Medullary sponge kidney (MSK) is a kidney malformation that normally occurs in one of the numerous steps characterizing renal morphogenesis. Abnormalities in genes fundamental for renal formation lead to diminished distal nephron development, causing cyst formation, nephrocalcinosis and distal renal tubular acidosis as subsequent consequence of urine concentration defects [1]. Dilatation of the collecting ducts in the renal medulla or in renal papillae produce the spongy appearance for which the disease is named [2]. MSK is generally considered a sporadic disorder, but an apparently autosomal dominant inheritance has also been observed. The disease affects both genders, with a slight female prevalence, and is generally diagnosed in adulthood (20–30 years old). Clinical manifestations include nephrolithiasis, urinary tract infections, micro- or macro-hematuria, hypercalciuria and hypocitraturia, but it is worth saying that often it may be asymptomatic and it is diagnosed as an incidental finding during diagnostic follow-up [3]. When it becomes manifest in children, the disease is more severe and is accompanied by rickets-like symptoms. Recurrent calcium nephrolithiasis and nephrocalcinosis are the most common signs, hyperparathyroidism is frequently associated.

Although the pathogenesis of MSK is yet to be determined, the idea that it is a developmental disorder is supported by its association with different malformative conditions (Wilms tumor, Horse-shoe kidney, autosomal dominant polycystic kidney disease, other urinary tract malformations) especially with Beckwith–Wiedemann syndrome [4, 5].

The diagnosis of MSK is made through intravenous urography (gold standard), but nowadays it has almost been replaced by computed tomography urography (CTU) even though it did not present the same sensibility. Concerning ultrasound, MSK presents a peculiar tetrad: hypoechoic medullary areas, hyperechoic spots, microcystic dilatations of papillary zone and multiple calcifications (linear, small stones or calcified intracystic sediment) in each papilla that, added to laboratory data and clinical history, could be helpful to identify patients with MSK [6]. Though differential diagnosis for cystic renal disease in “young kidneys” is very challenging, Thomas et al. proposed a pragmatic reporting format for cystic renal diseases [7]. The most common differential diagnosis are with nephronophthisis (NPH) and/or medullary cystic kidney disease (ADTKD). In NPH, kidneys are small to normal in size, with increased echogenicity, reduced cortico-medullary differentiation, and renal cysts formation on the cortico-medullary border. In ADTKD, kidneys are normal to small in size, with multiple cysts at the cortico-medullary junction and sometimes in the renal medulla, but renal stones and microcalcifications are not usually detected in these cases [8].

Aside from recurrent episodes of nephrolithiasis, MSK can be considered a benign condition. Most treatment regimens are centered around prophylactic measures compared to symptomatic care and a proper therapy for stones formations [9]. Considering organ shortages, Cheungpasitporn W and colleagues aimed to assess the outcomes of living kidney donors with MSK, reporting the safety of MSK kidney donors with normal kidney function, low kidney stone risk and no significant comorbidity [10].