Which diagnostic test helps estimate the level of nitrogen waste in the blood, primarily to evaluate how well the kidneys are functioning?

Correct option is B

Blood Urea Nitrogen (BUN) is a common blood test used to check the amount of urea-derived nitrogen in the bloodstream. Since urea is excreted by the kidneys, BUN levels help assess kidney function. Elevated BUN may indicate kidney impairment, dehydration, or high protein metabolism. It is usually evaluated along with serum creatinine for a more complete picture of renal health.
Explanation of options: (a) ✘ Incorrect. Serum creatinine also tests kidney function but does not directly measure nitrogen from urea. (b) ✔ Correct. BUN measures nitrogen in the blood from urea, a key indicator of kidney function. (c) ✘ Incorrect. GFR estimates filtration efficiency but is calculated, not directly measured through nitrogen levels. (d) ✘ Incorrect. Urine specific gravity assesses urine concentration, not nitrogenous waste in blood.