**Question:** Which of the following is an X-linked dominant condition presenting as hyperpigmented lesions along lines of Blaschko?

A. Melasma
B. Nevoid Melanosis
C. Lentigo Migrans
D. Xeroderma Pigmentosum

**Core Concept:** X-linked dominant conditions are genetic disorders that are inherited through the X chromosome. They typically present with symptoms in males and are often milder in females due to the presence of a second X chromosome that can mask the effect of the mutated gene. In these conditions, the affected individual carries the mutated gene on the X chromosome and can pass it on to their offspring.

**Why the Correct Answer is Right:**

**Correct Answer: D - Xeroderma Pigmentosum**

Xeroderma Pigmentosum (XP) is an X-linked dominant condition caused by mutations in DNA repair genes. It results in an extreme sensitivity to ultraviolet (UV) radiation from sunlight, leading to the development of hyperpigmented skin lesions along the lines of Blaschko. These lines represent the zones of normal skin cell migration during embryonic development, and the lesions track along these lines due to the preferential damage of melanocytes in these areas. XP is characterized by a triad of symptoms: photosensitivity, skin and mucosal lesions, and an increased risk of skin cancers.

**Why Each Wrong Option is Incorrect:**

**Option A - Melasma:** Melasma is a common acquired skin disorder characterized by hyperpigmentation due to hormonal factors. It does not follow the lines of Blaschko and is not associated with an increased risk of skin cancers.

**Option B - Nevoid Melanosis:** Nevoid Melanosis is a term for congenital generalized hyperpigmentation disorders, such as Nevus of Ota and Nevus of Ito. It is not an X-linked condition and does not present with the Blaschko's lines.

**Option C - Lentigo Migrans:** Lentigo Migrans refers to the migration of melanocytes along the lines of Blaschko. It is not a clinical entity but a description of the pattern of hyperpigmented lesions in certain conditions.

**Clinical Pearl:** Understanding the lines of Blaschko is crucial for diagnosing skin disorders that follow these lines, as it helps in differentiating them from generalized hyperpigmentation disorders like vitiligo or nevus disorders like Nevus of Ota and Ito.

**Why the Correct Answer is Right:**

Xeroderma Pigmentosum (XP) is an X-linked dominant disorder caused by mutations in DNA repair genes. It results in increased sensitivity to UV radiation, leading to the development of hyperpigmented lesions along the lines of Blaschko due to the preferential damage to melanocytes in these zones during embryonic development. XP is associated with an increased risk of skin cancers.

**Why Each Wrong Option is Incorrect:**

**Option A - Melasma:** Melasma is a common acquired skin disorder characterized by hyperpigmentation due to hormonal