**Question:** Racemic mixture of two enantiomers with different pharmaeokinetic and pharmacodynamic propeies is seen in -

A. D-penicillamine
B. D-penicillamine and L-penicillamine
C. Levodopa and Carbidopa
D. Dextromethorphan and Levomenthol

**Correct Answer:** D. Dextromethorphan and Levomenthol

**Core Concept:** Enantiomers are non-superimposable mirror images of a molecule, which have the same molecular formula and connectivity but differ in spatial arrangement. Racemates are a mixture of two enantiomers. Pharmaeokinetic and pharmacodynamic properties refer to the characteristics of a drug's absorption, distribution, and action in the body.

**Why the Correct Answer is Right:** Dextromethorphan and Levomenthol are two enantiomers found in a racemic mixture. They exhibit different pharmacokinetic and pharmacodynamic properties. Dextromethorphan is a CNS depressant and has a high affinity for the N-methyl-D-aspartate (NMDA) receptor, leading to the psychoactive effects seen in recreational use. Levomenthol, on the other hand, is a weak local anesthetic and has little to no pharmacological activity.

**Why Each Wrong Option is Incorrect:**

A. D-penicillamine and L-penicillamine are two enantiomers of penicillamine, a drug used to treat Wilson's disease. They have similar pharmacodynamic properties and are not racemic mixtures.

B. D-penicillamine and L-penicillamine are two enantiomers of penicillamine, a drug used to treat Wilson's disease. They have similar pharmacodynamic properties and are not racemic mixtures.

C. Levodopa and Carbidopa are enantiomers involved in the treatment of Parkinson's disease. They are not racemic mixtures, and their properties are complementary, not different.

Dextromethorphan and Levomenthol are the correct answer, as they are enantiomers in a racemic mixture, displaying different pharmacokinetic and pharmacodynamic properties. This is why they are used in this context:

**Clinical Pearl:** Racemates are important in drug design, as they can be used to create drugs that selectively target one enantiomer to avoid side effects associated with the other. This is a common approach in the development of drugs like levodopa-carbidopa combination for Parkinson's disease treatment.