Medullary nephrocalcinosis refers to abnormal deposition of calcium in the renal medullary pyramids. Causes include renal tubular acidosis, medullary sponge kidney, hypervitaminosis D, hyperparathyroidism, and other hypercalcuric states. In the NICU, medullary nephrocalcinosis is frequently encountered in premature infants who have received long-term furosemide therapy. Furosemide limits reabsorption of calcium at the ascending limb of the loop of Henle, resulting in its deposition.

Sonographically, the renal medullary pyramids are hyperechoic. Posterior acoustic shadowing is present only when the calcifications are sufficiently dense.