A 58-year-old man presents with progressive hoarseness and dyspnea for 3 weeks. He has a history of hypertension managed with medications. On examination, the voice is breathy and weak. Laryngoscopy reveals the left vocal cord in a paramedian position with incomplete glottic closure. There is no history of intubation, thyroid surgery, or malignancy. What is the most likely etiology of his vocal cord paralysis?