Which of the following substances is freely filtered at the glomerulus and is NOT reabsorbed or secreted by the renal tubules?

Explanation

## Ideal Filtration Marker **Key Point:** Inulin is the gold standard substance for measuring glomerular filtration rate (GFR) because it meets all criteria for an ideal filtration marker. ### Criteria for Ideal GFR Marker | Property | Inulin | Glucose | Amino acids | Albumin | |----------|--------|---------|-------------|----------| | Freely filtered | ✓ | ✓ | ✓ | ✗ | | Not reabsorbed | ✓ | ✗ | ✗ | ✗ | | Not secreted | ✓ | ✗ | ✗ | ✗ | | Not metabolized | ✓ | ✗ | ✗ | ✗ | | Inert | ✓ | ✓ | ✓ | ✓ | | Molecular weight | 5200 Da | 180 Da | 75–204 Da | 66,000 Da | **High-Yield:** Inulin clearance = GFR because: 1. Amount filtered = Amount excreted (no reabsorption, secretion, or metabolism) 2. Formula: GFR = (U~In~ × V) / P~In~, where U~In~ = urinary inulin, P~In~ = plasma inulin, V = urine flow rate **Clinical Pearl:** Although inulin is the reference standard, it is impractical clinically (requires IV infusion, expensive). Creatinine is used instead in clinical practice, though it slightly overestimates GFR due to tubular secretion. ### Why Other Options Fail - **Glucose:** Freely filtered but completely reabsorbed in proximal tubule via SGLT2 and GLUT2 transporters → not excreted in normal urine - **Amino acids:** Freely filtered but reabsorbed in proximal tubule via amino acid transporters → not excreted in normal urine - **Albumin:** Large protein (MW 66 kDa) → NOT freely filtered due to size and charge selectivity of glomerular filtration barrier **Mnemonic:** **FINE** = **F**reely filtered, **I**nert, **N**ot reabsorbed/secreted, **E**xcretion = amount filtered (describes Inulin)