Approximately 5-month-old castrated male Lab mix

• There is right sided cardiomegaly which is causing a shift of the left ventricle/apex to the left on the VD view. Also on the VD view, the right atrium is markedly enlarged. The heart has a rounded appearance on the lateral views but not on the VD view.
• The left heart is normal.
• The pulmonary vasculature and parenchyma are normal.
• There is a sternal abnormality with shortened 5th and 6th sternebrae, with irregularity of the 5th sternebra.

• Severe right sided cardiomegaly with marked right atrial enlargement most likely due to tricuspid dysplasia. Cor triatriatum dexter is possible. An atrial septal defect would be much less likely.
• Other congenital cardiac abnormalities such as pulmonic stenosis or Tetralogy of Fallot are not likely due to the radiographic signs that are present.
• A right atrial mass or myxomatous degeneration of the tricuspid valve are not likely in a patient of this age.
• The sternal abnormality is likely congenital, or may be due to prior trauma.

Most candidates realized tricuspid dysplasia was a top rule-out. More successful candidates were able to reason through the other possibilities and kept the differential list short. They also recognized that, although there was a sternal anomaly, the radiographic findings did not otherwise fit with a PPDH, so left that off the rule-out list. Less successful candidates struggled with cardiovascular physiology and included rule-outs that were not likely, based on radiographic findings – these included Tetralogy of Fallot, PDA, PPDH. They sometimes over-interpreted the left heart as being enlarged, which led them to inappropriate differentials.