**Core Concept**
The patient's symptoms suggest a dermatological condition characterized by hypopigmented macules, fine scaling, and a predilection for the trunk and flexural areas. This clinical presentation is typical of a fungal infection, specifically pityriasis versicolor, caused by Malassezia yeast.

**Why the Correct Answer is Right**
Pityriasis versicolor is a superficial fungal infection caused by the lipophilic yeast Malassezia, which is a normal inhabitant of the skin. The infection occurs when the yeast overgrows, leading to the formation of hypopigmented macules with fine scaling. The presence of Malassezia on the skin surface can be confirmed through a potassium hydroxide (KOH) preparation or a fungal culture.

**Why Each Wrong Option is Incorrect**
**Option A:** Wood's lamp examination is not the most appropriate investigation for diagnosing pityriasis versicolor, as it may show a yellowish or coppery fluorescence, but this is not specific or sensitive enough for confirmation.

**Option B:** Skin biopsy may show the presence of Malassezia, but it is an invasive procedure and not the first line of investigation for a diagnosis of pityriasis versicolor.

**Option C:** Histopathological examination of a skin biopsy may show changes consistent with a fungal infection, but it is not as specific or sensitive as a KOH preparation or fungal culture for diagnosing Malassezia.

**Option D:** A complete blood count (CBC) is not relevant to the diagnosis of pityriasis versicolor, as it is a superficial fungal infection.

**Clinical Pearl / High-Yield Fact**
Pityriasis versicolor is a superficial fungal infection that can be treated with topical antifungal agents, such as ketoconazole or clotrimazole. It is essential to note that the infection can be recurrent, and treatment may need to be repeated to achieve clearance.

**Correct Answer:** C.