A 34-year-old woman is brought to the emergency department due to a 2-day history of progressive dyspnea and drowsiness. According to her husband, she had difficulty swallowing food and almost choked while having dinner yesterday. One week ago, she was treated with ciprofloxacin for a urinary tract infection. The patient volunteers in a local library and for the past several months has had to reduce her work hours due to difficulty lifting books and placing them on shelves. She drinks alcohol socially and does not smoke. Her blood pressure is 142/90 mm Hg, pulse is 92/min, and respirations are 20/min and shallow. Pulse oximetry shows 93% on room air. The patient's BMI is 32 kg/m². Her speech sounds nasal. Physical examination demonstrates clear lung fields with the use of accessory muscles of respiration and paradoxical abdominal wall motion with inspiration. Heart sounds are normal without a murmur. Muscle strength is bilaterally decreased with normal deep tendon reflexes. Babinski reflex is absent bilaterally. Arterial blood gas shows pH 7.2, pCO₂ 65 mm Hg, and pO₂ 90 mm Hg. Which of the following is the most likely diagnosis?

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11ec25c8_d1bd_17c7_bc81_4948feda7ec1_MD0003_00 This patient most likely has myasthenic crisis as an early manifestation of myasthenia gravis. Myasthenia gravis usually presents in the second to third decade in women (sixth to eighth decade in men) with fluctuating weakness that worsens over the course of the day and most often involves the extraocular (eg, ptosis, diplopia), bulbar (eg, fatigable chewing, dysphagia, nasal speech), proximal limb, and respiratory muscles. Myasthenic crisis is a life-threatening complication characterized by severe respiratory muscle weakness leading to respiratory failure (eg, accessory muscle use, hypoxemia, respiratory acidosis). Patients often have increasing generalized or bulbar weakness prior to the onset of crisis. The condition may be precipitated by infection (eg, urinary tract infection), surgery, pregnancy, or medications (eg, aminoglycosides, fluoroquinolones, macrolides, beta blockers). Patients with severe exacerbations and declining respiratory status should be monitored in the intensive care unit and intubated for airway protection. (Choice A) Foodborne botulism is characterized by the sudden onset of bilaterally symmetric weakness starting with the face/cranial nerves and descending to the trunk, the upper extremities, and then the lower extremities. It is usually preceded by a prodrome of gastrointestinal symptoms and may lead to respiratory failure if respiratory muscles are affected. (Choice B) Guillain-Barré syndrome is characterized by ascending symmetric paralysis associated with weak/absent deep tendon reflexes and autonomic dysfunction. It usually occurs after an antecedent respiratory tract or gastrointestinal (eg, Campylobacter jejuni) infection and can require respiratory support if it progresses to involve the respiratory muscles. (Choice D) Although this patient is obese (BMI >30 kg/m²), obesity hypoventilation syndrome is not associated with difficulty swallowing or muscle weakness and would not generally cause respiratory distress. Symptoms (eg, excessive daytime sleepiness, loud snoring) are often related to coexisting obstructive sleep apnea. ² (Choice E) Polymyositis typically presents with the insidious onset of symmetric, proximal limb muscle weakness and myalgias associated with elevated muscle enzymes (eg, creatine kinase). Respiratory muscle weakness and respiratory failure are not characteristic. (Choice F) West Nile encephalitis is characterized by fever, headache, and confusion. Neuroinvasive West Nile infection may also cause myelitis, resulting in an acute paralysis syndrome characterized by asymmetric flaccid weakness in the limbs. Educational objective: Myasthenic crisis is a life-threatening complication of myasthenia gravis that is characterized by severe respiratory muscle weakness leading to respiratory failure. Symptoms are often preceded by increasing generalized or bulbar muscle weakness. The condition may be precipitated by infection, surgery, and certain medications (eg, fluoroquinolones). References: Myasthenic crisis. (http://www.ncbi.nlm.nih.gov/pubmed/23983833)

A 34-Year-Old Woman Is Brought to the Emergency Department Due