Hi. Thanks for your query. Noted the history about your husband and understood your concerns. The commonest reasons for pain at the anus and pain on sitting too is fissure in ano that could have developed due to passing of hard stool or intestinal infection. He has recently taken Zithromax which can cause GI disturbance and bacterial flora imbalance. Another reason as you have mentioned is that he is suffering from prostatitis and diverticulitis and reactivation of these infection too can cause or add on the existing problems. Hence I would advise his the following: Consult a General surgeon for clinical examination, per-rectal digital examination and Ano-proctoscopy; give a proper medication as per the cause thus found. If there is not much of relief he may need a colonoscopy and CT scan of the abdomen including that of pelvis to know the present status of diverticulitis and prostatitis and get treated accordingly.