**Core Concept**
The child's symptoms suggest a condition characterized by hyperpigmented macules and a specific response to friction, indicating a mechanical or physical stimulus triggering a cutaneous reaction.

**Why the Correct Answer is Right**
The child's lesions are likely **fixed drug eruptions (FDEs)**, a type of cutaneous adverse drug reaction. When the lesions are rubbed with the rounded end of a pen, also known as the **Nikolsky's sign** or **Darier's sign**, it triggers the release of histamine from mast cells, leading to urticarial wheal formation. This phenomenon is due to the presence of mast cells in the dermal-epidermal junction, which are activated upon mechanical stimulation. The hyperpigmented macules are a result of post-inflammatory hyperpigmentation (PIH) following the initial reaction.

**Why Each Wrong Option is Incorrect**

* **Option A:** This option is incorrect because it does not describe the specific reaction to friction that is characteristic of FDEs.
* **Option B:** This option is incorrect because it does not account for the hyperpigmented macules and the specific response to rubbing, which is a hallmark of FDEs.

**Clinical Pearl / High-Yield Fact**
Fixed drug eruptions are a type of cutaneous adverse drug reaction that can occur in response to various medications, including antibiotics, anticonvulsants, and NSAIDs. The lesions typically appear as hyperpigmented macules that are itchy and may blister upon rubbing.

**Correct Answer: C. Fixed Drug Eruptions (FDEs)**