A 6-year-old girl presents to the clinic with scaly patches on the scalp. Primary smears and culture of the skin and hair were negative. A few weeks later, she returned and was found to have inflammatory lesions. The hair fluoresced under Wood’s light and primary smears of skin and hair contained septate hyphae. On speaking with the parents, it was discovered that there were several pets in the household. Which of the following is the most likely agent?

## **Core Concept**
The question describes a case of a child presenting with scaly patches on the scalp that later became inflammatory, with hair fluorescing under Wood's light and the presence of septate hyphae in skin and hair smears. This clinical presentation suggests a fungal infection of the scalp, known as **tinea capitis** or ringworm of the scalp.

## **Why the Correct Answer is Right**
The correct answer, **Microsporum canis**, is a common cause of tinea capitis, particularly in children. This fungus is known for causing infections that can lead to scaly patches on the scalp, and it is often associated with exposure to infected animals, which fits with the history of pets in the household. A key diagnostic feature of infections caused by Microsporum species is that the infected hairs fluoresce under Wood's light, which was observed in this case.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Trichophyton rubrum* is more commonly associated with skin infections (tinea corporis, tinea cruris) and onychomycosis, rather than tinea capitis. It does not typically cause hair fluorescence under Wood's light.
- **Option B:** *Microsporum audouinii* can cause tinea capitis but is more commonly seen in children in endemic areas and does not typically fluoresce under Wood's light.
- **Option D:** *Trichophyton schoenleinii* causes favus, a chronic form of tinea capitis characterized by the presence of yellowish, cup-shaped crusts (scutula) around the infected hairs, which was not described in the case.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the presence of fluorescence under Wood's light in hairs is highly suggestive of a **Microsporum** infection. This quick diagnostic tool can guide further management and treatment of suspected tinea capitis.

## **Correct Answer: C. Microsporum canis**

✓ Correct Answer: B. Microsporum canis