Heme Synthesis and Porphyrias MCQs

Biochemistry · 37 free questions with answers & explanations.

1. A 35-year-old woman develops acute abdominal pain, ascending motor neuropathy, and psychiatric symptoms after starting oral contraceptives. Her urine turns dark on exposure to light. Elevated urinary porphobilinogen (PBG) and delta-aminolevulinic acid (ALA) confirm acute intermittent porphyria (AIP). The deficient enzyme in AIP is:
2. Lead poisoning causes a hypochromic microcytic anemia with basophilic stippling of erythrocytes. The primary biochemical mechanism of lead-induced anemia involves inhibition of which two enzymes in heme synthesis?
3. A 32-year-old woman presents with recurrent episodes of severe abdominal pain, peripheral motor neuropathy, and psychiatric symptoms precipitated by oral contraceptive pill use. Urine porphobilinogen (PBG) and delta-aminolevulinic acid (ALA) are markedly elevated during attacks. The enzyme deficiency responsible is:
4. Porphyria cutanea tarda (PCT) is the most common porphyria, presenting with photosensitive blistering skin lesions and hypertrichosis. Urine shows elevated uroporphyrins. The deficient enzyme and its biochemical consequence are:
5. Acute intermittent porphyria (AIP) results from porphobilinogen deaminase (PBGD) deficiency. Acute attacks are precipitated by drugs (barbiturates, griseofulvin), fasting, and infections. Which mechanism links these precipitants to the acute attack?
6. A patient with blistering photosensitivity, hypertrichosis, hyperpigmentation, and friable skin presents in their 40s. Urine fluoresces pink-orange under Wood's lamp. Liver biopsy shows iron overload. Uroporphyrinogen decarboxylase (UROD) activity is markedly reduced. What is the most likely diagnosis, and what is the first-line treatment?
7. Acute intermittent porphyria (AIP) is caused by deficiency of porphobilinogen deaminase (PBGD, also called hydroxymethylbilane synthase). Attacks are precipitated by drugs, fasting, and hormones. A patient in an acute attack has abdominal pain, hyponatremia (SIADH), and peripheral neuropathy with preserved skin findings. Which laboratory finding is PATHOGNOMONIC during an AIP attack?
8. Delta-aminolevulinic acid synthase (ALAS) is the rate-limiting enzyme of heme synthesis and is feedback-inhibited by heme. In the liver, ALAS1 (housekeeping) mRNA contains an iron-responsive element (IRE) in the 5'-UTR. When iron stores are high, iron regulatory protein (IRP) releases the IRE, allowing translation. How does glucose (high carbohydrate intake) suppress hepatic ALAS1 expression in acute porphyria?
9. Lead poisoning inhibits two enzymes in the heme biosynthetic pathway: delta-aminolevulinic acid dehydratase (ALAD) and ferrochelatase. ALAD converts 2 molecules of ALA to porphobilinogen (PBG). Ferrochelatase inserts Fe2+ into protoporphyrin IX. Which laboratory findings would be EXPECTED in a child with significant lead exposure?
10. A 30-year-old woman presents with recurrent abdominal pain, psychosis, and peripheral neuropathy. Her urine turns dark red on standing. No skin photosensitivity. Urine and fecal porphyrins: markedly elevated ALA and PBG in urine. The most likely diagnosis and deficient enzyme are:
11. In lead poisoning, heme synthesis is impaired at two enzymatic steps. Which enzymes are inhibited by lead, and what accumulates as a diagnostic marker in erythrocytes?
12. Porphyria cutanea tarda (PCT) is the most common porphyria. The deficient enzyme causes cutaneous blistering in sun-exposed areas but NO neurological symptoms. Which drug is a classical precipitant of symptomatic PCT?
13. A 32-year-old woman presents with recurrent episodes of acute abdominal pain, psychiatric symptoms, and motor neuropathy triggered by fasting and oral contraceptive pills. Urine turns dark on standing. Urine aminolevulinic acid (ALA) and porphobilinogen (PBG) are markedly elevated. Which enzyme is deficient?
14. Lead poisoning inhibits two specific enzymes of heme synthesis. Which two enzymes are affected, and what laboratory findings does each inhibition produce?
15. A 30-year-old woman presents with acute attacks of severe abdominal pain, motor neuropathy, hyponatremia, and dark urine after starting oral contraceptives. Urine shows elevated porphobilinogen (PBG) and delta-aminolevulinic acid (ALA). The diagnosis is acute intermittent porphyria (AIP). What enzyme is deficient, and why do hormonal medications precipitate attacks?
16. Porphyria cutanea tarda (PCT) is the most common porphyria. It presents with photosensitivity, blistering skin lesions, and hypertrichosis. Urine shows elevated uroporphyrin. Which enzyme is deficient and which is the most common precipitant in adults?
