Case of ankylosing spondilitis, which is a seronegative spondyloarthropathy and HLA-B 27 positive ( >90% of cases).

Sacroilitis is the hallmark of this condition represented by fusion of S-I joints bilaterally. This finding is not well appreciated in the AP images do to overlying bowel gas. 

Xray taken after a bowel preparation would help

However the other findings and signs , consistent with the condition, are present.