A 76-Year-Old Man Is Seen in the Emergency Department with Cough

A 76-year-old man is seen in the emergency department with cough and worsening dyspnea over the last 2 days.  He awoke suddenly last night gasping for air and could not get comfortable in any position.  He has no fever or chills.  The patient's other medical problems include hypertension and type 2 diabetes mellitus.  He has a 40-pack-year smoking history and drinks beer occasionally.
His temperature is 37.2° C (99° F) , blood pressure is 150/90 mm Hg, pulse is 106/min, and respirations are 25/min.  Pulse oximetry is 94% on 2 L O₂.  Body mass index is 35 kg/m².  Jugular venous pressure is measured at 12 cm H₂O.  He appears in mild respiratory distress.  Heart sounds are distant but no murmurs are heard.  Lung examination shows bibasilar crackles and scattered bilateral wheezes.  There is 1+ pitting ankle edema seen bilaterally.
Laboratory results are as follows:

Chest x-ray reveals prominent vascular markings and blunting of costophrenic angles.  Echocardiogram shows a diffusely hypokinetic left ventricle with ejection fraction of 35%.
Which of the following best explains the normal brain natriuretic peptide value?

A) High body mass index
B) Hypertension
C) Hyponatremia
D) Noncardiac cause of dyspnea
E) Older age