In kidney disease, BUN is high in blood and high in urine; in liver disease, BUN is low in blood with a BUN ratio less than 10:1.

• A. Kidney disease: High in blood, high in urine
• B. Kidney disease: High in blood, low in urine
• C. Kidney disease: Low in blood, high in urine
• D. Kidney disease: Low in blood, low in urine

Answer: (A)

BUN reflects how much urea nitrogen is circulating (produced in the liver) and how effectively the kidneys clear it. When kidney function is impaired, clearance drops, so urea accumulates in the blood, raising BUN. If urine is still being produced, the filtered urea load reaches the tubules and can appear as elevated BUN in the urine as well, especially when the remaining nephrons are dumping urea into urine despite reduced filtration. In contrast, liver disease lowers the liver’s production of urea, so BUN in the blood falls, and with less urea being delivered to the kidneys, urine BUN remains low; the BUN:creatinine ratio also falls (less than 10:1). So the pattern of high BUN in blood and high BUN in urine aligns with kidney dysfunction.