In a healthy individual on a normal diet, which of the following ion is completely absorbed in tubules, except?
**Core Concept**
Potassium (K+) absorption in the renal tubules plays a crucial role in maintaining electrolyte balance and blood pressure. The majority of K+ reabsorption occurs in the proximal convoluted tubule (PCT) and the thick ascending limb of the loop of Henle.
**Why the Correct Answer is Right**
Potassium is almost completely reabsorbed in the PCT through a sodium-potassium pump (Na+/K+-ATPase). This process is driven by the energy from ATP hydrolysis and helps maintain the concentration gradient for sodium reabsorption. Additionally, some K+ is reabsorbed in the thick ascending limb of the loop of Henle through a different transport mechanism. However, the kidneys are not capable of reabsorbing a specific ion that is present in the diet.
**Why Each Wrong Option is Incorrect**
**Option A:** Sodium (Na+) is almost completely reabsorbed in the renal tubules, particularly in the PCT and the thick ascending limb of the loop of Henle, making this option incorrect.
**Option B:** Calcium (Ca2+) is also reabsorbed in the renal tubules, primarily in the PCT and the distal convoluted tubule, which makes this option incorrect.
**Option C:** Phosphate (PO43-) is reabsorbed in the renal tubules, mainly in the PCT and the proximal convoluted tubule, making this option incorrect.
**Clinical Pearl / High-Yield Fact**
The kidneys can excrete up to 20-30% of the filtered load of sodium, but this is not a concern for most individuals on a normal diet. However, individuals with certain medical conditions, such as congestive heart failure or cirrhosis, may require sodium restriction to manage fluid overload.
**Correct Answer: D.**