**Question:** Resting membrane potential is determined by

A. Sodium-potassium pump
B. Calcium channels
C. L-type calcium channels
D. L-type calcium channels and sodium-potassium pump

**Correct Answer:** A. Sodium-potassium pump

**Core Concept:**
The resting membrane potential is a fundamental concept in cellular physiology, representing the electrical gradient across the cell membrane that maintains the cell in a stable state between depolarization and repolarization. This potential is crucial for proper cellular function, nerve impulse transmission, and muscle contraction.

**Why the Correct Answer is Right:**
The resting membrane potential is primarily determined by the activity of the Sodium-Potassium Pump (Na+/K+-ATPase). This enzyme, localized in the cell membrane, uses energy from ATP hydrolysis to actively transport three sodium ions (Na+) out of the cell and two potassium ions (K+) into the cell. This process maintains a steep concentration gradient for both ions, keeping the intracellular concentration of K+ higher than extracellular and vice versa, thus stabilizing the membrane potential.

**Why Each Wrong Option is Incorrect:**

A. L-type calcium channels (Cav1.2) are involved in excitability and contraction in smooth, cardiac, and skeletal muscle cells, but they do not directly determine the resting membrane potential.
B. Calcium channels (CaV) are involved in the excitation-contraction coupling process, but they are not responsible for maintaining the resting membrane potential.
C. L-type calcium channels (Cav1.2) contribute to the excitability of cells but are not directly involved in setting the resting membrane potential.

**Why the correct answer (A) is right:**
The Sodium-Potassium Pump directly influences the membrane potential by actively transporting ions against their concentration gradient, while the wrong options involve other cellular processes or ion channels.

**Clinical Pearl:**
Understanding the resting membrane potential helps medical students grasp the fundamental principles of cellular physiology and membrane potential regulation, which are essential for understanding advanced topics such as nerve impulse transmission, muscle contraction, and ion channelopathies associated with abnormal membrane potential.