Which of the following agents is an alfa2–selective antagonist?

Correct Answer: A

Rationale: The correct answer is A: Yohimbine. Yohimbine is an alpha2-selective antagonist because it specifically blocks alpha2-adrenergic receptors. This leads to increased sympathetic activity since alpha2 receptors normally inhibit sympathetic activity when activated. B: Tamsulosin is an alpha1-selective antagonist used to treat benign prostatic hyperplasia. C: Tolazoline is a non-selective alpha-adrenergic antagonist, affecting both alpha1 and alpha2 receptors. D: Prazosin is an alpha1-selective antagonist used to treat hypertension and benign prostatic hyperplasia. In summary, yohimbine is the correct answer because it selectively blocks alpha2 receptors, while the other choices are either alpha1-selective or non-selective alpha antagonists.

Correct Answer: A

Rationale: The correct answer is A because phenytoin blocks sodium channels, inhibiting the abnormal firing of neurons seen in epilepsy. This action helps to stabilize neuronal membranes and prevent seizures. Options B, C, and D are incorrect as phenytoin does not bind to GABA receptors, affect Ca2+ currents, or inhibit GABA-transaminase. These mechanisms are associated with different antiepileptic drugs.

Correct Answer: B

Rationale: The correct answer is B because carbidopa inhibits dopa decarboxylase enzyme, preventing the conversion of levodopa into dopamine outside the brain. This allows more levodopa to reach the brain and be converted into dopamine, improving its efficacy in treating Parkinson's disease. Choice A is incorrect as carbidopa does not directly affect the synthesis, release, or reuptake of dopamine. Choice C is incorrect because carbidopa does not stimulate dopamine receptors but rather works on the enzymatic conversion of levodopa to dopamine. Choice D is incorrect as carbidopa does not inhibit catechol-O-methyltransferase (COMT), which is another enzyme involved in dopamine metabolism.

Correct Answer: D

Rationale: The correct answer is D because intravenous morphine in dyspnea from pulmonary edema helps in reducing the perception of shortness of breath, patient anxiety, and also reduces cardiac preload and afterload. Morphine acts as a vasodilator, reducing venous tone and decreasing peripheral resistance, which in turn reduces cardiac preload and afterload. This leads to improved cardiac function and decreased workload on the heart, resulting in relief from dyspnea. Choices A, B, and C are all interconnected mechanisms through which morphine exerts its effects in alleviating dyspnea in pulmonary edema.

Correct Answer: C

Rationale: The correct answer is C, the mesolimbic and mesofrontal systems, as they are directly involved in regulating behaviors related to motivation, reward, and decision-making. The mesolimbic system projects from the ventral tegmental area to the limbic system, influencing emotions and reinforcement. The mesofrontal system connects the prefrontal cortex to the ventral tegmental area, modulating executive functions and cognitive control of behavior. A: The hypothalamic-pituitary system primarily regulates hormone production and homeostasis, not behavior. B: The extrapyramidal system is involved in motor control and not directly linked to behavior. D: The chemoreceptor trigger zone of the medulla is related to vomiting reflex and not behavior.