**Core Concept**
The Na-K ATPase, also known as the sodium-potassium pump, is a crucial enzyme for maintaining the resting membrane potential and proper ion balance in cells. It actively transports sodium ions out of the cell and potassium ions into the cell against their concentration gradients, utilizing ATP as an energy source.

**Why the Correct Answer is Right**
The correct answer, **A. 3 Na out / 2K in**, accurately represents the stoichiometry of the Na-K ATPase. This enzyme transports three sodium ions out of the cell and two potassium ions into the cell for every molecule of ATP hydrolyzed. This process is essential for maintaining the high concentration of sodium outside the cell and the high concentration of potassium inside the cell, which is critical for the resting membrane potential. The Na-K ATPase is a cotransporter that uses the energy from ATP hydrolysis to pump ions against their concentration gradients.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because it suggests that the Na-K ATPase transports sodium ions into the cell and potassium ions out of the cell, which is opposite to the actual function of the enzyme.
**Option C:** This option is incorrect because it suggests an unequal exchange of sodium and potassium ions, which is not consistent with the stoichiometry of the Na-K ATPase.
**Option D:** This option is incorrect because it suggests that the Na-K ATPase transports a different number of sodium and potassium ions than the actual stoichiometry.

**Clinical Pearl / High-Yield Fact**
The Na-K ATPase is inhibited by cardiac glycosides, such as digoxin, which are used to treat heart failure and atrial fibrillation. Understanding the mechanism of the Na-K ATPase is essential for appreciating the clinical effects of these medications.

**✓ Correct Answer: A. 3 Na out / 2K in**