**Core Concept**
Vitiligo is a chronic autoimmune skin condition characterized by the loss of melanocytes, leading to white patches. The prognosis of vitiligo is influenced by various factors, including age, disease duration, and associated autoimmune conditions.

**Why the Correct Answer is Right**
The correct answer to this question is not provided. However, I will explain the poor prognostic factors of vitiligo for the other options. Vitiligo patients with younger age at onset, longer disease duration, and associated autoimmune conditions such as thyroid disease, diabetes mellitus, or pernicious anemia have a poorer prognosis. This is because these factors are associated with a higher risk of disease progression, increased severity, and reduced response to treatment.

**Why Each Wrong Option is Incorrect**
* **Option A:** Younger age at onset is a poor prognostic factor of vitiligo, as it is associated with a higher risk of disease progression and reduced response to treatment.
* **Option B:** Longer disease duration is also a poor prognostic factor, as it indicates a more established autoimmune process and reduced likelihood of spontaneous repigmentation.
* **Option D:** Associated autoimmune conditions such as thyroid disease, diabetes mellitus, or pernicious anemia are poor prognostic factors, as they indicate a higher risk of disease progression and increased severity.

**Clinical Pearl / High-Yield Fact**
A mnemonic to remember poor prognostic factors of vitiligo is "YOUNG": Younger age at onset, Uncontrolled autoimmune conditions, Longer disease duration, and Greater severity of associated autoimmune conditions.

**Correct Answer: Not Provided.**