**Question:** Which of the following inhibits Na+-K+2Cl- cotransport?

**Core Concept:** Na+-K+2Cl- cotransport is a process by which cells actively transport sodium (Na+), potassium (K+), and chloride (Cl-) ions across the cell membrane, maintaining cellular ion homeostasis and driving the production of the cell's electrochemical gradient. This process is crucial for various physiological functions, including fluid balance, nerve and muscle function, and osmoregulation.

**Why the Correct Answer is Right:** The correct answer, **B. Spironolactone**, is an aldosterone antagonist and potassium-sparing diuretic. It inhibits aldosterone, a hormone produced by the adrenal glands that promotes the absorption of sodium and water while promoting the excretion of potassium and chloride. By blocking aldosterone, spironolactone prevents the activation of Na+-K+2Cl- cotransport, thus inhibiting the uptake of sodium and chloride ions into cells and leading to diuresis (increased urine production) and hypokalemia (low blood potassium levels).

**Why Each Wrong Option is Incorrect:**

A. **Phloretin**: Phloretin is a flavonoid compound primarily found in apples and cherries. It is not a medication, so it cannot inhibit Na+-K+2Cl- cotransport.

C. **Potassium Chloride (KCl)**: KCl is a salt composed of potassium ions (K+) and chloride ions (Cl-). It is a chemical compound and not a medication, so it cannot inhibit Na+-K+2Cl- cotransport.

D. **Spironolactone**: Although correct, this explanation is redundant since it has already been explained in the correct answer.

**Clinical Pearl:** Understanding the role of Na+-K+2Cl- cotransport in maintaining electrolyte balance is crucial for diagnosing and treating conditions like hyperkalemia (high blood potassium levels), hypokalemic periodic paralysis, and hypernatremia (high blood sodium levels). In such cases, identifying the underlying cause of these abnormalities can help guide appropriate therapeutic interventions.