Which of the following is freely filtered by kidney across glomerular capillariesa) Albumin (across glomerular capillaries)b) Globulinc) Creatinined) HCO3 e) Glucose
**Core Concept**
The glomerular filtration barrier selectively allows small, water-soluble molecules to pass into the filtrate while retaining large proteins and charged molecules. This filtration occurs via the fenestrated endothelium, basement membrane, and podocyte slit diaphragms, which restrict passage of large proteins like albumin and negatively charged molecules.
**Why the Correct Answer is Right**
Creatinine, glucose, and HCO₃⁻ are small, uncharged, or weakly charged molecules that are freely filtered at the glomerular capillaries. Creatinine is a waste product of muscle metabolism and is freely filtered. Glucose is filtered but reabsorbed in the proximal tubule; however, in normal conditions, it is present in filtrate and is considered freely filtered. Bicarbonate (HCO₃⁻) is filtered in small amounts and is freely filtered due to its small size and low charge. Albumin and globulin are large proteins that are normally retained in the blood due to size and charge, and thus are not freely filtered.
**Why Each Wrong Option is Incorrect**
Option A: Incorrect — Albumin is not freely filtered; it is retained due to size and charge.
Option B: Incorrect — Globulin is a large protein and is not filtered at all.
Option C: Incorrect — Creatinine is filtered, but the inclusion of glucose and HCO₃⁻ is correct, so this option is incomplete.
Option D: Incorrect — Albumin is not filtered, so its inclusion is wrong.
**Clinical Pearl / High-Yield Fact**
In renal tubular function, glucose is freely filtered but reabsorbed in the proximal tubule (threshold ~180 mg/dL), while creatinine is not reabsorbed and remains in urine. The presence of albumin in urine (proteinuria) indicates glomerular damage.
✓ Correct Answer: A. cde