Complex sentence: This form occurs most often in boys older than 8, large joints of lower extremities are commonly affected; characteristically, it causes back pain, and is linked to ankylosing spondylitis and inflammatory bowel disease.
Split into simpler sentences: This form occurs most often in boys older than 8, large jts. Of lower extremities are commonly affected; characteristically causes back pain, and is linked to ankylosing spondylitis and inflammatory bowel disease.