Haemoglobin, porphyrins and bilirubin

39 Kerala MSc MLT Entrance practice questions on Haemoglobin, porphyrins and bilirubin, 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

   In the Malloy-Evelyn method for serum bilirubin, total bilirubin is made to react with the diazo reagent by adding:

   1. AAscorbic acid
   2. BAlkaline tartrate
   3. CCaffeine-sodium benzoate
   4. DMethanol

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   Answer: D · Methanol

   Conjugated bilirubin reacts with diazotised sulphanilic acid in water (the direct reaction), but unconjugated bilirubin reacts only after an accelerator is added. In the Malloy-Evelyn method the accelerator is 50% methanol, and the purple azobilirubin is read at about 540 nm. The difference between the total and direct readings gives the indirect (unconjugated) fraction.

   • A: Ascorbic acid destroys excess diazo reagent to stop the reaction in Jendrassik-Grof.
   • B: Alkaline tartrate turns azobilirubin blue in Jendrassik-Grof; it is not the accelerator.
   • C: Caffeine-sodium benzoate is the accelerator of the Jendrassik-Grof method.

2. Question 2 · Medium

   Fetal haemoglobin can be measured by the alkali denaturation (Singer) method because HbF:

   1. AResists denaturation by strong alkali
   2. BIs denatured by alkali faster than HbA
   3. CIs the only haemoglobin that binds cyanide
   4. DPrecipitates in phosphate buffer with dithionite

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   Answer: A · Resists denaturation by strong alkali

   HbF (alpha2 gamma2) is far more resistant to denaturation by sodium hydroxide than adult haemoglobin. After a timed exposure to alkali, the denatured HbA is precipitated with ammonium sulphate and the HbF left in the filtrate is measured as a percentage of total haemoglobin. The Kleihauer acid elution stain for fetal cells relies on the same resistance of HbF.

   • B: HbA is the one denatured faster; HbF resists alkali.
   • C: All haemoglobins are converted to cyanmethaemoglobin by Drabkin reagent.
   • D: Dithionite in phosphate buffer is the solubility test for HbS.

3. Question 3 · Hard

   A 19-year-old student in Kochi has had 5 days of fever, nausea and dark urine. Results: total bilirubin 6.8 mg/dL (reference up to 1.2), direct 3.9 mg/dL, ALT 1,640 U/L (up to 40), ALP 180 U/L (40-130); urine bilirubin positive and urine urobilinogen increased; stools normal in colour. The jaundice is best classified as:

   1. ACongenital (Gilbert) jaundice
   2. BComplete obstructive jaundice
   3. CHepatocellular jaundice
   4. DHaemolytic jaundice

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   Answer: C · Hepatocellular jaundice

   Both conjugated and unconjugated bilirubin are raised, with an ALT over 40 times the upper limit and only a small rise in ALP, the pattern of hepatocyte injury such as viral hepatitis. Conjugated bilirubin regurgitates into blood and appears in urine. Urobilinogen still reaches the gut, and the damaged liver fails to re-excrete the reabsorbed fraction, so urine urobilinogen rises. Haemolysis gives no bilirubinuria, complete obstruction gives pale stools and absent urobilinogen, and Gilbert syndrome raises only unconjugated bilirubin with normal enzymes.

   • A: Gilbert syndrome raises unconjugated bilirubin only, with normal liver enzymes.
   • B: Complete obstruction gives pale stools, absent urine urobilinogen and a high ALP.
   • D: Haemolytic jaundice is unconjugated, with no bilirubin in urine and normal ALT.

4. Question 4 · Medium

   Acute intermittent porphyria causes abdominal and nervous system attacks but no skin photosensitivity. Why?

   1. AThe precursors that build up, ALA and PBG, are not porphyrins
   2. BIts enzyme defect is in red cells, not in the liver
   3. CThe porphyrins formed are excreted at once in bile
   4. DHaem synthesis is increased rather than reduced

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   Answer: A · The precursors that build up, ALA and PBG, are not porphyrins

   In acute intermittent porphyria, porphobilinogen deaminase is deficient, so the pathway is blocked before the tetrapyrrole ring is formed. ALA and PBG accumulate, and these are colourless precursors that do not absorb light. Photosensitivity needs porphyrins, which absorb light near 400 nm and generate tissue-damaging oxygen radicals, as in porphyria cutanea tarda or erythropoietic protoporphyria. A raised urine PBG during an attack is the key laboratory finding.

