**Core Concept**
Acute pyelonephritis is an inflammatory infection of the kidney caused by bacterial invasion, typically resulting from ascending urinary tract infections. This condition involves the renal parenchyma, leading to various changes visible on imaging studies.

**Why the Correct Answer is Right**
In acute pyelonephritis, imaging typically shows increased renal cortical thickness, parenchymal striations, or a "striated nephrogram" due to the inflammatory response. Additionally, perinephric fat stranding and fluid collections may be observed. These changes reflect the body's immune response to the infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Not typically seen in acute pyelonephritis, but rather in conditions like polycystic kidney disease or chronic pyelonephritis, where there is atrophy and scarring of the renal parenchyma.
**Option B:** May be seen in acute pyelonephritis, as the inflammatory response can cause increased renal cortical thickness.
**Option C:** Also seen in acute pyelonephritis, as perinephric fat stranding is a common finding due to the inflammatory response.

**Clinical Pearl / High-Yield Fact**
Remember that acute pyelonephritis typically presents with systemic symptoms like fever and flank pain, and imaging findings like striated nephrogram or perinephric fat stranding. Distinguishing it from other renal conditions requires a thorough clinical and imaging evaluation.

**Correct Answer:** D.