**Core Concept**
Methicillin resistance in MRSA (Methicillin-resistant Staphylococcus aureus) is primarily due to the acquisition of the mecA gene, which encodes a modified penicillin-binding protein (PBP2a). This modified PBP has a low affinity for beta-lactam antibiotics, rendering them ineffective against the bacteria.

**Why the Correct Answer is Right**
The mecA gene is carried on a mobile genetic element called the staphylococcal cassette chromosome mec (SCCmec), which is integrated into the S. aureus chromosome. This SCCmec element is not a plasmid, but rather a chromosomal integration site that harbors the mecA gene and other associated genes. The mecA gene is responsible for the methicillin resistance phenotype in MRSA, and its expression is not dependent on plasmid-mediated resistance.

**Why Each Wrong Option is Incorrect**
**Option A:** Resistance is produced as a result of altered PBP's - **This statement is true**. The mecA gene encodes a modified PBP2a that has a low affinity for beta-lactam antibiotics, making this statement correct about MRSA resistance.

**Option B:** Resistance may be produced by hyperproduction of beta lactamase - **This statement is partially true**. While beta-lactamase production can contribute to resistance in some bacteria, in MRSA, the primary mechanism of resistance is the acquisition of the mecA gene and the production of modified PBP2a.

**Option D:** Resistance may be missed at incubation temperature of 37 oC during susceptibility testing - **This statement is true**. MRSA resistance can be missed if the susceptibility testing is performed at 37°C, as the bacteria may require a higher temperature (e.g., 45°C) to fully express the methicillin resistance phenotype.

**Clinical Pearl / High-Yield Fact**
The mecA gene is a key determinant of methicillin resistance in MRSA, and its detection is critical for guiding antibiotic therapy. The SCCmec element, which harbors the mecA gene, is a mobile genetic element that can be transmitted between S. aureus isolates, contributing to the spread of MRSA in healthcare settings.

✓ Correct Answer: C. Resistance is primarily mediated/transmitted plasmids