**Core Concept**
Ash leaf maculae are a type of hypopigmented skin lesion characterized by a circular, oval, or irregular shape with a sharp margin. They are a classic skin manifestation of Tuberous Sclerosis Complex (TSC), a genetic disorder that affects multiple organ systems.

**Why the Correct Answer is Right**
Ash leaf maculae result from the loss of melanocytes in the epidermis due to the mutations in the TSC1 or TSC2 genes. These mutations lead to the activation of the mammalian target of rapamycin (mTOR) pathway, causing the inhibition of melanocyte development and proliferation. Ash leaf maculae are typically seen in patients with TSC, especially in children, and can be a diagnostic clue for this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Nevus depigmentosus is a congenital hypopigmented skin lesion that is usually present at birth and is not associated with TSC.
**Option B:** Vitiligo is an autoimmune condition characterized by the loss of melanocytes, but it typically presents with well-demarcated, white patches and is not associated with TSC.
**Option C:** Hypomelanosis of Ito is a rare condition characterized by hypopigmentation and other systemic abnormalities, but it is not typically associated with TSC.

**Clinical Pearl / High-Yield Fact**
Ash leaf maculae are often used as a diagnostic clue for TSC, especially in children, and can be a key finding in the clinical examination.

**Correct Answer: D. Tuberous Sclerosis Complex (TSC)**