## **Core Concept**
The fractional excretion of sodium (FeNa) is a measure used to assess the cause of acute kidney injury, particularly to differentiate between prerenal and intrinsic renal causes. It represents the percentage of sodium filtered by the kidneys that is excreted in the urine. A decreased FeNa indicates that the kidneys are avidly reabsorbing sodium, often in response to decreased renal perfusion.

## **Why the Correct Answer is Right**
The correct answer, **B. Aldosterone**, causes a decreased fractional excretion of sodium. Aldosterone is a mineralocorticoid hormone produced by the adrenal cortex that plays a critical role in regulating sodium and potassium balance. It acts on the collecting ducts of the kidneys to increase sodium reabsorption and potassium secretion. When aldosterone levels are high, more sodium is reabsorbed back into the bloodstream, reducing the amount excreted in the urine. This results in a decreased FeNa.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is blank and cannot be evaluated.
- **Option C:** This option is blank and cannot be evaluated.
- **Option D:** This option is blank and cannot be evaluated.

## **Clinical Pearl / High-Yield Fact**
A low FeNa (<1%) typically indicates a prerenal cause of acute kidney injury, such as dehydration or heart failure, where the kidneys are trying to retain sodium and water. Aldosterone's role in promoting sodium retention is crucial in these states. Remember, conditions that increase aldosterone (like heart failure) or mimic its effects (like NSAIDs) can decrease FeNa. ## **Correct Answer:** B. Aldosterone.