Which medication is contraindicated during pregnancy due to risk of premature ductus arteriosus closure?

• A. Ampicillin
• B. Ketoconazole
• C. Indomethacin
• D. Tylenol

Answer: (C)

Premature closure of the fetal ductus arteriosus happens when prostaglandin signals that keep the ductus open are blocked. Indomethacin is an NSAID that inhibits cyclooxygenase, reducing prostaglandin synthesis, so it can force the ductus arteriosus to close before birth. Because this can disrupt fetal circulation and kidney function, it is contraindicated in pregnancy, especially later in gestation when the ductus is still patent and the fetus relies on prostaglandins to maintain blood flow. In contrast, ampicillin and acetaminophen are generally safe in pregnancy, and ketoconazole carries teratogenic risk but doesn’t work through this ductus-closure mechanism.