Carbohydrate metabolism and diabetes

58 Kerala MSc MLT Entrance practice questions on Carbohydrate metabolism and diabetes, 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 · Medium

   A 26-year-old woman at 26 weeks of pregnancy attends a Kerala antenatal clinic that follows India's national single-step (DIPSI) test. She is given 75 g of oral glucose without regard to her last meal, and her 2-hour venous plasma glucose is 148 mg/dL. How is this interpreted?

   1. AGestational diabetes mellitus
   2. BNormal glucose tolerance
   3. CInvalid, as she was not fasting
   4. DOvert pre-existing diabetes

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   Answer: A · Gestational diabetes mellitus

   India's national guideline uses a single 75 g glucose load given irrespective of the last meal, with one sample at 2 hours. A 2-hour plasma glucose of 140 mg/dL or more is diagnosed as gestational diabetes. The test is done at the first antenatal visit and repeated at 24-28 weeks if the first is normal. Her value of 148 mg/dL therefore meets the cut-off.

   • B: 148 mg/dL is above the 140 mg/dL cut-off, so the result is not normal.
   • C: The single-step test is designed to be done without fasting.
   • D: Overt diabetes in pregnancy needs a 2-hour value of 200 mg/dL or more (WHO).

2. Question 2 · Medium

   A 45-year-old alcoholic man on a polished-rice diet has lactic acidosis, with raised blood pyruvate and lactate. Which enzyme has lost its coenzyme?

   1. ALactate dehydrogenase
   2. BPyruvate dehydrogenase
   3. CPyruvate kinase
   4. DPyruvate carboxylase

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   Answer: B · Pyruvate dehydrogenase

   Pyruvate dehydrogenase needs thiamine pyrophosphate (with lipoate, CoA, FAD and NAD+) to convert pyruvate to acetyl-CoA. In thiamine deficiency, pyruvate cannot enter the TCA cycle, so it accumulates and is reduced to lactate. Giving glucose without thiamine can worsen the acidosis, so thiamine is given first. Alpha-ketoglutarate dehydrogenase and transketolase also need TPP.

   • A: Lactate dehydrogenase uses NAD(H), a niacin coenzyme, not thiamine.
   • C: Pyruvate kinase needs magnesium and potassium, not a vitamin coenzyme.
   • D: Pyruvate carboxylase needs biotin, not thiamine.

3. Question 3 · Hard

   On day 3 of primaquine for Plasmodium vivax, a 24-year-old passes cola-coloured urine. Haemoglobin has fallen from 14.5 to 9.8 g/dL, the DAT is negative, supravital staining shows Heinz bodies and the film shows bite cells. Which red cell function is deficient?

   1. ANADPH production by the HMP shunt
   2. BATP production by anaerobic glycolysis
   3. C2,3-BPG formation by the Rapoport-Luebering shunt
   4. DNADH-dependent methaemoglobin reduction

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   Answer: A · NADPH production by the HMP shunt

   Oxidant drug haemolysis with Heinz bodies, bite cells and a negative DAT points to G6PD deficiency. G6PD is the first enzyme of the HMP shunt and is the red cell's only source of NADPH. NADPH keeps glutathione reduced, and reduced glutathione removes hydrogen peroxide. Without it, oxidised haemoglobin precipitates as Heinz bodies and the cells are destroyed.

   • B: Reduced ATP production, as in pyruvate kinase deficiency, causes chronic haemolysis without drug-induced Heinz bodies.
   • C: Changes in 2,3-BPG alter oxygen affinity but do not cause oxidant haemolysis.
   • D: NADH-methaemoglobin reductase deficiency causes methaemoglobinaemia, not Heinz body haemolysis.

4. Question 4 · Easy

   The two main products of the oxidative phase of the HMP shunt are:

   1. AATP and pyruvate
   2. BNADPH and ribose 5-phosphate
   3. CNADH and ribose 5-phosphate
   4. DFADH2 and acetyl-CoA

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   Answer: B · NADPH and ribose 5-phosphate

   The HMP (pentose phosphate) pathway oxidises glucose 6-phosphate, using glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, to make NADPH and CO2. The pentose formed becomes ribose 5-phosphate for nucleotide synthesis. NADPH is needed for fatty acid and steroid synthesis and to keep glutathione reduced in red cells. The pathway makes no ATP.

   • A: The HMP shunt does not produce ATP or pyruvate directly.
   • C: The HMP shunt reduces NADP+, not NAD+.
   • D: FADH2 and acetyl-CoA come from fatty acid oxidation and the TCA cycle.

