**Core Concept**
Morbus caeruleus, also known as cyanotic heart disease, is a congenital condition characterized by right-to-left shunting of blood due to an atrial septal defect (ASD) or a patent foramen ovale (PFO). The foramen ovale is a natural opening in the interatrial septum that allows blood to bypass the lungs in fetal life.

**Why the Correct Answer is Right**
The closure of the foramen ovale is a normal process that occurs after birth as the child begins to breathe and the pressure in the left atrium increases. This increase in left atrial pressure causes the foramen ovale to close, preventing right-to-left shunting and allowing the child to adapt to life outside the womb. The closure is facilitated by the development of the interatrial septum, which eventually becomes a solid wall.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the closure of the foramen ovale is a normal process that occurs after birth, not before.
**Option B:** This option is incorrect because the closure of the foramen ovale is a physiological process that occurs due to the increase in left atrial pressure, not due to any pathological process.
**Option C:** This option is incorrect because the closure of the foramen ovale is not a result of the increased pressure in the right atrium, but rather the increased pressure in the left atrium.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the closure of the foramen ovale is a normal process that occurs after birth, and it's not a pathological process. This can help differentiate between congenital heart diseases and normal physiological changes.

**Correct Answer: A. After birth.**