**Core Concept**
The patient's presentation of multiple shiny pinhead-sized white papules over the dorsum of the head, forearm, and penis is suggestive of a condition characterized by the presence of milia, which are small, white bumps caused by keratin cysts. This condition is often associated with the use of certain medications, skin conditions, or as a result of trauma.

**Why the Correct Answer is Right**
The correct diagnosis is **Milia**. Milia are small, white bumps that appear on the skin due to the accumulation of keratin, a protein found in skin cells. This accumulation occurs when keratin is trapped beneath the skin, causing a small cyst to form. The shiny appearance of the papules is due to the reflection of light off the surface of the keratin cysts. The presence of milia in multiple locations, including the dorsum of the head, forearm, and penis, supports this diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be considered if the patient had a history of eczema or atopic dermatitis, but the presentation of milia is distinct from the typical skin lesions seen in these conditions.
**Option B:** This option might be considered if the patient had a history of acne, but the presence of shiny, white papules without the characteristic comedones or inflammatory lesions makes acne less likely.
**Option C:** This option might be considered if the patient had a history of skin trauma, but the distribution and appearance of the papules are not typical of traumatic lesions.

**Clinical Pearl / High-Yield Fact**
Milia can be distinguished from other skin lesions by their characteristic shiny appearance and small size. They are often found in areas where the skin is prone to keratin accumulation, such as the face, neck, and hands.

**Correct Answer:** C.