**Core Concept**
The child's presentation of hyperpigmented macules with urticarial wheal upon friction is indicative of a condition characterized by abnormal vascular permeability and mast cell degranulation. This is a classic example of a physical urticaria, specifically dermatographism.

**Why the Correct Answer is Right**
The child's symptoms are consistent with dermatographism, a condition where mild mechanical stimuli, such as rubbing or stroking, lead to the appearance of urticarial wheals. This is due to the release of histamine and other mediators from mast cells, causing increased vascular permeability and inflammation. The urticarial wheals are typically confined to the border of the lesion, as seen in this case. The mechanism involves the activation of mast cells through mechanical stimuli, leading to the release of histamine and other chemical mediators.

**Why Each Wrong Option is Incorrect**
**Option A:** **Mastocytosis** is a condition characterized by excessive mast cell proliferation and accumulation, often presenting with skin lesions, but it does not typically exhibit the characteristic response to friction seen in dermatographism.

**Option B:** **Erythema multiforme** is a skin condition characterized by target lesions, but it does not typically exhibit the urticarial response to friction seen in this case.

**Option C:** **Lichen planus** is a chronic inflammatory skin condition characterized by purplish, itchy, flat bumps, but it does not typically exhibit the characteristic response to friction seen in dermatographism.

**Clinical Pearl / High-Yield Fact**
Dermatographism is a common form of physical urticaria, and its diagnosis can be confirmed by demonstrating the characteristic response to friction, as seen in this case.

**Correct Answer:** C.