Propranolol is useful in the treatment of all of the following except
**Core Concept**
Propranolol is a non-selective beta-blocker that reduces heart rate, contractility, and peripheral vascular resistance by blocking the effects of epinephrine and norepinephrine on beta-1 and beta-2 adrenergic receptors. This results in decreased myocardial oxygen demand, which is beneficial in various cardiovascular conditions.
**Why the Correct Answer is Right**
Propranolol is useful in treating conditions like hypertension, angina pectoris, supraventricular tachycardia (SVT), and essential tremor due to its ability to reduce heart rate and contractility. In these conditions, the decreased myocardial oxygen demand and reduced sympathetic tone help alleviate symptoms.
**Why Each Wrong Option is Incorrect**
**Option A:** Propranolol is not useful in treating asthma as it can cause bronchoconstriction by stimulating beta-2 receptors in the airways, leading to increased airway resistance and decreased lung function.
**Option B:** Propranolol is not used to treat hyperkalemia as it can paradoxically increase potassium levels by reducing aldosterone secretion and increasing potassium reabsorption in the kidneys.
**Option C:** Propranolol is not effective in treating pheochromocytoma as it can worsen hypertension due to unopposed alpha-receptor stimulation, leading to increased peripheral vascular resistance.
**Clinical Pearl / High-Yield Fact**
When using beta-blockers like propranolol, it's essential to be aware of their potential to worsen asthma or other respiratory conditions due to beta-2 receptor stimulation.
**Correct Answer:** C.