An agonist is a substance that:

Correct Answer: B

Rationale: The correct answer is B because an agonist interacts with the receptor and initiates changes in cell function. Agonists bind to the receptor and activate it, leading to a cellular response. Choice A is incorrect because agonists do produce an effect. Choice C is too general and does not specifically define the role of an agonist. Choice D describes the mechanism of a potentiator, not an agonist. In summary, an agonist directly interacts with the receptor to elicit a cellular response, making choice B the correct answer.

Correct Answer: B

Rationale: The correct answer is B: Bronchodilation. Isofluorophate is an organophosphate compound that acts as a cholinesterase inhibitor, leading to increased acetylcholine levels. Lacrimation, muscle twitching, and salivation are all effects of increased acetylcholine activity due to cholinesterase inhibition. However, bronchodilation is not typically associated with increased acetylcholine levels. Therefore, the correct answer is B. Choices A, C, and D are incorrect because they are known effects of increased acetylcholine activity.

Correct Answer: C

Rationale: The correct answer is C: Cardiovascular collapse. Ganglion blocking drugs are not used for cardiovascular collapse because they can worsen the condition by further reducing blood pressure. These drugs are indicated for hypertensive crises, controlled hypotension, and pulmonary edema due to their ability to block ganglionic transmission and lower blood pressure. In cardiovascular collapse, the priority is to increase blood pressure and perfusion, which is not achieved by using ganglion blocking drugs. Therefore, using these drugs in cardiovascular collapse can be detrimental. Choices A, B, and D are incorrect because ganglion blocking drugs can be beneficial in managing hypertensive crises, controlled hypotension, and pulmonary edema, respectively.

Correct Answer: B

Rationale: Step 1: Methoxamine is an alpha1-selective agonist, meaning it specifically targets alpha1 receptors. Step 2: Alpha1 receptors are found in smooth muscles, and activation causes vasoconstriction. Step 3: Methoxamine's selective action on alpha1 receptors makes it suitable for conditions requiring vasoconstriction. Step 4: Norepinephrine (A) is a non-selective agonist for alpha and beta receptors, not specifically targeting alpha1 receptors. Step 5: Ritodrine (C) is a beta2-selective agonist used for tocolysis in preterm labor, not targeting alpha1 receptors. Step 6: Ephedrine (D) is a mixed-acting sympathomimetic that acts on both alpha and beta receptors, not alpha1-selective.

Correct Answer: B

Rationale: The correct answer is B: Bradycardia. Phentolamine is an alpha-adrenergic blocker, which typically causes tachycardia rather than bradycardia due to its vasodilatory effect. Diarrhea (choice A) can occur due to its effect on smooth muscle relaxation, arrhythmias (choice C) can result from its impact on cardiac conduction, and myocardial ischemia (choice D) can happen due to decreased cardiac output. Bradycardia is not a common adverse effect of phentolamine, making it the correct answer in this scenario.