**Question:** Racemic mixture of two enantiomers with different pharmacokinetic and pharmacodynamic properties is seen in

A. Aspirin
B. Penicillin
C. Morphine
D. Thalidomide

**Core Concept:** Enantiomers are stereoisomers that are non-superimposable mirror images of each other and have identical physicochemical properties but different biological activities. A racemic mixture contains an equal amount of both enantiomers.

**Why the Correct Answer is Right:** D-penicillin is an example of a racemic mixture, containing two enantiomers: R-penicillin and S-penicillin. Both enantiomers possess similar physicochemical properties but differ in their pharmacokinetic and pharmacodynamic properties. R-penicillin is more active against gram-positive bacteria, while S-penicillin is more active against gram-negative bacteria. This results in improved antimicrobial activity and reduced side effects when using a single racemic compound instead of separate R and S enantiomers.

**Why Each Wrong Option is Incorrect:**

A. Aspirin (salicylic acid) is a single enantiomer and does not exhibit racemization. It possesses a single active enantiomer that inhibits cyclooxygenase enzymes, leading to anti-inflammatory, analgesic, and antipyretic effects.

B. Penicillin is the correct answer, but the correct option is D, not B.

C. Morphine is an opioid analgesic with a single enantiomer. It possesses similar physiochemical properties but differential pharmacological effects due to its interaction with opioid receptors.

D. Thalidomide is the correct example of a racemic mixture, containing two enantiomers with distinct pharmacokinetic and pharmacodynamic properties.

**Clinical Pearl:** Understanding the concept of racemates (racemic mixtures) helps in optimizing drug therapy by selecting the appropriate enantiomer(s) based on their distinct pharmacological properties. This knowledge can guide the choice of drugs for treating specific infections or conditions, potentially reducing side effects and enhancing therapeutic efficacy.