**Core Concept**
Lithium is a medication used in the management of bipolar disorder. Its cardiac effects are a significant concern due to its narrow therapeutic index. The most common cardiac adverse effect of lithium is related to its impact on cardiac conduction and rhythm.

**Why the Correct Answer is Right**
Lithium can cause QT interval prolongation, which increases the risk of torsades de pointes, a potentially life-threatening arrhythmia. This occurs because lithium inhibits the sodium-potassium ATPase pump, leading to an increase in intracellular sodium and subsequently, a decrease in potassium levels. The prolonged QT interval is a hallmark of this effect, making it a significant concern in patients taking lithium.

**Why Each Wrong Option is Incorrect**
**Option A:** Bradycardia - While lithium can cause bradycardia, it is not the most common cardiac adverse effect. Bradycardia is a potential side effect, but it is not as frequently associated with lithium as QT interval prolongation.
**Option B:** Hypotension - Lithium can cause hypotension, but it is not the most common cardiac adverse effect. Hypotension is a potential side effect, but it is not as directly related to lithium's mechanism of action as QT interval prolongation.
**Option C:** Cardiac failure - Lithium can cause cardiac failure, but it is not the most common cardiac adverse effect. Cardiac failure is a potential long-term consequence of lithium use, but it is not as frequently associated with lithium as QT interval prolongation.

**Clinical Pearl / High-Yield Fact**
It is essential to monitor patients taking lithium for signs of QT interval prolongation, such as a prolonged QTc interval on an electrocardiogram (ECG). This can be done at regular intervals, especially when initiating or adjusting lithium therapy.

**Correct Answer:** C. QT interval prolongation.