**Core Concept**
Methicillin resistance in Staphylococcus aureus (MRSA) is a significant clinical concern, as it limits treatment options for serious infections. Methicillin resistance is primarily due to the production of a modified penicillin-binding protein (PBP2a), which has a low affinity for β-lactam antibiotics.

**Why the Correct Answer is Right**
The mecA gene, responsible for encoding PBP2a, is located on a mobile genetic element called the staphylococcal chromosomal cassette mec (SCCmec). SCCmec is a bacteriophage that inserts into the S. aureus chromosome, carrying the mecA gene with it. This allows S. aureus to become resistant to methicillin and other β-lactam antibiotics. The mecA gene is responsible for the production of PBP2a, which has a reduced affinity for β-lactam antibiotics, thereby allowing the bacteria to grow in the presence of these drugs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** This option is not provided.
**Option C:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
Methicillin resistance is not the same as penicillin resistance. While penicillin-resistant S. aureus may still be susceptible to methicillin, methicillin-resistant S. aureus is resistant to all β-lactam antibiotics, including methicillin, oxacillin, and nafcillin.

**Correct Answer:** D. The mecA gene is responsible for encoding PBP2a, which is the primary mechanism of methicillin resistance in S. aureus.