**Core Concept:** Potassium is an essential electrolyte in the human body, playing a crucial role in maintaining the electroneutral balance and regulating nerve and muscle function. The kidneys play a vital role in regulating potassium levels through excretion and reabsorption.

**Why the Correct Answer is Right:** In a normal individual, the highest concentration of potassium is found in the **kidneys**, specifically in the **nephron** and **collecting ducts**. Potassium is actively reabsorbed in the nephron, while chloride ions are actively secreted, helping maintain the renal potassium concentration gradient. This gradient allows for effective excretion of excess potassium into the urine, thus maintaining overall potassium levels in the body.

**Why Each Wrong Option is Incorrect:**

A. **Muscle tissue** has a moderate concentration of potassium but is not the highest among the given options.
B. **Salivary glands** have a lower concentration of potassium compared to the kidneys.
C. **Blood plasma** has a similar concentration of potassium to muscle tissue but is not the highest among the options.
D. **Skin** has a moderate concentration of potassium but is not the highest among the given options.

**Clinical Pearl:** The correct answer highlights the crucial role of the kidneys in maintaining electrolyte balance, including potassium homeostasis. This understanding is essential in diagnosing and managing conditions affecting electrolyte levels, such as hypokalemia and hyperkalemia.

**Correct Answer:** C. Blood plasma