**Core Concept**
Sebaceous glands produce sebum, an oily substance that lubricates the skin and hair. When sebum accumulates due to overactivity of these glands or obstruction of ducts, it leads to comedones and inflammatory lesions, which are hallmark features of acne vulgaris.

**Why the Correct Answer is Right**
Acne vulgaris is directly caused by the accumulation of sebum in hair follicles, leading to comedones (blackheads and whiteheads). This is exacerbated by increased sebum production, keratinization, and bacterial colonization (e.g., *Propionibacterium acnes*). The blockage and inflammation result in papules, pustules, nodules, and cysts—classic signs of acne. This process is especially common in adolescents due to hormonal fluctuations.

**Why Each Wrong Option is Incorrect**
Option A: Milia are small, white cysts caused by keratin accumulation beneath the skin, not sebum buildup. They result from blocked follicles with keratin, not sebum.
Option C: Epidermoid cysts arise from epidermal cells and are filled with keratin, not sebum. They are non-infectious, slow-growing, and unrelated to sebaceous gland function.
Option D: Miliaria (prickly heat) results from sweat duct obstruction, not sebaceous gland sebum. It occurs in hot, humid environments and involves sweat, not sebum.

**Clinical Pearl / High-Yield Fact**
Acne vulgaris is the most common skin disorder in adolescents and is directly linked to sebum overproduction, follicular hyperkeratinization, and bacterial infection—making it a key topic in dermatology for exams.

✓ Correct Answer: B. Acne