MLT GlobeChemistry: Renal function (urea, creatinine)
54 Chemistry MCQs on Renal function (urea, creatinine) with answers and explanations.
Which compound carries the first nitrogen into the urea cycle, condensing with ornithine to form citrulline?
Mitochondrial CPS-I makes carbamoyl phosphate from NH4+ and HCO3-. The second nitrogen comes from aspartate at the argininosuccinate synthetase step.
Creatinine clearance is used clinically to estimate which kidney function?
Creatinine is freely filtered and only slightly secreted, so its clearance approximates GFR. Renal plasma flow is estimated with PAH clearance.
The Jaffe method for creatinine uses:
Creatinine reacts with picrate in alkaline solution to form a red-orange complex. Ketones, glucose and some cephalosporins also react and give false high results; enzymatic methods avoid this.
The main analytical advantage of enzymatic creatinine methods over the Jaffe method is:
Enzymatic (creatininase-based) methods are more specific and are less affected by pseudo-chromogens such as ketones, glucose and some cephalosporins. They are usually more expensive than Jaffe methods.
Current KDIGO guidance recommends estimating GFR in adults with which equation?
KDIGO recommends CKD-EPI creatinine-based eGFR for adults; the 2021 race-free version is now widely adopted. Schwartz is for children, and Cockcroft-Gault estimates clearance, mainly for drug dosing.
Which condition is best known for markedly raised serum uric acid due to rapid cell breakdown?
Tumor lysis releases nucleic acids, whose purines are degraded to uric acid, which may crystallise in the tubules. Fanconi syndrome and Wilson disease often cause low uric acid through renal loss.
The preferred specimen for screening diabetic patients for albuminuria is:
A spot urine ACR, preferably first-morning, corrects for urine concentration and is recommended for screening. Standard dipsticks are insensitive to low levels of albumin.
A patient has eGFR 38 mL/min/1.73 m² that has been stable for 6 months. Which KDIGO GFR category is this?
KDIGO GFR categories: G3a 45–59, G3b 30–44, G4 15–29 mL/min/1.73 m². A value of 38 is G3b; persistence over 3 months satisfies the chronicity criterion.
In which organ is urea mainly synthesized?
Urea is made in hepatocytes by the urea cycle, which converts toxic ammonia from amino acid breakdown into urea. The kidney only excretes urea; it does not make it.
Serum creatinine is 2.0 mg/dL. What is this value in µmol/L?
Creatinine (mg/dL) × 88.4 = µmol/L, so 2.0 × 88.4 = 176.8, about 177 µmol/L. 88.4 µmol/L corresponds to only 1.0 mg/dL.
Creatinine in the blood is formed mainly by:
Creatine phosphate in muscle is converted to creatinine spontaneously at a fairly constant rate related to muscle mass. Purine breakdown produces uric acid, not creatinine.
According to KDIGO, kidney damage or reduced GFR must be present for how long to diagnose chronic kidney disease?
CKD is defined as abnormal kidney structure or function present for more than 3 months with health effects. Shorter changes may represent acute kidney injury or acute kidney disease.
Which measurement is most useful to check whether a 24-hour urine collection is complete?
Creatinine excretion is fairly constant for a person and depends on muscle mass, so a total far below the expected amount suggests under-collection. Volume varies with fluid intake and cannot confirm completeness.
Urea is the main nitrogen-containing end product of the breakdown of:
Amino acid nitrogen is converted to ammonia and then to urea in the liver. Purines give uric acid and creatine phosphate gives creatinine.
Uric acid is the end product of the breakdown of:
In humans, the purine bases adenine and guanine are broken down to uric acid. Pyrimidines are degraded to soluble products such as beta-alanine.
In a healthy young adult, the glomerular filtration rate is approximately:
Normal GFR in young adults is about 90–120 mL/min/1.73 m² or more. A GFR below 15 indicates kidney failure. About 1200 mL/min is renal blood flow, not GFR.
What is the basic functional unit of the kidney?
The nephron, made up of the glomerulus and its tubule, is the functional unit that filters blood and forms urine.
A persistently raised serum uric acid is most closely associated with which disease?
Hyperuricemia can lead to deposition of monosodium urate crystals in joints, causing gout.
To calculate creatinine clearance, which specimens are needed?
Clearance = (urine creatinine × urine volume per minute) ÷ plasma creatinine, so both a timed urine collection and a plasma or serum creatinine are required.
The term 'azotemia' means:
Azotemia refers to accumulation of nitrogenous waste products such as urea and creatinine in the blood, usually from reduced kidney function.