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What is Acute pyelonephritis is diagnosed by? and explain in detail?
Classically, acute pyelonephritis has an acute onset with chills, fever, loin pain, lumbar tenderness, dysuria and frequency of micturition. 
Urine will show abundance of bacteria, pus cells and pus cell casts in the urinary sediment. 
Institution of specific antibiotics, after identification of bacteria by culture followed by sensitivity test, eradicates the infection in majority of patients.

Mohan H. Textbook of pathology. Jaypee Brothers Medical Publishers; 2015. Edition 7. Page 669