## **Core Concept**
The free water deficit in a patient with hypernatremia can be calculated using a specific formula. This calculation is crucial for determining the amount of free water (or hypotonic fluids) needed to correct the serum sodium level to a normal range, typically around 140 mEq/L. The formula for calculating free water deficit in patients with hypernatremia is: Free water deficit = Total body water (TBW) x ((Current Na / Desired Na) - 1).

## **Why the Correct Answer is Right**
To calculate the free water deficit for this patient, we first need to estimate the total body water (TBW). In adults, TBW is approximately 60% of body weight in males and 50% in females. For a 70 kg female, TBW = 0.5 x 70 kg = 35 L. The patient's current sodium level is 160 mEq/L, and we aim to correct it to 140 mEq/L. Using the formula: Free water deficit = 35 L x ((160 / 140) - 1) = 35 L x (1.1429 - 1) = 35 L x 0.1429 = 5 L.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Without calculation, it's hard to verify, but typically, this option would not align with the calculated free water deficit based on the provided numbers.
- **Option B:** Similarly, this option does not match our calculated free water deficit of 5 L.
- **Option D:** This option suggests a significantly larger deficit than calculated.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the free water deficit calculation helps in guiding fluid replacement in patients with hypernatremia. Correction should be done gradually to avoid cerebral edema. The rate of correction depends on the severity and duration of hypernatremia.

## **Correct Answer:** B. 5L.