Final answer:
Pyelonephritis is an infectious inflammatory disease of the kidneys caused by gram-negative organisms, typically E. coli.
Step-by-step explanation:
Pyelonephritis is an infectious inflammatory disease involving the renal parenchyma and renal pelvis. It is usually caused by gram-negative organisms, particularly E. coli, which are commonly found in the intestinal tract. This infection can spread from the lower urinary tract or through the bloodstream to the kidneys. Common symptoms include back pain, fever, and nausea or vomiting.