Causative agent of Favus is
**Core Concept**
Favus is a chronic and contagious form of tinea capitis, also known as ringworm of the scalp. It is characterized by large, yellowish crusts on the scalp, often accompanied by hair loss and scarring. The causative agent is a dermatophyte fungus that infects the hair shaft.
**Why the Correct Answer is Right**
The correct answer is Trichophyton schoenleinii, a specific species of dermatophyte fungus that exclusively infects the hair shaft, leading to the characteristic symptoms of Favus. This fungus produces enzymes such as keratinase, which breaks down keratin in the hair shaft, facilitating its entry and proliferation within the hair. The infection spreads through direct contact with an infected individual or contaminated objects.
**Why Each Wrong Option is Incorrect**
**Option A:** Trichophyton tonsurans is a common causative agent of tinea capitis, but it is not the primary cause of Favus. It typically infects the scalp, causing inflammation and hair loss, but does not produce the characteristic yellowish crusts of Favus.
**Option B:** Microsporum canis is a common cause of tinea capitis in children, but it is not associated with Favus. It typically infects the scalp, causing inflammation and hair loss, but does not produce the characteristic yellowish crusts of Favus.
**Option C:** Epidermophyton floccosum is a dermatophyte fungus that typically infects the skin and nails, but is not associated with Favus. It is not adapted to infect the hair shaft, which is the hallmark of Favus.
**Clinical Pearl / High-Yield Fact**
Favus is a highly contagious and chronic form of tinea capitis that requires prompt treatment to prevent scarring and hair loss. It is essential to identify the causative agent accurately to choose the most effective treatment.
**Correct Answer:** D. Trichophyton schoenleinii