Tachyphylaxis is:

Correct Answer: B

Rationale: Tachyphylaxis refers to a rapid decrease in response to a drug upon repeated or continuous administration. The correct answer is B because it specifically mentions "very rapidly developing tolerance," which accurately describes tachyphylaxis. Option A is incorrect because tachyphylaxis is not related to drug interactions. Option C is incorrect because tachyphylaxis is characterized by a quick onset of decreased responsiveness, not taking days or weeks to develop. Option D is incorrect as tachyphylaxis is a recognized phenomenon in pharmacology.

Correct Answer: A

Rationale: The correct answer is A: Scopolamine. Scopolamine is a muscarinic receptor-blocking drug that antagonizes the action of acetylcholine at muscarinic receptors. Scopolamine is commonly used for its anticholinergic effects, such as reducing salivation and mucous secretion. B: Pipecuronium is a neuromuscular blocking agent that acts at the neuromuscular junction, not at muscarinic receptors. C: Trimethaphan is a ganglionic blocker that acts at the autonomic ganglia, not at muscarinic receptors. D: Pilocarpine is a muscarinic receptor agonist, not a muscarinic receptor blocker. It is used to stimulate muscarinic receptors for various therapeutic purposes.

Correct Answer: C

Rationale: The correct answer is C (Pancuronium). Pancuronium is a non-depolarizing neuromuscular blocker that can cause tachycardia by blocking muscarinic receptors leading to the inhibition of the parasympathetic system. This results in the dominance of the sympathetic system, leading to tachycardia. Tubocurarine (A) and Atracurium (B) are also non-depolarizing neuromuscular blockers, but they do not directly cause tachycardia. Succinylcholine (D) is a depolarizing neuromuscular blocker that can cause bradycardia due to its effects on acetylcholine receptors at the neuromuscular junction.

Correct Answer: D

Rationale: The correct answer is D because ephedrine actually increases arterial pressure by stimulating alpha-adrenergic receptors, leading to vasoconstriction. A is incorrect as ephedrine works by directly stimulating adrenergic receptors, not by releasing stored catecholamines. B is correct as ephedrine is indeed a mild CNS stimulant. C is incorrect as tachyphylaxis, or a rapid decrease in response with repeated administration, is not commonly associated with ephedrine.

Correct Answer: B

Rationale: The correct answer is B: Metoprolol. Metoprolol is a cardioselective beta blocker, meaning it primarily targets beta-1 receptors in the heart and has minimal effect on beta-2 receptors in the lungs. This makes it preferable in patients with asthma as it reduces the risk of bronchoconstriction compared to non-selective beta blockers like Propranolol (choice A). Metoprolol also has a lower risk of masking hypoglycemic symptoms in diabetic patients compared to non-selective agents. Nadolol (choice C) and Timolol (choice D) are non-selective beta blockers and may worsen asthma and peripheral vascular diseases due to their effects on beta-2 receptors.