You have been asked to see an in-patient on the stroke ward by one of the nurses as she is concerned about an ulcer on the patient’s leg. On examination, you note the presence of an arterial ulcer on the dorsum of the foot which is not bleeding. The base of the ulcer contains greyish granulation tissue and there is pain at night which is relieved by dangling the leg out of the bed. You check the pulses in the foot, and you note they are absent. What is the most likely diagnosis?