**Question:** The penicillin G preparation with the longest duration of action is:

A. Benzylpenicillin
B. Procaine penicillin
C. Procaine penicillin G
D. Penicillin G potassium

**Core Concept:** Penicillin is an antibiotic derived from the fungus Penicillium chrysogenum. It works by inhibiting bacterial cell wall synthesis, causing the bacteria to lyse. Penicillin G is the most commonly used penicillin and comes in different preparations with varying durations of action due to differences in their pharmacokinetic properties.

**Why the Correct Answer is Right:** Penicillin G potassium has the longest duration of action among the given options. This is because it is a potassium salt, which is more soluble in water and gets rapidly absorbed into the bloodstream upon injection. As a result, it reaches a higher peak concentration and has a longer duration of action compared to other penicillin G preparations.

**Why Each Wrong Option is Incorrect:**

A. Benzylpenicillin: This option refers to benzylpenicillin, which is a prodrug that requires enzymatic conversion to the active form, benzylpenicillin potassium, for its therapeutic action. This conversion process slows down its absorption and prolongs the time to peak concentration, resulting in a shorter duration of action.

B. Procaine penicillin: Procaine is a local anesthetic that is ester-linked to penicillin G, which impairs its solubility and absorption. This leads to a slower and less predictable absorption rate, ultimately reducing the duration of action compared to penicillin G potassium.

C. Procaine penicillin G: Procaine is again linked to penicillin G, similar to option B, which results in reduced absorption rate and shorter duration of action.

**Clinical Pearl:** Penicillin G potassium is commonly used for treating severe infections, especially those with high inoculum size or caused by resistant bacteria, due to its extended duration of action. However, it should not be used in patients with known penicillin allergy or severe renal impairment, as it is not suitable for these patient populations.