Maximum K+ concentration is seen in ___________
**Core Concept**
The question is testing the knowledge of the distribution of potassium ions (K+) in the human body. Potassium is an essential electrolyte that plays a crucial role in maintaining fluid balance, nerve function, and muscle contraction.
**Why the Correct Answer is Right**
The maximum concentration of potassium ions is found in the cells, particularly in the intracellular fluid (ICF). The high concentration of potassium in the ICF is due to the active transport of potassium ions into the cells through the sodium-potassium pump (Na+/K+ ATPase). This pump uses ATP to pump three sodium ions out of the cell and two potassium ions into the cell, resulting in a high concentration of potassium ions in the ICF. The intracellular concentration of potassium is approximately 150-160 mmol/L, while the extracellular concentration is around 3.5-5 mmol/L.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the extracellular fluid (ECF) has a lower concentration of potassium ions compared to the intracellular fluid.
**Option B:** This option is incorrect because the blood plasma has a lower concentration of potassium ions compared to the intracellular fluid.
**Option C:** This option is incorrect because the interstitial fluid has a lower concentration of potassium ions compared to the intracellular fluid.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the sodium-potassium pump plays a critical role in maintaining the balance of electrolytes in the body. Abnormalities in this pump can lead to various diseases, such as hypertension, heart failure, and kidney disease.
**Correct Answer: D. Intracellular fluid.