**Core Concept:** Acne is a common chronic inflammatory skin disorder, primarily affecting the pilosebaceous units (hair follicles and sebaceous glands). The pathogenesis involves the accumulation of sebum, keratin, and bacteria in the follicle, leading to inflammation and formation of comedones.

**Why the Correct Answer is Right:** In acne, the primary change involves the overproduction of sebum, which is an oily substance secreted by the sebaceous glands for the lubrication and protection of the skin. Sebum is composed mainly of triglycerides, free fatty acids, squalene, and cholesterol esters. The correct answer is **D** - sebum, because the increased levels of triglycerides and free fatty acids contribute to the plugging of the pilosebaceous unit and subsequent inflammation in acne.

**Why Each Wrong Option is Incorrect:**

A. **Keratin:** Although keratin accumulation can occur in acne, the question asks for the acids, and keratin is a protein, not an acid.

B. **Squalene:** This is a lipid, not an acid, and although it is present in sebum, it does not contribute to the acidic environment that triggers inflammation in acne.

C. **Cholesterol esters:** These are lipids, not acids, and do not play a significant role in the pathogenesis of acne.

**Clinical Pearl:** To understand acne pathogenesis, it is essential to comprehend the role of sebum composition, specifically the increased triglycerides and free fatty acids, leading to the formation of comedones and inflammation.

**Correct Answer:** D. Sebum