Drugs that are receptor agonists may demonstrate what property?

Correct Answer: C

Rationale: The correct answer is C: Desensitization or down-regulation with continuous use. Receptor agonists continuously activate receptors, leading to desensitization or down-regulation where the receptors become less responsive or decrease in number. This occurs due to the body's attempt to maintain homeostasis in response to constant stimulation. Choices A and D are incorrect because receptor agonists do not typically bind irreversibly or exhibit an inverse relationship between drug concentration and action. Choice B is incorrect as up-regulation is usually observed in response to receptor antagonists, not agonists.

Correct Answer: B

Rationale: The correct answer is B because the blood-brain barrier is designed to restrict the passage of many drugs into and out of brain cells, regulating the entry of substances to protect the brain from harmful agents. Explanation: 1. The blood-brain barrier is a specialized structure that tightly regulates the passage of substances from the bloodstream into the brain. 2. Many drugs, especially water-soluble and ionized ones (choice A), face difficulty crossing this barrier due to their properties. 3. The fetal-placental barrier (choice C) primarily protects the fetus from harmful substances but does not slow drug entry into brain cells. 4. Lipid-soluble drugs (choice D) can actually pass through these barriers easily due to their chemical properties, posing risks for pregnant women. In summary, choice B is correct as it accurately describes the role of the blood-brain barrier in controlling drug entry into brain cells, while the other choices present inaccurate statements regarding these distribution barriers.

Correct Answer: D

Rationale: Step-by-step rationale for why choice D is correct: 1. Unbinding a nonvolatile drug from plasma proteins increases the drug's free concentration in the blood. 2. Increased free drug concentration enhances the drug's ability to be filtered by the kidneys. 3. Higher filtration of the drug leads to increased excretion through the renal system. 4. Therefore, unbinding a nonvolatile drug from plasma proteins enhances the body's ability to excrete the drug efficiently. Summary of why other choices are incorrect: A: Reduced circulation and perfusion of the kidney would decrease the excretion of drugs. B: Chronic renal disease impairs renal function, reducing drug excretion. C: Competition for a transport site by another drug may affect distribution but not necessarily increase drug excretion.

Correct Answer: C

Rationale: The correct answer is C because prescribing a third drug to counteract the adverse reaction of the combination does not address the root cause of the drug-drug interaction. It may lead to further complications and potentially more interactions. Reducing the dose of one drug (A) or both drugs (D) and scheduling their administration at different times (B) are effective strategies to minimize drug-drug interactions by adjusting the dosing regimen.

Correct Answer: D

Rationale: The correct answer is D: Down-regulate the numbers of that specific receptor. Step 1: When an agonist continuously activates a receptor, the body may reduce the number of receptors to maintain homeostasis. Step 2: Down-regulation is a feedback mechanism to decrease receptor sensitivity due to excessive stimulation. Step 3: This helps prevent overstimulation and desensitization of the receptor. Summary: A: Up-regulating the total number of receptors is unlikely as the body typically down-regulates in response to prolonged agonist exposure. B: Blocking the receptor with a partial agonist is not a typical response to repeated agonist exposure. C: Altering the drug's metabolism is not directly related to receptor down-regulation in response to agonist stimulation.