17. A 28-year-old woman presents with acute abdominal pain, confusion, hyponatremia, and reddish urine after starting oral contraceptive pills. Urine porphobilinogen (PBG) is markedly elevated. The molecular mechanism by which OCP triggers an acute attack involves:
18. Erythropoietic protoporphyria (EPP) is caused by ferrochelatase deficiency. The accumulated photosensitising compound and the characteristic feature distinguishing it from other porphyrias is:
19. In porphyria cutanea tarda (PCT), the diagnostic finding on urine fluorescence/Woods lamp examination is pink-red fluorescence due to accumulation of which compound?
20. Lead poisoning inhibits two enzymes in heme synthesis, causing accumulation of ALA and zinc protoporphyrin (ZPP). The two enzymes inhibited by lead are:
21. Congenital erythropoietic porphyria (CEP, Gunther's disease) presents with severe photosensitivity, haemolytic anaemia, and reddish-brown urine in infancy. The enzyme deficient in CEP and the accumulating porphyrin are:
22. A 30-year-old woman presents with recurrent episodes of severe abdominal pain, peripheral neuropathy, and confusion, precipitated by oral contraceptives. Urine turns dark on standing. Urine porphobilinogen (PBG) is markedly elevated. Which enzyme is deficient?
23. Acute intermittent porphyria (AIP) is caused by porphobilinogen deaminase (HMBS) deficiency. Why do AIP attacks not cause photosensitivity despite other porphyrias causing it?
24. A 35-year-old man presents with acute abdominal pain, neuropsychiatric symptoms, and dark port-wine urine after starting phenobarbital. Urine Watson-Schwartz test is positive for porphobilinogen (PBG). The diagnosis is acute intermittent porphyria (AIP). The enzyme deficient is:
25. Lead poisoning inhibits two enzymes in heme synthesis: delta-aminolevulinate dehydratase (ALAD) and ferrochelatase. The resulting accumulation of protoporphyrin IX (measured as erythrocyte protoporphyrin or EP/ZPP) is used as a screening test. Which laboratory finding is specifically caused by ALAD inhibition?
26. Acute Intermittent Porphyria (AIP) presents with abdominal pain, neuropsychiatric symptoms, and autonomic dysfunction. Urine analysis shows elevated ALA and PBG. Why does glucose infusion and heme arginate alleviate acute attacks?
27. A patient presents with acute abdominal pain, peripheral neuropathy, and red-brown urine precipitated by sulphonamide use. Urine shows markedly elevated ALA and PBG. The diagnosis is Acute Intermittent Porphyria (AIP). The enzyme deficient in AIP is:
28. Lead poisoning inhibits two enzymes in the haem biosynthetic pathway. Which pair of enzymes is inhibited, and what diagnostic metabolites accumulate?
29. A 32-year-old woman presents with recurrent episodes of severe abdominal pain, ascending motor neuropathy, dark urine, and confusion. Episodes are precipitated by oral contraceptive pills. Urinary ALA and PBG are markedly elevated. This is most consistent with acute intermittent porphyria (AIP). The molecular rationale for OCP being a trigger is:
30. Acute intermittent porphyria (AIP) causes acute neurovisceral attacks. Which of the following correctly describes the enzyme defect and the compound accumulating to toxic levels?
31. Acute intermittent porphyria (AIP) is caused by porphobilinogen deaminase (PBGD) deficiency. Drug-precipitated attacks cause abdominal pain and neuropsychiatric symptoms. The MECHANISM by which certain drugs (e.g., barbiturates) precipitate AIP attacks is:
32. Acute intermittent porphyria (AIP) presents with abdominal pain, neuropsychiatric features, and autonomic dysfunction but NO skin photosensitivity. The enzymatic defect is in porphobilinogen deaminase (PBGD). Why is there no photosensitivity in AIP?
33. Erythropoietic protoporphyria (EPP) causes severe burning photosensitivity in children exposed to sunlight. The enzyme defect leads to accumulation of protoporphyrin IX in red blood cells. Which enzyme is deficient in EPP?
34. Acute intermittent porphyria (AIP) is caused by deficiency of porphobilinogen (PBG) deaminase. Attacks are precipitated by drugs that induce hepatic ALA synthase (ALAS1). Which drug is SAFEST in a patient with known AIP?
35. Porphyria cutanea tarda (PCT) is characterised by skin photosensitivity and blistering. The enzyme deficient is:
36. A 30-year-old woman presents with acute attacks of abdominal pain, autonomic dysfunction, and peripheral neuropathy after starting an oral contraceptive pill. Urine is dark red. The enzyme deficient in this condition is:
37. In porphyria cutanea tarda (PCT), the enzyme deficiency leads to accumulation of which compound that causes photosensitisation?