Carbohydrate metabolism and its disorders

26 NEET-PG practice questions on Carbohydrate metabolism and its disorders, part of Biochemistry. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Easy

   A 10-day-old exclusively breastfed baby in the NICU has vomiting, jaundice, hepatomegaly and E. coli sepsis. Urine is positive for reducing sugar on Benedict's test but negative for glucose on a dipstick, and bilateral oil-drop cataracts are seen. Deficiency of which enzyme is most likely?

   1. AGalactokinase (GALK)
   2. BUridyltransferase (GALT)
   3. CGlucose-6-phosphatase
   4. DAldolase B

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   Answer: B · Uridyltransferase (GALT)

   Classic galactosaemia is caused by deficiency of galactose-1-phosphate uridyltransferase (GALT). Lactose in milk releases galactose, and galactose-1-phosphate builds up in liver, kidney and brain, while galactitol formed by aldose reductase causes cataracts. The reducing sugar in urine is galactose, which the glucose-specific dipstick misses, and E. coli sepsis is a classic association. Breastfeeding must be stopped and a lactose-free (soy-based) formula started; the parents need counselling that this is lifelong.

   • A: Galactokinase deficiency causes cataracts alone, without liver disease or sepsis.
   • C: Glucose-6-phosphatase deficiency causes fasting hypoglycaemia and lactic acidosis, not cataracts.
   • D: Aldolase B deficiency (fructose intolerance) appears after fructose or sucrose is introduced at weaning.

2. Question 2 · Medium

   A 28-year-old woman is admitted to the ICU three days after taking 40 tablets of paracetamol 500 mg. She is drowsy; ALT is 6,800 U/L, INR 5.2, blood glucose 42 mg/dL and lactate 8 mmol/L despite adequate fluids and blood pressure. Which failure best explains both the hypoglycaemia and the raised lactate?

   1. ABlock of muscle glycogen breakdown by paracetamol
   2. BLoss of hepatic gluconeogenesis from lactate
   3. CIncreased glucose loss in urine from tubular damage
   4. DThiamine deficiency blocking pyruvate dehydrogenase

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   Answer: B · Loss of hepatic gluconeogenesis from lactate

   In acute liver failure the hepatocytes can no longer release glucose by glycogenolysis or make it by gluconeogenesis, so hypoglycaemia is common and needs regular checks and dextrose infusions. The liver normally takes up much of the body's lactate and turns it back into glucose in the Cori cycle, so failure of this uptake also raises blood lactate. Hypoglycaemia and a high lactate are both markers of severity in paracetamol-induced liver failure and count towards the decision to refer for transplantation.

   • A: Paracetamol does not block muscle glycogen breakdown, and muscle cannot release glucose anyway.
   • C: Glycosuria from tubular damage is small and does not cause this degree of hypoglycaemia.
   • D: Thiamine deficiency is not the cause here; the liver itself has failed.

3. Question 3 · Hard

   A 9-month-old boy is admitted with developmental delay, hypotonia and episodes of rapid breathing that worsen after carbohydrate-rich feeds. Blood lactate is 7.8 mmol/L (reference 0.5-2.2) with a lactate:pyruvate ratio of 14 (reference below 25); MRI shows bilateral basal ganglia lesions. Cultured skin fibroblasts show low pyruvate dehydrogenase complex activity. The registrar lists four points: 1. The normal lactate:pyruvate ratio fits a block before the respiratory chain. 2. A trial of high-dose thiamine is worthwhile. 3. A high-carbohydrate diet will reduce the lactic acidosis. 4. A ketogenic diet supplies acetyl-CoA that bypasses the block. Which points are correct?

   1. A2, 3 and 4 only
   2. B1, 2 and 4 only
   3. C2 and 4 only
   4. D1, 3 and 4 only

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   Answer: B · 1, 2 and 4 only

   Points 1, 2 and 4 are correct. In pyruvate dehydrogenase deficiency, pyruvate and lactate rise together, so their ratio stays normal; a high ratio points instead to a respiratory chain defect, where NADH cannot be reoxidised. Thiamine pyrophosphate is a PDH coenzyme and some patients respond to high doses, so a trial is standard. Ketone bodies are converted to acetyl-CoA in the brain without passing through PDH, so a ketogenic (high-fat, low-carbohydrate) diet is the main dietary treatment; point 3 is false because extra carbohydrate increases pyruvate and therefore lactate.

   • A: Includes high-carbohydrate feeding and omits the correct reading of the ratio.
   • C: Omits point 1, the correct reading of the normal lactate:pyruvate ratio.
   • D: Includes high-carbohydrate feeding, which raises lactate, and omits the thiamine trial.

