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

A. D-Amphetamine and L-Amphetamine
B. Dextrorphan and Levorphan
C. D-Methadone and L-Methadone
D. D-Aspartame and L-Aspartame

**Correct Answer:** D. D-Aspartame and L-Aspartame

**Core Concept:** Enantiomers are non-superimposable mirror images of each other, and their pharmacological and pharmacokinetic properties can differ due to their interaction with chiral receptors and enzymes.

**Why the Correct Answer is Right:** In the case of aspartame, the two enantiomers are D-Aspartame (active enantiomer) and L-Aspartame. These two enantiomers have different pharmacokinetic and pharmacodynamic properties. D-Aspartame is rapidly absorbed, while L-Aspartame is not, and D-Aspartame is the active component responsible for the sweet taste.

**Why Each Wrong Option is Incorrect:**

A. D-Amphetamine and L-Amphetamine: Although these enantiomers have different pharmacokinetic properties, they are neurotransmitters and not sweeteners. Their pharmacodynamic effects are similar, as they act on the same receptors.

B. Dextrorphan and Levorphan: These are the dextrorotatory and levorotatory forms of codeine, respectively. They have different pharmacokinetic properties due to their interaction with the O-demethylation enzyme CYP2D6, but their pharmacodynamic effects are similar as they both produce analgesia and sedation.

C. D-Methadone and L-Methadone: These are the dextrorotatory and levorotatory forms of methadone, respectively. They have different pharmacokinetic properties due to their interaction with the O-demethylation enzyme CYP2B6, but their pharmacodynamic effects are similar as they both produce opioid effects.

D. D-Aspartame and L-Aspartame: As explained above, D-Aspartame is the active enantiomer responsible for the sweet taste, while L-Aspartame is not absorbed and is excreted.

**Clinical Pearl:** Understanding the concept of enantiomers is crucial for appreciating the pharmacokinetic and pharmacodynamic differences between drugs, particularly when assessing drug interactions or selecting the appropriate dosage form for a patient.