1. A researcher is investigating the effect of an enzyme whose reaction product is crucial for de novo fatty acid synthesis. Which of the following enzymes is most likely the focus of this study?
A. Acyl CoA Dehydrogenase
B. Glucose-6-Phosphatase
C. Glucose-6-Phosphate Dehydrogenase (G-6PD)
D. Lactate Dehydrogenase
E. Phosphofructokinase-1 (PFK-1)
Correct Answer – C. Glucose-6-Phosphate Dehydrogenase (G-6PD): G-6PD is a key enzyme in the Hexose Monophosphate (HMP) pathway, producing NADPH as a reaction product. NADPH is essential for de novo fatty acid synthesis, providing the reducing power needed for lipid biosynthesis.
Incorrect options
A. Acyl CoA Dehydrogenase: This enzyme is involved in fatty acid β-oxidation, not synthesis, and produces FADH2 as a byproduct. It does not supply NADPH or directly contribute to fatty acid synthesis.
B. Glucose-6-Phosphatase: Involved in gluconeogenesis and glucose homeostasis, this enzyme does not contribute to the HMP pathway or fatty acid synthesis.
D. Lactate Dehydrogenase: Lactate dehydrogenase is part of anaerobic glycolysis, converting pyruvate to lactate, and does not play a role in NADPH production or fatty acid synthesis.
E. Phosphofructokinase-1 (PFK-1): A glycolytic enzyme, PFK-1 regulates carbohydrate breakdown but does not produce NADPH or link directly to fatty acid synthesis.
2. A 5-year-old boy is brought to the clinic with symptoms of persistent fatigue, pallor, and easy bruising. His parents report he has been unusually tired and less active over the past few weeks. On examination, the child is found to have mild splenomegaly. Laboratory tests reveal anemia, a high reticulocyte count, and elevated white blood cell count, consistent with increased cellular turnover. Which of the following enzymes is most likely supplying phosphorylated ribose to meet this demand?
A. 6-Phosphogluconate Dehydrogenase
B. Transaldolase
C. Transketolase
D. Glucose-6-Phosphate Dehydrogenase
E. Phosphopentose Isomerase
F. Phosphopentose Epimerase
Correct Answer – E. Phosphopentose Isomerase: Phosphopentose isomerase is a crucial enzyme in the pentose phosphate pathway, converting ribulose-5-phosphate to ribose-5-phosphate. Ribose-5-phosphate is a phosphorylated ribose necessary for nucleotide synthesis, essential for the rapid cell division occurring in the bone marrow of this patient.
Incorrect options
A. 6-Phosphogluconate Dehydrogenase: This enzyme is involved in producing NADPH in the pentose phosphate pathway but does not directly provide ribose-5-phosphate.
B. Transaldolase: Transaldolase participates in sugar rearrangement in the non-oxidative branch of the pentose phosphate pathway but does not produce ribose-5-phosphate directly.
C. Transketolase: Transketolase assists in rearranging sugars within the pentose phosphate pathway but does not directly produce ribose-5-phosphate needed for nucleotide synthesis.
D. Glucose-6-Phosphate Dehydrogenase: This rate-limiting enzyme in the pentose phosphate pathway generates NADPH but does not directly produce ribose-5-phosphate, crucial for nucleotide synthesis in rapidly dividing cells.
F. Phosphopentose Epimerase: Phosphopentose epimerase converts ribulose-5-phosphate to xylulose-5-phosphate, another intermediate in the pentose phosphate pathway, but does not produce ribose-5-phosphate directly.
3. The oxidation of 3 mol of glucose by the pentose phosphate pathway may result in the production of which of the following?
A. 2 mol of pentose, 4 mol of NADPH, and 8 mol of CO₂
B. 3 mol of pentose, 4 mol of NADPH, and 3 mol of CO₂
C. 3 mol of pentose, 6 mol of NADPH, and 3 mol of CO₂
D. 4 mol of pentose, 3 mol of NADPH, and 3 mol of CO₂
E. 4 mol of pentose, 6 mol of NADPH, and 6 mol of CO₂
Correct Answer – C. 3 mol of pentose, 6 mol of NADPH, and 3 mol of CO₂. In the pentose phosphate pathway, each molecule of glucose produces 2 mol of NADPH and 1 mol of CO₂ during its oxidative phase. Therefore, the oxidation of 3 mol of glucose would yield:
• 3 mol of pentose (ribulose-5-phosphate) as the end product of the pathway,
• 6 mol of NADPH (since each glucose provides 2 mol of NADPH),
• 3 mol of CO₂ (one per glucose molecule oxidized).