4. Question 4 · Hard

   A 10-month-old girl with hereditary fructose intolerance, confirmed by ALDOB gene testing, is brought to casualty sweating and drowsy 40 minutes after relatives fed her sweets at a festival. Glucose is 36 mg/dL, serum phosphate is low, and uric acid and lactate are raised. Intramuscular glucagon given by the casualty officer does not raise her glucose. Which mechanism best explains the hypoglycaemia?

   1. AAbsent liver glycogen stores from a glycogen storage defect
   2. BGalactose-1-phosphate inhibiting phosphoglucomutase
   3. CFructose stimulating excess insulin secretion from the pancreas
   4. DFructose-1-phosphate trapping phosphate and blocking glucose release

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   Answer: D · Fructose-1-phosphate trapping phosphate and blocking glucose release

   Fructokinase phosphorylates fructose rapidly, but without aldolase B the fructose-1-phosphate cannot be split and accumulates. This traps inorganic phosphate, so ATP falls, AMP is broken down to uric acid, and low phosphate slows glycogen phosphorylase; fructose-1-phosphate also inhibits phosphorylase and the aldolase step of gluconeogenesis. Glucose release from glycogen and from gluconeogenesis are both blocked, which is why glucagon fails. Treatment is IV dextrose now and strict avoidance of fructose, sucrose and sorbitol.

   • A: Her glycogen stores are present; their release is blocked.
   • B: Galactose-1-phosphate is the metabolite of galactosaemia, not fructose intolerance.
   • C: Fructose is a weak insulin secretagogue, and insulin is not the cause here.

5. Question 5 · Easy

   A 4-month-old girl is admitted with poor feeding, a weak cry and breathlessness. She is floppy, with a large tongue and a liver palpable 3 cm below the costal margin. Chest X-ray shows massive cardiomegaly, and the ECG shows a short PR interval with very tall QRS complexes. Blood glucose is normal. Deficiency of which enzyme is most likely?

   1. AGlycogen debranching enzyme
   2. BGlucose-6-phosphatase
   3. CGlycogen branching enzyme
   4. DLysosomal acid alpha-glucosidase

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   Answer: D · Lysosomal acid alpha-glucosidase

   Infantile Pompe disease (glycogen storage disease type II) is caused by deficiency of lysosomal acid alpha-glucosidase (acid maltase). Glycogen builds up inside lysosomes of heart, skeletal muscle and liver, giving hypertrophic cardiomyopathy, hypotonia and macroglossia. Blood glucose stays normal because the cytosolic pathways of glycogen breakdown are intact. Enzyme replacement with recombinant alglucosidase alfa works best when started early, so the diagnosis must not be delayed.

   • A: Debranching enzyme deficiency can involve heart and muscle but causes fasting ketotic hypoglycaemia.
   • B: Glucose-6-phosphatase deficiency causes fasting hypoglycaemia, not cardiomyopathy.
   • C: Branching enzyme deficiency (Andersen disease) causes progressive cirrhosis in infancy.

6. Question 6 · Medium

   During a pre-employment check-up, a healthy 30-year-old software engineer's urine gives a positive Benedict test, but a glucose oxidase dipstick is negative. Fasting glucose and HbA1c are normal, and Seliwanoff's test on the urine is positive. Consider four statements: 1. The reducing sugar is fructose. 2. The likely deficient enzyme is fructokinase. 3. He must avoid fruit and sucrose for life. 4. The condition is benign and needs no treatment. Which statements are correct?

   1. A1, 2 and 3 only
   2. B1 and 2 only
   3. C2 and 4 only
   4. D1, 2 and 4 only

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7. Question 7 · Medium

   A 45-year-old man from a village in North 24 Parganas, West Bengal, has drunk tube-well water for many years. He has raindrop pigmentation over the trunk, thick horny skin on his palms and soles, and tingling in his feet. His blood lactate is mildly raised. Arsenite binds the dithiol group of a coenzyme and inhibits which enzyme complex, explaining the raised lactate?

   1. APyruvate carboxylase
   2. BGlyceraldehyde-3-phosphate dehydrogenase
   3. CPyruvate dehydrogenase complex
   4. DCytochrome c oxidase

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8. Question 8 · Easy

   A 19-year-old college footballer from Pune has had cramps and stiffness in his thighs within minutes of sprinting since school. If he slows down for a few minutes and then carries on, he can continue more comfortably. After a heavy weight-training session he passed dark red-brown urine, and serum CK was 9,800 U/L. Deficiency of which enzyme is most likely?

   1. AMuscle phosphofructokinase
   2. BMuscle glycogen phosphorylase
   3. CMyoadenylate deaminase
   4. DCarnitine palmitoyltransferase II

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9. Question 9 · Medium

   A 6-year-old boy from Odisha has had pallor and spells of jaundice since he needed phototherapy as a newborn. He has splenomegaly. Hb is 7.4 g/dL, reticulocytes 12%, MCHC normal; the smear shows a few echinocytes but no spherocytes, osmotic fragility is normal and the G6PD screen is normal. He plays football without breathlessness despite his low haemoglobin. What best explains his good exercise tolerance?

