**Question:** A five day old, full term male infant was severely cyanotic at birth. Prostaglandin E was administered initially and later balloon atrial septostomy was done which showed improvement in oxygenation. The most likely diagnosis of this infant is:

A. Patent ductus arteriosus (PDA)
B. Persistent pulmonary hypertension of the newborn (PPHN)
C. Transposition of great arteries (TGA)
D. Complete atrioventricular septal defect (AVSD)

**Correct Answer:** D. Complete atrioventricular septal defect (AVSD)

**Core Concept:**
Atrioventricular septal defects are a group of congenital heart defects in which the atrioventricular valves and the muscular walls separating the atria and ventricles are not properly formed during fetal development. AVSDs are typically classified into three types:

1. Type A: Combined AVSD (also known as "common AVSD") - Involves defects in both atrioventricular valves and the muscular walls separating the atria and ventricles.
2. Type B: Single ventricle AVSD (also known as "unicuspid AVSD") - Involves defects in both atrioventricular valves, but only one ventricle.
3. Type C: Dobutamine-induced AVSD - This is a rare condition caused by the administration of dobutamine during pregnancy.

**Why the Correct Answer is D (Complete AVSD):**
In this case, the infant is severely cyanotic at birth, and the administration of prostaglandin E (PGE) initially improves oxygenation due to the dilation of ductus arteriosus, leading to increased oxygenated blood flow to the systemic circulation. However, the improvement does not persist, indicating that the underlying defect is not a PDA or TGA.

**Why Balloon Atrial Septostomy is Indicated:**
After the initial treatment with PGE, balloon atrial septostomy is performed to create a larger opening in the atrial septum, allowing for increased flow from the left atrium to the right ventricle and subsequently to the pulmonary circulation. This helps to improve oxygenation further.

**Why Balloon Atrial Septostomy Leads to Improved Oxygenation:**
Balloon atrial septostomy improves oxygenation by increasing the flow of oxygenated blood from the left atrium to the right ventricle, which pumps it to the pulmonary circulation. This procedure allows for better distribution of oxygenated blood to the systemic circulation, leading to improved oxygen saturation.

**Why an Infant with Complete AVSD Needs Further Management:**
In infants with complete AVSD, balloon atrial septostomy is performed to improve oxygenation, but they require additional management, including:

1. Ventricular septostomy for optimal oxygenation and hemodynamics.
2. Congenital heart defect repair surgery (e.g., Ross procedure or Norwood procedure) to correct the structural heart defect.

**Why Balloon Atrial Septostomy is a Temporary Solution:**
Balloon atrial septostomy is a temporary intervention that