Repeated USMLE Questions Step 1 (18)


A 34-year-old man newly immigrant from Kenya presents to your office for fever, sore throat and general aches. He has a long history of malaria and takes daily primaquine. Throat swab reveals streptococcal infection. You decide to prescribe an antibiotic.

Which of these antibiotics is contraindicated for this patient?

A- Ampicillin

B- Amoxicillin

C- Ciprofloxacin

D- Erythromycin


Erythromycin blocks the metabolism of primaquine by the liver through depressing the P450 system. This leads to accumulation of primaquine causing toxicity.

The correct answer is D


By | 2016-10-17T05:28:34+00:00 July 29th, 2016|USMLE Step 1|3 Comments

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  1. Ankush Dargopatil December 17, 2016 at 1:23 pm - Reply

    Sir could u help me with pharmacology & also guide me for u usmle preparation pls

  2. Andrea March 4, 2017 at 1:36 am - Reply

    why not ciprofloxacin? thats also a CYP450 inhibitor??

    • Dr. Ray Makar March 4, 2017 at 1:55 am - Reply

      You are right but erythromycin has more effect than ciprofloxacin. It is not uncommon to see some questions in USMLE that has more than one correct answer but you choose the answer according to the most likely fitting the whole situation. Erythromycin is usually the first drug of choice used in this case and the effect of erythromycin on hepatic P450 is more prominent than ciprofloxacin. Thank you for this good question.

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