Aspirin prolongs bleeding by inhibiting the synthesis of which of the following?
**Core Concept**
Aspirin's mechanism of action involves the inhibition of an enzyme crucial for the synthesis of a key molecule in the coagulation cascade, leading to an increased risk of bleeding.
**Why the Correct Answer is Right**
Aspirin inhibits the enzyme cyclooxygenase (COX), specifically COX-1, which is responsible for the production of thromboxane A2 (TXA2) in platelets. TXA2 promotes platelet aggregation and vasoconstriction, which are essential steps in the formation of a blood clot. By inhibiting COX-1, aspirin reduces the formation of TXA2, leading to impaired platelet function and an increased risk of bleeding. This is particularly relevant in individuals with aspirin-induced asthma or those taking anticoagulant medications.
**Why Each Wrong Option is Incorrect**
**Option A:** Factor VII is a vitamin K-dependent clotting factor involved in the extrinsic pathway of coagulation. Aspirin does not affect the synthesis of factor VII.
**Option B:** Prostacyclin is a vasodilatory and antiplatelet agent produced by endothelial cells. While aspirin can affect prostacyclin production, it is not the primary mechanism by which aspirin prolongs bleeding.
**Option C:** Fibrinogen is a glycoprotein that serves as a precursor to fibrin, which forms the clot. Aspirin does not directly affect the synthesis of fibrinogen.
**Clinical Pearl / High-Yield Fact**
Mnemonic: "COX-1, TXA2, and platelets - aspirin's mechanism of action is anti-platelet, not anti-coagulant."
**Correct Answer:** C. Thromboxane A2