A 60-year-old man who is receiving warfarin for recurrent deep vein thrombosis and has had stable, therapeutically appropriate anticoagulation for several months is treated by his physician for a urinary tract infection. Twelve days later he presents to the emergency department with lower gastrointestinal bleeding and is found to have a markedly prolonged prothrombin time. What is the most likely cause?

Question

Quizplus · Accepted Answer

The most likely cause of the prolonged prothrombin time is reduced production of vitamin K by gut flora due to the antibiotic treatment. Antibiotics can disrupt the gut microbiome, leading to decreased synthesis of vitamin K, which is essential for the synthesis of clotting factors. This effect is particularly significant in patients on warfarin, as warfarin works by inhibiting vitamin K-dependent clotting factors.

A 60-Year-Old Man Who Is Receiving Warfarin for Recurrent Deep