   • B: AIP is a hepatic porphyria; the defect shows in the liver.
   • C: No porphyrins accumulate in AIP; the block is before porphyrin formation.
   • D: Haem synthesis is reduced; ALA synthase is induced because haem feedback is lost.

5. Question 5 · Easy

   The first enzyme of haem synthesis, ALA synthase, needs which coenzyme?

   1. ABiotin
   2. BPyridoxal phosphate
   3. CThiamine pyrophosphate
   4. DTetrahydrofolate

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   Answer: B · Pyridoxal phosphate

   ALA synthase condenses glycine with succinyl-CoA in the mitochondria to form delta-aminolevulinic acid, and pyridoxal phosphate is its coenzyme. It is the rate-limiting enzyme and is repressed by haem. Vitamin B6 deficiency or isoniazid therapy can therefore reduce haem synthesis and cause a sideroblastic anaemia.

   • A: Biotin is the coenzyme of carboxylases.
   • C: Thiamine pyrophosphate serves oxidative decarboxylases and transketolase.
   • D: Tetrahydrofolate carries one-carbon units, not used by ALA synthase.

6. Question 6 · Hard

   A 5-year-old boy has had blistering and scarring of sun-exposed skin since infancy. His nappies were stained pink-red, his teeth glow red under ultraviolet light, and he has a mild haemolytic anaemia with splenomegaly. Urine uroporphyrin I and coproporphyrin I are markedly raised; urine PBG is normal. Which enzyme is deficient?

   1. APorphobilinogen deaminase
   2. BUroporphyrinogen III synthase
   3. CUroporphyrinogen decarboxylase
   4. DFerrochelatase

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7. Question 7 · Hard

   A 3-month-old girl is fed formula made with water from a farm well. She has blue lips but no breathlessness. Her arterial blood is chocolate-brown and stays dark when shaken in air; pO2 is 96 mm Hg, yet pulse oximetry reads 84%. A haemolysate shows an absorption peak near 630 nm that disappears when cyanide is added. The most likely cause of the cyanosis is:

   1. ANitrate-induced methaemoglobinaemia
   2. BDrug-induced sulphaemoglobinaemia
   3. CCarbon monoxide poisoning
   4. DCyanotic congenital heart disease

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8. Question 8 · Medium

   HbS and HbC both carry a substitution at position 6 of the beta chain. Why does HbC move more slowly toward the anode than HbS at pH 8.6?

   1. AHbC is a much larger molecule than HbS
   2. BThe valine in HbS adds an extra negative charge
   3. CHbC carries gamma chains in place of beta chains
   4. DLysine adds a positive charge where glutamate was lost

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9. Question 9 · Medium

   After a gallstone blocking the common bile duct is removed, serum direct bilirubin may stay raised for 2-3 weeks. This is mainly due to:

   1. AContinued haemolysis of red cells
   2. BA delay in the rise of UGT1A1 activity
   3. CDelta bilirubin bound covalently to albumin
   4. DBilirubin stored in the skin returning

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10. Question 10 · Medium

    Fetal haemoglobin takes up oxygen from maternal blood at the placenta because HbF:

    1. ABinds 2,3-bisphosphoglycerate weakly
    2. BLacks the proximal histidine of haem
    3. CHas four alpha chains instead of two
    4. DContains iron in the ferric state

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11. Question 11 · Easy

    When haem oxygenase opens the haem ring in macrophages, iron is released together with which gas?

    1. ANitric oxide
    2. BHydrogen sulphide
    3. CCarbon monoxide
    4. DCarbon dioxide

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12. Question 12 · Hard

    Serum bilirubin by a diazo method: a 10 mg/dL standard gives absorbance 0.400. A patient's total tube reads 0.260 and the direct tube reads 0.100 (both blank-corrected). What is the indirect bilirubin?

    1. A6.5 mg/dL
    2. B1.6 mg/dL
    3. C4.0 mg/dL
    4. D2.5 mg/dL

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13. Question 13 · Hard

    A 40-year-old man has worked for 12 years in a battery recycling unit. He has colicky abdominal pain and wrist weakness. Hb 9.8 g/dL, MCV 76 fL, blood film shows coarse basophilic stippling, serum ferritin 160 ng/mL (reference 30-300). Urine ALA is raised and urine porphobilinogen is normal. What is the most likely cause?

    1. AIron deficiency anaemia
    2. BLead poisoning
    3. CAcute intermittent porphyria
    4. DBeta-thalassaemia trait

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14. Question 14 · Easy

    Which enzyme of the haem pathway is the most sensitive to inhibition by lead?

