What's causing this patient's back pain and rectal bleeding?

Proctoscopy shows moderate inflammation and ulcers in the rectum
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Matthew is a 19-year-old apprentice plumber who presents complaining of low back pain and rectal bleeding.

He reports a 2-3 month history of low back pain that he has been self-managing with rest and paracetamol when needed.

He feels his heavy schedule of daily tasks — which include bending, lifting and squatting — might be the cause.