**Core Concept**
Acne vulgaris is a chronic inflammatory skin condition caused by a combination of factors, including excess sebum production, keratinization, and bacterial colonization by Propionibacterium acnes (P. acnes). Effective treatment of acne involves addressing these underlying factors, often with a combination of topical and systemic therapies.

**Why the Correct Answer is Right**
Minocycline is a broad-spectrum antibiotic belonging to the tetracycline class, which is effective against P. acnes. It works by inhibiting protein synthesis in bacteria, thereby reducing inflammation and preventing the formation of acne lesions. Minocycline also has anti-inflammatory properties, which help to reduce the severity of acne.

**Why Each Wrong Option is Incorrect**
**Option A:** Penicillin is not typically used to treat acne vulgaris, as it is not effective against P. acnes. Penicillin is generally used to treat bacterial infections caused by other types of bacteria.
**Option B:** Co-triamoxazole is a combination of sulfamethoxazole and trimethoprim, which is used to treat a variety of bacterial infections. However, it is not typically used to treat acne vulgaris, as it can have side effects such as skin rash and photosensitivity.
**Option D:** Gentamicin is an aminoglycoside antibiotic that is effective against a wide range of bacterial infections. However, it is not typically used to treat acne vulgaris, as it can cause skin irritation and other side effects.
**Option E:** Tetracycline is a broad-spectrum antibiotic that is effective against P. acnes and is often used to treat acne vulgaris. However, minocycline is a more effective and better-tolerated option, making it the preferred choice.

**Clinical Pearl / High-Yield Fact**
When treating acne vulgaris, it's essential to use a combination of therapies that address the underlying factors contributing to the condition. Topical retinoids and benzoyl peroxide can help to reduce inflammation and prevent the formation of acne lesions, while systemic antibiotics like minocycline can help to control bacterial colonization.

**Correct Answer: C. Minocycline**