Most diffusible ion in excitable tissue is

**Core Concept**
The most diffusible ion in excitable tissue is a critical factor in the generation and propagation of action potentials. Excitable tissues, such as neurons and muscle cells, rely on the rapid movement of ions across their cell membranes to facilitate electrical signaling. Among the various ions, one type is particularly adept at crossing cell membranes, enabling the rapid transmission of signals.

**Why the Correct Answer is Right**
The correct answer is **Potassium (K+)**. Potassium ions are the most diffusible in excitable tissue due to their small size and high concentration gradient across the cell membrane. During the resting state, potassium ions tend to leak out of the cell, creating a negative resting membrane potential. This outward flow of potassium ions is facilitated by the presence of potassium channels, which are highly permeable to K+ ions. As a result, potassium ions play a crucial role in the initiation and propagation of action potentials.

**Why Each Wrong Option is Incorrect**
* **Option A:** Sodium (Na+) ions are not the most diffusible in excitable tissue, despite their role in the rising phase of the action potential. Sodium channels are voltage-gated and open briefly during the action potential, allowing a rapid influx of Na+ ions. However, the high concentration of potassium ions and their rapid efflux from the cell make them more diffusible.
* **Option B:** Calcium (Ca2+) ions are not the most diffusible in excitable tissue, as they are tightly regulated and play a crucial role in muscle contraction and neurotransmitter release. Calcium channels are often voltage-gated and have a lower density than potassium channels.
* **Option D:** Chloride (Cl-) ions are not the most diffusible in excitable tissue, as they are less permeable than potassium ions and play a more minor role in the generation and propagation of action potentials.

**Clinical Pearl / High-Yield Fact**
The high permeability of potassium ions to excitable tissues is a critical factor in the development of certain cardiac arrhythmias, such as torsades de pointes. This arrhythmia is often associated with a prolonged QT interval, which can be caused by a delay in the repolarization of cardiac myocytes, leading to an accumulation of potassium ions and a subsequent risk of arrhythmia.

**Correct Answer:** C. Potassium (K+)