**Core Concept**
The rim-sign on nephrogram refers to a radiographic finding where the contrast material outlines the renal cortex, creating a "rim" around the renal medulla. This phenomenon occurs due to the differential excretion of contrast material by the renal tubules and the renal cortex.

**Why the Correct Answer is Right**
The rim-sign on nephrogram is typically seen in conditions where the renal tubules are damaged or impaired, such as acute tubular necrosis (ATN). In ATN, the renal tubules are unable to reabsorb or secrete contrast material, leading to a delayed excretion of contrast material by the renal tubules. As a result, the contrast material accumulates in the renal cortex, creating a "rim" around the renal medulla.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the rim-sign on nephrogram is not typically seen in pyelonephritis, where the inflammation is usually limited to the renal cortex and medulla.
* **Option B:** This option is incorrect because the rim-sign on nephrogram is not typically seen in kidney stones, where the obstruction is usually localized to the renal pelvis or ureter.
* **Option C:** This option is incorrect because the rim-sign on nephrogram is not typically seen in polycystic kidney disease, where the renal cysts are usually asymptomatic and do not cause significant impairment of renal function.

**Clinical Pearl / High-Yield Fact**
The rim-sign on nephrogram is a valuable radiographic finding that can help diagnose acute tubular necrosis (ATN) in patients with acute kidney injury (AKI). It is essential to consider this finding in the context of the patient's clinical presentation and laboratory results.

**Correct Answer: D. Acute Tubular Necrosis (ATN)**