**Core Concept**
The patient's presentation suggests a diagnosis of a dermatological condition characterized by oval scaly hypopigmented macules over the chest and back. This condition is often associated with a defect in the epidermal melanin unit, leading to a reduction in melanin production and transfer.

**Why the Correct Answer is Right**
The correct diagnosis is **Pityriasis Alba**. Pityriasis Alba is a common, benign condition characterized by oval-shaped, scaly, hypopigmented patches on sun-exposed areas such as the chest and back. The pathophysiology is thought to involve a combination of factors including impaired melanin transfer, decreased melanin production, and an increase in the turnover of epidermal cells. The condition is often seen in children but can also occur in adults.

**Why Each Wrong Option is Incorrect**
* **Option A:** This could be a reference to Pityriasis Versicolor, a fungal infection that presents with scaly patches on the chest and back. However, Pityriasis Versicolor typically presents with hyperpigmented patches, not hypopigmented macules.
* **Option B:** This could be a reference to Pityriasis Rosea, a skin condition characterized by a herald patch followed by a secondary eruption of scaly patches. However, Pityriasis Rosea typically presents with hyperpigmented patches, not hypopigmented macules.
* **Option C:** This could be a reference to Vitiligo, a condition characterized by white patches on the skin due to a loss of melanocytes. However, Vitiligo typically presents with sharply demarcated white patches, not scaly, hypopigmented macules.

**Clinical Pearl / High-Yield Fact**
Pityriasis Alba is often associated with atopic dermatitis and can be a trigger for atopic march, a process where atopic dermatitis progresses to other atopic conditions such as asthma and allergic rhinitis.

**Correct Answer:** D.