WBR0519

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Pharmacology
Sub Category	SubCategory::Gastrointestinal, SubCategory::Hematology
Prompt	[[Prompt::A 62-year-old man with a past medical history significant for bioprosthetic mitral valve replacement presents to the physician’s office for tar-colored stools. The patient is maintained on metoprolol, ramipril, warfarin, and simvastatin. Upon further questioning, the patient explains that he recently recovered from a bacterial pneumonia and is currently continuing his antibiotics regimen. On physical examination, you notice several bruises on the forearms and shins. Work-up reveals an INR of 8 (therapeutic range: 2-3). Which of the following antibiotics is the most likely contributor to the patient’s 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::