Regarding the fetal circulatory shunts and their closure after birth, all of the following statements are correct EXCEPT:

Explanation

## Fetal Circulatory Shunts and Closure Mechanisms ### Overview of Fetal Shunts The fetal circulation contains three major shunts that bypass the non-functional lungs and liver: 1. **Foramen ovale** — right-to-left atrial shunt 2. **Ductus venosus** — umbilical venous shunt 3. **Ductus arteriosus** — pulmonary arterial shunt ### Closure Timeline and Mechanisms | Shunt | Functional Closure | Anatomical Closure | Mechanism | |-------|-------------------|-------------------|----------| | **Foramen ovale** | Minutes to hours | 3–12 months | ↑ LA pressure pushes septum primum against septum secundum | | **Ductus venosus** | Days to weeks | Weeks to months | ↓ Umbilical flow; becomes ligamentum venosus | | **Ductus arteriosus** | 24–72 hours | 2–3 weeks | ↑ O₂ tension; ↓ PGE₂; becomes ligamentum arteriosum | ### Key Point: The Ductus Venosus **Key Point:** The ductus venosus does NOT remain patent after birth. It closes functionally within days to weeks as umbilical blood flow ceases, and becomes a fibrous cord called the **ligamentum venosus**. This is the INCORRECT statement. ### Why Each Statement Is Correct (Except One) **Foramen ovale closure (Option A):** Correct. At birth, clamping of the umbilical cord and expansion of the lungs cause a sudden drop in pulmonary vascular resistance and an increase in left atrial pressure. The septum primum is pushed against the septum secundum, achieving functional closure within minutes to hours. [cite:Moore Clinically Oriented Anatomy 8e Ch 1] **Ductus arteriosus closure (Option C):** Correct. Increased arterial oxygen tension (PaO₂ rises from ~30 mmHg to ~90 mmHg) and decreased prostaglandin E₂ levels trigger smooth muscle contraction in the ductal wall. Functional closure occurs within 24–72 hours; anatomical closure (fibrosis) follows within 2–3 weeks, forming the ligamentum arteriosum. [cite:Langman Embryology 13e Ch 12] **Anatomical closure of foramen ovale (Option D):** Correct. Over weeks to months, the septum primum gradually fuses with the septum secundum, creating a permanent seal. In ~25% of adults, a probe patent foramen ovale (PFO) persists but remains functionally closed due to the valve-like overlap. ### Clinical Pearl: Patent Ductus Arteriosus **Clinical Pearl:** Failure of the ductus arteriosus to close results in a patent ductus arteriosus (PDA), a left-to-right shunt causing a continuous "machinery" murmur. Risk factors include prematurity, maternal rubella infection, and hypoxia. Indomethacin or ibuprofen (inhibit PGE₂ synthesis) can promote closure; surgical ligation is reserved for cases unresponsive to medical management. ### High-Yield Mnemonic: Fetal Shunt Closure **Mnemonic:** **FOD** — **F**oramen ovale (minutes–hours), **O**xygenation closes ductus arteriosus (24–72 hrs), **D**uctus venosus (days–weeks). All three close functionally at birth; anatomical closure follows over weeks to months.