A patient is prescribed diphenhydramine for allergic rhinitis. Diphenhydramine works by:

Correct Answer: A

Rationale: Rationale: Diphenhydramine is a first-generation antihistamine that works by blocking histamine receptors in the respiratory tract. Histamine is released during an allergic reaction and causes symptoms like a runny nose. By blocking histamine receptors, diphenhydramine effectively reduces these symptoms. Therefore, option A is correct. Option B is incorrect because diphenhydramine does not stimulate beta-2 receptors. Option C is incorrect as diphenhydramine does not increase mucus production. Option D is incorrect as diphenhydramine does not block acetylcholine receptors in the brain.

Correct Answer: B

Rationale: The therapeutic index value of 2 indicates that the toxic dose is 2 times the effective dose. To have a 50% chance of developing toxic side effects, the patient would need to take a dose that is half of the toxic dose. This means the patient would need to take 1/2 of the toxic dose at each administration. Since the therapeutic index is 2, the patient would need to divide the toxic dose into 2 equal parts, which results in 2 doses. Therefore, the correct answer is B: Two doses. Summary: A: Incorrect - Taking one dose would exceed the therapeutic index and increase the risk of toxic side effects. C: Incorrect - Not providing a clear answer. D: Incorrect - Incomplete option.

Correct Answer: A

Rationale: The correct answer is A: Cyproheptadine. Cyproheptadine is a histamine and serotonin receptor antagonist, making it suitable for treating intestinal hypermobility of carcinoid tumors. Histamine and serotonin play a role in intestinal motility, and blocking their receptors can help alleviate symptoms. Summary of other choices: B: Ergonovine is a uterine stimulant and is not indicated for treating intestinal hypermobility. C: Loratadine is a selective histamine H1 receptor antagonist used for allergies, not for intestinal hypermobility. D: Promethazine is an antihistamine and antiemetic, but it is not commonly used for treating intestinal hypermobility associated with carcinoid tumors.

Correct Answer: A

Rationale: The correct answer is A: Reversible binding to H1 receptors. Antihistamines work by competitively binding to H1 receptors, blocking the action of histamine. This reversible binding prevents histamine from exerting its effects, such as inflammation and itching. Choice B is incorrect because irreversible binding would lead to prolonged effects and potential toxicity. Choice C is incorrect as H2 receptors are targeted by a different class of drugs used for conditions like acid reflux. Choice D is also incorrect for the same reason as choice B.

Correct Answer: C

Rationale: Step-by-step rationale for why choice C (Diphenhydramine) is correct: 1. Diphenhydramine is a first-generation antihistamine known for causing sedation. 2. The patient was experiencing daytime sleepiness, a common side effect of diphenhydramine. 3. The combination with diazepam (a sedative) likely potentiated the sedative effects. 4. Loratadine (choice A) is a non-sedating antihistamine. 5. Prednisone (choice B) is a corticosteroid and not typically associated with sedation. 6. Zolpidem (choice D) is a sedative-hypnotic used for insomnia but not taken by the patient.