Clinical Reasoning: A 75-year-old man with parkinsonism, mood depression, and weight loss.

A 75-year-old man presented to the emergency department with a 1-year history of 66-pound weight loss and alternating bowel habits. He was admitted to the hospital, where he underwent several examinations to investigate the presence of a malignancy. A colonoscopy, a gastroscopy, an ultrasound of the abdomen, and a contrast-enhanced CT scan of thorax and abdomen did not detect any neoplasia. The only findings consisted of a prostatic hypertrophy and a basal pleural–parenchymal hyperdensity in the left lung, which was described as the result of an infective process. Neoplastic markers CA19.9, carcinoembryonic antigen, neuron-specific enolase, and α-fetoprotein were also negative. Wide-spectrum blood tests were unremarkable, except for hypogammaglobulinemia and elevated β2 microglobulin.