## **Core Concept**
The Darier's sign is a clinical test used to assess for mast cell disorders, particularly urticaria pigmentosa (cutaneous mastocytosis). It involves gently rubbing the skin with a blunt instrument, which leads to a localized urticarial reaction if the test is positive. This reaction occurs due to the release of histamine and other mediators from mast cells.

## **Why the Correct Answer is Right**
The correct answer, urticaria pigmentosa, produces a positive Darier's sign because it is characterized by the abnormal accumulation of mast cells in the skin. When these mast cells are mechanically stimulated, as during the Darier's sign test, they degranulate and release histamine, leading to localized erythema, edema, and pruritus. This reaction is a hallmark of mast cell activation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While other mast cell disorders could theoretically produce a positive Darier's sign, urticaria pigmentosa is the most classic and commonly associated condition.
- **Option B:** Conditions not primarily associated with mast cell accumulation or abnormal mast cell function would not produce a positive Darier's sign.
- **Option C:** Similarly, conditions not related to mast cell disorders would not exhibit this reaction.
- **Option D:** This option is also incorrect for similar reasons as options A, B, and C, as the conditions listed are not primarily associated with a positive Darier's sign.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that a positive Darier's sign is highly suggestive of mastocytosis, particularly urticaria pigmentosa. This test can be used as a diagnostic tool to help confirm the condition. It's also important to note that patients with mastocytosis can have systemic symptoms, and the condition can range from mild to severe.

## **Correct Answer:** C. Urticaria pigmentosa.