WBR0519

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Pharmacology
Sub Category	SubCategory::Gastrointestinal, SubCategory::Hematology
Prompt	[[Prompt::A 62 year old male patient on immunosuppressive therapy with a past medical history significant for a recent bioprosthetic mitral valve replacement, systolic heart failure, hypertension, and hyperlipidemia, presents to the physician’s office for black-colored stools. The patient’s medications are metoprolol, ramipril, warfarin, simvastatin, fenofibrate. He has stopped taking aspirin against medical advice one year ago. The patient explains that he recently had a bacterial infection and had started antibiotic therapy. During work-up, INR was measured to be 8 (therapeutic range: 2-3). Which of the following antibiotics most likely contributed to the patient’s gastrointestinal bleeding symptoms?]]
Answer A	AnswerA::Ciprofloxacin
Answer A Explanation	AnswerAExp::Ciprofloxacin is an inhibitor of P-450 system, thus causing less warfarin elimination and more systemic effect and possible bleeding.
Answer B	AnswerB::Amoxicillin
Answer B Explanation	AnswerBExp::Amoxicillin is not classically associated with P-450 inhibition.
Answer C	AnswerC::Rifampin
Answer C Explanation	AnswerCExp::Rifampin is an inducer of the P-450 system. As a result, warfarin will be eliminated faster; the patient will be predisposed to thrombotic events.
Answer D	AnswerD::Chloramphenicol
Answer D Explanation	AnswerDExp::Chloramphenicol is not classically associated with P-450 inhibition.
Answer E	AnswerE::Clindamycin
Answer E Explanation	AnswerEExp::Clindamycin is not classically associated with P-450 inhibition.
Right Answer	RightAnswer::A
Explanation	[[Explanation::Patients receiving warfarin therapy, such as the case in mechanical valve replacement, must be monitored for possible bleeding at all times using INR. Introduction of new medications must always take into account the possible interaction with warfarin. P-450 inhibitors will prevent the elimination of warfarin in the liver and cause prolonged effects and possible bleeding, On the other hand, P-450 inducers will hasten warfarin elimination and predispose the patient to thrombotic events. Among the list of antibiotics, ciprofloxacin is an inhibitor of P-450 system. Drugs that inhibit P-450 (and cause more warfarin bleeding effect): Macrolides, amiodarone, isoniazid, cimetidine, ritonavir, ciprofloxacin, ketoconazole, sulfonamides, gemfibrozil, quinidine. Educational Objective: Ciprofloxacin is an inhibitor of P-450 system References: First Aid 2014 page 255]]
Approved	Approved::No
Keyword	WBRKeyword::Ciprofloxacin, WBRKeyword::Warfarin, WBRKeyword::Bleeding, WBRKeyword::Bleed, WBRKeyword::Drug interaction, WBRKeyword::P-450, WBRKeyword::P450, WBRKeyword::GI bleeding, WBRKeyword::Black stools, WBRKeyword::Gastrointestinal
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::