A drug undergoes extensive first-pass metabolism in the liver. Its oral bioavailability is 20% despite complete GI absorption. The hepatic extraction ratio (ER) is:
- A 0.80 ✓
- B 0.20
- C 0.50
- D 0.40
Correct answer: A. 0.80
Explanation
Bioavailability (F) = 1 - ER (for a drug with complete GI absorption and no gut wall metabolism). Therefore ER = 1 - F = 1 - 0.20 = 0.80. An extraction ratio of 0.80 means 80% of the drug is extracted and metabolized during a single pass through the liver. High-extraction drugs (ER > 0.7) include lidocaine, propranolol, morphine, and verapamil; their oral bioavailability is markedly reduced by first-pass metabolism and is highly sensitive to changes in hepatic blood flow (e.g., cirrhosis, congestive heart failure).
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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