A 42-year old woman presents with intermittent per-rectal bleeding for two weeks. She has no other symptoms. She had a subtotal colectomy with an ileostomy six months ago, due to fulminant ulcerative colitis refractory to medical treatment. She was found to be human immunodeficiency virus (HIV) positive six years ago, and has been on antiretroviral therapy since then. Her last CD4 cell count is from two weeks ago; and is within normal limits. The rest of her history is unremarkable. A full blood count is within normal parameters.