**Core Concept**
Succinylcholine is a depolarizing neuromuscular blocking agent that acts by mimicking the action of acetylcholine at the neuromuscular junction, leading to muscle contraction and eventual blockade. However, prolonged exposure to succinylcholine can lead to a unique phenomenon known as phase II or desensitization blockade.

**Why the Correct Answer is Right**
The correct answer is **B. Phase 2 blockade produced by succinylcholine**. Succinylcholine can induce a phase I blockade, characterized by initial depolarization and muscle contraction, followed by a phase II blockade, where the muscle is unable to respond to acetylcholine due to desensitization of the nicotinic receptors. This phase II blockade is more pronounced with repeated doses of succinylcholine, leading to prolonged paralysis. In this case, the patient received a total of 640 mg of succinylcholine, which is significantly higher than the recommended dose, leading to phase II blockade.

**Why Each Wrong Option is Incorrect**
**Option A:** Pseudocholinesterase deficiency would lead to prolonged action of succinylcholine, but it would not cause phase II blockade. The patient's pseudocholinesterase levels were not mentioned in the scenario.

**Option C:** Undiagnosed muscular dystrophy would increase the risk of hyperkalemia and rhabdomyolysis with succinylcholine, but it would not cause phase II blockade.

**Option D:** Fasciculation produced by succinylcholine is a common side effect, but it is not responsible for the prolonged paralysis seen in this patient.

**Clinical Pearl / High-Yield Fact**
Succinylcholine should be used with caution in patients with a history of previous neuromuscular blockade or repeated doses, as it can lead to phase II blockade. A maximum dose of 400-600 mg is recommended to minimize this risk.

**✓ Correct Answer: B. Phase 2 blockade produced by succinylcholine**