Complex sentence: Hydronephrosis that occurs acutely with sudden onset (as caused by a kidney stone) can cause intense pain in the flank area (between the hips and ribs), while a chronic occurrence that develops gradually will present with no pain or attacks of a dull discomfort.
Split into simpler sentences: Hydronephrosis that occurs acutely with sudden onset (as caused by a kidney stone) can cause intense pain in the flank area (between the hips and ribs). Conversely, hydronephrosis that develops gradually will generally cause no pain or attacks of a dull discomfort.