## **Core Concept**
The resting state of excitable tissues, such as neurons and muscle cells, is characterized by a resting membrane potential that is primarily maintained by the selective permeability of the cell membrane to various ions, notably potassium (K+), sodium (Na+), and chloride (Cl-). The cell membrane's permeability to these ions is crucial for generating and maintaining the resting membrane potential.

## **Why the Correct Answer is Right**
The correct answer, , indicates that the excitable tissue at rest is least permeable to sodium ions (Na+). This is accurate because, at rest, the cell membrane is more permeable to potassium ions (K+) than to sodium ions (Na+). The resting membrane potential is largely determined by the movement of K+ ions out of the cell, down their concentration gradient, which creates a negative charge inside the cell. The cell membrane is relatively impermeable to Na+ at rest due to a low number of open sodium channels, which helps maintain the resting membrane potential.

## **Why Each Wrong Option is Incorrect**
* **Option A:** - This option is incorrect because the cell membrane at rest is somewhat permeable to chloride ions, and chloride channels are present in many types of cells, contributing to the stability of the resting membrane potential.
* **Option B:** - This option is incorrect because potassium ions (K+) are the ions to which the cell membrane is most permeable at rest, primarily due to the presence of open potassium leak channels.
* **Option C:** - This option might seem plausible but is not the correct answer based on the context of the question.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the resting membrane potential is primarily maintained by the high permeability to K+ and low permeability to Na+. This is crucial for understanding how action potentials are generated and how various drugs and conditions can affect neuronal and muscle excitability.

## **Correct Answer:** .