**Core Concept**
Urticaria pigmentosa is a rare skin disorder characterized by the presence of hyperpigmented macules that can cause urticarial wheal upon mechanical stimulation. This condition is a form of cutaneous mastocytosis, where mast cells accumulate in the skin, leading to an excessive release of histamine upon physical contact.

**Why the Correct Answer is Right**
The correct answer is urticaria pigmentosa due to the characteristic histological finding of increased mast cells in the affected skin. When the skin is rubbed with the rounded end of a pen (a mechanical stimulus), the mast cells in the affected area release histamine, leading to the development of urticarial wheal. This phenomenon is known as Darier's sign, which is a classic diagnostic feature of urticaria pigmentosa.

**Why Each Wrong Option is Incorrect**
**Option A:** Fixed drug eruption is a type of skin reaction that presents with well-defined lesions, but it does not typically exhibit the characteristic urticarial wheal upon mechanical stimulation.

**Option B:** Lichen planus is an autoimmune disease that affects the skin, mucous membranes, and other areas, but it does not typically present with the urticarial wheal response to mechanical stimulation.

**Option D:** Urticarial vasculitis is a type of vasculitis that presents with urticarial lesions, but it is not associated with the characteristic histological finding of increased mast cells in the skin.

**Clinical Pearl / High-Yield Fact**
Urticaria pigmentosa can be a challenging diagnosis, but the presence of Darier's sign (urticarial wheal upon mechanical stimulation) is a key diagnostic clue. It is essential to consider this condition in the differential diagnosis of children presenting with hyperpigmented macules and urticarial lesions.

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