1. C. The placenta can serve as a site of metabolism for some drugs passing through it. The placenta can carry out a number of drug metabolizing reactions, including dealkylation and hydroxylation. Lipophilic drugs readily diffuse across the placenta and enter the fetal circulation; drugs with a molecular weight of under 500 generally can easily cross the placenta; and no known active transport systems play any important role in drug secretion in the placenta.

2. B. Indomethacin's action as a potent nonselective COX inhibitor appears to be important in speeding up ductus closure. The other drugs do not have such an action, and prostaglandin E1 given by infusion would cause the ductus to remain open.

3. E. The kidney, the most important organ involved in drug clearance, especially of highly water-soluble drugs, shows an age-related decline in function. Lean body mass decreases, percentage of body fat increases, production of plasma proteins decrease, and drug metabolism decreases for most drugs as patients reach their seventh and eighth decade.

4. C. The large amounts of drug B that are given can displace the almost completely bound drug A from its albumin binding site and lead to an increase in free drug. The latter is then available to be distributed outside the blood compartment and reach tissues where its concentrations will increase.

5. D. The elderly are frequently sensitive to the pulmonary depressing actions of opioid analgesics. These agents should be used with caution in the elderly until an adequate dosage has been determined for a particular patient. This does not mean that the older patient should not be given opioid analgesics for pain, however, since appropriate pain management should always be part of any total clinical treatment plan.

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