Okay, let's tackle this question. The patient is an 8-year-old boy with a boggy swelling on the scalp and easily pluckable hair. The key here is to figure out the most likely diagnosis and the appropriate investigation.

First, boggy swelling and easily pluckable hair make me think of conditions affecting the hair follicles. Common pediatric scalp conditions include tinea capitis, which is a fungal infection. Tinea capitis typically presents with patches of hair loss, broken hairs, and sometimes a boggy swelling called a kerion. The hair can be easily plucked because the infection weakens the follicle.

The investigation of choice for tinea capitis is usually a potassium hydroxide (KOH) preparation of the plucked hairs. This allows visualization of fungal elements under the microscope. Another possibility is a fungal culture, but KOH is more rapid and often sufficient for diagnosis.

Looking at the options, if the correct answer is KOH preparation, then the other options might include things like skin biopsy, Gram stain, or PCR. A skin biopsy might be used for more chronic or atypical cases, but it's not the first-line test. Gram stain is for bacterial infections, not fungal. PCR is more advanced and less commonly used in routine settings. So the correct answer should be KOH preparation.

**Core Concept**
The clinical presentation of boggy scalp swelling with easily pluckable hair points to **tinea capitis**, a fungal infection of the hair follicles. The investigation of choice for this condition is **potassium hydroxide (KOH) mount**, which detects fungal hyphae or spores in hair samples.

**Why the Correct Answer is Right**
**KOH preparation** is the first-line diagnostic test for tinea capitis. It involves dissolving non-fungal tissue with KOH, allowing visualization of fungal elements (hyphae, spores) under microscopy. This method is rapid, cost-effective, and highly specific for dermatophyte infections. The "easily pluckable hair" suggests a fungal etiology, as tinea capitis weakens the hair shaft, facilitating removal.

**Why Each Wrong Option is Incorrect**
**Option A:** *Skin biopsy* is unnecessary for typical tinea capitis and reserved for atypical cases or suspected malignancy.
**Option B:** *Gram stain* detects bacterial infections (e.g., folliculitis), not fungal infections.
**Option C:** *PCR testing* is not routinely used due to high cost and availability issues in resource-limited settings.

**Clinical Pearl / High-Yield Fact**
Never forget: **KOH mount** is the gold standard for diagnosing superficial fungal infections like tinea. For tinea capitis, **culture** may also be used but takes longer (7–14 days) and is less sensitive than KOH in early infections.

**Correct Answer: D. Potassium hydroxide (KOH) preparation**

✓ Correct Answer: C. KOH smear