Hexamethonium blocks the action of acethylcholine and similar agonists at:

Correct Answer: C

Rationale: Hexamethonium is a ganglionic blocker that acts specifically at the autonomic ganglia. It inhibits the action of acetylcholine at the ganglionic nicotinic receptors, leading to the blockade of autonomic functions. This results in the inhibition of sympathetic and parasympathetic responses. Therefore, the correct answer is C: Autonomic ganglia. Muscarinic receptor site (A) is not directly affected by Hexamethonium. Neuromuscular junction (B) is not the primary site of action for this drug. Axonal transmission (D) is not the specific target of Hexamethonium.

Correct Answer: A

Rationale: The correct answer is A because phentolamine is an alpha-adrenergic antagonist that blocks presynaptic alpha2 receptors. By inhibiting these receptors, it increases norepinephrine release, leading to stimulation of unblocked beta receptors on the heart, resulting in tachycardia. This mechanism directly links the action of phentolamine to the observed tachycardia. Choice B (Baroreflex mechanism) is incorrect because phentolamine-induced tachycardia is primarily mediated by the alpha2 receptor antagonism rather than the baroreflex mechanism. Choice C (Direct effect on the heart by stimulation of beta1 receptors) is incorrect because phentolamine does not directly stimulate beta1 receptors on the heart; rather, it indirectly causes cardiac stimulation through enhanced norepinephrine release. Choice D (Inhibition of transmitter reuptake at noradrenergic synapses) is incorrect because phentolamine does not inhibit

Correct Answer: B

Rationale: The correct answer is B: Cimetidine. Cimetidine is a cytochrome P450 inhibitor that can inhibit hepatic metabolism of drugs like hypnotics, leading to increased drug levels and potential toxicity. Flumazenil (A) is a benzodiazepine receptor antagonist, not a metabolic inhibitor. Phenytoin (C) induces hepatic enzymes, increasing metabolism of other drugs. Theophylline (D) is metabolized by cytochrome P450, but it doesn't inhibit metabolism of hypnotics.

Correct Answer: D

Rationale: The correct answer is D. Phenobarbital is a barbiturate medication that can cause physical and psychological dependence, leading to addiction (Choice A). It can also exacerbate petit mal epilepsy by lowering seizure threshold (Choice B). Additionally, sedation is a common side effect of phenobarbital due to its central nervous system depressant properties (Choice C). Therefore, all of the above choices are correct, making option D the correct answer.

Correct Answer: D

Rationale: The correct answer is D: Selegiline. Selegiline is a selective inhibitor of monoamine oxidase B (MAO-B), which helps increase dopamine levels in the brain. Levodopa is a precursor to dopamine, Amantadine is an antiviral drug used to treat Parkinson's symptoms, and Tolcapone is a COMT inhibitor. Selegiline specifically targets MAO-B without affecting MAO-A, making it the most suitable choice for selective inhibition of MAO-B in the context of Parkinson's disease treatment.