This Frontal Radiograph Suggests an Increase in Lung Density Associated

This frontal radiograph suggests an increase in lung density associated with an apparent silhouette sign along the right border of the heart. However, after a lateral view was also examined, the radiologist concluded that the patient's lungs were normal. Why?

A) The radiograph is an AP rather than the normal PA view.
B) The patient has fibrous granulomas.
C) The patient has pectus excavatum.
D) The patient is recovering from heart bypass surgery.