A fistula-in-ano is a hollow tract lined with granulation tissue connecting a primary opening inside the anal canal to a secondary opening in the perianal skin. Secondary tracts may be multiple and from the same primary opening
Fistula-In-Ano: External opening of fistulus tract is apparent in photo above. Proximal opening wouldbe at level of crypts, within the anal canal. Fistulas are frequently associated with perirectal abscesses,though none are present in this case.
Anatomy of the anal canal and perianal space: