A competitive antagonist is a substance that:

Correct Answer: B

Rationale: The correct answer is B because a competitive antagonist binds to the same receptor site as the agonist, thereby inhibiting the agonist response. This results in a decrease in the efficacy of the agonist without affecting its potency. Choice A is incorrect because a substance producing submaximal effects is a partial agonist, not a competitive antagonist. Choice C is incorrect as competitive antagonists specifically bind to receptor sites, not nonspecific sites. Choice D is incorrect as it describes a partial agonist, which can act as an agonist or antagonist depending on the receptor subtype.

Correct Answer: C

Rationale: Rationale: Edrophonium is the correct answer as it is an organophosphate cholinesterase inhibitor used in the diagnosis of myasthenia gravis. It is water-soluble and can be prepared in an aqueous solution for ophthalmic use. Physoctigmine is not an organophosphate, and Echothiophate is not water-soluble and requires reconstitution before use.

Correct Answer: D

Rationale: The correct answer is D: Succinylcholine. Succinylcholine has a "double-acetylcholine" structure because it contains two acetylcholine molecules linked together. This structure allows succinylcholine to act as a depolarizing neuromuscular blocker by mimicking the action of acetylcholine at the neuromuscular junction. Rocuronium (A) and Atracurium (C) are nondepolarizing neuromuscular blockers that do not have a double-acetylcholine structure. Carbachol (B) is a cholinergic agonist but does not have a double-acetylcholine structure like succinylcholine.

Correct Answer: A

Rationale: Rationale: Terbutaline is a beta2-selective agonist because it primarily targets beta2-adrenergic receptors in the lungs, leading to bronchodilation. Xylometazoline is an alpha-adrenergic agonist, Isoproterenol is a non-selective beta agonist, and Dobutamine primarily targets beta1-adrenergic receptors. Therefore, Terbutaline is the correct choice for beta2-selective agonist based on its pharmacological action.

Correct Answer: D

Rationale: The correct answer is D because the statement that the ALFA1a subtype mediates both vascular and prostate smooth muscle contraction is incorrect. The ALFA1a subtype primarily mediates prostate smooth muscle contraction, while the ALFA1b subtype is responsible for vascular smooth muscle contraction. This is a key distinction in understanding the roles of different subtypes of alfa1 receptors. Choices A, B, and C are all correct statements as there are indeed at least three subtypes of alfa1 receptors (alfa1a, alfa1b, and alfa1d), ALFA1a subtype does mediate prostate smooth muscle contraction, and ALFA1b subtype does mediate vascular smooth muscle contraction, respectively.