5. Question 5 · Hard

   A 7-month-old girl began vomiting, sweating and refusing feeds soon after fruit juice and sugar were added to her diet. Results: plasma glucose 38 mg/dL after a feed, enlarged liver, raised ALT, urine Benedict positive with a rapid red Seliwanoff reaction. She had thrived on breast milk alone. Which enzyme is deficient?

   1. AFructokinase
   2. BFructose 1,6-bisphosphatase
   3. CGalactose 1-phosphate uridyltransferase
   4. DAldolase B

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   Answer: D · Aldolase B

   Symptoms that start with fructose and sucrose, not with milk, point to fructose metabolism. In hereditary fructose intolerance, aldolase B deficiency traps fructose 1-phosphate in the liver, using up phosphate and ATP and blocking glycogenolysis and gluconeogenesis, so hypoglycaemia follows. Fructose spills into urine, giving a reducing sugar with a positive Seliwanoff test. Fructokinase deficiency also causes fructosuria but is harmless.

   • A: Fructokinase deficiency (essential fructosuria) causes no symptoms or hypoglycaemia.
   • B: Fructose 1,6-bisphosphatase deficiency causes fasting hypoglycaemia with lactic acidosis, without fructosuria.
   • C: Galactosaemia begins with milk feeding in the first weeks of life.

6. Question 6 · Medium

   In the liver, glucagon slows glycolysis and favours gluconeogenesis mainly by lowering the concentration of:

   1. AAcetyl-CoA
   2. BCitrate
   3. CFructose 2,6-bisphosphate
   4. DGlucose 6-phosphate

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7. Question 7 · Easy

   Gerhardt's ferric chloride test on urine detects which ketone body?

   1. AAcetoacetate only
   2. BAll three ketone bodies
   3. CBeta-hydroxybutyrate only
   4. DAcetone only

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8. Question 8 · Hard

   A 50-year-old woman with poorly controlled diabetes collects a 24-hour urine of 3,000 mL. The urine is diluted 1 in 10 for glucose by GOD-POD, and the diluted sample reads 150 mg/dL. How much glucose did she lose in 24 hours?

   1. A4.5 g
   2. B450 g
   3. C15 g
   4. D45 g

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9. Question 9 · Hard

   A 19-year-old man has muscle cramps and dark urine after sprints, and his cramps ease if he slows down and keeps going for a few minutes. On a forearm exercise test, venous ammonia rises fivefold but lactate does not rise. Haemoglobin, reticulocytes and bilirubin are normal. Which enzyme is deficient?

   1. AAcid alpha-glucosidase
   2. BCarnitine palmitoyltransferase II
   3. CMuscle glycogen phosphorylase
   4. DMuscle phosphofructokinase

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10. Question 10 · Medium

    When liver glycogen is broken down, most glucose units are released as glucose 1-phosphate, but about one in ten appears directly as free glucose. This free glucose comes from:

    1. AAlpha-1,6 branch points cut by the debranching enzyme
    2. BGlucose 6-phosphate cleaved by glucose 6-phosphatase
    3. CNon-reducing chain ends cut by salivary amylase
    4. DAlpha-1,4 bonds split by glycogen phosphorylase

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11. Question 11 · Medium

    In gluconeogenesis, the irreversible pyruvate kinase step of glycolysis is bypassed by which pair of enzymes?

    1. APyruvate carboxylase and PEP carboxykinase
    2. BPyruvate dehydrogenase and citrate synthase
    3. CFructose 1,6-bisphosphatase and glucose 6-phosphatase
    4. DPhosphoglycerate kinase and enolase

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12. Question 12 · Easy

    Which enzyme of the TCA cycle forms a high-energy phosphate (GTP or ATP) by substrate-level phosphorylation?

    1. AAlpha-ketoglutarate dehydrogenase
    2. BIsocitrate dehydrogenase
    3. CSuccinyl-CoA synthetase (succinate thiokinase)
    4. DSuccinate dehydrogenase

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13. Question 13 · Hard

    A 2-year-old boy has a large liver, mild fasting hypoglycaemia and weak muscles. ALT is 240 U/L and CK 1,100 U/L; lactate and uric acid are normal. Glucagon raises glucose 2 hours after a meal but not after an overnight fast. A liver biopsy shows glycogen with abnormally short outer branches. Which enzyme is deficient?

