A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 56-year-old man with chronic kidney failure is brought to to the emergency department by ambulance after he passed out during dinner. On presentation, he is alert and complains of shortness of breath as well as chest palpitations. An EKG is obtained demonstrating an irregular rhythm consisting of QT amplitudes that vary in height over time. Other findings include uncontrolled contractions of his muscles. Tapping of his cheek does not elicit any response. Over-repletion of the serum abnormality in this case may lead to which of the following?
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A
BradycardiaCorrect. Magnesium over-repletion (hypermagnesemia) causes bradycardia and AV block by blocking calcium channels at SA/AV nodes; loss of DTRs and respiratory depression follow at higher levels.
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B
Diffuse calcificationsIncorrect. Diffuse calcifications occur with hypercalcemia/hyperphosphatemia (metastatic calcification in ESRD), not from magnesium over-repletion.
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C
Kidney stonesIncorrect. Kidney stones reflect hypercalciuria/hypercalcemia, not magnesium toxicity.
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D
SeizuresIncorrect. Seizures occur with HYPOmagnesemia or hypocalcemia (neuromuscular irritability), not from over-replacement of magnesium.
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E
TetanyIncorrect. Tetany results from HYPOcalcemia or HYPOmagnesemia (increasing neuromuscular excitability); over-repletion of magnesium causes the opposite - decreased neuromuscular activity and loss of reflexes.
↑ Tap an answer to reveal the reasoning
Answer: A. An ESRD patient with syncope, palpitations, an irregular rhythm with varying QT amplitude (TORSADES de pointes - polymorphic VT, often confused here with TdP-like polymorphic VT secondary to long QT), tetany/muscle contractions, and a NEGATIVE Chvostek sign ("tapping cheek does not elicit response") - wait, classic Chvostek would be POSITIVE in hypocalcemia. Let's re-read: "tapping of his cheek does not elicit any response" - this is actually describing a negative Chvostek. However, the combination of long QT, muscle twitching, and ESRD points most strongly to HYPOMAGNESEMIA (or hypocalcemia/hypokalemia, all common in ESRD).
Given the constellation - QT prolongation/polymorphic VT, neuromuscular irritability, and ESRD - the most likely serum abnormality is HYPOMAGNESEMIA. Treatment is IV magnesium sulfate (also given as first-line for torsades).
Over-repletion of MAGNESIUM (hypermagnesemia) causes neuromuscular depression: loss of deep tendon reflexes first, then respiratory depression and BRADYCARDIA/cardiac arrest at higher levels. Mg blocks calcium influx at the SA node and AV node, slowing conduction. Hence bradycardia is the answer.
Alternative interpretation if the abnormality is CALCIUM: hypercalcemia causes shortened QT (not the QT changes here) and can cause kidney stones and constipation, but the cardiac findings here fit Mg better. The classic hypermagnesemia toxicity = bradycardia, hyporeflexia, respiratory depression.
Distractors: diffuse calcifications occur in hypercalcemia/hyperphosphatemia (metastatic calcification, important in ESRD - but this is over-repletion of CALCIUM/phosphate, not Mg); kidney stones suggest hypercalciuria/hypercalcemia; seizures suggest HYPOmagnesemia or HYPOcalcemia, not over-repletion.
**Why each option:**
**A.** Magnesium over-repletion (hypermagnesemia) causes bradycardia and AV block by blocking calcium channels at SA/AV nodes; loss of DTRs and respiratory depression follow at higher levels.
**B.** Diffuse calcifications occur with hypercalcemia/hyperphosphatemia (metastatic calcification in ESRD), not from magnesium over-repletion.
**C.** Kidney stones reflect hypercalciuria/hypercalcemia, not magnesium toxicity.
**D.** Seizures occur with HYPOmagnesemia or hypocalcemia (neuromuscular irritability), not from over-replacement of magnesium.
_Note: this question is flagged as `ambiguous` and may benefit from review._