130 mmol/L sodium is associated with the onset of hypokalemia due to renal threshold. Which option reflects this level?

• A. 120 mmol/L
• B. 130 mmol/L
• C. 140 mmol/L
• D. 110 mmol/L

Answer: (B)

Potassium balance in the kidney is shaped by how much sodium reaches the distal nephron and by aldosterone. The distal tubule and collecting duct exchange Na+ for K+, so when there is a shift in tubular sodium handling, potassium excretion tends to increase. There is a threshold at which this shift becomes enough to lower serum potassium—the point where renal mechanisms start to drive more potassium loss. In this scenario, that threshold is around 130 mmol/L. So at this level, the kidney begins to excrete potassium more aggressively, leading to the onset of hypokalemia. The other levels—being lower or higher than this threshold—don’t capture the initial change in renal potassium handling that marks the onset.