Question:medium
Which among the following is an early sign of magnesium toxicity?
Which among the following is an early sign of magnesium toxicity?
Show Hint
Loss of the knee jerk is the bedside trigger to stop magnesium before breathing or the heart is hit.
Updated On: Jun 24, 2026
- Depression of deep tendon reflexes
- Respiratory depression
- Cardiac arrest
- Decreased urine output
Show Solution
The Correct Option is A
Solution and Explanation
Magnesium toxicity is best understood as a dose-dependent escalation of neuromuscular and cardiac blockade, because magnesium competes with calcium at the neuromuscular junction and the cardiac conduction system. As the serum concentration creeps up, the body gives warning signs in a predictable order, and clinicians exploit that order for safety monitoring.
The very first thing to vanish is the deep tendon reflex arc, since neuromuscular transmission is the most sensitive to magnesium excess. Only at higher concentrations do you see drowsiness, then depression of the respiratory drive, and at the highest levels heart block and arrest. This is exactly why, when magnesium sulphate is run for seizure prophylaxis in pre-eclampsia and eclampsia, the nurse checks the knee jerk repeatedly; an absent patellar reflex is the trigger to halt the infusion long before breathing or the heart is threatened.
So among the choices, respiratory depression and cardiac arrest are late and ominous, and low urine output is what predisposes to magnesium accumulation rather than an early effect of it. The earliest sign is reflex depression.\[\boxed{\text{Depression of deep tendon reflexes}}\]
The very first thing to vanish is the deep tendon reflex arc, since neuromuscular transmission is the most sensitive to magnesium excess. Only at higher concentrations do you see drowsiness, then depression of the respiratory drive, and at the highest levels heart block and arrest. This is exactly why, when magnesium sulphate is run for seizure prophylaxis in pre-eclampsia and eclampsia, the nurse checks the knee jerk repeatedly; an absent patellar reflex is the trigger to halt the infusion long before breathing or the heart is threatened.
So among the choices, respiratory depression and cardiac arrest are late and ominous, and low urine output is what predisposes to magnesium accumulation rather than an early effect of it. The earliest sign is reflex depression.\[\boxed{\text{Depression of deep tendon reflexes}}\]
Was this answer helpful?
0
