**Core Concept**
The foramen ovale is a fetal shunt that allows blood to bypass the lungs and enter the systemic circulation. It is a flap-like opening in the interatrial septum that closes shortly after birth. The closure of the foramen ovale is essential to prevent paradoxical embolism and maintain efficient blood circulation.

**Why the Correct Answer is Right**
The foramen ovale closes due to the increased pressure in the left atrium and the decreased pressure in the right atrium after birth. As the lungs expand and begin to oxygenate blood, the pressure in the left atrium increases, and the pressure in the right atrium decreases. This pressure gradient causes the flap-like septum to fuse, closing the foramen ovale. The fusion of the septum is facilitated by the formation of a fibrous tissue that eventually obliterates the opening.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because the foramen ovale does not close due to the contraction of the interatrial septum muscles. The septum is a passive structure that relies on pressure gradients to close.
* **Option B:** Incorrect because the closure of the foramen ovale is not triggered by the increase in pulmonary blood flow. While the lungs do expand and increase blood flow, it is the pressure gradient between the atria that drives closure.
* **Option C:** Incorrect because the foramen ovale does not close due to the formation of a septal defect. The septum actually fuses to close the opening.

**Clinical Pearl / High-Yield Fact**
The foramen ovale typically closes within the first few months of life, but it may remain patent in some individuals. An open foramen ovale can be a risk factor for paradoxical embolism, which occurs when a blood clot enters the systemic circulation through the foramen ovale.

**Correct Answer:** C.