Which antiemetic selectively blocks levodopa-induced vomiting without blocking its antiparkinsonian action?

Correct Answer: C

Rationale: Rationale: 1. Domperidone selectively blocks levodopa-induced vomiting by acting on peripheral dopamine receptors in the chemoreceptor trigger zone. 2. It does not cross the blood-brain barrier, hence does not interfere with levodopa's antiparkinsonian action. 3. Metoclopramide (A) can cross the blood-brain barrier and may diminish the therapeutic effects of levodopa. 4. Cisapride (B) is a prokinetic agent and does not have a specific antiemetic effect on levodopa-induced vomiting. 5. Ondansetron (D) is a serotonin receptor antagonist that targets different pathways and does not specifically address levodopa-induced vomiting.

Correct Answer: A

Rationale: The correct answer is A: Partial agonist at Mu receptor. Buprenorphine is a partial agonist at the Mu opioid receptor, meaning it activates the receptor to produce a partial response compared to a full agonist. This property allows for reduced risk of respiratory depression and overdose compared to full agonists. Choices B, C, and D are incorrect because buprenorphine does not act as a full agonist at either the Mu or Kappa receptors. It is important to understand the pharmacological properties of buprenorphine to optimize its use in clinical practice.

Correct Answer: A

Rationale: The correct answer is A: Prostaglandins. Prostaglandins are most associated with labor as they play a key role in promoting uterine contractions. They are synthesized in the uterus and stimulate smooth muscle contractions, leading to the initiation and progression of labor. Thromboxanes primarily regulate platelet function and vasoconstriction, not uterine contractions. Leukotrienes are involved in inflammatory responses and immune regulation, not specifically labor. NSAIDs inhibit prostaglandin synthesis, which can hinder labor progression, making them an unlikely choice for promoting uterine contractions.

Correct Answer: D

Rationale: The correct answer is D because the first pass effect refers to the amount of drug that is lost due to hepatic metabolism when a drug is absorbed from the gastrointestinal tract after oral administration. This process occurs before the drug reaches systemic circulation, reducing the bioavailability of the drug. Option A is incorrect as hepatic artery does not eliminate the drug. Option B is incorrect as it refers to drug excretion via stool. Option C is incorrect as it refers to drug destruction by stomach acidity, not hepatic metabolism.

Correct Answer: D

Rationale: The correct answer is D because drugs exhibiting zero-order kinetics have a linear plot of drug concentration versus time. In zero-order kinetics, the rate of drug elimination remains constant regardless of drug concentration, leading to a linear decrease in drug concentration over time. Choice A is incorrect as first-order kinetics are more common. Choice B is incorrect as zero-order kinetics show a constant rate of elimination, not exponential decrease. Choice C is incorrect as the half-life of drugs exhibiting zero-order kinetics is dose-dependent.