Okay, let's tackle this question. The patient is a 24-year-old male with multiple small hypopigmented macules on the upper chest and back that have been present for three months. The description mentions they're circular, follicular, and some have coalesced with fine scaling. He had similar lesions a year ago that resolved with treatment. The question is asking for the most appropriate investigation to confirm the diagnosis.

First, I need to think about differential diagnoses for hypopigmented macules. Common possibilities include pityriasis versicolor (tinea versicolor), vitiligo, pityriasis alba, and maybe even other fungal infections. The key features here are the location, the circular arrangement around follicles, scaling, and previous treatment that worked. Pityriasis versicolor is a fungal infection caused by Malassezia species. It often presents with hypopigmented or hyperpigmented patches that are scaly, especially in warm, moist areas. The fact that the lesions subsided with treatment before suggests a fungal cause, as antifungal therapy would be effective.

Now, the investigation to confirm this would be a potassium hydroxide (KOH) preparation of skin scrapings. This test is used to detect fungal elements. When you scrape the scales from the lesion and apply KOH, it dissolves the keratin, allowing visualization of hyphae or yeast under the microscope. If the previous treatment was antifungal, and the lesions are recurring, KOH would help confirm the presence of the fungus again.

Looking at the options, even though they're not listed, typical distractors might include tests like a skin biopsy, which is more for conditions like vitiligo or lichen planus. A biopsy might not be the first step here. Another option could be a fungal culture, but that's more time-consuming and less commonly used than KOH. Blood tests like ANA for vitiligo or others might be considered, but they're not specific for fungal infections.

So the correct answer should be the KOH test. The other options are either for different conditions or not the most appropriate first step. The clinical pearl here is that KOH is the gold standard for diagnosing superficial fungal infections like tinea versicolor. Students should remember that when they see scaly, hypopigmented patches that respond to antifungals, KOH is the go-to test.

**Core Concept**
This question tests the diagnostic approach to hypopigmented, scaly skin lesions likely caused by *Malassezia* species, a common cause of tinea versicolor. The key differentials include fungal infections, vitiligo, and pityriasis alba. Diagnosis relies on identifying fungal elements via microscopic examination.

**Why the Correct Answer is Right**
The potassium hydroxide (KOH) mount is the gold standard for diagnosing superficial fungal infections like tinea versicolor. *Malassezia* yeast forms appear as short hyphae or "spaghetti and meatball" structures under microscopy. The clinical history of prior resolution with antifungal treatment and follicular, scaly, hypopigmented macules strongly suggests this diagnosis. KOH dissolves keratin, allowing visualization of fungal elements.

**Why Each Wrong Option is Incorrect**
**Option A:** Skin biopsy would be used for conditions like vitiligo or lichen planus, not

✓ Correct Answer: A. KOH preparation