**Core Concept**
Beta blockers are a class of medications that primarily act by inhibiting the effects of epinephrine (adrenaline) and norepinephrine (noradrenaline) on beta receptors, thereby decreasing heart rate, contractility, and cardiac output. In contrast, pulmonary hypertension is characterized by elevated pulmonary artery pressure, which can be exacerbated by decreased cardiac output and increased pulmonary vascular resistance.

**Why the Correct Answer is Right**
Beta blockers are generally contraindicated in pulmonary hypertension due to their potential to decrease cardiac output, which can worsen pulmonary hypertension. This is particularly concerning in patients with pre-existing right ventricular dysfunction, as beta blockers can lead to further right ventricular failure. In contrast, endothelin receptor antagonists (e.g., bosentan), phosphodiesterase inhibitors (e.g., sildenafil), and calcium channel blockers (e.g., nifedipine) are all used to treat pulmonary hypertension by reducing pulmonary vascular resistance and improving right ventricular function.

**Why Each Wrong Option is Incorrect**
**Option A:** Endothelin receptor antagonists are actually used to treat pulmonary hypertension by blocking the action of endothelin-1, a potent vasoconstrictor that contributes to increased pulmonary vascular resistance.
**Option B:** Phosphodiesterase inhibitors, such as sildenafil, are used to treat pulmonary hypertension by increasing cyclic guanosine monophosphate (cGMP) levels, leading to vasodilation and reduced pulmonary vascular resistance.
**Option C:** Calcium channel blockers, such as nifedipine, are used to treat pulmonary hypertension by inhibiting calcium influx into vascular smooth muscle cells, leading to vasodilation and reduced pulmonary vascular resistance.

**Clinical Pearl / High-Yield Fact**
When initiating therapy for pulmonary hypertension, it is essential to carefully assess cardiac function and right ventricular reserve to avoid exacerbating right ventricular failure.

**✓ Correct Answer: D. Beta blockers. They are generally contraindicated in pulmonary hypertension due to their potential to decrease cardiac output and worsen pulmonary hypertension.**