Medullary nephrocalcinosis is a clinical variant of chronic kidney disease characterized by diffuse deposition of calcium and phosphate salts in the parenchyma, leading to inflammatory-dystrophic changes and the development of chronic renal failure. Medullary nephrocalcinosis is divided into primary (develops in previously unaltered kidneys and initially affects the proximal part of the nephron) and secondary (simultaneously affects all parts of the renal glomeruli, causing nephrosclerosis and increasing the risk of chronic kidney disease). The article describes two clinical cases of medullary nephrocalcinosis with different etiological factors. In the first case, the disease developed against the background of regular use of third-generation cephalosporins (ceftriaxone), while in the second case, it resulted from lipid and purine metabolism disorders. The genetic and etiopathogenetic aspects of medullary nephrocalcinosis are discussed. Given the importance of determining the causes and pathogenetic mechanisms of medullary nephrocalcinosis, the need for thorough diagnostics in adult patients with changes in the renal parenchyma accompanied by hyperparathyroidism, hyper- or normocalcemia, hypophosphatemia, hypomagnesemia, and hypercalciuria is emphasized. Diagnostic workup should include genetic, metabolic, and iatrogenic components of kidney damage.