Incorrect options
The other options do not match this stoichiometry. A, B, D, and E: These options have incorrect amounts of NADPH, CO₂, or pentose production based on the pathway’s stoichiometry for 3 mol of glucose.
4. A 52-year-old woman presents to the clinic for evaluation of fatigue and easy bruising. Laboratory tests reveal a deficiency in NADPH-dependent glutathione reductase activity, leading to oxidative stress in her red blood cells. To compensate, her body has upregulated the pentose phosphate pathway to increase NADPH production. Given this metabolic shift, C-1 of glucose in her red blood cells would most likely contribute to which of the following products?
A. Carbon dioxide
B. Glyceraldehyde-3-Phosphate
C. Fructose-6-Phosphate
D. Sedoheptulose-7-Phosphate
E. Ribulose-5-Phosphate
Correct Answer – A. Carbon dioxide: In the oxidative phase of the pentose phosphate pathway, C-1 of glucose is specifically oxidized and released as carbon dioxide. This reaction also generates NADPH, which is essential for maintaining the redox balance in red blood cells and supporting glutathione reduction to combat oxidative stress.
Incorrect Options:
B. Glyceraldehyde-3-Phosphate: Glyceraldehyde-3-phosphate is produced in the non-oxidative phase of the pentose phosphate pathway but does not involve the release of C-1 as CO₂.
C. Fructose-6-Phosphate: Fructose-6-phosphate is another product of the non-oxidative phase, and it does not involve the oxidative loss of C-1 as CO₂.
D. Sedoheptulose-7-Phosphate: This sugar phosphate is also produced in the non-oxidative phase of the pathway and does not directly involve the oxidative decarboxylation of C-1.
E. Ribulose-5-Phosphate: Ribulose-5-phosphate is generated in the pentose phosphate pathway, but it forms after the oxidation of C-1 to CO₂ and does not directly involve the release of CO₂ itself.
5. Which of the following is the main function of the pentose phosphate pathway?
A. Provide the cell an alternative pathway should glycolysis fail
B. Provide a mechanism for the direct oxidation of pentoses
C. Salvage of nucleotides
D. Interconversion of hexoses
E. Support reductive biosynthesis and cellular turnover
Correct Answer – E. Support reductive biosynthesis and cellular turnover: The pentose phosphate pathway (PPP) provides NADPH, essential for various biosynthetic reactions and for maintaining cellular antioxidant defenses. Additionally, it supplies ribose-5-phosphate, which is necessary for nucleotide synthesis in dividing cells.
Incorrect Options:
A. Provide the cell an alternative pathway should glycolysis fail: The PPP does not serve as an alternative to glycolysis but functions parallel to it for specific needs.
B. Provide a mechanism for direct oxidation of pentoses: The PPP begins with glucose-6-phosphate, a hexose, and does not primarily oxidize pentoses.
C. Salvage of nucleotides: While the PPP provides ribose-5-phosphate for nucleotide synthesis, it is not involved in the salvage pathway.
D. Interconversion of hexoses: Although sugar interconversions occur in the PPP’s non-oxidative phase, the pathway’s main roles are NADPH production and ribose-5-phosphate generation.
5. A 19-year-old African-American male military recruit is preparing for deployment to Iraq. As a preventive measure against malaria, he receives a prophylactic dose of primaquine. A few days later, he develops fatigue and symptoms of hemolytic anemia. Laboratory tests confirm the presence of hemolysis. Which of the following enzymes is most likely deficient?
A. Fructokinase
B. Aldolase B
C. Glucose-6-Phosphate Dehydrogenase (G6PD)
D. Galactokinase
E. Pyruvate Kinase
Correct Answer – C. Glucose-6-Phosphate Dehydrogenase (G6PD): G6PD deficiency is common among individuals of African, Mediterranean, and Middle Eastern descent. G6PD is an enzyme in the pentose phosphate pathway responsible for producing NADPH, which protects red blood cells from oxidative stress. Certain drugs, like primaquine, can increase oxidative stress, leading to hemolysis in individuals with G6PD deficiency.
