When a patent ductus arteriosus is hemodynamically significant, the usual first-line approach is medical closure using nonsteroidal anti-inflammatory drugs like indomethacin or ibuprofen. These medications inhibit cyclooxygenase, which lowers prostaglandin E2 levels that normally keep the ductus arteriosus open. With reduced prostaglandins, the ductus constricts and closes, addressing the root cause and often avoiding surgery. If NSAIDs are contraindicated (for example, due to renal issues or risk factors for necrotizing enterocolitis) or if the ductus remains open after a course of medical therapy, surgical closure becomes the next step. This can be achieved by ligation or, in some centers, transcatheter closure. Other options don’t close the defect themselves. Observation alone is not appropriate when the PDA is causing significant symptoms, and high-dose diuretics only manage fluid overload without addressing the underlying ductal patency.

Preventing VKDB hinges on rapidly raising a newborn’s vitamin K level so the liver can produce adequate clotting factors. Newborns have very low vitamin K stores and limited gut bacteria to synthesize it, so they are at risk for bleeding due to insufficient factors II, VII, IX, and X. Administering a single intramuscular dose of vitamin K at birth reliably delivers vitamin K, achieves protective levels quickly, and avoids issues with absorption that can occur with oral dosing. The standard dose is 0.5–1 mg given intramuscularly, providing strong protection against early, classic, and late VKDB. While oral regimens exist, they’re less dependable due to variability in absorption and adherence, making the intramuscular injection the preferred approach.

In HFOV optimization, the goal is to minimize oxygen exposure and injury from sustained airway pressures as the infant’s lung status improves. Once oxygenation is adequate, you reduce the fraction of inspired oxygen to lower oxygen toxicity and decrease the mean airway pressure to lessen lung stretch, while keeping gas exchange stable by adjusting amplitude and frequency as needed. If oxygenation worsens after lowering MAP, you can adjust other settings, but the typical approach during optimization is to decrease both FiO2 and MAP. Increasing FiO2 or MAP would raise oxygen exposure or pressure, which is not consistent with the optimization goal. Reducing FiO2 with a higher MAP would maintain oxygenation but at greater pressure, also not ideal for optimization. Decreasing FiO2 and MAP together aligns with stepping down support as the infant improves.

In fetal life, high pulmonary vascular resistance keeps the pulmonary vessels constricted, so blood flow to the lungs is very small. Oxygenation occurs via the placenta, and blood is directed away from the lungs through the fetal shunts—the foramen ovale and the ductus arteriosus—toward the systemic circulation. When PVR is increased, this pulmonary flow is further reduced, matching the description of a constricted circulation with very little blood passing through the lungs. The other statements don’t fit because they imply lower PVR with high flow, no effect of PVR on fetal flow, or a circulation identical to after birth.