Which of the following is the most common cause of acute pyelonephritis in patients without Urological abnormalities or urinary calculi?

In general, gram-negative organisms are the most common causes of acute urinary tract infections in patients who do not have complicating abnormalities of the urinary tract, such as obstruction from calculi or other causes. Among the gram-negative organisms, Escherichia coli is the bacterium that is most frequently isolated from urine cultures in these patients. Different species of Pseudomonas, Proteus, Serratia, and Klebsiella may also be responsible for so-called uncomplicated urinary tract infections, including acute pyelonephritis, but are less commonly implicated than Escherichia coli. Ref: Lerma E.V. (2009). Chapter 37. Chronic Tubulointerstitial Nephritis. In E.V. Lerma, J.S. Berns, A.R. Nissenson (Eds), CURRENT Diagnosis & Treatment: Nephrology & Hypeension.