Identify the fibrous capsule.

Correct answer: B

Renal medulla: The renal medulla is the deepest (innermost) region of the kidney. It is composed of renal pyramids, which are cone-shaped structures containing loops of Henle and collecting ducts. These structures are essential for the countercurrent mechanism, which allows the kidney to concentrate urine by reabsorbing water and solutes. The medulla lies beneath the renal cortex and surrounds the renal pelvis, making it central to urine formation and concentration.

A. It indicates increased tubular secretion and higher GFR: Cystatin C is freely filtered and not secreted by the renal tubules, making it an accurate marker for filtration. An increase in its blood concentration reflects a decrease in filtration, not an increase. Elevated levels always correlate with reduced clearance.

B. It suggests early decline in GFR despite normal BUN: Cystatin C is a more sensitive marker for GFR than BUN or creatinine, especially in elderly patients with reduced muscle mass. It remains stable regardless of diet or age. An isolated rise often identifies stage 2 or 3 chronic kidney disease.

C. It reflects dehydration with preserved GFR: Dehydration typically causes a disproportionate rise in BUN (prerenal azotemia) due to increased urea reabsorption. Cystatin C is not significantly affected by hydration status or tubular flow rates. Its elevation specifically points toward a functional reduction in glomerular filtration.

D. It shows liver dysfunction unrelated to GFR: Blood urea nitrogen (BUN) levels can be affected by liver function since the liver produces urea. Cystatin C, however, is produced by all nucleated cells at a constant rate and is not influenced by hepatic health. Its primary clinical utility is renal assessment.