**Core Concept**
The magnitude of an action potential is primarily determined by the rapid depolarization phase, which is driven by the influx of positively charged ions into the cell. In the context of neuronal physiology, this is largely attributed to the movement of sodium (Na+) ions.

**Why the Correct Answer is Right**
During the initial phase of an action potential, voltage-gated sodium channels open, allowing a large influx of Na+ ions into the neuron. This rapid increase in positive charge leads to a significant depolarization of the membrane potential, ultimately resulting in the initiation of an action potential. The movement of Na+ ions is facilitated by the sodium-potassium pump, which maintains a concentration gradient across the cell membrane. The rapid depolarization phase is largely attributed to the sodium channels, which are responsible for the initial influx of Na+ ions.

**Why Each Wrong Option is Incorrect**
* **Option B:** Magnesium (Mg++) ions do play a role in neuronal excitability, but they are not primarily responsible for determining the magnitude of an action potential. Mg++ ions can block NMDA receptors and modulate the activity of voltage-gated calcium channels.
* **Option C:** Potassium (K++) ions are involved in the repolarization phase of an action potential, where they flow out of the cell, contributing to the return of the membrane potential to its resting state. However, they do not primarily determine the magnitude of an action potential.
* **Option D:** Calcium (Ca++) ions do play a role in the later phases of an action potential, particularly in the release of neurotransmitters and the modulation of synaptic plasticity. However, they are not primarily responsible for determining the magnitude of an action potential.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the rapid depolarization phase of an action potential is largely driven by the movement of sodium ions, and this is a critical concept in understanding various neurological disorders, such as epilepsy and multiple sclerosis.

✓ Correct Answer: A. Na+