USMLE® Step 1 style questions USMLE
A 78-year-old man is brought to the emergency department with a 3-day history of fever and abdominal pain. Examination and urinalysis findings are consistent with pyelonephritis, causing sepsis and acute renal failure on a background of pre-existing chronic kidney disease (CKD). His temperature is 38.7°C (101.6°F), pulse is 110 beats/min, respirations are 22/min, and blood pressure is 98/65mmHg. A systems examination reveals a loss of deep tendon reflexes and ECG shows prolonged atrioventricular conduction. On further questioning, the patient also reveals that he has been feeling generally weak and nauseous, and has had two episodes of vomiting today. His past medical history is significant for CKD with an eGFR of 24 mL/min, and GERD for which he has recently been taking antacids. Considering the history and examination findings, which of the following electrolyte abnormalities is most likely to have caused this patient's presentation?
Content Reviewers:Rishi Desai, MD, MPH
‘Hyper-’ means ‘over’ and ‘-magnes-’ refers to magnesium, and -emia refers to the blood, so hypermagnesemia means higher than normal magnesium levels in the blood, and symptoms typically develop at a level over 4 mEq/L.
An average adult has about 25 grams of magnesium in their body.
About half is stored in the bones, and most of the other half is found within cells.
In fact, magnesium is a really common positively charged ion found within the cell, second only to king potassium.
A very tiny fraction, roughly 1% of the total magnesium in the body, is in the extracellular space which includes both the intravascular space - the blood and lymphatic vessels, and the interstitial space - the space between cells.
About 20% of the magnesium in the extracellular space, which would be about 0.2% of the total magnesium, is bound to negatively charged proteins like albumin, but the other 80% or 0.8% of the total magnesium, can be filtered into the kidneys.
So in the kidney, that magnesium gets filtered into the nephron, andi about 30% gets reabsorbed at the proximal convoluted tubule, 60% gets reabsorbed in the ascending loop of Henle, and 5% get reabsorbed at the distal convoluted tubule.
That leaves only 5% to get excreted by the kidneys.
So, in order for there to be too much magnesium in the blood, this normal balance has to be disturbed.
Another cause of hypermagnesemia is ingesting more magnesium than the kidneys can excrete.
Sometimes this can be due to an intravenous infusion of magnesium that isn’t prepared correctly.
If these medications are taken in excess over a long period of time, it can lead to hypermagnesemia.
There are some less common causes of hypermagnesemia.