A 68-year-old man is brought to the emergency department with 3 days of severe, watery dia… — USMLE Step 2 CK

A 68-year-old man is brought to the emergency department with 3 days of severe, watery diarrhea and poor oral intake. He reports feeling lightheaded upon standing. His temperature is $37.1^\circ$ C ( $98.8^\circ$ F), blood pressure is $112/72$ mm Hg supine and $90/60$ mm Hg standing, and pulse is 92/min supine and 116/min standing. Physical examination reveals dry mucous membranes and delayed capillary refill.

Laboratory results are as follows:

Serum studies:
Sodium: $140$ mEq/L
Potassium: $3.5$ mEq/L
Chloride: $110$ mEq/L
Bicarbonate: $20$ mEq/L
Blood urea nitrogen: $42$ mg/dL
Creatinine: $1.4$ mg/dL

Urine studies:
Sodium: $20$ mEq/L
Creatinine: $40$ mg/dL

Based on these findings, what is the patient's fractional excretion of sodium?

Answer: 0.5 %

Answer

0.5

The correct calculation of the Fractional Excretion of Sodium (FENa) requires the formula:

FENa = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100

. Plugging in the patient's values yields

(20 \times 1.4) / (140 \times 40) \times 100

, which simplifies to

28 / 5600 \times 100 = 0.5\%

. In the context of this patient's severe diarrhea, poor oral intake, and orthostatic hypotension, a FENa

< 1\%

accurately supports the diagnosis of a prerenal state (hypovolemia) with an appropriate, intact renal tubular reabsorption of sodium.

Step-by-Step Solution

Identify the formula for Fractional Excretion of Sodium (FENa).

FENa = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100

This formula is used to differentiate prerenal acute kidney injury from intrinsic renal disease by measuring the fraction of filtered sodium that is excreted.

Substitute the patient's laboratory values into the equation.

U_{Na} = 20

mEq/L,

P_{Cr} = 1.4

mg/dL,

P_{Na} = 140

mEq/L,

U_{Cr} = 40

mg/dL

Accurate variable placement is required for correct calculation.

Calculate the final percentage.

(20 \times 1.4) / (140 \times 40) \times 100 = (28 / 5600) \times 100 = 0.5\%

Mathematical simplification yields the final FENa percentage.

Key Concept

Calculation of Fractional Excretion of Sodium (FENa)

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