A 24-year-old man presents to the emergency department after being the victim of a gunshot wound during a fight. On arrival, the paramedics report that the patient is breathing spontaneously and saturating well on a non-rebreather mask. During the primary survey, the patient is found to have equal bilateral breath sounds and stable vital signs. On secondary survey, a single gunshot wound is identified in the left fourth intercostal space, 1 cm to the left of the left sternal border. Which of the following cardiac structures is most likely injured in this patient?
