The principal adverse effects of phentolamine include all of the following EXCEPT:

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.

Correct Answer: C

Rationale: The correct answer is C: Vigabatrin. Vigabatrin irreversibly inhibits GABA-T by irreversibly binding to its active site, leading to a sustained increase in GABA levels. This results in enhanced GABAergic neurotransmission. Diazepam (A) and Phenobarbital (B) are not inhibitors of GABA-T. Felbamate (D) acts by blocking NMDA receptors and enhancing GABAergic transmission, but it does not directly inhibit GABA-T.

Correct Answer: D

Rationale: The correct answer is D: Amantadine. Amantadine is an antiviral agent used in the prophylaxis of influenza A2 by inhibiting the viral replication process. Selegiline (A) is a monoamine oxidase inhibitor used in Parkinson's treatment. Sinemet (B) is a combination of carbidopa and levodopa for Parkinson's symptoms. Pergolide (C) is a dopamine agonist used in Parkinson's disease but has been withdrawn due to side effects. Amantadine is the only antiparkinsonian drug in the list with antiviral properties specifically used for influenza A2.

Correct Answer: B

Rationale: The correct answer is B: Disulfiram. Disulfiram inhibits the enzyme aldehyde dehydrogenase, leading to accumulation of acetaldehyde when alcohol is consumed. This causes a noxious reaction (e.g., nausea, vomiting) as acetaldehyde builds up. Naltrexone (A) is an alcohol antagonist that reduces alcohol cravings. Diazepam (C) is a benzodiazepine used for anxiety and alcohol withdrawal symptoms. Morphine (D) is an opioid analgesic and not used for causing a noxious reaction to alcohol.

Correct Answer: B

Rationale: The correct answer is B: Chlorpromazine. Chlorpromazine has significant peripheral alpha-adrenergic blocking activity, leading to side effects like orthostatic hypotension. This is due to its antagonistic action on alpha-1 adrenergic receptors in the peripheral nervous system. Haloperidol (A) primarily acts on dopamine receptors in the brain. Clozapine (C) is known for its anticholinergic effects and does not have significant alpha-adrenergic blocking activity. Risperidone (D) primarily acts on dopamine and serotonin receptors, with minimal alpha-adrenergic blocking activity.