## **Core Concept**
The condition described suggests a fungal infection of the scalp, commonly known as **tinea capitis** or ringworm of the scalp. This condition is characterized by circular, scaly, patchy hair loss with itching. The diagnosis is primarily clinical but can be confirmed with specific investigations.

## **Why the Correct Answer is Right**
The investigation of choice for confirming a fungal infection like tinea capitis is a **Wood's lamp examination** or **KOH (potassium hydroxide) mount**. However, among the given options, the most specific and commonly used investigation would likely involve microscopic examination or culture. A **KOH mount** is often the first step, where a sample from the lesion is treated with KOH to dissolve keratin and other cellular material, making fungal elements like hyphae and spores more visible under a microscope. This is a quick, bedside test that can provide immediate results.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Without specifics, it's hard to judge, but if it's not related to fungal diagnostics, it's incorrect.
- **Option B:** Similarly, without specifics, if it doesn't relate to fungal diagnostics or is less specific than the correct answer, it's incorrect.
- **Option D:** This would be incorrect if it suggests an investigation not typically used for diagnosing fungal infections of the scalp.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that a **Wood's lamp** can help diagnose certain fungal infections; for example, *Microsporum* species fluoresce under UV light. However, not all fungi causing tinea capitis fluoresce, making **KOH mount** a universally useful test.

## **Correct Answer:** . KOH mount.