## **Core Concept**
Magnesium toxicity, particularly in the context of eclampsia treatment with magnesium sulphate, is a critical concern due to its narrow therapeutic index. The condition arises when serum magnesium levels exceed 2.5 mEq/L, leading to neuromuscular and cardiovascular effects. Monitoring for early signs of toxicity is crucial to prevent progression to more severe complications.

## **Why the Correct Answer is Right**
The correct answer, , refers to diminished deep tendon reflexes. This is an early sign of magnesium toxicity because magnesium sulphate acts as a neuromuscular blocker. At therapeutic levels, it helps in seizure prophylaxis in eclampsia. However, as toxicity develops, one of the first noticeable effects is the reduction or disappearance of deep tendon reflexes (DTRs). This occurs due to the blockade of acetylcholine release at the neuromuscular junction and the stabilization of the nerve membrane. Monitoring DTRs is a standard practice to assess for early signs of magnesium toxicity.

## **Why Each Wrong Option is Incorrect**
- **Option A:** is incorrect because respiratory depression is a later and more severe sign of magnesium toxicity. It occurs when the neuromuscular blockade becomes more pronounced, affecting the diaphragm and other respiratory muscles.
- **Option B:** is incorrect because cardiac arrest is a critical and life-threatening consequence of severe magnesium toxicity, not an early sign. It occurs when magnesium levels are significantly elevated, affecting cardiac conduction and contractility.
- **Option C:** is incorrect because decreased urine output is more related to renal effects or volume status rather than a direct effect of magnesium toxicity on the nervous system or muscles.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that deep tendon reflexes should be regularly checked in patients receiving magnesium sulphate for eclampsia. The disappearance or significant diminution of these reflexes is an early warning sign for potential magnesium toxicity, necessitating a review of magnesium dosing.

## **Correct Answer:** . Diminished deep tendon reflexes.