<prompt> Gene responsible for semisynthetic penicillin resistance in Methicillin resistant Staph aureus MRSA?Mec AMec BMec DMec C  <response>: Methicillin resistance in S. aureus is mediated by a chromosomally coded gene called mec A gene, which alters penicillin-binding protein PBP present on S. aureus cell membranetoPBP-2a. PBP is an essential protein needed for cell wall synthesis of bacteria. Beta lactam drugs bind and inhibit this protein, thereby inhibit the cell wall synthesis. The altered PBP2a of MRSA strains has less affinity for beta lactam antibiotics; hence, MRSA strains are resistant to all beta lactam antibiotics. 