    1. AALA dehydratase
    2. BALA synthase
    3. CUroporphyrinogen decarboxylase
    4. DPBG deaminase

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15. Question 15 · Easy

    The first reaction of haem synthesis condenses glycine with:

    1. AAcetyl-CoA
    2. BMalonyl-CoA
    3. CSuccinyl-CoA
    4. DOxaloacetate

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16. Question 16 · Medium

    Why is the final blue azobilirubin of the Jendrassik-Grof procedure read at about 600 nm rather than in the red-purple range?

    1. ACaffeine absorbs strongly below 600 nm
    2. BThe diazo reagent itself is blue at that wavelength
    3. CUnconjugated bilirubin reacts only above 600 nm
    4. DHaemoglobin and carotenes absorb little there

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17. Question 17 · Medium

    Phototherapy lowers bilirubin in newborns because blue light:

    1. ADestroys bilirubin bound in the brain
    2. BSpeeds up the breakdown of fetal red cells
    3. CConverts bilirubin into water-soluble isomers
    4. DInduces UDP-glucuronyltransferase in the liver

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18. Question 18 · Hard

    A 34-year-old woman is admitted with severe abdominal pain, constipation, tachycardia and confusion three days after starting a sulphonamide. Serum sodium is 124 mmol/L. Her fresh urine looks normal but turns dark red-brown after standing in sunlight. The Ehrlich colour stays in the aqueous layer after chloroform extraction. Deficiency of which enzyme is most likely?

    1. AFerrochelatase
    2. BGlucose-6-phosphate dehydrogenase
    3. CPorphobilinogen deaminase
    4. DUroporphyrinogen decarboxylase

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19. Question 19 · Medium

    Physiological jaundice appears in many healthy newborns on day 2-3 mainly because:

    1. AHepatic UGT1A1 activity is still low at birth
    2. BBreast milk contains conjugated bilirubin
    3. CThe neonatal kidney excretes no urobilinogen
    4. DBiliary atresia is present in most neonates

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20. Question 20 · Hard

    A 45-year-old man is brought from a closed room where a charcoal stove was burning. He has headache and confusion. Pulse oximetry shows SpO2 98% on room air, but arterial blood gas shows lactate 5.2 mmol/L. Which test will best confirm the cause?

    1. ARepeat pulse oximetry on another finger
    2. BCarboxyhaemoglobin by CO-oximetry
    3. CSerum cyanide by gas chromatography
    4. DMethaemoglobin by Evelyn-Malloy method

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21. Question 21 · Easy

    Yellow colour of the sclera and skin usually becomes visible once serum total bilirubin rises above about:

    1. A5-6 mg/dL
    2. B0.5-1 mg/dL
    3. C2-3 mg/dL
    4. D10-12 mg/dL

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22. Question 22 · Hard

    A 5-week-old baby girl is still jaundiced. Her stools have been pale since birth and her urine stains the nappy yellow. Total bilirubin 9.4 mg/dL, direct 6.8 mg/dL, GGT 420 U/L (reference up to 160), Hb 11.2 g/dL, reticulocytes 1.5%. What is the most important next step?

    1. AStart phototherapy at home for a week
    2. BUrgent referral to exclude biliary atresia
    3. CRepeat bilirubin after two months
    4. DChange from breast milk to formula

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23. Question 23 · Hard

    A 9-year-old boy with G6PD deficiency has a sudden fall of Hb from 12.4 to 7.8 g/dL after eating fava beans. Reticulocytes 8%, LDH raised, haptoglobin undetectable, total bilirubin 4.6 mg/dL, direct 0.4 mg/dL. Which urine finding fits this picture?

    1. ABilirubin positive, urobilinogen absent
    2. BBilirubin positive, urobilinogen increased
    3. CBilirubin negative, urobilinogen increased
    4. DBilirubin negative, urobilinogen absent

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24. Question 24 · Hard

    A 6-year-old boy with dapsone-induced methaemoglobinaemia (MetHb 32%) is given intravenous methylene blue. MetHb does not fall, and his Hb drops from 11.5 to 8.9 g/dL over the next day with bite cells on the film. Which underlying condition best explains this?

    1. AG6PD deficiency
    2. BHaemoglobin M disease
    3. CPyruvate kinase deficiency
    4. DNADH-cytochrome b5 reductase deficiency

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25. Question 25 · Easy

    Conjugation of bilirubin in the liver cell is carried out by:

    1. AHaem oxygenase
    2. BGlutathione S-transferase
    3. CBiliverdin reductase
    4. DUDP-glucuronyltransferase

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