**Core Concept:** Gram-positive cocci are cocci that stain purple in the Gram stain due to the presence of peptidoglycan in the cell wall. Penicillin is an antibiotic that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs). Resistance to penicillin indicates that the bacteria has developed mechanisms to prevent the antibiotic from binding to PBPs, such as increased penicillinase production, altered PBPs, or reduced cell wall permeability.

**Why the Correct Answer is Right:** The correct answer is **D. Staphylococcus aureus** because it is a Gram-positive coccus that can develop resistance to penicillin through various mechanisms mentioned above. It is a common cause of post-operative infections, especially in cases like this patient's cardiac surgery.

**Why Each Wrong Option is Incorrect:**

A. **Staphylococcus epidermidis**: Although also a Gram-positive coccus, S. epidermidis is less likely to cause post-operative infections and is generally more resistant to beta-lactam antibiotics due to its ability to form a biofilm.

B. **Streptococcus pyogenes**: While S. pyogenes is a Gram-positive coccus and causes post-operative infections, it is less likely to cause fever and chills following cardiac surgery compared to S. aureus.

C. **Streptococcus pneumoniae**: Similar to S. pyogenes, S. pneumoniae is less likely to cause post-operative fever and chills following cardiac surgery compared to S. aureus. Additionally, it is less commonly resistant to penicillin due to its different penicillin-binding proteins.

**Clinical Pearl:** It is crucial to consider antibiotic resistance mechanisms when diagnosing post-operative infections in patients with fever and chills. Infections with resistant Gram-positive cocci like S. aureus can pose a therapeutic challenge and prompt a change in antibiotic therapy.