**Question:** All of the following statements regarding ductus arteriosus are true except

A. Closes after birth due to fall in placental oxygenation
B. Functions as an alternative pathway for oxygenation of fetal blood
C. Contributes to 20% of the total blood flow in the newborn period
D. Closes automatically after birth

**Correct Answer:** D. Closes automatically after birth

**Core Concept:** Ductus arteriosus is a fetal shunt that connects the pulmonary artery and the aorta, allowing oxygenated blood to bypass the lungs and reach the systemic circulation. It plays a crucial role in maintaining adequate oxygenation in the fetus during the period when the placenta is the primary source of oxygenation.

**Why the Correct Answer is Right:**

The ductus arteriosus closes automatically after birth due to the rise in oxygenation provided by the lungs. The closure of the ductus arteriosus is a key event in the transition from fetal to neonatal circulation. This process is primarily mediated by the release of endothelin-1 (ET-1), a potent vasoconstrictor, in response to increased oxygenation and decreased placental oxygenation. This leads to narrowing and eventual closure of the ductus arteriosus, ensuring a smooth transition from the fetal to the postnatal circulation.

**Why Each Wrong Option is Incorrect:**

A. Closing due to decreased placental oxygenation is partially correct, but the primary reason is the increased oxygenation after birth, which triggers the closure process.
B. Although the ductus arteriosus provides an alternative oxygenation pathway, it closes after birth due to the physiological changes, not oxygenation directly.
C. The ductus arteriosus contributes to only 2% of total blood flow in the newborn period, which is not relevant to the primary reason for its closure.

**Clinical Pearl:**

A smooth transition from fetal to neonatal circulation is crucial for the newborn's survival and well-being. The closure of the ductus arteriosus is a well-regulated physiological response to environmental changes, ensuring that the newborn can adapt to the increased oxygenation and decreased placental oxygenation after birth. A thorough understanding of these physiological changes is essential for medical students and healthcare professionals, particularly in pediatrics, neonatology, and obstetrics.