A 63 years old man sustained a cut on his hand while gardening. He presents with signs of generalized muscle stiffness and trismus, and appears to be grimacing. Intermittent tonic contraction of his skeletal muscles causes intensely painful spasms, which last for minutes, during which he retains consciousness. During a generalized spasm the patient arches his back, extends his legs, flexes his arms in abduction, and, and clenches his fists. He also experiences dysphagia and apneic periods. Clinical exam reveals hyperpyrexia, cardiac arrhythmias, and hypotension. What is the most likely diagnosis?