**Core Concept:** Pulmonary embolism (PE) is a life-threatening condition where a blood clot blocks the pulmonary artery, resulting in inadequate oxygen delivery to the lungs, and right heart strain and failure. The treatment focuses on thrombolysis, anticoagulation, and vasodilation.

**Why the Correct Answer is Right:**
The correct answer is **D** - Heparin. Heparin is a potent anticoagulant that directly inhibits clot formation, prevents platelet aggregation, and prolongs the clotting cascade. By reducing clot formation and improving blood flow, heparin can alleviate right ventricular strain and failure in pulmonary embolism.

**Why Each Wrong Option is Incorrect:**
A. Option A (Furosemide) is a diuretic used to reduce fluid accumulation, not directly address the clot in pulmonary embolism.
B. Option B (Nitroglycerin) is a vasodilator used for angina relief, not pulmonary embolism treatment.
C. Option C (Aspirin) is an antiplatelet agent used for preventing secondary thrombosis, not treating established pulmonary embolism.

**Why Each Wrong Option is Incorrect:**
A. Option A (Furosemide) is a diuretic used to reduce fluid accumulation, not directly address the clot in pulmonary embolism.
B. Option B (Nitroglycerin) is a vasodilator used for angina relief, not pulmonary embolism treatment.
C. Option C (Aspirin) is an antiplatelet agent used for preventing secondary thrombosis, not treating established pulmonary embolism.

**Clinical Pearl:** Pulmonary embolism is a medical emergency that needs prompt intervention. Heparin is the primary drug choice for treating established pulmonary embolism, as it addresses the clot directly and improves right ventricular function. However, it must be used carefully and under close monitoring, as its anticoagulant nature carries risks of bleeding complications. In severe cases, a pulmonary embolectomy (surgical removal of the clot) or catheter-directed thrombolysis (clot-busting therapy) might be necessary.