**Core Concept**
The presentation of a lesion with peripheral scaling and central clearing, along with scarring, suggests a diagnosis of a dermatological condition that requires histopathological examination for definitive diagnosis. This is a key principle in dermatology, where clinical appearance alone may not be sufficient for accurate diagnosis.

**Why the Correct Answer is Right**
The peripheral scaling and central clearing with scarring in the young boy's lesion are suggestive of a condition like discoid lupus erythematosus (DLE) or a form of granulomatous dermatitis. Biopsy is the investigation of choice for these conditions as it allows for the examination of the histopathological features, which are critical for diagnosis. The biopsy can be performed on a skin sample obtained from the lesion, and it will provide information on the inflammatory cell infiltrate, the presence of granulomas, and the extent of scarring.

**Why Each Wrong Option is Incorrect**
**Option A:** A Tzank smear is a diagnostic tool used to examine cells from a lesion for the presence of viral inclusions, such as those seen in herpes simplex or varicella-zoster infections. It is not typically used to diagnose conditions characterized by scarring and peripheral scaling.
**Option B:** A KOH preparation is used to diagnose fungal infections by demonstrating the presence of fungal hyphae or spores. While it may be used to diagnose fungal infections that present with scaling, it is not the investigation of choice for conditions characterized by scarring and central clearing.
**Option D:** Sabouraud's agar is a type of culture medium used to grow fungi. While it may be used to diagnose fungal infections, it is not the investigation of choice for conditions characterized by scarring and central clearing.

**Clinical Pearl / High-Yield Fact**
When confronted with a skin lesion that presents with scarring and peripheral scaling, remember that biopsy is often the investigation of choice, as it will provide the necessary information for a definitive diagnosis. Be aware of the classic clinical presentation of conditions like discoid lupus erythematosus, which may mimic other dermatological conditions.

**Correct Answer Line**
✓ Correct Answer: C. Biopsy