Filtered glucose load begins appearing in the urine at a plasma concentration near 200 mg/dL even though the theoretical maximum transport capacity for glucose (TmG) is about 375 mg/min, which corresponds to a higher plasma level. This early appearance of glycosuria before TmG is reached is called the splay phenomenon. What accounts for it?

• A Nephron-to-nephron variation means some individual nephrons reach their own transport maximum at lower plasma glucose concentrations than the population average ✓
• B Glucose is actively secreted by a minority of nephrons once its plasma level exceeds the renal threshold
• C Back-leak of glucose across the tight junctions of the thick ascending limb increases as plasma glucose rises
• D Glucosuric plasma levels suppress ADH, raising urine flow and washing unreabsorbed glucose into the bladder before equilibration

Explanation

TmG is a population average, but individual nephrons differ in SGLT2 transporter number and length of proximal tubule. Nephrons with lower capacities hit their personal maximum first and spill glucose while others still have reserve, producing a rounded transition curve (the splay) between the threshold and the true Tm. Secretion of glucose does not occur in humans, ruling out option B.

Reference: Vander's Renal Physiology, 8th ed.

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