Which of the following conditions is Heparin primarily used to prevent?

Heparin is primarily used to prevent deep vein thrombosis (DVT) because it is an anticoagulant that reduces the formation of blood clots. DVT commonly occurs in patients who are immobile for extended periods, such as those undergoing surgery or those with certain medical conditions that increase clotting risk. By inhibiting clotting factors in the coagulation pathway, heparin effectively decreases the likelihood of blood clots forming in the deep veins, thus preventing DVT.

While heparin can also play a role in the prevention of pulmonary embolism (PE), which can arise from a DVT, its primary indication in the context of prophylaxis is clearly for DVT. Additionally, although heparin may be used during acute situations like myocardial infarction or in patients with atrial fibrillation to prevent thromboembolic events, these uses are more about managing existing conditions rather than prevention in the context of a high-risk situation like prolonged immobility or surgery. Thus, the most accurate answer regarding heparin's primary preventative use is DVT.