A 70-year-old retired postman has an aching right thigh and says his hats no longer fit. Serum ALP is 820 U/L (reference 40-130 U/L), GGT 28 U/L (reference below 55 U/L), and calcium, phosphate, bilirubin and ALT are normal. X-ray shows a thickened, coarse cortex of the femur. Where does the raised ALP come from?

Osteoblasts in remodelling bone

Epithelial cells of the small intestine

Hepatocytes lining bile canaliculi

Tumour cells making a placental form

A 58-year-old man with lung cancer has a serum ALP of 410 U/L (reference 40-130 U/L). Bone scan and liver imaging show no metastases, and GGT is normal. In the laboratory, his ALP activity is unchanged after heating the serum at 65 °C for 10 minutes. Which ALP isoenzyme is this?

Regan (placental-like) isoenzyme

Bone isoenzyme from osteoblasts

Liver isoenzyme from canaliculi

Intestinal isoenzyme from enterocytes

A 50-year-old man with recurrent gouty arthritis and tophi is started on allopurinol. Xanthine oxidase converts allopurinol to oxypurinol, which then stays tightly bound to the reduced molybdenum centre of the same enzyme. Which type of enzyme inhibition does this illustrate?

Suicide (mechanism-based) inhibition

Simple reversible competitive inhibition

A 40-year-old man reaches a district hospital 4 days after epigastric pain radiating to the back began during a drinking binge. Serum amylase is 108 U/L (reference 30-110 U/L) and serum lipase 1,450 U/L (reference below 60 U/L). CT shows an oedematous pancreas. Why are the two enzyme results discordant?