   1. ARaised HbF increasing haemoglobin's O2 affinity
   2. BHigh NADPH protecting haemoglobin from oxidation
   3. CRaised 2,3-BPG lowering haemoglobin's O2 affinity
   4. DLow red cell ATP increasing O2 binding

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10. Question 10 · Hard

    A 6-year-old boy from Mangaluru develops pallor and dark urine two days after swallowing a naphthalene mothball. Hb is 6.8 g/dL, reticulocytes 14%, and the smear shows bite cells and blister cells; the direct Coombs test is negative. A G6PD fluorescent spot test done on day 4 of the illness is reported as normal. What should be done to settle whether he has G6PD deficiency?

    1. ARepeat the G6PD assay about 3 months later
    2. BDo an osmotic fragility test now
    3. CMeasure red cell pyruvate kinase now
    4. DAccept the result and exclude G6PD deficiency

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11. Question 11 · Hard

    A 2-year-old boy has a protuberant abdomen with a large liver, fasting hypoglycaemia, raised lactate and triglycerides, and recurrent mouth ulcers, skin abscesses and perianal abscesses. His absolute neutrophil count is about 600/µL on repeated tests. Glucose-6-phosphatase activity measured in a liver microsomal preparation that has been disrupted with detergent is normal. Which defect is most likely?

    1. AGlucose-6-phosphate transporter of the ER
    2. BLysosomal acid alpha-glucosidase
    3. CGlycogen debranching enzyme
    4. DGlucose-6-phosphatase catalytic subunit

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12. Question 12 · Medium

    A 2-year-old girl has a large liver and a doll-like face. She becomes hypoglycaemic after an overnight fast, with ketones in her urine, but blood lactate and uric acid are normal. ALT is 180 U/L and CK 950 U/L. Liver biopsy shows excess glycogen with abnormally short outer branches. Deficiency of which enzyme is most likely?

    1. AGlucose-6-phosphatase
    2. BGlycogen branching enzyme
    3. CLiver glycogen phosphorylase
    4. DGlycogen debranching enzyme

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13. Question 13 · Hard

    A 24-year-old woman has avoided fruit and sweets since early childhood because they made her vomit, and she has never had a dental cavity. On the first day after an appendicectomy she is given an IV infusion containing sorbitol. Within hours she becomes sweaty and confused, with glucose 40 mg/dL, rising ALT, a low serum phosphate and a raised uric acid. What is the most appropriate immediate step?

    1. AGive IV insulin with potassium
    2. BGive IM glucagon and continue the infusion
    3. CChange to an invert sugar infusion
    4. DStop the infusion and give IV dextrose

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14. Question 14 · Easy

    After a carbohydrate-rich meal, insulin rises and the liver speeds up glycolysis while switching off gluconeogenesis. Which compound is the most potent allosteric activator of hepatic phosphofructokinase-1 in this fed state?

    1. AGlucose-6-phosphate
    2. BFructose-2,6-bisphosphate
    3. CFructose-1,6-bisphosphate
    4. DCitrate

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15. Question 15 · Hard

    A 22-year-old man from Ahmedabad has had muscle cramps and fatigue with short bursts of intense exercise since childhood, and once passed dark urine after a cricket match. Unlike a friend with a similar problem, he feels worse if he eats sweets before playing. CK is 1,200 U/L, Hb 12.4 g/dL, reticulocytes 4.5%, indirect bilirubin 2.1 mg/dL and uric acid 8.9 mg/dL. On forearm exercise testing, lactate does not rise while ammonia rises normally. Deficiency of which enzyme is most likely?

    1. AMuscle phosphofructokinase
    2. BRed cell pyruvate kinase
    3. CCarnitine palmitoyltransferase II
    4. DMuscle glycogen phosphorylase

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16. Question 16 · Medium

    A 3-year-old boy with glucose-6-phosphatase deficiency (glycogen storage disease Ia), confirmed by gene testing, is reviewed in the paediatric clinic with his parents. The resident explains four points: 1. Glucagon given during fasting will not raise his blood glucose. 2. Uncooked cornstarch at bedtime helps prevent night-time hypoglycaemia. 3. Fruit sugar and milk sugar (fructose, sucrose and lactose) should be limited. 4. His high uric acid is partly due to lactate competing with urate for renal excretion. Which points are correct?

