A 70 kg young athlete was planned for surgery. During anesthesia, vecuronium was not available, so repeated doses of succinylcholine was given intermittently up to 640 mg. During recovery, the patient was not able to spontaneously respire and move limbs. What is the cause?

**Core Concept**
Succinylcholine, a depolarizing neuromuscular blocker, can cause hyperkalemia and subsequent cardiac arrest when administered in large doses. This is due to the prolonged depolarization of muscle cells, leading to an efflux of potassium ions into the bloodstream.

**Why the Correct Answer is Right**
The excessive administration of succinylcholine (640 mg) in this young athlete led to a significant increase in serum potassium levels. This hyperkalemia caused a severe cardiac arrhythmia, resulting in the inability to spontaneously respire and move limbs. The mechanism behind this is the depolarization of muscle cells, which releases potassium ions into the bloodstream, leading to cardiac arrest.

**Why Each Wrong Option is Incorrect**
**Option A:** Muscle weakness is a common side effect of succinylcholine but is not the cause of respiratory paralysis in this scenario. Muscle weakness would not lead to the severe cardiac arrhythmia observed.
**Option B:** Malignant hyperthermia is a rare but life-threatening complication of succinylcholine administration. However, it is not directly related to the high dose of succinylcholine causing cardiac arrest in this case.
**Option C:** Anaphylaxis is a severe allergic reaction that can occur with succinylcholine administration. However, it is not the cause of the cardiac arrhythmia and respiratory paralysis in this scenario.

**Clinical Pearl / High-Yield Fact**
Succinylcholine should be used with caution in patients with burns, spinal cord injuries, or muscle denervation, as these conditions can increase the risk of hyperkalemia.

**Correct Answer:** D.

✓ Correct Answer: B. Phase II blockade