A 62-year-old woman is brought to the emergency department due to progressive weakness and dyspnea. The patient is visiting her niece, who says the patient forgot to bring her regular medications and has not been taking them for the past week. The patient began feeling weak and fatigued 3 days ago and has had shortness of breath since yesterday. Temperature is 37 C (98.6 F), blood pressure is 122/88 mm Hg, pulse is 90/min, and respirations are 24/min and shallow. On physical examination, the patient appears dyspneic with drooping at the eyelids and corners of the mouth. Her speech has a nasal quality. Arterial blood gas obtained on room air shows pH 7.32; PaCO₂ is 52 mm Hg, PaO₂ is 72 mm Hg, and HCO₃ is 26 mEq/L. Bedside spirometry demonstrates decreased forced vital capacity. A pathologic process involving which of the following structures is the most likely cause of this patient's shortness of breath?

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11ec160b_3b3d_58c6_9249_5f6b2c308959_MD0001_00 This patient has hypercapnic and hypoxic respiratory failure (low pH, high CO₂, low O₂) indicating global hypoventilation. In association with the ptosis, bulbar weakness, and low forced vital capacity, this presentation suggests myasthenic crisis (severe weakness and respiratory depression due to an exacerbation of myasthenia gravis [MG]). MG is characterized by autoantibodies against nicotinic acetylcholine receptors on the postsynaptic membrane of the neuromuscular junction, resulting in receptor degradation. This reduces the sensitivity of the postsynaptic membrane to acetylcholine stimulation, leading to reduced muscular response despite normal acetylcholine release. Muscle weakness worsens with repetition as acetylcholine stores within the presynaptic nerve terminal become progressively depleted. Patients with MG typically have extraocular (eg, ptosis, diplopia), bulbar (eg, dysphonia, difficulty chewing), and facial (eg, myasthenic snarl) weakness. In addition, neck and proximal muscle weakness may occur, and in severe cases the respiratory muscles may be affected, leading to respiratory failure (as in this patient). Acetylcholinesterase inhibitors (eg, pyridostigmine, neostigmine) are used for symptomatic treatment, and withdrawal can trigger a myasthenic crisis. (Choice A) Impairment of the respiratory control centers in the brain stem causes hypoventilation during sleep (Ondine curse); however, voluntary breathing is unaffected. (Choice C) Pulmonary embolism can result in dyspnea and gas exchange abnormalities (eg, hypoxia). However, significant muscular weakness (eg, ptosis, nasal speech, decreased vital capacity) would be unexpected. (Choices D and E) Patients with diseases involving the terminal airways and alveoli (eg, pulmonary fibrosis, emphysema) and large airways (eg, asthma, chronic bronchitis) can present with respiratory failure subsequent to muscle fatigue. However, these diseases are not associated with non-respiratory muscle weakness (eg, dysphonia, bulbar weakness). Educational objective: Myasthenia gravis is characterized by autoantibodies against postsynaptic nicotinic acetylcholine receptors at the neuromuscular junction, which result in receptor degradation. Patients typically have extraocular, bulbar, and facial weakness that worsens with activity. In severe cases, the respiratory muscles may be affected, leading to hypoventilation and respiratory failure.

A 62-Year-Old Woman Is Brought to the Emergency Department Due