All of the following are risk factors for deep vein thrombosis (DVT) except:
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.
Obesity
Age less than 40 years
Duration of surgery more than thiy minutes
Use of the oestrogen-progesterone contraceptive pills