A 3-pound neonate has a continuous machine-like murmur along the left sternal border due to a patent ductus arteriosus. Indomethacin closes the PDA by inhibiting production of PGE2, which is produced by cyclooxygenase. Which enzyme is inhibited?

• A. Phospholipase A2
• B. Cyclooxygenase
• C. Lipoxygenase
• D. Adenylate cyclase

Answer: (A)

The key idea is how prostaglandin synthesis keeps the ductus arteriosus open and how blocking that pathway can cause closure. Indomethacin works by inhibiting cyclooxygenase, the enzyme that converts the prostaglandin precursor into prostaglandin H2, a step that leads to production of PGE2. Since PGE2 maintains ductal patency in neonates, inhibiting cyclooxygenase reduces PGE2 levels and promotes closure of the ductus. So the enzyme being inhibited is cyclooxygenase. The other options don’t fit because phospholipase A2 acts earlier in the pathway to release arachidonic acid, but indomethacin doesn’t inhibit it; lipoxygenase leads to leukotrienes, not prostaglandins; and adenylate cyclase is not part of this prostaglandin synthesis route.