Which of the following is NOT a risk factor for developing DVT?

High levels of physical activity are generally associated with a lower risk of developing deep vein thrombosis (DVT). Increased physical activity promotes better circulation and helps prevent the stasis of blood flow, which is one of the contributing factors to the formation of blood clots.

In contrast, the other listed factors—obesity, age over 40, and major surgery—are known risk factors for DVT. Obesity can lead to increased pressure in the veins, contributing to clot formation. Age over 40 is significant as the risk of DVT increases with age due to changes in blood vessel function and blood coagulability. Major surgery often involves immobilization and trauma, both of which can increase the likelihood of developing DVT due to reduced mobility and various physiological changes that accompany surgical interventions.

Hence, high levels of physical activity stand out as a protective factor against DVT, making it the correct choice for what is NOT a risk factor for its development.