Amylase is cleared from blood faster than lipase

Lipase is falsely raised by alcohol in serum

Lipase also rises in salivary gland disease

Amylase rises only in gallstone pancreatitis

Enzymes, kinetics and clinical enzymology

16 NEET-PG practice questions on Enzymes, kinetics and clinical enzymology, part of Biochemistry. The first 16 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
A 70-year-old retired postman has an aching right thigh and says his hats no longer fit. Serum ALP is 820 U/L (reference 40-130 U/L), GGT 28 U/L (reference below 55 U/L), and calcium, phosphate, bilirubin and ALT are normal. X-ray shows a thickened, coarse cortex of the femur. Where does the raised ALP come from?
1. AOsteoblasts in remodelling bone
2. BEpithelial cells of the small intestine
3. CHepatocytes lining bile canaliculi
4. DTumour cells making a placental form
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Answer: A · Osteoblasts in remodelling bone
A very high ALP with normal GGT, bilirubin and ALT means the enzyme is not from the liver, and in an elderly man with bone pain, an enlarging skull and a coarse cortex it comes from bone in Paget disease. Osteoblasts make ALP while laying down new bone, and Paget disease has intense, disorganised osteoclastic resorption followed by osteoblastic repair. Calcium is usually normal unless the patient is immobilised. ALP is used to follow the response to bisphosphonates.
- B: Intestinal ALP rises only modestly, for example after fatty meals.
- C: Liver ALP would usually rise with GGT, which is normal here.
- D: Placental-like ALP from a tumour does not fit the bone findings.
Question 2 · Hard
A 66-year-old man with dilated cardiomyopathy is admitted in cardiogenic shock (BP 70/40 mmHg for about 6 hours) and stabilised on inotropes. On day 2: ALT 3,400 U/L, AST 4,100 U/L, LDH 5,200 U/L (reference up to 250 U/L), bilirubin 1.8 mg/dL and INR 1.9. On day 5: ALT 900 U/L and AST 220 U/L. Hepatitis A, E and B markers are negative and he has taken no paracetamol. What is the most likely cause of the enzyme rise?
1. AAutoimmune hepatitis
2. BChronic congestive hepatopathy
3. CIschaemic hepatitis
4. DAcute viral hepatitis
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Answer: C · Ischaemic hepatitis
Aminotransferases in the thousands within a day of prolonged hypotension, with a very high LDH, that fall by more than half within a few days are typical of ischaemic hepatitis (shock liver): centrilobular hepatocytes die from low flow and recover quickly once perfusion returns. An ALT/LDH ratio below 1.5 favours ischaemic or toxic injury, whereas viral hepatitis gives a higher ratio with a slower fall. Here ALT/LDH is 3,400/5,200, about 0.65. Management is to restore cardiac output; the liver usually recovers.
- A: Autoimmune hepatitis does not appear abruptly after shock and fall within days.
- B: Chronic congestion gives mild enzyme changes, not values in the thousands.
- D: Viral hepatitis falls more slowly and has a higher ALT/LDH ratio; viral markers are negative.
Question 3 · Medium
A 58-year-old man with lung cancer has a serum ALP of 410 U/L (reference 40-130 U/L). Bone scan and liver imaging show no metastases, and GGT is normal. In the laboratory, his ALP activity is unchanged after heating the serum at 65 °C for 10 minutes. Which ALP isoenzyme is this?
1. ABone isoenzyme from osteoblasts
2. BLiver isoenzyme from canaliculi
3. CRegan (placental-like) isoenzyme
4. DIntestinal isoenzyme from enterocytes
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Answer: C · Regan (placental-like) isoenzyme
Placental ALP and the placental-like Regan isoenzyme are the most heat-stable forms, surviving heating that destroys the others. The Regan isoenzyme is an oncofetal protein expressed by some carcinomas, such as those of lung, ovary and testis, so a heat-stable ALP without bone or liver disease suggests tumour production. Bone ALP is the most heat-labile ('bone burns'). Normal GGT and clear liver imaging also argue against a hepatic source.
- A: Bone ALP is the most heat-labile and is lost on heating.
- B: Liver ALP is less heat-stable than placental forms, and GGT would usually rise with it.
- D: Intestinal ALP is moderately heat-labile and rises after fatty meals in some blood groups.
Question 4 · Medium
A 50-year-old man with recurrent gouty arthritis and tophi is started on allopurinol. Xanthine oxidase converts allopurinol to oxypurinol, which then stays tightly bound to the reduced molybdenum centre of the same enzyme. Which type of enzyme inhibition does this illustrate?
1. ASimple reversible competitive inhibition
2. BSuicide (mechanism-based) inhibition
3. CAllosteric feedback inhibition
4. DUncompetitive inhibition
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Answer: B · Suicide (mechanism-based) inhibition
Allopurinol is a hypoxanthine analogue that the enzyme itself processes into oxypurinol, which binds very tightly to the active site and inactivates the enzyme: the enzyme brings about its own inhibition, so this is called suicide or mechanism-based inhibition. Uric acid formation falls and the more soluble hypoxanthine and xanthine are excreted. Because xanthine oxidase also breaks down 6-mercaptopurine, the dose of azathioprine or mercaptopurine must be cut sharply if allopurinol is added. Febuxostat is a non-purine inhibitor of the same enzyme.
- A: Allopurinol resembles the substrate, but its tight binding after conversion goes beyond simple reversible competition.
- C: Allosteric feedback inhibition is by an end product at a separate regulatory site.
- D: Uncompetitive inhibitors bind only the enzyme-substrate complex.
Question 5 · Medium
A 40-year-old man reaches a district hospital 4 days after epigastric pain radiating to the back began during a drinking binge. Serum amylase is 108 U/L (reference 30-110 U/L) and serum lipase 1,450 U/L (reference below 60 U/L). CT shows an oedematous pancreas. Why are the two enzyme results discordant?
1. ALipase is falsely raised by alcohol in serum
2. BLipase also rises in salivary gland disease
3. CAmylase rises only in gallstone pancreatitis
4. DAmylase is cleared from blood faster than lipase
