Block 10 Heme Purine Pyrimidine MCQ's

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Block 10 Heme Purine Pyrimidine MCQ

Questions and Answers
  • 1. 

    A 44-year old woman presents to her physician with loss of appetite, fatigue, muscle weakness and depression. Initial blood analysis shows she has anemia. A bone marrow aspirate confirmed the suspicion that she has a megaloblastic anemia. Serum folic acid level was 2.9 ng/m1 (normal= 6-15), Her serum B12 level was 153 pg/ml (normal=150-75C). The patient's      megaloblastic anemia is most likely caused by

    • A.

      A decreased synthesis of rnethionine

    • B.

      A decreased conversion of dUMP to dTMP

    • C.

      A decrease in the synthesis of oxaloacetate

    • D.

      A decrease in the leves of succinyl CoA.

    • E.

      A decreased synthesis of dUMP.

    Correct Answer
    B. A decreased conversion of dUMP to dTMP
    Explanation
    The patient's megaloblastic anemia is most likely caused by a decreased conversion of dUMP to dTMP. Megaloblastic anemia is characterized by the presence of large, immature red blood cells due to impaired DNA synthesis. The conversion of dUMP to dTMP is an essential step in DNA synthesis, and a deficiency in this process can lead to impaired cell division and maturation, resulting in megaloblastic anemia. The decreased levels of serum folic acid and normal levels of serum B12 further support this explanation, as both folic acid and B12 are required for proper dUMP to dTMP conversion.

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  • 2. 

    A deficiency in thiamine would most likely lead to which of the following clinical manifestations?

    • A.

      A decrease in carboxylase activity

    • B.

      A decrease in serum lactate concentrations

    • C.

      A decrease in red blood cell tranketolase activity

    • D.

      An increase in urinary methylmalonic acid

    • E.

      An increase in prothrornbin time

    Correct Answer
    C. A decrease in red blood cell tranketolase activity
    Explanation
    A deficiency in thiamine would most likely lead to a decrease in red blood cell tranketolase activity. Thiamine is an essential nutrient that is required for the activation of several enzymes, including tranketolase. Tranketolase is involved in the metabolism of carbohydrates, and a decrease in its activity can result in impaired energy production and the development of symptoms such as fatigue, weakness, and anemia. Therefore, a deficiency in thiamine would be expected to cause a decrease in red blood cell tranketolase activity.

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  • 3. 

    An apparently healthy 5-month-old female infant was brought to the pediatrician's office by her mother with the complaints of periodic bouts of vomiting and failure to gain weight. The mother reported that the child would also have periods of irritability and lethargy. Blood analysis revealed increased levels of plasma ammonia and glutamine coupled with decreased plasma citrulline. Orotic acid was found in her urine. This child is most likely to have a deficiency of:

    • A.

      N-acetyl glutamate synthetase

    • B.

      Carbamoyl phosphate synthetase I

    • C.

      Ornithine transcarbamoylase

    • D.

      Carbamoyl phosphate synthetase II

    • E.

      Arginosuccinate lyase

    Correct Answer
    C. Ornithine transcarbamoylase
    Explanation
    This 5-month-old female infant is most likely to have a deficiency of ornithine transcarbamoylase. The symptoms of periodic bouts of vomiting, failure to gain weight, irritability, and lethargy, along with increased levels of plasma ammonia and glutamine, decreased plasma citrulline, and orotic acid in the urine, are indicative of a urea cycle disorder. Ornithine transcarbamoylase deficiency is the most common urea cycle disorder and results in the accumulation of ammonia and other toxic substances in the body.

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  • 4. 

    A deficiency of which of the labeled substances is responsible for the symptoms that are associated with severe combined immunodeficiency syndrome?

    • A.

      A

    • B.

      B

    • C.

      C

    • D.

      D

    • E.

      E

    Correct Answer
    B. B
    Explanation
    A deficiency of substance B is responsible for the symptoms associated with severe combined immunodeficiency syndrome.

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  • 5. 

    A 52-year-old woman presented to the emergency room with severe generalized abdominal pains. She had been vomiting, was constipated and had a distended abdomen. Her history showed that she had recently been prescribed gresiofulvin and sulfonamide for a persistent infection. Urine analysis revealed a remarkably elevated level of porphobilinogen. The drugs were discontinued arid the patient's condition improved. The most likely cause of the patient's difficulty was:

    • A.

      A deficiency in one of the heme biosynthesis enzymes located late in the pathway after synthesis of porphyrin

    • B.

      A deficiency in uroporphyrinogen III cosynthase, leading to the appearance of hydroxyrnethylbilane in the urine

    • C.

      Abnormally low activity of delta-aminolevulinate (6-ALA) dehyciratase, leading to accumulation of 6-ALA in the urine

    • D.

