A patient with Fanconi syndrome has glycosuria at normal plasma glucose levels, phosphaturia, aminoaciduria, and RTA. The defect in Fanconi syndrome primarily involves:

• A Generalized dysfunction of proximal tubular reabsorption (all proximal transporters impaired) ✓
• B Glomerular filtration of large molecules normally retained in plasma
• C Distal tubular H+ secretion — causing Type 1 RTA specifically
• D Aldosterone resistance in the collecting duct

Explanation

Fanconi syndrome is a generalized defect of proximal tubular function, impairing reabsorption of glucose (despite normal plasma glucose), amino acids, phosphate, uric acid, bicarbonate (causing Type 2 proximal RTA), potassium, and sodium. Causes include Wilson's disease, cystinosis, multiple myeloma, tenofovir toxicity, and lead poisoning — all affecting proximal tubular cell energy metabolism (mitochondrial dysfunction). Type 1 (distal) RTA is a separate entity involving the collecting duct H+-ATPase. Aldosterone resistance causes type 4 RTA (hyperkalaemic).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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