QT interval is shoened in-
**Core Concept**
The QT interval represents the time from the start of the Q wave to the end of the T wave in the cardiac cycle, reflecting the ventricular depolarization and repolarization phases. A shortened QT interval is associated with a faster ventricular repolarization.
**Why the Correct Answer is Right**
The correct answer is related to the effect of certain medications on the ventricular action potential. Quinidine is a Class I anti-arrhythmic medication that blocks sodium channels, thereby prolonging the depolarization phase but also shortening the repolarization phase due to the upstroke velocity being reduced. This results in a shortened QT interval.
**Why Each Wrong Option is Incorrect**
* **Option A:** Digitalis, a cardiac glycoside, primarily acts by inhibiting the Na+/K+-ATPase pump, leading to an increase in intracellular calcium and a prolongation of the QT interval.
* **Option B:** Beta-blockers, such as propranolol, slow the heart rate and prolong the QT interval by blocking beta-adrenergic receptors, which are involved in the regulation of heart rate and contractility.
* **Option C:** Amiodarone, a Class III anti-arrhythmic, prolongs the QT interval by blocking potassium channels and increasing the duration of the ventricular action potential.
**Clinical Pearl / High-Yield Fact**
It is essential to recognize that medications with Class I anti-arrhythmic properties, such as quinidine, can lead to a paradoxical shortening of the QT interval due to their effect on ventricular repolarization.
**Correct Answer: C. Quinidine.