## Core Concept
Methicillin resistance in Methicillin-resistant Staphylococcus aureus (MRSA) is primarily attributed to the acquisition of the **mecA gene**, which encodes penicillin-binding protein 2a (PBP2a). This protein has a low affinity for β-lactam antibiotics, rendering them ineffective. The mecA gene is carried on a mobile genetic element known as the **staphylococcal cassette chromosome mec (SCCmec)**.

## Why the Correct Answer is Right
The correct answer involves understanding that MRSA's resistance to methicillin and other β-lactam antibiotics is mainly due to the mecA gene. This gene leads to the production of PBP2a, which has a significantly reduced affinity for β-lactam antibiotics. As a result, MRSA strains are resistant to all β-lactam antibiotics, including methicillin, oxacillin, and penicillin.

## Why Each Wrong Option is Incorrect
- **Option A:** Typically, statements about MRSA being resistant to methicillin and other β-lactams due to the mecA gene are correct.
- **Option B:** If an option suggests that MRSA's resistance is not related to the mecA gene or involves another mechanism, it would be incorrect because the mecA gene is the primary determinant of methicillin resistance.
- **Option C:** Similar to Option B, if it inaccurately describes the role of mecA or suggests an alternative primary mechanism, it would be incorrect.
- **Option D:** Without specific details on the options, generally, any statement contradicting the established role of mecA in MRSA resistance or suggesting an incorrect alternative would be wrong.

## Clinical Pearl / High-Yield Fact
A crucial point to remember is that **MRSA infections are treated with antibiotics that are not β-lactams**, such as vancomycin, linezolid, or daptomycin. The presence of the mecA gene makes MRSA resistant to all β-lactam antibiotics, necessitating the use of alternative antimicrobial agents.

## Correct Answer: D.