Valvular Heart Disease — Pathology — Topic Test

A 55-year-old man has chronic watery diarrhoea, flushing and right-sided valvular heart disease. Urinary 5-hydroxyindoleacetic acid is markedly elevated. A small-bowel tumour biopsy shows nests and trabeculae of uniform cells with salt-and-pepper chromatin that stain strongly for chromogranin A and synaptophysin. The systemic syndrome appears only because the tumour has spread to which site?