Which of the following urine findings is pathognomonic for acute pyelonephritis?

• A. Red blood cell casts
• B. Hyaline casts
• C. White blood cell casts
• D. Renal tubular epithelial casts

Answer: (C)

White blood cell casts are the most specific sign among these options for kidney involvement from infection. Casts form when inflammatory cells travel through and shed within the renal tubules; in acute pyelonephritis you have inflammation of the renal parenchyma with neutrophil migration into the tubules, producing these leukocyte-containing casts. They point to tubulointerstitial inflammation in the kidney, rather than changes confined to the glomerulus or to nonspecific tubule injury.

RBC casts would suggest glomerular bleeding and glomerulonephritis, not pyelonephritis. Hyaline casts are nonspecific and can appear with concentrated urine or various states of renal stress. Renal tubular epithelial casts indicate tubular injury such as acute tubular necrosis or toxicity, not infection of the kidney parenchyma.

So, the presence of white blood cell casts best supports acute pyelonephritis among the choices.