A 62-year-old woman presents with progressively worsening shortness of breath on exertion over the past year. She is now unable to walk more than one block on level ground without stopping. She also reports generalized fatigue and occasional lightheadedness with activity. Two years ago, she was diagnosed with a pulmonary embolism (PE) after a long-haul flight. She received treatment with rivaroxaban for six months. Her only other significant past medical history is hypertension. It was diagnosed five years ago and is well-controlled with lisinopril. She denies chest pain, palpitations, cough, or recent illness. She is a lifelong nonsmoker and consumes alcohol socially. A review of systems is positive for bilateral ankle swelling in the evenings.

Initial laboratory studies are as follows:

WBC: 8,500/mm³ (4,500-11,000)

Hemoglobin: 13.8 g/dL (12.0-15.5)

Hematocrit: 41% (36-46)

Platelets: 250,000/mm³ (150,000-400,000)

PT: 12.5 seconds (11-15)

aPTT: 32 seconds (25-40)

Na⁺: 138 mEq/L (136-146)

K⁺: 4.2 mEq/L (3.5-5.0)

Cl^-: 102 mEq/L (98-107)

HCO₃^-: 25 mEq/L (22-28)

Glucose: 95 mg/dL (70-100)

BUN: 24 mg/dL (7-18)

Creatinine: 1.1 mg/dL (0.6-1.1)

Albumin: 3.8 g/dL (3.5-5.5)

ALT: 55 U/L (7-55)

AST: 60 U/L (8-48)

Alkaline phosphatase: 130 U/L (40-129)

Bilirubin, total: 1.5 mg/dL (0.1-1.2)

BNP: 1200 pg/mL (<100)