The ultrasound image demonstrates a longitudinal view of the right kidney. Within the renal pyramids, there are multiple echogenic foci, some showing posterior acoustic shadowing—findings characteristic of medullary nephrocalcinosis. This sonographic appearance is strongly associated with medullary sponge kidney (MSK).

Medullary sponge kidney is a congenital disorder of the renal tubules that results in ectatic (dilated) collecting ducts in the renal medulla. The dilated ducts frequently become calcified, leading to the "paintbrush" or "bouquet of flowers" appearance seen in the renal pyramids on ultrasound. These calcifications are most often bilateral and symmetrical, further aiding the diagnosis.

Comparison of answer choices:

A. Fungal balls (mycetomas) typically appear as mobile, non-shadowing echogenic masses within the collecting system, often in immunocompromised patients.

B. Renal cell carcinoma usually presents as a solid mass with irregular borders and variable echogenicity—this image does not show a mass.

C. Acute pyelonephritis may show renal enlargement, decreased echogenicity, or loss of corticomedullary differentiation—but not calcification of the pyramids.

D. Medullary sponge kidney is correct due to the punctate echogenic foci within the medullary pyramids, consistent with nephrocalcinosis.