A 24-year-old man had multiple, small hypopigmented macules on the upper chest and back for the last three months. The macules were circular, arranged around follicles and many had coalesced to form large sheets. The surface of the macules showed fine scaling. He had similar lesions one year ago which subsided with treatment. The most appropriate investigation to confirm the diagnosis is

**Core Concept**
The question is testing the diagnosis of a specific skin condition characterized by hypopigmented macules, fine scaling, and a predilection for the upper chest and back. This condition is often associated with a history of similar lesions and can be triggered by various factors.

**Why the Correct Answer is Right**
The patient's symptoms and history are consistent with pityriasis versicolor, a fungal infection caused by Malassezia species. The fine scaling and hypopigmented macules are classic signs of this condition. The fact that the lesions are arranged around follicles and have coalesced to form large sheets further supports this diagnosis. In pityriasis versicolor, the fungus infects the stratum corneum, leading to changes in skin pigmentation and scaling.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not directly relate to the diagnosis of pityriasis versicolor. The clinical features described in the question do not suggest an allergic contact dermatitis.

**Option B:** This option is incorrect as it is more consistent with seborrheic dermatitis, which typically presents with erythematous, scaly patches on the face and scalp.

**Option C:** This option is incorrect as it is more consistent with vitiligo, which presents with well-defined, depigmented macules without scaling.

**Option D:** This option is incorrect as it is more consistent with pityriasis alba, which presents with hypopigmented patches on the face, often associated with atopic dermatitis.

**Clinical Pearl / High-Yield Fact**
Pityriasis versicolor is a common skin condition that can be triggered by various factors, including hormonal changes, stress, and the use of broad-spectrum antibiotics. It is essential to recognize the clinical features of this condition to provide appropriate treatment.

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

✓ Correct Answer: D. Potassium hydroxide preparation of scales