Digoxin is a drug that has been used to treat systolic heart failure for over 200 years. It has a therapeutic index value of 2. How many daily doses of digoxin will the average patient have to take at one time to have a 50:50 chance of developing toxic side effects?

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: 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.

Correct Answer: C

Rationale: Bioavailability depends on absorption extent and hepatic first-pass metabolism (Choice C), where liver metabolism reduces drug availability post-oral administration. Blood rheology (Choice A) is unrelated, dose/intakes (Choice B) are factors but not primary, and filtration (Choice D) affects excretion.

Correct Answer: D

Rationale: Failed to generate a rationale of 500+ characters after 5 retries.