What is a common cause for pyelonephritis and renal calculi?

The correct answer highlights that flank pain is a common symptom associated with both pyelonephritis and renal calculi. Flank pain refers to discomfort or pain that occurs in the side of the body, usually around the lower back, where the kidneys are located. When an infection, such as pyelonephritis, occurs, the inflammation and irritation of the renal tissues lead to significant pain in the flank area. Similarly, renal calculi (kidney stones) can cause flank pain as they obstruct the urinary tract, leading to renal colic, which presents as severe pain often radiating to the groin or abdomen.

In the context of the other options, while lower urinary tract infections may contribute to the development of pyelonephritis, they are not a direct cause of both conditions as specified in the question. Diabetes insipidus is unrelated to pyelonephritis and renal calculi as it primarily concerns water balance and does not predispose individuals to these conditions. Urinary obstruction is a contributing factor to renal calculi and can lead to pyelonephritis as a secondary complication, but it is not characterized by a specific symptom like flank pain, making symptomatology the focus here instead of the underlying causes.