**Question:** Decrease number of melanocyte is seen in:

A. Vitiligo
B. Erythema nodosum
C. Porphyria cutanea tarda
D. Pernicious anaemia

**Core Concept:**

Melanocytes are the pigment-producing cells found in the basal layer of the epidermis, responsible for producing melanin, the pigment responsible for skin color. In physiological conditions, their number remains stable, ensuring consistent skin pigmentation. However, under certain pathological conditions, melanocyte population can decrease, leading to hypopigmentation.

**Why the Correct Answer is Right:**

A. Vitiligo: Vitiligo is an autoimmune disorder where the immune system mistakenly attacks melanocytes, causing their destruction and melanin depletion, leading to the loss of pigmentation in affected areas.

B. Erythema nodosum: This dermatological condition is characterized by subcutaneous inflammation and is unrelated to melanocytes. The correct answer should be B.

C. Porphyria cutanea tarda: This is a group of rare genetic disorders involving the liver and heme synthesis. Hypopigmentation is not a typical feature of this condition.

D. Pernicious anaemia: This is a type of anaemia caused by vitamin B12 deficiency, which is unrelated to melanocytes and skin pigmentation.

**Why Each Wrong Option is Incorrect:**

B. Erythema nodosum: This condition involves subcutaneous inflammation and is unrelated to melanocytes and skin pigmentation.

C. Porphyria cutanea tarda: Although porphyrias can cause skin manifestations, hypopigmentation is not a typical feature.

D. Pernicious anaemia: Pernicious anaemia is an anaemia caused by vitamin B12 deficiency, unrelated to melanocytes and skin pigmentation.

**Clinical Pearl:**

Erythema nodosum should be remembered as a separate entity with distinct clinical features and pathogenesis. Recalling the correct answer (A) and understanding the differences between correct and incorrect options is essential for clinical reasoning and differential diagnosis.

In conclusion, understanding the relationship between melanocytes, skin pigmentation, and the correct answer (A) Vitiligo is crucial for recognizing skin disorders and making accurate diagnoses. This knowledge aids in differentiating between various skin conditions and providing appropriate treatment strategies.