Review Question - QID 104021

A 70-year-old man presents with a complaint of progressive dyspnea on minimal exertion. The patient reports being quite active and able to climb 3 flights of stairs in his building 10 years ago, whereas now he feels extremely winded when climbing a single flight. At first, he attributed this to old age but has more recently begun noticing that he feels similarly short of breath when lying down. He denies any recent fevers, cough, chest pain, nausea, vomiting, or diarrhea. He denies any past medical history except for two hospitalizations over the past 10 years for "the shakes." Family history is negative for any heart conditions. Social history is significant for a 10 pack-year smoking history. He currently drinks "a few" drinks per night. On exam, his vitals are: BP 120/80, HR 85, RR 14, and SpO2 97%. He is a mildly obese man who appears his stated age. Physical exam is significant for a normal heart exam with a few crackles heard at the bases of both lungs. Abdominal exam is significant for an obese abdomen and a liver edge palpated 2-3 cm below the costal margin. He has 2+ edema present in both lower extremities. Lab results reveal a metabolic panel significant for a sodium of 130 mEq/L but otherwise normal. Complete blood count, liver function tests, and coagulation studies are normal as well. An EKG reveals signs of left ventricular enlargement with a first degree AV block. A cardiac catheterization report from 5 years ago reveals a moderately enlarged heart but patent coronary arteries. Which of the following is the most likely cause of this individual's symptoms?