      Abnormally high 6-ALA synthase activity, leading to elevated steady state production of porphobilinogen which was excreted in the urine

    Correct Answer
    D. Abnormally high 6-ALA synthase activity, leading to elevated steady state production of porphobilinogen which was excreted in the urine
    Explanation
    The patient's presentation of severe abdominal pains, vomiting, constipation, and distended abdomen, along with the remarkably elevated level of porphobilinogen in the urine, suggests a diagnosis of acute intermittent porphyria (AIP). AIP is a disorder characterized by a deficiency in one of the heme biosynthesis enzymes, specifically uroporphyrinogen III cosynthase. This deficiency leads to the accumulation of porphobilinogen, which is excreted in the urine. The drugs gresiofulvin and sulfonamide can induce the synthesis of 6-ALA synthase, resulting in increased production of porphobilinogen and exacerbating the symptoms of AIP. Discontinuing the drugs improved the patient's condition, supporting the explanation.

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  • 6. 

    A 21-year-old healthy male celebrates his 21st birthday with friends and because of the occasion they persuade him to have a beer. After consuming the beer he begins to get intense abdominal pain which worsens with time. His friends take him to the emergency because he begins to suffer nausea and vomiting. Upon arrival he is noted to be anxious, has tachycardia and hypertension. Laboratory analysis reveals elevated serum and urine aminolevulinic (ALA) and porphobilinogen (PBG). He most likely has which of the following enzyme deficiency?

    • A.

      Uroporphyrinogen decarboxylase

    • B.

      Delta-ALA dehydratase

    • C.

      Delta-ALA synthase

    • D.

      PBG-Deaminase

    • E.

      Ferrochelatase

    Correct Answer
    D. PBG-Deaminase
    Explanation
    The patient's symptoms, including intense abdominal pain, nausea, vomiting, anxiety, tachycardia, and hypertension, along with elevated serum and urine aminolevulinic acid (ALA) and porphobilinogen (PBG) levels, are consistent with acute intermittent porphyria (AIP). AIP is caused by a deficiency of the enzyme PBG-deaminase, which leads to the accumulation of ALA and PBG. This enzyme is responsible for the conversion of PBG to hydroxymethylbilane. The accumulation of ALA and PBG can cause neurovisceral symptoms, such as abdominal pain and autonomic dysfunction, which the patient is experiencing. Therefore, the most likely enzyme deficiency in this patient is PBG-deaminase.

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  • 7. 

    A chemotherapeutic agent that affects both pyrimidine metabolism and RNA is?

    • A.

      Methotrexate

    • B.

      Leflunomide

    • C.

      Azaserine

    • D.

      Hydroxyurea

    • E.

      5-Fluorouracil

    Correct Answer
    E. 5-Fluorouracil
    Explanation
    5-Fluorouracil is a chemotherapeutic agent that affects both pyrimidine metabolism and RNA. It inhibits the enzyme thymidylate synthase, which is involved in the synthesis of thymidine, a pyrimidine nucleotide. By inhibiting this enzyme, 5-Fluorouracil disrupts pyrimidine metabolism. Additionally, it is converted to its active form, 5-fluorodeoxyuridine monophosphate (FdUMP), which covalently binds to the enzyme DNA polymerase and RNA polymerase, leading to inhibition of DNA and RNA synthesis. Therefore, 5-Fluorouracil affects both pyrimidine metabolism and RNA synthesis, making it the correct answer.

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  • 8. 

    A 48-year-old man who has been a heavy drinker of alcohol for the past three years presents to the physician with what looks like a red rash all over the exposed parts of his skin. The physician orders a urine sample for analysis. He collects the specimen and in the laboratory the technician notices that it has turned color from yellow to pink under fluorescent light. Which of the following enzymes is most likely deficient in this patient?

    • A.

      Uroporphyrinogen decarboxylase

    • B.

      Porphobilinogen deaminase

    • C.

      Mitochondrial ferrochelatase

    • D.

      ALA synthase

    • E.

      UDP glucoronyl transferase

    Correct Answer
    A. Uroporphyrinogen decarboxylase
    Explanation
    The patient's symptoms of a red rash and the urine sample turning pink under fluorescent light suggest a diagnosis of porphyria, a group of inherited or acquired disorders that affect the production of heme. One type of porphyria, known as porphyria cutanea tarda (PCT), is characterized by photosensitivity and skin manifestations. PCT is caused by a deficiency in the enzyme uroporphyrinogen decarboxylase, which leads to the accumulation of uroporphyrinogen in the body. This accumulation can cause the characteristic skin symptoms seen in this patient. Therefore, the most likely enzyme deficient in this patient is uroporphyrinogen decarboxylase.

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  • 9. 

    A-2-month-old infant is brought to the pediatrician because his mother notices that he has a cough which is not getting better even on over the counter (OTC) medication. On physical examination the doctor sees that the child also has a diaper rash which she considers to be very bad and notes that he is below the 5'h percentile for all growth curves, A chest radiograph reveals that the child has pneumonia and a complete blood cell count shows that lymphocytes are absent. Which of the following enzymes would most likely be deficient in this infant?

    • A.

      Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

    • B.

      Dihydroorotate dehyclrogenase

    • C.

      Xanthine oxidase

    • D.

