Final answer:
Acute pyelonephritis is most commonly caused by bacterial infections, predominantly Escherichia coli (E. coli), originating from the lower parts of the urinary tract.
Step-by-step explanation:
Acute pyelonephritis is a serious kidney infection that is most commonly caused by bacteria that have spread from other parts of the urinary tract such as the bladder. The most frequent bacterium responsible for this condition is Escherichia coli (E. coli), which is known to thrive in fecal matter. In addition to bacteria that travel from lower urinary tract infections, bacteria can sometimes reach the kidneys via the bloodstream. Diagnosing pyelonephritis involves microscopic examination of urine, culture, and testing for leukocyte esterase and nitrite levels. Treatment typically includes broad-spectrum antibiotics like penicillins, cephalosporins, and fluoroquinolones.