**Core Concept:** Quinidine is a class Ic antiarrhythmic drug used in the treatment of certain types of arrhythmias, particularly atrial fibrillation. It acts by blocking voltage-gated sodium channels in the heart, leading to a decrease in cardiac excitability and refractoriness.

**Why the Correct Answer is Right:**
Option C (hyperkalemia) is not an extracardiac manifestation of quinidine because it is a direct consequence of quinidine's action on the heart. Quinidine increases potassium secretion in the distal renal tubule, leading to a decrease in plasma potassium levels. However, this effect is primarily beneficial for treating arrhythmias rather than causing a manifestation.

**Why Each Wrong Option is Incorrect:**

A. Convulsions (hypotension): Quinidine has a low risk of causing seizures due to its antiarrhythmic properties.

B. Torsades de pointes (QT prolongation): While QT interval prolongation is a known side effect of quinidine, it does not directly cause the manifestation listed in the options.

D. Hyperkalemia: As mentioned earlier, quinidine causes hyperkalemia indirectly by increasing potassium secretion in the distal renal tubule, not a direct extracardiac manifestation.

**Clinical Pearl / High-Yield Fact:**

Quinidine is generally safe in patients with renal or hepatic impairment due to its low protein binding and low first-pass effect. However, it should be used cautiously in patients with severe bradycardia, hypokalemia, or QT interval prolongation.

**Correct Answer:** C (hyperkalemia) is not an extracardiac manifestation of quinidine. Quinidine does not directly cause hyperkalemia but indirectly affects potassium levels through renal potassium secretion.