**Core Concept**
Mastitis is an inflammatory condition of the breast, often caused by bacterial infection. The primary goal of treatment is to address the underlying infection and alleviate symptoms.

**Why the Correct Answer is Right**
The first-line treatment for mastitis typically involves antibiotics to target the causative bacteria. **Penicillin** is a broad-spectrum antibiotic effective against a wide range of bacteria, including Staphylococcus aureus, a common cause of mastitis. Penicillin works by inhibiting cell wall synthesis in bacteria, ultimately leading to their death. It is often administered orally or intravenously, depending on the severity of the infection.

**Why Each Wrong Option is Incorrect**
**Option A:** While **Cephalosporins** are also effective antibiotics, they are not typically the first-line choice for mastitis due to potential resistance patterns and side effects.
**Option B:** **Macrolides** are a class of antibiotics that can be used in mastitis, but they are generally reserved for patients with penicillin allergies or when penicillin is not effective.
**Option C:** **Antifungals** are not typically used to treat mastitis, as this condition is usually caused by bacterial infection, not fungal infection.

**Clinical Pearl / High-Yield Fact**
When treating mastitis, it's essential to consider the patient's overall health, potential allergies, and the local resistance patterns of the causative bacteria to choose the most effective antibiotic.

**Correct Answer: D. Penicillin.**