**Core Concept**
The question is testing the student's knowledge of beta-lactam antibiotic resistance mechanisms, specifically the production of beta-lactamase enzymes by certain bacteria. The patient's symptoms and gram-positive cocci in clusters are indicative of Staphylococcus aureus infection.

**Why the Correct Answer is Right**
Staphylococcus aureus produces a beta-lactamase enzyme called penicillinase, which breaks down penicillin and other beta-lactam antibiotics, rendering them ineffective. This is a common mechanism of resistance in S. aureus, particularly in methicillin-sensitive S. aureus (MSSA) strains. The production of penicillinase is encoded by the blaZ gene, which is responsible for the breakdown of penicillin.

**Why Each Wrong Option is Incorrect**
* **Option A:** Penicillin G is a narrow-spectrum beta-lactam antibiotic that is effective against many gram-positive cocci, including some strains of S. aureus that are not resistant to beta-lactamase.
* **Option B:** Ampicillin is a broad-spectrum penicillin that is effective against a wider range of bacteria, including some beta-lactamase-producing strains of S. aureus. However, it is not the most effective option in this scenario.
* **Option D:** Nafcillin is a penicillinase-resistant penicillin that is effective against beta-lactamase-producing strains of S. aureus.

**Clinical Pearl / High-Yield Fact**
It's essential to note that methicillin-resistant S. aureus (MRSA) strains produce a different enzyme, PBP2a, which confers resistance to all beta-lactam antibiotics, including penicillinase-resistant penicillins.

**Correct Answer:** C. Amoxicillin