**Core Concept**
Tricuspid regurgitation (TR) is a valvular heart disease characterized by the leakage of the tricuspid valve, allowing blood to flow back from the right ventricle into the right atrium during systole. The etiology of TR can be primary (due to intrinsic valve abnormalities) or secondary (due to extrinsic factors that affect the valve's function).

**Why the Correct Answer is Right**
The most common cause of secondary tricuspid regurgitation is right ventricular pressure or volume overload, which can occur due to pulmonary hypertension. This condition leads to right ventricular hypertrophy and dilation, ultimately causing tricuspid valve annular dilation and leaflet tethering. The increased pressure in the pulmonary artery can also lead to right ventricular strain, further exacerbating the tricuspid regurgitation. The tricuspid valve's normal functioning is reliant on the proper coaptation of its leaflets, which is compromised in the setting of right ventricular overload.

**Why Each Wrong Option is Incorrect**
* **Option A:** Primary tricuspid valve abnormalities, such as tricuspid valve prolapse or endocarditis, are less common causes of tricuspid regurgitation.
* **Option B:** Left-sided heart failure is not a direct cause of tricuspid regurgitation, although it can lead to secondary pulmonary hypertension, which in turn can cause TR.
* **Option D:** Congenital tricuspid atresia is a rare congenital heart defect that is not the most common cause of secondary tricuspid regurgitation.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with tricuspid regurgitation, it is essential to assess for underlying causes of right ventricular pressure or volume overload, such as pulmonary hypertension or left-sided heart failure, as these conditions can significantly impact the severity and prognosis of TR.

**Correct Answer: C. Pulmonary hypertension.**