    1. ADebranching enzyme (amylo-1,6-glucosidase)
    2. BLysosomal acid alpha-glucosidase
    3. CLiver glycogen phosphorylase
    4. DGlucose-6-phosphatase

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14. Question 14 · Hard

    A 70-year-old woman with type 2 diabetes is drowsy after a week of fever and poor fluid intake. Na+ 150 mmol/L, glucose 1,080 mg/dL, urea 120 mg/dL, HCO3- 21 mmol/L, urine ketones trace. Using 2Na+ + glucose/18 + urea/6, what is her calculated serum osmolality?

    1. A360 mOsm/kg
    2. B320 mOsm/kg
    3. C230 mOsm/kg
    4. D380 mOsm/kg

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15. Question 15 · Medium

    Which pair of glycolytic enzymes forms ATP by substrate-level phosphorylation?

    1. AHexokinase and phosphofructokinase-1
    2. BEnolase and phosphoglycerate mutase
    3. CPhosphoglycerate kinase and pyruvate kinase
    4. DAldolase and triose phosphate isomerase

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16. Question 16 · Hard

    A farm worker swallows a rodenticide containing sodium fluoroacetate. He develops seizures and cardiac arrhythmia, with metabolic acidosis and a markedly raised serum citrate. Which enzyme is blocked?

    1. ASuccinate dehydrogenase
    2. BIsocitrate dehydrogenase
    3. CCitrate synthase
    4. DAconitase

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17. Question 17 · Easy

    Insulin increases glucose uptake into skeletal muscle and adipose tissue by moving which transporter to the cell membrane?

    1. ASGLT1
    2. BGLUT4
    3. CGLUT1
    4. DGLUT2

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18. Question 18 · Easy

    Glycogen phosphorylase, the rate-limiting enzyme of glycogenolysis, requires which coenzyme?

    1. AThiamine pyrophosphate
    2. BBiotin
    3. CPyridoxal phosphate
    4. DFlavin adenine dinucleotide

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19. Question 19 · Easy

    Classic galactosaemia, which presents in a breast-fed newborn with vomiting, jaundice and cataract, is due to deficiency of:

    1. AUDP-galactose 4-epimerase
    2. BLactase of the intestinal brush border
    3. CGalactose-1-phosphate uridyltransferase
    4. DGalactokinase

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20. Question 20 · Medium

    Why can even-chain fatty acids not give net synthesis of glucose in humans?

    1. AAcetyl-CoA cannot enter the TCA cycle
    2. BLiver cells lack glucose-6-phosphatase
    3. CFatty acids cannot enter liver mitochondria
    4. DThe pyruvate dehydrogenase reaction is irreversible

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21. Question 21 · Medium

    Ketone body production in liver mitochondria is controlled mainly at the step catalysed by:

    1. ACytosolic HMG-CoA reductase
    2. BAcetoacetyl-CoA thiolase
    3. CHMG-CoA lyase
    4. DMitochondrial HMG-CoA synthase

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22. Question 22 · Medium

    A clinic screening pregnant women for gestational diabetes with the 50 g glucose challenge test should collect blood:

    1. AAt any time of day, with plasma glucose at 3 hours
    2. BAfter an overnight fast, with plasma glucose at 2 hours
    3. CAfter an overnight fast, with plasma glucose at 1 hour
    4. DAt any time of day, with plasma glucose at 1 hour

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23. Question 23 · Medium

    After a carbohydrate meal, insulin increases the oxidation of pyruvate to acetyl-CoA by switching on the pyruvate dehydrogenase complex. This activation occurs through:

    1. APhosphorylation by PDH kinase
    2. BA rise in the NADH/NAD+ ratio
    3. CDephosphorylation by PDH phosphatase
    4. DA rise in mitochondrial acetyl-CoA

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24. Question 24 · Hard

    A 35-year-old bodybuilder has eaten about 12 raw egg whites a day for a year. He has scaly dermatitis, hair loss and fatigue. Fasting glucose is 62 mg/dL and blood lactate is mildly raised. Deficiency of which coenzyme, and failure of which gluconeogenic enzyme, best explain these results?

    1. AThiamine, with failure of pyruvate dehydrogenase
    2. BBiotin, with failure of pyruvate carboxylase
    3. CBiotin, with failure of PEP carboxykinase
    4. DPyridoxine, with failure of alanine transaminase

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25. Question 25 · Medium

    After a meal, insulin increases glycogen storage in the liver mainly by:

    1. ADephosphorylating and activating glycogen synthase
    2. BActivating phosphorylase kinase in the liver
    3. CPhosphorylating and activating glycogen synthase
    4. DRaising cAMP to activate protein kinase A

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