Situation: A 64-year-old man with chronic atrial fibrillation and hypertension takes digoxin 0.125 mg and hydrochlorothiazide 25 mg orally every morning. He has no heart failure or kidney disease. He is admitted with generalized weakness, leg cramps, and palpitations. His serum potassium is 2.9 mEq/L (3.5–5.0 mEq/L). The hydrochlorothiazide is held, and he has a peripheral intravenous (IV) catheter in his forearm. On day 3 he has received 120 mEq of potassium chloride, and the hydrochlorothiazide has stayed on hold since admission. His results are: Potassium: 3.1 mEq/L (3.5–5.0 mEq/L) Magnesium: 1.2 mg/dL (1.7–2.2 mg/dL) Sodium: 137 mEq/L (135–145 mEq/L) Calcium: 9.0 mg/dL (8.6–10.2 mg/dL) What is the MOST likely reason his potassium remains low?

His magnesium is low (1.2 mg/dL), and hypomagnesemia makes hypokalemia resistant to treatment because the kidneys keep wasting potassium until magnesium is replaced. The thiazide has been held for 3 days, so its effect has worn off. Magnesium replacement is needed for the potassium to correct.

Why the potassium stays low
This patient’s serum magnesium is 1.2 mg/dL (reference 1.7–2.2 mg/dL), which is the key to the persistent hypokalemia. Hypomagnesemia causes the kidneys to continue wasting potassium in the urine, so potassium replacement alone cannot correct the serum potassium until magnesium is repleted. Even after 120 mEq of potassium chloride over three days, the potassium rose only from 2.9 mEq/L to 3.1 mEq/L because the renal potassium leak remained active.

Mechanism linking magnesium and potassium
In the distal nephron, potassium secretion through the renal outer medullary potassium (ROMK) channel is normally inhibited by intracellular magnesium. When magnesium is depleted, this magnesium-dependent inhibition is lost, and ROMK channels remain open, allowing ongoing urinary potassium excretion. The result is refractory hypokalemia: potassium given intravenously is rapidly excreted in the urine rather than retained. This is why hypomagnesemia is a well-recognized cause of potassium wasting that does not respond to potassium supplementation alone.

Why the other options are less likely
Watch out! The thiazide diuretic was held on admission and has remained held for three days. Thiazide-induced kaliuresis resolves within that timeframe, so ongoing drug effect is not a plausible explanation for the persistently low potassium on day 3. Digoxin inhibits the Na⁺/K⁺-ATPase pump, which can raise extracellular potassium slightly; it does not cause hypokalemia by shifting potassium into cells. A 0.9% sodium chloride carrier does not dilute serum potassium enough to explain a value of 3.1 mEq/L after aggressive replacement.

Clinical application for the licensure exam
Key point! Whenever hypokalemia does not correct despite adequate potassium replacement, check the serum magnesium. The sequence of correction is important: magnesium replacement should be given first or concurrently, because repleting magnesium restores the kidney’s ability to retain potassium. This principle appears across electrolyte imbalance, diuretic therapy, and arrhythmia-related questions.

Finding	Value	Interpretation
Serum potassium	3.1 mEq/L	Still low despite 120 mEq KCl
Serum magnesium	1.2 mg/dL	Hypomagnesemia driving renal K⁺ wasting
Serum sodium	137 mEq/L	Normal
Serum calcium	9.0 mg/dL	Normal

Connecting to the evidence
The case reports describe patients with refractory hypokalemia accompanied by hypomagnesemia, where potassium levels remained low until the underlying renal magnesium and potassium wasting was addressed ^[1][2][3][4]. Although these reports focus on Gitelman syndrome, they illustrate the same physiologic principle seen in this patient: persistent renal potassium loss in the setting of hypomagnesemia makes potassium replacement ineffective until magnesium is corrected. In this patient, the combination of chronic diuretic use and hypomagnesemia created a similar state of renal potassium wasting, and the low magnesium on day 3 is the most direct explanation for the failure of potassium to normalize.