    1. A2 and 3 only
    2. B1, 2 and 4 only
    3. C1, 2 and 3 only
    4. D1, 2, 3 and 4

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17. Question 17 · Medium

    A 26-year-old gym trainer from Delhi has eaten about fifteen raw egg whites a day for a year to build muscle. He now has thinning hair, a scaly red rash around the nose and mouth, low mood and muscle aches. His blood lactate is mildly raised. Reduced activity of which enzyme best explains the raised lactate?

    1. AFructose-1,6-bisphosphatase
    2. BPhosphoenolpyruvate carboxykinase
    3. CPyruvate carboxylase
    4. DGlucose-6-phosphatase

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18. Question 18 · Medium

    A 25-year-old marathon runner from Bengaluru asks the sports medicine doctor why the large glycogen store in his leg muscles cannot keep up his blood glucose during an overnight fast, while the much smaller store in his liver can. What is the reason?

    1. AMuscle lacks glycogen phosphorylase
    2. BMuscle has no glucose transporters
    3. CMuscle glycogen has no branch points
    4. DMuscle lacks glucose-6-phosphatase

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19. Question 19 · Medium

    A 32-year-old daily-wage labourer is brought to casualty unresponsive after two days of heavy drinking with almost no food. Capillary glucose is 34 mg/dL and blood alcohol is high; he wakes after IV thiamine and dextrose. Why does alcohol cause hypoglycaemia in a fasting person?

    1. AAcetaldehyde inhibiting hepatic glucose-6-phosphatase
    2. BHigh NADH/NAD+ ratio diverting gluconeogenic substrates
    3. CEthanol increasing glucose uptake by skeletal muscle
    4. DEthanol directly stimulating insulin secretion

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20. Question 20 · Medium

    A 38-year-old woman observing a three-day religious fast attends the OPD feeling weak. Her blood glucose is 68 mg/dL and urine ketones are positive. Her liver is making glucose from several substrates released by other tissues. Which substrate cannot give a net yield of glucose?

    1. ALactate from red blood cells
    2. BPalmitate from adipose tissue
    3. CGlycerol from adipose tissue
    4. DAlanine from skeletal muscle

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21. Question 21 · Medium

    A 58-year-old man with diabetes attends a PHC at 8 am. His fasting blood is drawn into a plain tube, but the sample reaches the district laboratory unseparated at 2 pm. The laboratory reports glucose as 88 mg/dL, although a glucometer reading taken at the time of sampling was 136 mg/dL. A grey-top sodium fluoride tube would have prevented this error by inhibiting which enzyme?

    1. AEnolase
    2. BPhosphofructokinase-1
    3. CPyruvate kinase
    4. DHexokinase

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22. Question 22 · Medium

    A 24-year-old man from Kutch, Gujarat, treated at a PHC for vivax malaria, develops dark urine, jaundice and breathlessness on the third day of primaquine. His Hb has fallen from 13.1 to 8.2 g/dL; the smear shows bite cells, and a supravital stain shows Heinz bodies. Lack of which product of the affected pathway explains his haemolysis?

    1. ANADPH for keeping glutathione reduced
    2. BNADH for methaemoglobin reductase
    3. C2,3-BPG for oxygen unloading
    4. DATP for the sodium-potassium pump

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23. Question 23 · Medium

    A 4-year-old girl from Kannur is referred from the eye OPD with bilateral cataracts. She has grown and developed normally, has no jaundice or enlarged liver, and drinks milk daily. Her urine contains galactose, and red cell galactose-1-phosphate uridyltransferase activity is normal. Which substance accumulating in her lenses causes the cataracts?

    1. AGalactitol
    2. BFructose-1-phosphate
    3. CGalactose-1-phosphate
    4. DSorbitol

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24. Question 24 · Easy

    A 5-month-old boy from Kozhikode is brought to the paediatric OPD with a protuberant abdomen, a round doll-like face and spells of sweating and irritability 3-4 hours after feeds. The liver is palpable 6 cm below the costal margin; the spleen is not felt. Fasting glucose is 38 mg/dL, with raised blood lactate, uric acid and triglycerides. Deficiency of which enzyme is most likely?

    1. ALysosomal acid alpha-glucosidase
    2. BLiver glycogen phosphorylase
    3. CGlucose-6-phosphatase
    4. DGlycogen debranching enzyme

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25. Question 25 · Medium

    A 67-year-old man with type 2 diabetes has taken metformin 1 g twice daily for years. He is admitted with vomiting and drowsiness. Creatinine is 4.1 mg/dL (eGFR 14 mL/min/1.73 m2), pH 7.08, bicarbonate 8 mmol/L, anion gap 28 mmol/L and lactate 11 mmol/L; blood ketones are negative. Which action of the drug explains the lactic acidosis?

    1. AStimulation of insulin release from beta cells
    2. BInhibition of glucose reabsorption in the kidney
    3. CInhibition of hepatic gluconeogenesis from lactate
    4. DBlockade of lactate dehydrogenase in muscle

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