Review Question - QID 214330

A 47-year-old woman with a history of left breast HER2+ ductal carcinoma presents to the emergency department with 2 weeks of worsening shortness of breath. She becomes short of breath while climbing stairs and exercising. She denies fever, weight loss, cough, and chest pain. She had bilateral mastectomies performed 6 months prior and is currently receiving adjuvant targeted biological therapy. She has no other past medical history and has no allergies. She has 2 children at home that currently attend daycare. She does not drink alcohol and has smoked 1 pack per day for 20 years. Her temperature is 99.0°F (37.2°C), blood pressure is 145/80 mmHg, pulse is 85/min, respirations are 20/min, and O2 saturation is 98% on room air. On exam, she has no conjunctival pallor and her mucous membranes are moist. Her heart rate and rhythm are regular with a third heart sound following S2. Crackles are heard in the bilateral lower lobes. The mastectomy scars are healed with no erythema. Her abdomen is soft and nontender and the remainder of the exam is unremarkable. Chest radiograph is shown in Figure A. Which of the following is the most likely cause of this patient's presentation?