Primary anal canal syphilis can be easily missed as a diagnosis.
Historical factors:
(1) receptive anal intercourse (more often male but can also be female)
(2) syphilis in insertive partner (may not be known or shared)
Clinical findings:
(1) ulceration in the anal canal (chancre)
(2) inflammatory polyp in the anal canal
(3) anal discharge
Laboratory findings:
(1) serologic tests for syphilis are often positive but can be negative (if suspicious of the diagnosis both RPR and antitreponemal antibody testing should be performed)
(2) other sexually transmitted disease, including HIV
Biopsy findings:
(1) bandlike infiltrate of plasma cells together with lymphocytes and histiocytes
(2) perivascular chronic inflammatory infiltrate rich in plasma cells
(3) poorly-formed granulomas
(4) spirochetes on silver impregnation or immunoperoxidase stains
