What on earth is pyelonephritis? Let’s understand what it actually is by taking a look at its etymological breakdown.

Pyelonephritis
Origin: Greek, pyelos (basin, taken to mean pelvis) + nephros (kidney) + –itis (inflammation of)
Inflammation of the basin and the kidney

At first, this seems cryptic. What does basin refer to, and what exactly does kidney refer to? Starting with the root pyelo-, it denotes “pelvis,” but its base etymology comes from the Greek pyelos (basin). There is in fact such a thing as pyelitis, which would be inflammation localized to the renal pelvis. As for the nephr- root, it of course means kidney, but to be more precise, it refers to the kidney parenchyma. Nephritis on its own denotes the inflammation of the kidney’s functional tissue.

At the end of the day, pyelonephritis is the joint inflammation of the renal pelvis and the renal parenchyma. A vast majority of the cases are caused by a bacterial agent such as E. coli., and as such, they proliferate in a large portion of the kidney instead of being localized to only the pelvis or the parenchyma.

There are three types of pyelonephritis: acute, chronic, and xanthogranulomatous. Let’s look at the last one:

Xanthogranulomatous
Origin [1]: Greek, xanthos (yellow)
Origin [2]: Latin, granum (grain) + –ula (diminutive) + –oma (mass/proliferation/tumor)
Forming yellow granulomas

Indeed, under the microscope, one would find granulomas—collections of macrophages that try to wall off infections they cannot eradicate. On gross appearance, these granulomas appear yellow because they are macrophages that have been filled to the brim with lipids.