A two-year-old boy presented to the emergency department with new-onset fever. The boy's mother mentioned that a mass in the boy's left arm present since birth had grown larger several days previously. Of note, the boy had never had workup for the mass. The boy was examined by a surgeon, who noted a bulge just above the elbow with focal warmth and fluctuation, and referred him for soft tissue ultrasound of the arm.

On commencement of the ultrasound examination, the arm was noted to be uniformly swollen, without any skin discolouration or a well-defined mass.

The ultrasonographic findings are consistent with a macrocystic lymphatic malformation of the arm. The single hypoechoic structure is either hemorrhagic or infected.