A medical student ingests glucose until glycosuria appears. Her plasma glucose at onset of glucosuria is 200 mg/dL, yet laboratory measurement of her maximal tubular transport capacity (TmG) later shows 350 mg/min. What explains glucose appearing in urine at a plasma level well below that predicted from TmG alone?
- A Glucose is secreted by the distal tubule at high plasma concentrations
- B Nephron heterogeneity causes splay: some nephrons reach their individual transport maxima earlier than others ✓
- C The filtered load of glucose falls as plasma glucose rises because of osmotic diuresis
- D High plasma glucose reduces the number of functional SGLT2 transporters per nephron
Correct answer: B. Nephron heterogeneity causes splay: some nephrons reach their individual transport maxima earlier than others
Explanation
Individual nephrons differ in GFR and in SGLT2/SGLT1 transporter density, so each reaches its own transport maximum at a different plasma glucose concentration. This population spread flattens the titration curve near TmG, creating the splay region, and glycosuria begins around 180 mg/dL even though average TmG corresponds to a higher filtered load. Secretion does not occur, and transporter numbers are not acutely reduced by hyperglycemia.
Reference: Vander's Renal Physiology, 9th ed.
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