**Core Concept**
Exocytosis is a cellular process where vesicles fuse with the plasma membrane to release their contents into the extracellular space. This process is critically dependent on calcium ions (Ca²⁺) as a key second messenger that triggers membrane fusion.

**Why the Correct Answer is Right**
Calcium ions (Ca²⁺) initiate exocytosis by binding to synaptotagmins on the vesicle membrane, inducing conformational changes that promote vesicle docking and fusion with the plasma membrane. This is especially vital in neurons (neurotransmitter release) and endocrine cells (hormone secretion). The influx of Ca²⁺ into the cytoplasm occurs via voltage-gated calcium channels, which open in response to depolarization.

**Why Each Wrong Option is Incorrect**
Option B: Mg²⁺ is a cofactor in enzyme reactions and involved in muscle relaxation, but it does not trigger vesicle fusion.
Option C: Na⁺ contributes to membrane depolarization but is not directly involved in the fusion step of exocytosis.
Option D: K⁺ helps maintain resting membrane potential and hyperpolarizes the cell, but it does not initiate exocytosis.

**Clinical Pearl / High-Yield Fact**
In clinical settings, calcium channel blockers (e.g., diltiazem) reduce neurotransmitter release by blocking Ca²⁺ entry, which is why they are used in hypertension and arrhythmias — a direct link between Ca²⁺ and exocytosis.

✓ Correct Answer: A. Ca