In a teaching case recently published in the American Journal of Kidney Diseases, Dimke et al present a case of hypomagnesemia and hypokalemia. The following questions will test your knowledge on renal magnesium handling and the evaluation of hypomagnesemia.
1. Which nephron segment reabsorbs the majority of filtered magnesium?
A. Proximal tubule
B. Thick ascending limb of the loop of Henle
C. Distal convoluted tubule (DCT)
D. Connecting tubule
E. Cortical collecting duct
2. What is the primary transporter of magnesium in the apical membrane of DCT cells?
B. Claudin 16
3. A 66-year old man presents to the emergency department with seizures. The patient has a history of metastatic colorectal cancer and has been receiving weekly cetuximab infusions over the last 3 months. Imaging of the head is unremarkable. His serum Mg2+ is found to be 0.5 mg/dL. Which of following is the most likely mechanism for the profound hypomagnesemia seen in this patient?
A. Competitive inhibition of epidermal growth factor receptor (EGFR)
B. Impaired basolateral sorting of Pro-EGF
C. Impaired paracellular Mg2+ transport via Claudin 16
D. Impaired basolateral Mg2+ extrusion via CNNM2
E. Impaired Na+-K+-ATPase activity due to decreased HNF1B expression
4. A 45 year-old man with chronic diarrhea presents to the emergency department complaining of generalized weakness. His serum potassium is found to be 1.8 mEq/L. His serum potassium cannot be corrected despite aggressive supplementation with oral and intravenous potassium chloride for several days. His serum magnesium is then checked and found to be 0.6 mg/dL. What is the most likely mechanism of hypomagnesemia-induced hypokalemia?
A. Hypomagnesemia causes low intracellular Mg2+ levels, which inhibit the NKCC2 transporter in the thick ascending limb of the loop of Henle.
B. Hypomagnesemia causes K+ shift to the intracellular compartment.
C. Hypomagnesemia causes low intracellular Mg2+ levels which relieve inhibition of K+ secretion via ROMK channels.
D. Hypomagnesemia causes secondary hyperaldosteronism which stimulates renal K+ secretion.
E. Hypomagnesemia causes low intracellular Mg2+ levels, which cause impairment of the Na+/K+-ATPase pump, decreasing cellular uptake of potassium leading to potassium wasting.
5. Which one of the following medications is effective as an adjuvant therapy for chronic hypomagnesemia?
Post prepared by Dr. Helbert Rondon, eAJKD Contributor, University of Pittsburgh.
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