1. A. When one inhibits the action of a drug-metabolizing enzyme (A), one would expect an increase instead of a decrease in drug concentrations, since less is being metabolized. Induction of an enzyme (B) would have the opposite effect, since there would be more enzyme available to metabolize the drug. C is correct, since the most common mechanism of enzyme induction is through synthesis of new enzyme protein, which does not occur immediately. Finally, mechanism-based inactivation (D) is also correct, since this is irreversible, leaving the enzyme inactive and eventually it is degraded by the body.
2. A. CYP3A4 is the predominant cytochrome P450 drug-metabolizing enzyme in the body, both in terms of amount of enzyme and the number of drugs that it metabolizes. It has been estimated to carry out approximately 50% of the cytochrome P450-mediated reactions observed. The other enzymes have been reported to carry out 30% (CYP2D6), 15-20% (CYP2C9) and 1-2% (both CYP2E1 and CYP1A2).
3. E. Most glucuronic acid conjugates are less effective than the parent drug. The conjugate, however, usually maintains the same pharmacological mechanism of action, although frequently of a lesser magnitude. Conjugation with glucuronic acid makes a drug molecule more water soluble (A), and glu-curonic acid conjugates are more likely to be eliminated by secretion into the bile (C) than are uncon-jugated compounds. These glucuronide conjugates, once secreted into the bile, may be cleaved by (3-glucuronidases to liberate the parent compound, which can then be reabsorbed (D). Several glu-curonic acid conjugates of drugs (e.g., morphine 6-glucuronide) possess pharmacological activity (B).
4. C. Plasma proteins are too large to be filtered by the glomerulus, so that any drug molecules bound to these plasma proteins will not undergo filtration. A is not correct: ionized drugs are less likely to undergo reabsorption, since this is generally thought to be a passive process. B is also not correct: low-molecular-weight drugs are more likely to be filtered, since they can easily pass through the glomerulus filter. Finally, weakly acidic drugs will be un-ionized at a low (acidic) pH, hence more likely to undergo reabsorption, thus reducing the net elimination (D).
5. B. Lipid-soluble molecules are more likely to be excreted in breast milk because it is primarily a passive diffusion process. A, C, and D are not correct because they are opposite of the typical characteristics of drugs excreted into breast milk.
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