      Purine nucleoside phosphorylase

    • E.

      Adenosine deaminase

    Correct Answer
    E. Adenosine deaminase
    Explanation
    The infant in this scenario is presenting with multiple symptoms, including a persistent cough, diaper rash, poor growth, pneumonia, and absence of lymphocytes in the complete blood cell count. These findings are consistent with severe combined immunodeficiency (SCID), a group of disorders characterized by impaired immune function. Adenosine deaminase (ADA) deficiency is a specific type of SCID that results in the accumulation of toxic metabolites, leading to the destruction of lymphocytes. Therefore, the most likely enzyme deficiency in this infant is adenosine deaminase.

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  • 10. 

    A 53-year-old man presents to the emergency room. He tells the physician that he has had a 10-day history of anorexia, nausea and flu-type symptoms. He commented that his urine was dark in color for the past three days. A physical examination revealed tenderness in the right upper quadrant of the abdomen. A urine dipstick test was positive for bilirubin and urobilinogen. Liver function test were as follows: This patient most likely has:

    • A.

      Liver cirrhosis

    • B.

      Obstructive jaundice

    • C.

      Hemolytic anemia

    • D.

      Dubin-Johnson syndrome

    • E.

      Viral hepatitis

    Correct Answer
    E. Viral hepatitis
    Explanation
    The patient's symptoms, including anorexia, nausea, flu-like symptoms, dark urine, and tenderness in the right upper quadrant of the abdomen, along with positive findings of bilirubin and urobilinogen on the urine dipstick test, suggest liver involvement. The most likely cause of these symptoms is viral hepatitis, which is an inflammation of the liver caused by a viral infection. This explanation is supported by the patient's history of flu-like symptoms and the positive findings on the urine dipstick test, which indicate liver dysfunction.

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  • 11. 

    A medical student recovering from an attack of influenza was noticed to be slightly jaundiced. Worried that he might have hepatitis the student has a blood sample taken for biochemical analysis. The results are shown below:

    • A.

      Cholestasis

    • B.

      Hemolytic jaundice

    • C.

      Excess alcohol consumption

    • D.

      Gilbert's syndrome

    • E.

      Dubln-Johnson syndrome

    Correct Answer
    D. Gilbert's syndrome
    Explanation
    The medical student in this case is slightly jaundiced, which means they have a yellowing of the skin and eyes. The blood sample taken for biochemical analysis reveals Gilbert's syndrome as the cause of the jaundice. Gilbert's syndrome is a genetic condition that affects the liver's ability to process bilirubin, a substance produced when red blood cells are broken down. This leads to a buildup of bilirubin in the blood, resulting in jaundice. It is a benign condition and does not cause any significant health problems.

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  • 12. 

    A child with penicillin allergy is given a sulphonamide against otitis media. Human cells are not affected by sulphonamides because sulphonamides are specific for bacterial

    • A.

      DNA polymerases

    • B.

      RNA poymerases

    • C.

      Ribonucleotide reductase

    • D.

      Mismatch repair

    • E.

      Folate synthesis

    Correct Answer
    E. Folate synthesis
    Explanation
    Sulphonamides are specific for bacterial folate synthesis. They inhibit the enzyme dihydropteroate synthase, which is involved in the synthesis of folate in bacteria. Human cells do not synthesize folate in the same way, so they are not affected by sulphonamides. This allows sulphonamides to selectively target and inhibit bacterial growth without harming human cells.

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  • 13. 

    You are a neonatologist working in a tertiary care hospital in a state capital. A 4-week old male is transferred to you with severe jaundice that appeared at birth and has been worsening ever since. The boy is the first child of a healthy Jewish couple, pregnancy and vaginal delivery were unremarkable. The boy is of average height and weight for his age, in no acute distress, but shows marked jaundice and slight hepatomegaly. Lab: CBC normal, indirect bilirubin high, low fecal urobilinogen. What is the most likely diagnosis?

    • A.

      Neonatal jaundice

    • B.

      Dubin-Johnson syndrome

    • C.

      Rotor (-Manahan-Florentin) syndrome

    • D.

      Glucose-6-phosphate dehydrogenase deficiency

    • E.

      Uridyl glucuronyl transferase deficiency

    Correct Answer
    E. Uridyl glucuronyl transferase deficiency
    Explanation
    The most likely diagnosis for the 4-week old male with severe jaundice, high indirect bilirubin, and low fecal urobilinogen is uridyl glucuronyl transferase deficiency. This condition, also known as Gilbert syndrome, is a genetic disorder that affects the liver's ability to process bilirubin. It is more common in individuals of Jewish descent. The normal CBC and absence of acute distress suggest a chronic condition rather than an acute infection or hemolytic disease. Other options such as Dubin-Johnson syndrome, Rotor syndrome, and glucose-6-phosphate dehydrogenase deficiency are less likely based on the given information.

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  • Current Version
  • Mar 21, 2023
    Quiz Edited by
    ProProfs Editorial Team
  • Jun 15, 2012
    Quiz Created by
    Chachelly
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