**Core Concept**
Beta blockers are a class of medications that act by blocking the effects of the hormone epinephrine (adrenaline) and by decreasing the heart rate and the heart's workload. They are commonly used to treat cardiovascular diseases, including hypertension, angina, and arrhythmias.

**Why the Correct Answer is Right**
Beta blockers work by competitively inhibiting the beta-1 adrenergic receptors in the heart, leading to decreased heart rate and contractility. They also have a negative chronotropic effect, which means they slow down the heart rate. This makes them useful in treating conditions such as hypertension, angina, and certain arrhythmias. However, beta blockers are not indicated for conditions that require increased sympathetic tone, such as pheochromocytoma.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypertension - Beta blockers are actually indicated for hypertension as they help to decrease the heart rate and the heart's workload, which in turn lowers blood pressure.
**Option B:** Angina - Beta blockers are also indicated for angina as they reduce the heart rate and myocardial oxygen demand, which helps to alleviate symptoms.
**Option C:** Arrhythmias - Beta blockers are indicated for certain arrhythmias, such as supraventricular tachycardia, as they help to slow down the heart rate and restore a normal rhythm.
**Option D:** Pheochromocytoma - Beta blockers are contraindicated in pheochromocytoma as they can worsen the condition by blocking the beta-2 adrenergic receptors, which can lead to unopposed alpha-1 receptor stimulation and a subsequent increase in blood pressure.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that beta blockers can worsen conditions that require increased sympathetic tone, such as pheochromocytoma. Therefore, it's crucial to carefully evaluate the patient's condition before initiating beta blocker therapy.

**Correct Answer: D. Pheochromocytoma**