A 68-year-old man has had worsening dyspnea and orthopnea for the past 3 years with increased production of frothy sputum. On examination, crackles are auscultated at lung bases. A chest radiograph shows bilateral interstitial infiltrates, distinct Kerley B lines, and a prominent left heart border. Laboratory studies show Na+, 135 mmol/L; K+, 3.8 mmol/L; Cl-, 99 mmol/L; CO2, 25 mmol/L; glucose, 76 mg/ dL; creatinine, 1.5 mg/dL; and urea nitrogen, 30 mg/dL. Fractional excretion of sodium is less than 1%. Plasma renin, aldosterone, and antidiuretic hormone levels all are increased. B-type natriuretic peptide (BNP) is 200 pg/mL (normal <100 pg/mL). Which of the following pathologic findings is this man most likely to have?