**Core Concept**
Magnesium toxicity, also known as hypermagnesemia, occurs when serum magnesium levels exceed 2.5-3 mg/dL, leading to a range of clinical manifestations. The earliest signs of magnesium toxicity are often subtle and can be easily overlooked, making prompt recognition crucial for effective management.

**Why the Correct Answer is Right**
The earliest sign of magnesium toxicity is typically a decrease in deep tendon reflexes (DTRs). This occurs due to the inhibition of acetylcholine release at the neuromuscular junction, leading to a reduction in excitatory neurotransmission. As magnesium levels continue to rise, this effect becomes more pronounced, and other signs such as muscle weakness, flaccidity, and respiratory depression may ensue.

**Why Each Wrong Option is Incorrect**
**Option A:** Confusion and lethargy can be seen in magnesium toxicity, but they are not the earliest signs.
**Option B:** While muscle weakness is a common feature of magnesium toxicity, it is not the earliest sign.
**Option C:** Respiratory depression is a severe manifestation of magnesium toxicity, but it is not the first sign to appear.

**Clinical Pearl / High-Yield Fact**
Remember that the "dead giveaway" for magnesium toxicity is decreased DTRs, especially in a patient with a history of renal failure or excessive magnesium intake.

**Correct Answer:**
Decreased deep tendon reflexes.