**Core Concept**
The concentration of ions in a cell compared to the surrounding extracellular fluid is crucial for maintaining proper cellular function, particularly in the context of nerve and muscle excitability. The cell membrane's selective permeability and ion pumps work together to regulate the concentration of ions such as potassium (K+), sodium (Na+), and chloride (Cl-) inside the cell.

**Why the Correct Answer is Right**
Potassium ions (K+) are the most abundant cation inside a cell, with a concentration of approximately 150-160 mmol/L. This high concentration is maintained by the sodium-potassium pump (Na+/K+-ATPase), which actively transports three sodium ions out of the cell and two potassium ions into the cell in exchange for the energy from ATP hydrolysis. This process helps to establish a negative resting membrane potential, which is essential for nerve and muscle function. The high concentration of potassium inside the cell also helps to regulate the cell's osmotic balance and maintain proper cellular volume.

**Why Each Wrong Option is Incorrect**
**Option A:** Sodium (Na+) concentration is typically higher outside the cell (approximately 140 mmol/L) than inside, due to the active transport by the sodium-potassium pump.

**Option B:** Chloride (Cl-) concentration is lower inside the cell (approximately 4-6 mmol/L) than outside, where it is approximately 100 mmol/L.

**Option D:** Calcium (Ca2+) concentration is typically very low inside the cell (approximately 10-20 nmol/L) compared to the surrounding extracellular fluid, where it is approximately 1.2 mmol/L.

**Clinical Pearl / High-Yield Fact**
Remember that the high concentration of potassium inside the cell helps to maintain a negative resting membrane potential, which is essential for nerve and muscle excitability. This is why hypokalemia (low potassium levels) can lead to muscle weakness and other symptoms related to impaired nerve function.

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