**Core Concept**
Piperacillin is a broad-spectrum cephalosporin with enhanced activity against Gram-negative bacteria, including *Pseudomonas aeruginosa*, due to its ability to penetrate the outer membrane of this organism. It is often used in combination with tazobactam (piperacillin-tazobactam) to overcome beta-lactamase resistance.

**Why the Correct Answer is Right**
Piperacillin, especially in combination with tazobactam, is effective against *Pseudomonas aeruginosa* because it can penetrate the outer membrane of Gram-negative rods and remains active despite beta-lactamase production. Unlike other penicillins, piperacillin has intrinsic activity against *Pseudomonas*, making it a first-line agent in severe infections involving this pathogen.

**Why Each Wrong Option is Incorrect**
Option B: Methicillin is a penicillinase-resistant penicillin, but it has no activity against *Pseudomonas aeruginosa* and is primarily effective against Gram-positive organisms.
Option C: Nafcillin and Option D: Cloxacillin are also penicillinase-resistant penicillins, used mainly for Staphylococcus aureus infections, and lack activity against Gram-negative bacteria like *Pseudomonas*.

**Clinical Pearl / High-Yield Fact**
Piperacillin-tazobactam is the most commonly used beta-lactam agent for empiric coverage of *Pseudomonas aeruginosa* in hospital-acquired infections, especially in patients with severe sepsis or pneumonia.

✓ Correct Answer: A. Piperacillin