**Core Concept**

Mastitis is an inflammation of the breast tissue, often caused by bacterial infection. The drug of choice for treating severe mastitis should ideally be effective against the common pathogens responsible for the infection, have minimal side effects, and be easy to administer.

**Why the Correct Answer is Right**

The drug of choice for severe mastitis is usually a broad-spectrum antibiotic that covers Staphylococcus aureus, Streptococcus pyogenes, and other common breast pathogens. **Penicillinase-resistant penicillins**, such as Flucloxacillin, are effective against S. aureus, which is a common cause of mastitis. Flucloxacillin also has a low incidence of side effects and is easily administered orally or intravenously.

**Why Each Wrong Option is Incorrect**

* **Option A:** Amoxicillin is not the first choice for mastitis because it is not effective against methicillin-resistant S. aureus (MRSA), which is becoming increasingly common.
* **Option B:** Cephalexin is a cephalosporin antibiotic, but it is not as effective as flucloxacillin against S. aureus.
* **Option D:** Erythromycin is a macrolide antibiotic, but it is not the first choice for mastitis due to its lower efficacy against S. aureus compared to penicillinase-resistant penicillins.

**Clinical Pearl / High-Yield Fact**

When treating mastitis, it is essential to consider the severity of the infection, the patient's allergies, and the local antibiotic resistance patterns to choose the most effective antibiotic.

**Correct Answer: C. Flucloxacillin**