In a quantal dose-response study of a new anticonvulsant, 50% of epileptic patients achieve complete seizure freedom at 200 mg/day. This value represents:

• A The EC50 of the graded concentration-response curve
• B The therapeutic index of the drug
• C The median toxic dose (TD50) of the drug
• D The median effective dose (ED50) of the drug ✓

Explanation

Quantal dose-response curves plot the frequency of a defined all-or-none response against dose in a population. The dose producing the response in 50% of subjects is the median effective dose (ED50). It differs fundamentally from the EC50, which describes graded intensity of effect in a single subject or tissue. Therapeutic index requires both ED50 and TD50 (or LD50), so neither parameter alone constitutes the TI.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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