MLT GlobeChemistry: Enzymes & cardiac markers – page 5
132 Chemistry MCQs on Enzymes & cardiac markers with answers and explanations.
A 25-year-old completes a marathon. The next day CK is 3500 U/L and high-sensitivity troponin is below the 99th percentile. The best interpretation is:
Strenuous exercise damages skeletal muscle and raises CK markedly; a normal troponin argues against myocardial injury. Macro-CK usually gives a persistent, unexplained mild rise.
For a troponin assay to be called 'high-sensitivity', it must have which analytical performance at the 99th percentile upper reference limit?
High-sensitivity assays have a CV ≤10% at the 99th percentile and measure troponin in at least half of healthy people. Detection in healthy people is expected, not zero.
A patient presents 6 days after an episode of severe chest pain. Which marker is most likely to still be elevated?
Troponins remain raised for about 7–14 days after MI, allowing late diagnosis. CK-MB returns to normal in 2–3 days and myoglobin within about a day.
Compared with BNP, NT-proBNP:
Pro-BNP is split into active BNP (half-life about 20 min) and inactive NT-proBNP (half-life 1–2 h). NT-proBNP is more affected by reduced renal clearance.
A breathless patient with a BMI of 42 kg/m² has echocardiographic heart failure but a lower-than-expected NT-proBNP. The best explanation is:
Natriuretic peptide concentrations are lower in obese patients, so a 'normal' value does not fully exclude heart failure. Renal failure, not obesity, typically raises them.
A 22-year-old is admitted 2 days after a paracetamol (acetaminophen) overdose. ALT is 4500 U/L and AST 5200 U/L. This enzyme pattern indicates:
Aminotransferases above about 1000 U/L indicate massive hepatocyte necrosis, seen in toxic (paracetamol), ischemic or acute viral hepatitis. Cholestasis mainly raises ALP and GGT.
After heating serum at 56 °C for 10 minutes, most ALP activity remains. Which isoenzyme is most likely present?
Placental ALP is the most heat-stable isoenzyme; bone ALP is the most heat-labile ('bone burns'). Liver ALP is intermediate.
An ALP result is almost undetectable in a patient with no clinical signs. The sample was collected in a lavender-top tube. The best explanation is:
ALP is a metalloenzyme that requires Zn2+ and Mg2+; EDTA removes them and inactivates the enzyme. Serum or heparinized plasma must be used.
In the common ALP method, p-nitrophenyl phosphate is hydrolysed at alkaline pH. What is measured?
ALP removes phosphate from colourless p-nitrophenyl phosphate, producing yellow p-nitrophenoxide read at 405 nm. The rate of colour formation reflects activity.
An elderly man with Paget disease of bone has markedly raised ALP. Which test result best confirms that the ALP is not from the liver?
GGT is raised in hepatobiliary disease but not in bone disease, so raised ALP with normal GGT suggests a bone source. AST and LD are not specific for the liver.
A child with swollen parotid glands has raised serum amylase and normal lipase. The amylase is mainly the:
Mumps parotitis releases salivary amylase; lipase is not produced by salivary glands, so it stays normal. Pancreatic amylase would usually rise together with lipase.
Why are kinetic (multipoint rate) assays preferred over single fixed-time assays for enzyme activity?
Multiple readings allow the analyzer to confirm a constant (zero-order) rate and flag lag phase or substrate depletion. A fixed-time assay assumes linearity without checking it.
On electrophoresis at pH 8.6, which creatine kinase isoenzyme moves fastest toward the anode?
CK-BB (CK-1) is the most negatively charged and moves furthest toward the anode, CK-MB is intermediate, and CK-MM (CK-3) stays nearest the cathode.
In the CK-MB immunoinhibition method, antibody to the M subunit is added and the remaining activity is multiplied by 2. Why?
Anti-M blocks CK-MM completely and the M half of CK-MB. The activity left is the B subunit of CK-MB (assuming no CK-BB), so doubling gives CK-MB activity.
In the IFCC CK method (creatine phosphate + ADP → creatine + ATP), ATP is measured through hexokinase and G6PD coupled reactions. What is read?
ATP phosphorylates glucose (hexokinase); G6PD then oxidizes glucose-6-phosphate while reducing NADP+ to NADPH, so absorbance at 340 nm rises in proportion to CK activity.
CK activity falls in stored serum as its sulfhydryl groups oxidize. Which reagent component restores this activity?
N-acetylcysteine is a thiol reagent that reduces oxidized sulfhydryl groups and reactivates CK. Diadenosine pentaphosphate inhibits adenylate kinase; pyridoxal phosphate is the cofactor for aminotransferases.
Which troponin subunit binds calcium and is identical in cardiac and slow-twitch skeletal muscle, so it is not used as a cardiac marker?
Troponin C is the calcium-binding subunit and has the same sequence in heart and slow skeletal muscle. Cardiac troponin I and T have heart-specific forms, which makes them useful markers.
By the Fourth Universal Definition, acute myocardial infarction requires a rise and/or fall of cardiac troponin with at least one value above the 99th percentile URL, plus:
Troponin changes alone define myocardial injury. Infarction also needs evidence of ischemia: symptoms, new ECG changes, imaging findings or a coronary thrombus.
A patient taking high-dose biotin supplements has a troponin result much lower than expected on a streptavidin–biotin sandwich immunoassay. Why?
In sandwich assays, excess free biotin occupies streptavidin sites on the solid phase so the labeled complex is not captured, giving falsely low results. Competitive assays give falsely high results instead.
After a crush injury, a patient has CK 45 000 U/L and red-brown urine. Dipstick blood is 3+ but urine microscopy shows no red cells, and the plasma is not pink. The urine pigment is most likely:
Myoglobin from damaged muscle is small, filtered quickly and gives a positive peroxidase (blood) reaction. Free hemoglobin would usually make the plasma pink; the very high CK points to rhabdomyolysis.