**Core Concept**
Acne vulgaris is a common skin condition characterized by the occurrence of comedones, inflammatory papules, pustules, and nodules. The pathogenesis involves the obstruction of pilosebaceous units due to hyperkeratinization, sebum production, and the proliferation of Propionibacterium acnes (P. acnes).

**Why the Correct Answer is Right**
The pathognomonic lesion of acne vulgaris is the **comedone**, particularly the open comedone or **blackhead**. This occurs when the pilosebaceous unit becomes obstructed, leading to the accumulation of keratin, sebum, and P. acnes. As the comedone grows, it becomes visible on the skin's surface as a dark, open pore. The presence of a blackhead is a hallmark of acne vulgaris and is often used as a diagnostic criterion.

**Why Each Wrong Option is Incorrect**
* **Option A:** Milia are small, white bumps that occur when keratin becomes trapped in the skin, but they are not pathognomonic for acne.
* **Option B:** Papules and pustules are inflammatory lesions that occur in acne, but they are not the defining characteristic of the condition.
* **Option D:** Nodules are large, painful lesions that occur in severe acne, but they are not the primary lesion that defines acne vulgaris.

**Clinical Pearl / High-Yield Fact**
To remember the pathognomonic lesion of acne vulgaris, recall that **"Blackheads are the hallmark of acne"**. This can help you quickly identify the key feature of this common skin condition.

**Correct Answer: C. Comedone (blackhead)**