Okay, let's tackle this question. The question is about the most readily detected clinical sign of hypermagnesemia. First, I need to recall what hypermagnesemia is. Magnesium is an essential electrolyte, and its levels are tightly regulated. Hypermagnesemia means elevated magnesium levels in the blood, which can be due to kidney dysfunction, excessive intake (like from medications), or other causes.

Now, the signs and symptoms of hypermagnesemia. I remember that magnesium affects neuromuscular transmission, the central nervous system, and cardiac function. Early signs might be subtle. The question asks for the most readily detected clinical sign, so I need to think about what's most obvious and detectable during a physical exam.

Common symptoms include muscle weakness, hypotension, decreased deep tendon reflexes, lethargy, and in severe cases, respiratory depression or cardiac arrest. The most detectable sign would be something that a clinician can notice without needing a lab test. Decreased deep tendon reflexes are a classic sign and are part of the neurological exam. Hypotension could be another, but maybe it's less specific. Lethargy might be subjective.

Wait, the options aren't listed here, but the correct answer is supposed to be the most readily detected. So if the options include decreased deep tendon reflexes, that's likely the correct answer. Let me confirm: hypermagnesemia leads to a decrease in neuromuscular excitability, so reflexes are diminished. This is a key point. Also, in early stages, the first sign might be decreased reflexes, which is easily checked during a physical exam.

Other signs like hypotension or cardiac changes might come later and require more specific monitoring. So the most readily detected would be the decreased DTRs. Therefore, the correct answer should be decreased deep tendon reflexes.

**Core Concept**
Hypermagnesemia (serum Mg >2.6 mg/dL) impairs neuromuscular transmission by blocking acetylcholine release at the neuromuscular junction. The earliest detectable sign is **decreased deep tendon reflexes (DTRs)** due to impaired motor neuron excitability.

**Why the Correct Answer is Right**
Magnesium competes with calcium for binding sites on NMDA receptors and inhibits voltage-gated calcium channels, reducing neurotransmitter release. This leads to progressive neuromuscular depression: decreased DTRs are the **first objective sign**. As levels rise, flaccid paralysis, hypotension, and cardiac conduction defects follow. DTRs are easily assessed via physical exam, making them the most readily detected clinical manifestation.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypotension occurs in moderate hypermagnesemia but is less specific and often secondary to vasodilation.
**Option B:** Lethargy or confusion is a later CNS manifestation, less detectable in early stages.
**Option C:** Hyporeflexia (not hyporeflexia) is the correct term; hyporeflexia is not a primary feature of hypermagnesemia.
**Option D:** Cardiac changes (e.g., prolonged PR interval) require ECG monitoring and are not "readily" detected clinically.

**Clinical Pearl / High-Yield Fact**
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✓ Correct Answer: D. Disappearance of deep tendon reflexes