This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
All of the following are risk factors for deep vein thrombosis (DVT) except: 
Select and explain. 
A. Obesity, B. Age less than 40 years, C. Duration of surgery more than thiy minutes, D. Use of the oestrogen-progesterone contraceptive pills
B: Age less than 40 years
Answer: B: Age less than 40 years 
Explanation: Patients over the age of 40 years is a risk factor for DVT. High dose estrogen therapy is also associated with DVT. Ref: Bailey and Love's Sho Practice of Surgery, 24th Edition, Page 970.