**Core Concept**
Antipseudomonal penicillins are broad-spectrum penicillins specifically effective against *Pseudomonas aeruginosa*, a gram-negative bacillus commonly implicated in hospital-acquired infections. These drugs possess enhanced activity against pseudomonas due to structural modifications that improve penetration through its outer membrane and resistance to beta-lactamase degradation.

**Why the Correct Answer is Right**
Piperacillin is a broad-spectrum penicillin with intrinsic activity against *Pseudomonas aeruginosa*. It is structurally related to ticarcillin and has a side chain that enhances penetration into gram-negative bacteria, including pseudomonas. Piperacillin is often used in combination with tazobactam (as piperacillin-tazobactam) to provide beta-lactamase inhibition and improve efficacy against resistant strains.

**Why Each Wrong Option is Incorrect**
Option A: Cephalexin is a first-generation cephalosporin with limited activity against *Pseudomonas* and is primarily effective against gram-positive organisms and some gram-negative bacteria.
Option B: Dicloxacillin is a penicillinase-resistant penicillin used against staphylococcal infections, but it lacks activity against *Pseudomonas*.
Option D: Cloxacillin is also a penicillinase-resistant penicillin, effective against MSSA but inactive against pseudomonas.

**Clinical Pearl / High-Yield Fact**
Piperacillin-tazobactam is a first-line empiric therapy for severe infections involving *Pseudomonas aeruginosa*, especially in immunocompromised or ICU patients. Remember: "Piperacillin = antipseudomonal penicillin."

✓ Correct Answer: C. Piperacillin