You are correct. Mumps IgM and IgG were initially negative, but he seroconverted on day 9 after presentation. He also developed radiological evidence of pancreatitis and L orchitis.

Presternal edema in mumps is a distinguishing clinical feature, also known as Gellis sign. Lymphatic drainage obstruction from the anterior superior chest wall, by the enlarged salivary glands, is the likely cause. Concurrent dysphagia, hoarseness, and tenderness may occur, but most are asymptomatic.

Following routine MMR vaccination in children, mumps has become relatively rare. Dx may require imaging and bx to rule out other causes of sternal masses.

This case is likely secondary to vaccine failure, but the possibility of waning mumps immunity in adolescence is a concern.

Archives of Disease in Childhood 2022;107:839