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Answer: D · Amylase is cleared from blood faster than lipase
In acute pancreatitis amylase rises within hours but returns to normal in about 3-5 days because it is small and cleared by the kidneys, whereas lipase stays raised for longer. In a patient who presents late, amylase may already be normal while lipase is still high. Lipase is also more specific for the pancreas, because amylase rises in salivary gland disease and other conditions. A level above three times the upper limit, with typical pain, meets the biochemical criterion.
- A: Alcohol does not cause a false lipase rise; the lipase reflects the pancreatitis.
- B: Salivary disease raises amylase, not lipase.
- C: Amylase rises in pancreatitis of any cause, including alcohol.
Question 6 · Hard
A 32-year-old healthy man receives suxamethonium for rapid-sequence intubation before an emergency appendicectomy. Three hours later he is still paralysed and is ventilated until he recovers. Plasma cholinesterase activity is low and the dibucaine number is 20 (normal about 80). What is the most likely explanation?
1. AMalignant hyperthermia susceptibility
2. BHeterozygous atypical plasma cholinesterase
3. CHomozygous atypical plasma cholinesterase
4. DAcquired deficiency from liver disease
Answer and explanation in LearnFree account
Question 7 · Hard
A 9-year-old boy with acute lymphoblastic leukaemia receives high-dose IV methotrexate in a regional cancer centre. At 48 hours his methotrexate level is still high, creatinine has doubled and mouth ulcers are appearing. Folinic acid (leucovorin) rescue is increased. Why does folinic acid rescue his normal cells when folic acid would not?
1. AIt displaces methotrexate from dihydrofolate reductase
2. BIt cleaves methotrexate to inactive products
3. CIt speeds renal excretion of methotrexate
4. DIt bypasses the blocked dihydrofolate reductase step
Answer and explanation in LearnFree account
Question 8 · Easy
The coenzyme required by ALT and AST to transfer amino groups is:
1. ATetrahydrofolate
2. BThiamine pyrophosphate
3. CLipoic acid
4. DPyridoxal phosphate
Answer and explanation in LearnFree account
Question 9 · Medium
A 45-year-old man who drinks about 180 mL of whisky daily is admitted with jaundice and a tender liver. AST 210 U/L, ALT 85 U/L, GGT 460 U/L, bilirubin 3.1 mg/dL and MCV 104 fL. The unit's resident lists four statements: 1. An AST/ALT ratio above 2 favours alcoholic liver disease. 2. Aminotransferases above 1,000 U/L are typical of alcoholic hepatitis. 3. The high GGT partly reflects enzyme induction by alcohol. 4. The relatively low ALT partly reflects pyridoxal phosphate deficiency. Which statements are correct?
1. A1 and 3 only
2. B1, 3 and 4 only
3. C2, 3 and 4 only
4. D1, 2 and 3 only
Answer and explanation in LearnFree account
Question 10 · Medium
A 62-year-old man on atorvastatin 80 mg daily was given clarithromycin for pneumonia a week ago. He now has severe aching of the thighs and dark urine. Serum CK is 14,500 U/L and the CK-MB fraction is 2% of the total. Which CK isoenzyme accounts for most of this rise?
1. ACK-MM (CK3)
2. BCK-BB (CK1)
3. CCK-MB (CK2)
4. DMitochondrial CK (CK-Mt)
Answer and explanation in LearnFree account
Question 11 · Medium
A 24-year-old woman has a fasting plasma glucose of 118 mg/dL at her first antenatal visit; HbA1c is 6.2%. Her mother and grandfather have the same mild, stable fasting hyperglycaemia without complications. Genetic testing finds a heterozygous inactivating mutation of the glucokinase (GCK) gene. Which pair of properties lets glucokinase act as the glucose sensor of the beta cell?
1. ALow Km, not inhibited by glucose-6-phosphate
2. BHigh Km, inhibited by glucose-6-phosphate
3. CLow Km, inhibited by glucose-6-phosphate
4. DHigh Km, not inhibited by glucose-6-phosphate
Answer and explanation in LearnFree account
Question 12 · Easy
A 62-year-old man reaches a hospital in Nashik 30 hours after severe chest pain at home. As part of a teaching exercise, the biochemistry department runs LDH isoenzyme electrophoresis on his serum and reports a 'flipped' pattern. Which isoenzyme now exceeds LDH2?
1. ALDH4
2. BLDH1
3. CLDH3
4. DLDH5
Answer and explanation in LearnFree account
Question 13 · Medium
A postgraduate tests a new compound on a purified enzyme and draws a Lineweaver-Burk plot. With the compound present, the line has a higher y-intercept, but it meets the x-axis at the same point as the control line. What kind of inhibition does this show?
1. AMixed inhibition
2. BUncompetitive inhibition
3. CNon-competitive inhibition
4. DCompetitive inhibition
Answer and explanation in LearnFree account
Question 14 · Medium
A 30-year-old farmer from Warangal reaches a medical college hospital 36 hours after drinking an organophosphate. He is being atropinised. The senior resident explains that pralidoxime is now likely to help much less than if given early. What is the reason?
1. ADownregulation of nicotinic receptors
2. BAgeing of the phosphorylated enzyme
3. CRapid renal excretion of the poison
4. DSaturation of muscarinic receptors by atropine
Answer and explanation in LearnFree account
Question 15 · Medium
A 35-year-old mechanic in Ludhiana drank radiator coolant (ethylene glycol) and is treated with fomepizole. The resident notes four points about the action of fomepizole on alcohol dehydrogenase: 1. It binds only to the enzyme-substrate complex. 2. It raises the apparent Km of the enzyme for ethylene glycol. 3. It leaves the Vmax of the enzyme unchanged. 4. Very high substrate concentrations can overcome its effect. Which statements are correct?
1. A2 and 3 only
2. B1, 2 and 3 only
3. C2, 3 and 4 only
4. D3 and 4 only
Answer and explanation in LearnFree account
Question 16 · Easy
A 48-year-old woman has generalised itching for three months. Serum ALP is 540 U/L (reference 40-130 U/L), bilirubin 1.1 mg/dL and ALT 52 U/L. Her physician wants to know whether the ALP comes from the liver or from bone. A raised level of which enzyme supports a hepatobiliary source?
1. ATartrate-resistant acid phosphatase
2. B5'-nucleotidase
3. CLactate dehydrogenase
4. DCreatine kinase
Answer and explanation in LearnFree account

