## Core Concept
The Na+-K+ pump, also known as the sodium-potassium pump, is an **enzyme** (an **ATPase**) that moves **sodium ions out** and **potassium ions into** the cell, using **energy from ATP**. This process is crucial for maintaining the **resting membrane potential** and regulating the balance of fluids and electrolytes within the cell.

## Why the Correct Answer is Right
The correct answer states that the Na+-K+ pump **moves 3 sodium ions out and 2 potassium ions into the cell per ATP molecule hydrolyzed**. This is accurate because the pump uses the energy from one ATP molecule to transport three sodium ions out of the cell and two potassium ions into the cell. This **3:2 ratio** is essential for establishing and maintaining the concentration gradients of sodium and potassium across the cell membrane.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because it inaccurately describes the stoichiometry of the Na+-K+ pump.
* **Option B:** This option is incorrect because it suggests an incorrect direction of ion movement or an incorrect stoichiometry.
* **Option D:** This option is incorrect because it either misrepresents the energy source or the ion transport mechanism of the Na+-K+ pump.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the Na+-K+ pump is **electrogenic**, meaning it contributes to the resting membrane potential by moving more positive charges out of the cell than into it. This pump is **vital for neuronal function** and **muscle contraction**, and its dysfunction can lead to various diseases, including **heart failure** and **neurodegenerative disorders**.

## Correct Answer Line
**Correct Answer: C. 3 Na+ out, 2 K+ in.**