the skin, hair, and nails.

**Clinical Pearl:** Athlete's foot (tinea pedis) is primarily caused by dermatophytes