Low molecular proteinuria is caused by impairment of renal tubular function, resulting in impaired reabsorption of protein filtered through the normal glomerulus. Low molecular protein generally refers to proteins with a molecular weight of 40,000 daltons. Because renal tubules play an important role in the reabsorption and catabolism of low-molecular protein, the excretion of low-molecular proteinuria increases in renal tubular lesions. A. Etiology of low molecular proteinuria 1. Dent’s disease: It is an X-linked genetic renal tubular disease. It was first reported in the United Kingdom. This disease is characterized by low-molecular proteinuria, hypercalciuria, renal calcification, and renal calculi, but normal urinary acidification function. Some patients have rickets and progressive renal failure, along with high urinary growth hormone excretion levels. Renal pathology reveals mild glomerular thylakoid hyperplasia and minor tubulointerstitial lesions. Male onset, female carriers. Female carriers have low-molecular proteinuria, and half have hypercalciuria but no obvious symptoms. 2. X-linked recessive nephrolithiasis: It is a disease in which proximal tubular dysfunction is predominant. It was first reported in North America. It develops in males and is carried by females. Females often have mild urinalysis abnormalities, varying degrees of low molecular proteinuria, and 1/3 of patients have hypercalciuria. Patients often present with proteinuria and microscopic hematuria within 1 year of age and kidney stones between 3 and 6 years of age. There are also patients with onset in adolescence. The clinical presentation is characterized by kidney stones, renal calcification and progressive renal failure. The stones are often composed of calcium oxalate and calcium phosphate. Not all patients progress to renal failure. Renal transplantation is effective, suggesting that the disease is tubular per se rather than systemic. 1 case of rickets in 30 children. Renal pathology was seen as glomerulosclerosis, interstitial fibrosis and tubular atrophy. 3. Low molecular proteinuria with hypercalciuria and renal calcification: first reported in Japan. The patient is often a male child in a family. These patients have low-molecular proteinuria, hypercalciuria, renal calcification, and urinary concentration dysfunction. Lack of renal colic and rickets is not a point of differentiation from XRN and Dent. However, the presence of renal calcification at the age of 3-5 years suggests a poor prognosis. In urine screening of school-age and preschool children conducted in Japan, it was found that most of the asymptomatic patients younger than 14 years had normal blood creatinine. 4. X-linked recessive hypophosphatasia rickets: It was first found in an Italian family. All male patients in this family presented with rickets, osteomalacia, hypophosphatemia, renal calcification, and early progressive renal failure. They differed from X-linked dominantly inherited hypophosphatemic rickets in that they had hypercalciuria, increased 1,25(OH)2D and proteinuria, and also developed renal calcification and progression to renal failure. The disease develops in males and is carried by females. Female carriers have normal biochemical tests except for hypercalciuria. In terms of treatment, it is generally advocated that dietary sodium intake should be restricted and diets containing high levels of oxalate should be limited, as oxalate is prone to crystallization which can further promote stone formation. Absorptive idiopathic hypercalciuria should be treated by avoiding excessive calcium intake or taking phosphates, especially effective in those with hypophosphatemia. Renal hypercalciuria is not affected by dietary calcium intake. Thiazide diuretics promote calcium reabsorption from the distal tubules and lower urinary calcium. Amiloride (aminoclopramide) also promotes calcium reabsorption by the renal tubules and inhibits potassium excretion. The mechanism is not known. It may be used in combination with thiazide diuretics in addition to its application alone. Attention should be paid to electrolyte disturbances when applying these drugs. Kidney transplantation can be performed in end-stage renal failure. II. Prevention of kidney stones After treatment to remove stones, treatment should be continued to prevent recurrence of stones. The preventive measures depend on the primary cause of the stone and the results of the analysis of the stone’s composition. Drinking large amounts of water is suitable for patients with any type of stone. High fluid intake can reduce the concentration of calcium oxalate and calcium phosphate crystals in the urine and can increase the concentration threshold for calcium oxalate crystals to precipitate. However, there are no study values to show that the amount of water consumed is effective in preventing stone recurrence. Since stones do not grow uniformly, but are concentrated in certain specific time frames, such as after meals and at night when the urine is concentrated. Therefore, it is important to drink plenty of water within 3 hours after meals, at night, and during heavy activity, and patients should be instructed to record their urine volume daily and adjust their water intake accordingly to ensure a daily urine volume of 2 to 3 L.