A nurse is teaching a group of nursing students about pyelonephritis. Which of the following statements should the nurse include in the teaching?

A. Pyelonephritis is most often caused by a single organism, most commonly Escherichia coli (E. coli), not a variety of different microorganisms. B. Pyelonephritis is an infection of the upper urinary tract, specifically affecting the kidneys, not the lower urinary tract. C. Pyelonephritis typically presents with symptoms, such as flank pain, fever, chills, and urinary urgency or frequency. D. Upward spread of bacteria from the bladder (ascending infection) is the most common cause of pyelonephritis, making this the correct statement.