## **Core Concept**
Penicillinase-resistant penicillins are a class of antibiotics designed to resist degradation by beta-lactamase enzymes (penicillinases) produced by certain bacteria. These drugs are used to treat infections caused by bacteria that are resistant to other penicillins.

## **Why the Correct Answer is Right**
The correct answer, **. Methicillin**, is a penicillinase-resistant penicillin. It works by inhibiting bacterial cell wall synthesis, but its beta-lactam ring is stable against hydrolysis by beta-lactamase enzymes. This property makes methicillin effective against bacteria that produce penicillinases, such as some strains of Staphylococcus aureus.

## **Why Each Wrong Option is Incorrect**
* **Option A:** is incorrect because is primarily used to treat infections caused by susceptible gram-negative bacteria and some gram-positive bacteria but is not notable for penicillinase resistance.
* **Option B:** is incorrect because, although it has a broad spectrum of activity, its resistance to penicillinase is not its most distinguishing feature.
* **Option D:** is incorrect because, while it is a type of penicillin with a broad spectrum of activity, it is not specifically known for being penicillinase-resistant.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that methicillin (and other penicillinase-resistant penicillins like **. Nafcillin** and **. Oxacillin**) is used specifically for treating infections caused by methicillin-sensitive Staphylococcus aureus (MSSA). The development of methicillin-resistant Staphylococcus aureus (MRSA) has limited the use of these drugs.

## **Correct Answer:** . Methicillin.