An ill-defined area of sclerosis (arrows) is noted over the posterior trabecular bone of the calcaneal tuberosity. The non-anatomic distribution for this sclerosis should raise a red flag in your head and make you think of the possibility of a stress fracture. If there is any doubt in your mind that this is a real finding, the prior film below was taken about a year before the initial film. It shows that the sclerosis wasn't present at that time.
This patient's history of vague ankle pain is a bit of a red herring. This patient is a street person, and was not a terribly good historian. Once the sclerotic area was noted on the initial film, closer questioning revealed that his heel was indeed the area that really hurt.
Why did he have a stress fracture? We don't know. His exercise pattern did not seem to have changed, as far as we could elicit from him. Most likely, this fracture was due to the chronic, repetitive stress he experienced while walking around on the streets where he lived.