USMLE Step 1 Practice Test 15

A male newborn delivered at 28 weeks' gestation is being treated in the neonatal intensive care unit. On day 4 of life, his oxygen requirement increases. Pulse is 168/min, and blood pressure is 58/24 mm Hg. Peripheral pulses are bounding, and the precordium is hyperdynamic. A continuous murmur is heard best below the left clavicle. Echocardiography shows left-to-right flow from the descending aorta into the pulmonary artery through a vessel that normally closes after birth. A drug is administered to close this vessel. Which of the following is the mechanism of action of this drug?