Question

Which among the following is an early sign of magnesium toxicity?

• A. Depression of deep tendon reflexes
• B. Respiratory depression
• C. Cardiac arrest
• D. Decreased urine output

Hint:

Loss of the knee jerk is the bedside trigger to stop magnesium before breathing or the heart is hit.

Answer 1

The Correct Option is A

Step 1: Recall the toxic effects of hypermagnesemia in order of rising serum levels. Magnesium acts as a physiological calcium antagonist and progressively blocks neuromuscular transmission as its level rises.
Step 2: Sequence the clinical signs. At mildly elevated levels the first detectable sign is loss or depression of the deep tendon reflexes. As levels climb further, somnolence, respiratory depression and finally cardiac conduction block and cardiac arrest occur.
Step 3: Apply the clinical rule. Because loss of deep tendon reflexes appears before respiratory or cardiac compromise, it is used as the earliest bedside warning sign, for example during magnesium sulphate therapy in eclampsia. Disappearance of the patellar reflex signals that infusion should be stopped.
Step 4: Compare options. Respiratory depression and cardiac arrest are late, dangerous effects; decreased urine output is a cause of magnesium retention rather than an early toxic sign.
Conclusion: The correct answer is option A, depression of deep tendon reflexes.