**Core Concept**
The Na+ – K+ ATPase is a crucial enzyme responsible for maintaining the electrochemical gradient across the cell membrane, particularly in neurons and muscle cells. This enzyme pumps sodium ions out of the cell and potassium ions into the cell, using energy from ATP hydrolysis. The Na+ – K+ ATPase is essential for nerve conduction, muscle contraction, and maintaining the resting membrane potential.

**Why the Correct Answer is Right**
The correct answer is related to the mechanism of action of the Na+ – K+ ATPase. This enzyme is a transmembrane protein that spans the cell membrane, with one binding site for ATP and another for the sodium-potassium ion complex. The enzyme hydrolyzes ATP to produce energy, which is then used to drive the transport of three sodium ions out of the cell and two potassium ions into the cell. This process is essential for maintaining the electrochemical gradient across the cell membrane.

**Why Each Wrong Option is Incorrect**

* **Option A:** This option is incorrect because it does not specify the correct location of the Na+ – K+ ATPase. While the enzyme is indeed found in the plasma membrane, it is not the primary location where it is found.
* **Option B:** This option is incorrect because it is not specific to the Na+ – K+ ATPase. Other enzymes, such as the Ca2+ – ATPase, also use ATP hydrolysis to pump ions across the cell membrane.
* **Option C:** This option is incorrect because it is not a correct description of the Na+ – K+ ATPase. The enzyme does not pump sodium ions into the cell and potassium ions out of the cell; instead, it pumps sodium ions out of the cell and potassium ions into the cell.

**Clinical Pearl / High-Yield Fact**
The Na+ – K+ ATPase is an essential enzyme for maintaining the resting membrane potential, and its activity is crucial for nerve conduction and muscle contraction. Disruptions in Na+ – K+ ATPase activity can lead to conditions such as hypokalemia, hypernatremia, and muscle weakness.

**Correct Answer:** D.