Pyelonephritis is usually caused by __________, a normal flora organism in the large intestine.

Pyelonephritis is usually caused by Escherichia coli (E. coli), which is a bacterium commonly found in the intestines of humans and animals. This organism is part of the normal gut flora, but it can lead to infections when it enters the urinary tract. In cases of pyelonephritis, E. coli travels up the ureters to the kidneys, which can result in inflammation and infection.

The pathogenic strains of E. coli, particularly those that possess specific virulence factors, are often responsible for urinary tract infections that can ascend to the kidneys. Proper diagnosis and treatment are crucial since untreated pyelonephritis can lead to serious complications, including kidney damage or sepsis. Understanding the source of the infection can help in devising effective preventive measures, such as proper hygiene practices and timely medical intervention.