Which of the following is a risk factor for deep vein thrombosis (DVT)?

Prolonged immobility is indeed a significant risk factor for deep vein thrombosis (DVT). When a person remains stationary for long periods, such as during long flights, bed rest after surgery, or extended desk work, the natural flow of blood in the veins can be disrupted. This stagnation increases the likelihood of clot formation, as blood pools in the veins, especially in the lower extremities. Factors like muscle contractions, which help facilitate blood flow, are diminished during periods of inactivity, thereby raising the risk of DVT.

In contrast, regular exercise promotes blood circulation and helps maintain healthy blood flow, which can reduce the risk of clot formation. Thin blood, typically referred to as having lower viscosity, may actually decrease risk rather than increase it; it doesn't create the same conditions for clotting that thickened blood does. High fluid intake helps to maintain blood fluidity and can contribute to proper hydration, further minimizing clot risk. Thus, out of the options provided, prolonged immobility stands out as a definitive risk factor for DVT.