Which of the following is least absorbable in renal tubules?
**Core Concept:** Renal tubular function, reabsorption of solutes and water, and the role of specific transport proteins in this process.
**Why the Correct Answer is Right:** Glucose is poorly reabsorbed in the renal tubules due to its large size and inability to cross the tight junctions of the nephron epithelium. Glucose is actively transported back into the bloodstream by the sodium-glucose co-transporter (SGLT) proteins, specifically SGLT1 and SGLT2. In contrast to glucose, urea is small enough to easily pass through the tight junctions and can be reabsorbed passively.
**Why Each Wrong Option is Incorrect:**
A. Urea is the correct answer because it is small enough to passively diffuse through the renal tubule and is actively reabsorbed via urea transporter proteins (UT-A1 and UT-A3).
B. This is incorrect because glucose is actively transported back into the bloodstream by SGLT proteins (SGLT1 and SGLT2), making it more likely to be reabsorbed compared to urea.
C. This is incorrect because phosphate is actively transported back into the bloodstream via sodium-phosphate co-transporters (NaPi2a and NaPi2b), making it more likely to be reabsorbed compared to urea.
D. This is incorrect because creatinine is only slightly reabsorbed via the creatinine transporter (CT) protein, and its reabsorption is not as significant as that of glucose, phosphate, or urea.
**Clinical Pearl:** Understanding renal tubular reabsorption is crucial for understanding disorders like Fanconi syndrome, where failure of glucose reabsorption leads to glycosuria, and tubulopathies like hypophosphatemic rickets, where phosphate reabsorption is impaired, causing hypophosphatemia and rickets.