Review Question - QID 214391

A 56-year-old man presents to his physician with worsening shortness of breath. He has also noticed some difficulty with swallowing and generalized weakness. He denies any fevers, chills, cough, chest pain, palpitations, or lightheadedness. He reports recovering from a gastrointestinal illness approximately 2 weeks ago. He has a medical history of hypertension and hyperlipidemia, for which he takes amlodipine and atorvastatin. He has smoked 1 pack of cigarettes daily for the past 35 years. Cardiopulmonary auscultation demonstrates normal heart sounds and bilateral lower lobe atelectasis. In the supine position, the abdominal wall retracts during inspiration. Neurological examination is notable for bilateral lower extremity atrophy and ankle clonus. His forced vital capacity is 65% while upright. Which of the following is most likely the cause of this patient's shortness of breath?