While receiving mechanical ventilation, a 57-year-old man contracts pneumonia caused by Pseudomonas aeruginosa. The patient is given antibiotic therapy with ceftazidime and gentamicin. Assuming the half-life of gentamicin is 3 hours in this paicular patient, what percentage of the initial dose will most likely remain in his body 6 hours later?

**Core Concept**
Pharmacokinetics of gentamicin, a nephrotoxic aminoglycoside antibiotic, involves understanding its elimination half-life and its impact on drug concentration over time. In this scenario, the patient's half-life of gentamicin is 3 hours, which determines how quickly the drug is eliminated from the body.

**Why the Correct Answer is Right**
After one half-life (3 hours), 50% of the initial dose of gentamicin remains in the body. After another half-life (6 hours total), 50% of the remaining 50% is eliminated, leaving 25% of the initial dose. This is calculated using the formula for exponential decay: A = P(1/2)^t/T, where A is the amount remaining, P is the initial dose, t is time elapsed, and T is the half-life.

**Why Each Wrong Option is Incorrect**
**Option A:** This option suggests that 100% of the initial dose remains, which contradicts the concept of exponential decay and the patient's half-life.

**Option B:** This option implies that 75% of the initial dose remains, which is incorrect because after the first half-life, 50% remains, and after the second half-life, 25% of the initial dose remains.

**Option C:** This option suggests that 50% of the initial dose remains, which is incorrect because this would be the amount remaining after one half-life, not two.

**Clinical Pearl / High-Yield Fact**
The half-life of a drug is crucial in determining the dosing interval for aminoglycosides like gentamicin. In this case, the dosing interval should be at least 8-12 hours to avoid accumulation and toxicity.

**Correct Answer: D. 25%**