**Core Concept**
Calcium channel blockers (CCBs) are a class of medications that inhibit the influx of calcium ions into cardiac and smooth muscle cells, leading to vasodilation and decreased cardiac contractility. In the event of an overdose, a specific antidote is required to counteract the effects of CCBs.

**Why the Correct Answer is Right**
Glucagon is the antidote of choice for calcium channel blocker overdose. It works by increasing intracellular cyclic AMP levels, which in turn activates protein kinase A and leads to an increase in calcium release from the sarcoplasmic reticulum. This ultimately results in an increase in cardiac contractility and a decrease in the negative inotropic effects of CCBs. Glucagon also has a direct effect on the heart, where it increases the slope of phase 4 of the action potential, leading to an increase in the rate of depolarization.

**Why Each Wrong Option is Incorrect**
**Option A:** Atropine is a muscarinic antagonist used primarily for bradycardia and heart block, but it is not effective in reversing the effects of CCB overdose.
**Option B:** Sodium bicarbonate may be used to treat hyperkalemia associated with CCB overdose, but it does not directly counteract the negative inotropic effects of CCBs.
**Option C:** Calcium gluconate may be used to treat hypocalcemia, but in the context of CCB overdose, it can actually worsen cardiac contractility by increasing the blockage of calcium channels.

**Clinical Pearl / High-Yield Fact**
In the setting of a CCB overdose, be aware that glucagon is the antidote of choice and should be administered promptly to reverse the negative inotropic effects. Also, note that calcium channel blockers can cause a "rebound" effect when the overdose is treated with calcium gluconate, leading to a worsening of cardiac contractility.

**Correct Answer:** C. Glucagon.