**Core Concept**
Acute pyelonephritis refers to the inflammation of the kidney caused by a bacterial infection, typically ascending from the lower urinary tract. The condition involves the renal parenchyma and is characterized by an intense inflammatory response, leading to various morphological changes visible on imaging studies, including ultrasound.

**Why the Correct Answer is Right**
In acute pyelonephritis, ultrasound findings may include increased renal cortical thickness, echogenicity, and cortical scarring, which are due to the inflammatory response and tissue damage. The renal pelvis and calyces may also appear dilated due to the obstruction caused by the inflammatory process. Additionally, the presence of perinephric fluid (indicative of abscess formation) or pyelonephritic tracks (indicative of the spread of infection) can be visualized on ultrasound. However, the typical echogenic foci seen in pyonephrosis, which are indicative of a urinary tract obstruction with resultant renal pelvic or calyceal dilatation and sloughing, are not characteristic of acute pyelonephritis.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased renal cortical thickness is indeed seen in acute pyelonephritis, making this option incorrect.
**Option B:** Echogenic foci may be seen in pyonephrosis, but not typically in acute pyelonephritis, making this option a plausible choice. However, in this case, the question asks which option is NOT seen on ultrasound in acute pyelonephritis.
**Option C:** This option is not provided, so it cannot be evaluated.
**Option D:** This option is not provided, so it cannot be evaluated.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate between acute pyelonephritis and pyonephrosis on ultrasound, as the latter often requires urgent intervention to prevent further renal damage.

**Correct Answer: B.**