Complex sentence: Both acute and chronic tubulointerstitial nephritis can be caused by a bacterial infection in the kidneys known as pyelonephritis, but the most common cause is by an adverse reaction to a drug.
Split into simpler sentences: Both acute and chronic tubulointerstitial nephritis can be caused by a bacterial infection in the kidneys, known as pyelonephritis. The most common cause is by an allergic reaction to a drug.