## **Core Concept**
Magnesium toxicity, also known as hypermagnesemia, occurs when the serum magnesium levels exceed 2.5 mEq/L (1.25 mmol/L). This condition can lead to serious clinical manifestations, primarily affecting the nervous and cardiovascular systems. Early recognition of its signs is crucial for prompt management.

## **Why the Correct Answer is Right**
The correct answer, , is an early sign of magnesium toxicity. One of the earliest and most notable signs of hypermagnesemia is the loss of deep tendon reflexes. This occurs because magnesium acts as a neuromuscular blocker, and elevated levels can lead to neuromuscular weakness and depressed reflexes. The loss of deep tendon reflexes is often seen before more severe symptoms such as respiratory muscle weakness or cardiac arrest.

## **Why Each Wrong Option is Incorrect**
- **Option A:** is incorrect because, although muscle weakness is a symptom of magnesium toxicity, it typically presents after the loss of deep tendon reflexes.
- **Option B:** is incorrect as respiratory depression is a more severe and later manifestation of magnesium toxicity.
- **Option C:** is incorrect because, while cardiac arrest can occur in severe magnesium toxicity, it is a late and life-threatening complication, not an early sign.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the loss of deep tendon reflexes is an early and sensitive indicator of magnesium toxicity. This highlights the importance of monitoring neurological status in patients at risk of hypermagnesemia, such as those with renal failure or those receiving magnesium-containing medications.

## **Correct Answer:** . Loss of deep tendon reflexes