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A 42-year-old male with a past medical history of substance use presents to the ED for chest pain, shortness of breath, nausea, and vomiting. He reported snorting fentanyl two days ago and has a remote history of IV drug use. Vital signs include 98.8°F, HR 150, RR 23, SpO2 98% on RA, and BP of 180/100. He is anxious-appearing and diaphoretic, with dilated pupils (6mm, equal, round, and reactive) and full body rigors; ankle clonus is absent. Labs demonstrate a potassium of 2.8 mEq/L and a lactate of 3 mg/dL. He receives electrolyte repletion, fluid resuscitation, and broad-spectrum antibiotics. Concerned for opioid withdrawal, the treating physicians orders multiple doses of buprenorphine and full opioid agonists, without evidence of clinical improvement. What is the diagnosis?