**Core Concept**
Aspirin's mechanism of action involves the inhibition of an enzyme crucial for the synthesis of prostanoids, which are key mediators of inflammation and pain. This enzyme is a crucial target for anti-inflammatory, analgesic, and antipyretic effects.

**Why the Correct Answer is Right**
Aspirin exerts its effect by irreversibly inhibiting the enzyme cyclooxygenase (COX), specifically COX-1 and COX-2 isoforms. This inhibition prevents the conversion of arachidonic acid to prostaglandin H2 (PGH2), thereby reducing the production of prostanoids such as prostaglandins, thromboxanes, and prostacyclins. The reduced levels of these prostanoids contribute to the anti-inflammatory, analgesic, and antipyretic effects of aspirin. COX enzymes are present in platelets, where they are responsible for the synthesis of thromboxane A2, a potent vasoconstrictor that promotes platelet aggregation.

**Why Each Wrong Option is Incorrect**
**Option A:** Lipooxygenase decreased - Aspirin's mechanism of action does not involve the inhibition of lipoxygenase, which is involved in the synthesis of leukotrienes from arachidonic acid.

**Option C:** Phospholipase - Phospholipase A2 is involved in the hydrolysis of membrane phospholipids to release arachidonic acid, but aspirin does not inhibit phospholipase.

**Option D:** Increased Lipoxygenase - Aspirin's mechanism of action does not involve the increase of lipoxygenase activity.

**Clinical Pearl / High-Yield Fact**
Aspirin's irreversible inhibition of COX enzymes leads to a prolonged effect, which is why aspirin can have a significant impact on platelet aggregation and bleeding risk.

**✓ Correct Answer: B. Cyclooxygenase decreased**