Enzymes, kinetics and clinical enzymology

A 58-year-old man with lung cancer has a serum ALP of 410 U/L (reference 40-130 U/L). Bone scan and liver imaging show no metastases, and GGT is normal. In the laboratory, his ALP activity is unchanged after heating the serum at 65 °C for 10 minutes. Which ALP isoenzyme is this?

A 50-year-old man with recurrent gouty arthritis and tophi is started on allopurinol. Xanthine oxidase converts allopurinol to oxypurinol, which then stays tightly bound to the reduced molybdenum centre of the same enzyme. Which type of enzyme inhibition does this illustrate?

The coenzyme required by ALT and AST to transfer amino groups is:

A 62-year-old man on atorvastatin 80 mg daily was given clarithromycin for pneumonia a week ago. He now has severe aching of the thighs and dark urine. Serum CK is 14,500 U/L and the CK-MB fraction is 2% of the total. Which CK isoenzyme accounts for most of this rise?

A 62-year-old man reaches a hospital in Nashik 30 hours after severe chest pain at home. As part of a teaching exercise, the biochemistry department runs LDH isoenzyme electrophoresis on his serum and reports a 'flipped' pattern. Which isoenzyme now exceeds LDH2?

A postgraduate tests a new compound on a purified enzyme and draws a Lineweaver-Burk plot. With the compound present, the line has a higher y-intercept, but it meets the x-axis at the same point as the control line. What kind of inhibition does this show?

A 30-year-old farmer from Warangal reaches a medical college hospital 36 hours after drinking an organophosphate. He is being atropinised. The senior resident explains that pralidoxime is now likely to help much less than if given early. What is the reason?

A 48-year-old woman has generalised itching for three months. Serum ALP is 540 U/L (reference 40-130 U/L), bilirubin 1.1 mg/dL and ALT 52 U/L. Her physician wants to know whether the ALP comes from the liver or from bone. A raised level of which enzyme supports a hepatobiliary source?