What is the most common bacteria to cause pyelonephritis?

• A. Klebsiella Pneumoniae
• B. Staphylococcus Aureus
• C. Proteus Mirabilis
• D. Escherichia Coli

Answer: (D)

Escherichia coli is the most common cause of pyelonephritis because kidney infection usually starts as an ascent from a lower urinary tract infection, and E. coli is by far the most frequent organism in community-acquired UTIs. It’s part of the normal gut flora and possesses virulence factors—such as fimbriae that help it adhere to the uroepithelium and resist flushing—that enable it to ascend and establish infection in the kidney. While other bacteria like Proteus, Klebsiella, or Staphylococcus aureus can cause urinary infections, they are less common overall, and Staphylococcus aureus more often indicates hematogenous spread rather than a typical ascending infection.