I am going to answer the sample USMLE Step 1 Questions with Explanation. These questions are available at USMLE.org
A 78-year-old man comes to the office for a follow-up examination. He was discharged from the hospital 1 week ago after being treated for a nontuberculous mycobacterial infection. He started treatment with ciprofloxacin and rifampin at that time. He also has hypertension and underwent placement of a mechanical aortic valve 6 years ago for aortic stenosis. Other current medications are hydrochlorothiazide, lisinopril, and warfarin. His warfarin dose was doubled 4 days ago. He says that he is trying to follow a healthier diet. He drinks two 12-oz beers daily. Results of laboratory studies done 4 days ago and today are shown
| 4 Days Ago | Today | |
| Prothrombin time | 11 sec (INR=1) | 11.2 sec (INR=1.1) |
| Partial thromboplastin time | 29 sec | 27 sec |
Which of the following is the most likely cause of this patient’s laboratory findings?
(A) Decreased protein binding
(B) Eradication of gut flora
(C) Increased alcohol intake
(D) Increased vegetable consumption
(E) Induction of cytochrome enzymes
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Answer
Rifampin, one of the medications the patient is taking for his nontuberculous mycobacterial infection, is known to induce cytochrome enzymes, including cytochrome P450 enzymes. These enzymes play a role in the metabolism of warfarin. Induction of these enzymes can increase the metabolism of warfarin, leading to a decrease in its effectiveness and a decrease in the INR value. This option is the most likely explanation for the patient’s laboratory findings, given his recent initiation of rifampin therapy.
(A) Decreased protein binding:
- Warfarin is highly protein-bound in the blood, and changes in protein binding can affect its concentration and activity. However, decreased protein binding typically leads to an increase in the INR, not a decrease. Therefore, this option is unlikely.
(B) Eradication of gut flora:
- Gut flora can affect the metabolism and absorption of certain medications, but there’s no direct evidence to suggest that eradication of gut flora would lead to a decrease in the INR value. This option is less likely.
(C) Increased alcohol intake:
- While taking warfarin, it is recommended to not have more than 2 drinks (can of beer, glass of wine, shot of spirit) per day for men or 1 drink per day for women. This patient alcohol intake is considered within this range so, it doesn’t affect warfarin metabolism. This option is less likely.
(D) Increased vegetable consumption:
- Increased vegetable consumption is generally recommended for patients on warfarin due to its high vitamin K content. However, this would not likely lead to a decrease in the INR value. This option is unlikely.
The correct answer is (E) Induction of cytochrome enzymes
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