Which of the following direct-acting cholinomimetics is mainly muscarinic in action?

Correct Answer: A

Rationale: Bethanechol is the correct answer as it is mainly muscarinic in action. Bethanechol is a selective muscarinic receptor agonist with minimal nicotinic receptor activity. It is used to stimulate the bladder and gastrointestinal smooth muscle. Carbachol, while also a direct-acting cholinomimetic, has both muscarinic and nicotinic actions. Acetylcholine, the natural neurotransmitter, acts on both muscarinic and nicotinic receptors. "None of the above" is incorrect as Bethanechol is mainly muscarinic in action.

Correct Answer: D

Rationale: The correct answer is D: Beta 2 receptor. Bronchial smooth muscle relaxation is mainly mediated by Beta 2 adrenergic receptors when activated by agonists like salbutamol. These receptors stimulate adenylate cyclase leading to increased cAMP levels, which in turn promotes muscle relaxation. Alpha 1 receptors (Choice A) and Alpha 2 receptors (Choice B) are associated with vasoconstriction and decreased sympathetic outflow, respectively, not bronchial smooth muscle relaxation. Beta 1 receptors (Choice C) are primarily found in the heart and play a role in cardiac function, not bronchial smooth muscle relaxation.

Correct Answer: A

Rationale: Rationale: Labetalol is a reversible nonselective alpha-beta antagonist. It blocks both alpha and beta receptors. It is reversible due to its competitive binding, making it effective in managing hypertension. Phentolamine is an alpha blocker, not affecting beta receptors. Metoprolol and Propranolol are both beta blockers, targeting beta receptors only, making them selective and not reversible nonselective alpha-beta antagonists.

Correct Answer: C

Rationale: The correct answer is C: Phenobarbital. Phenobarbital is a long-acting barbiturate that can lead to cumulative and residual effects due to its long half-life and potential for drug accumulation in the body over time. This can result in prolonged sedative effects and increased risk of side effects. Zolpidem (A), Temazepam (B), and Triazolam (D) are shorter-acting hypnotic drugs with less potential for cumulative and residual effects compared to Phenobarbital.

Correct Answer: C

Rationale: Carbamazepine is an antiepileptic drug that acts by blocking voltage-gated sodium channels, similar to Phenytoin. Both drugs inhibit sustained high-frequency repetitive firing of action potentials in neurons. Benzodiazepines (A) enhance inhibitory neurotransmission via GABA receptors, Valproate (B) inhibits GABA transaminase and increases GABA levels, and Ethosuximide (D) blocks T-type calcium channels, making them incorrect choices.