Risky Business

Dr. Bob, a radiologists, is approaching retirement and he too often preaches to his young residents how good the old times were. He expounds on the superior merits of an upper GI series over current CT imaging for revealing an enlarged heart chamber. To demonstrate, Dr. Bob pulls out a two liter bottle of barium and proceeds to drink it. Next, he puts Bob Seger’s Old Time Rock n Roll  on his CD player and begins to mimic Tom Cruises dance in Risky Business. The residents are amazed at Dr. Bob’s agility and rhythm,  when all of a sudden Dr. Bob lunges forward, grabs his chest and passes out. He is rushed to the ED where a lateral chest film is performed and reveals an exceedingly enlarged chamber of the heart. The barium still present in his esophagus is used to confirm the enlarged heart chamber. Which of the indicated chambers could have been most readily detected in the lateral chest film of Dr. Bod?

Enlargement of which chamber would impinge/displace the esophagus?