Incorrect Options:
A. Fructokinase: Fructokinase deficiency would lead to issues with fructose metabolism but does not cause hemolytic anemia.
B. Aldolase B: Aldolase B deficiency leads to hereditary fructose intolerance and would not cause hemolysis after primaquine administration.
D. Galactokinase: Galactokinase deficiency results in galactosemia, causing cataracts but not hemolytic anemia.
E. Pyruvate Kinase: Pyruvate kinase deficiency can cause hemolytic anemia, but it is not triggered by oxidative drugs like primaquine. Instead, G6PD deficiency is most commonly linked to drug-induced hemolysis.
6. A 50-year-old male with a history of chronic alcohol use presents with pain, numbness, tingling, and weakness in his feet. He is diagnosed with thiamine deficiency. Thiamine, as Thiamine pyrophosphate (TPP), is crucial for an enzyme that catalyzes the oxidative decarboxylation of α-ketoglutarate in the TCA cycle and is also required at one step in the pentose phosphate pathway. Which of the following conversions might be affected?
A. Ribulose-5-phosphate to Ribose-5-phosphate
B. Glucose-6-phosphate to 6-Phosphogluconolactone
C. 6-Phosphogluconate to Ribulose-5-phosphate
D. Xylulose-5-phosphate + Ribose-5-phosphate to Sedoheptulose-7-phosphate + Glyceraldehyde-3-phosphate
E. Fructose-6-phosphate to Glucose-6-phosphate
Correct Answer – D. Xylulose-5-phosphate + Ribose-5-phosphate to Sedoheptulose-7-phosphate + Glyceraldehyde-3-phosphate: This reaction in the non-oxidative phase of the pentose phosphate pathway is catalyzed by transketolase, an enzyme that requires thiamine as a cofactor. Thiamine deficiency would impair transketolase function, affecting this conversion.
Incorrect Options:
A. Ribulose-5-phosphate to Ribose-5-phosphate: This reaction is catalyzed by phosphopentose isomerase, which does not require thiamine.
B. Glucose-6-phosphate to 6-phosphogluconolactone: This step is catalyzed by glucose-6-phosphate dehydrogenase, an enzyme that does not require thiamine.
C. 6-Phosphogluconate to Ribulose-5-phosphate: Catalyzed by 6-phosphogluconate dehydrogenase, this step generates NADPH and CO₂ but does not depend on thiamine.
E. Fructose-6-phosphate to Glucose-6-phosphate: This conversion is part of glycolysis and gluconeogenesis, not the HMP pathway, and does not require thiamine.
7. In the context of the pentose phosphate (HMP) pathway, higher activity of which of the following best meets the metabolic demands of rapidly dividing cancer cells, specifically to supply ribose-5-phosphate for nucleotide synthesis?
A. 6-Phosphogluconate Dehydrogenase
B. Glucose-6-Phosphate Dehydrogenase
C. Non-Oxidative Phase (Reversed)
D. Transaldolase
E. Transketolase
Correct Answer – C. Non-Oxidative Phase (Reversed): Rapidly dividing cancer cells often require large amounts of ribose-5-phosphate for nucleotide synthesis. To meet this demand without excessive NADPH production, they frequently use a reversal of the non-oxidative phase of the pentose phosphate pathway, converting glycolytic intermediates to ribose-5-phosphate.
Incorrect Options:
A. 6-Phosphogluconate Dehydrogenase: This enzyme is part of the oxidative phase, generating NADPH, which is important for redox balance but not the primary need for nucleotide synthesis in cancer cells.
B. Glucose-6-Phosphate Dehydrogenase: This rate-limiting enzyme in the oxidative phase produces NADPH rather than ribose-5-phosphate, so it does not directly support the increased nucleotide synthesis needs.
D. Transaldolase: This enzyme interconverts sugars in the non-oxidative phase, but the phase itself (when reversed) is more critical for nucleotide synthesis.
E. Transketolase: While transketolase plays a role in the non-oxidative phase, the reversal of the entire non-oxidative phase is essential for redirecting glycolytic intermediates toward ribose-5-phosphate production.
8. The pentose phosphate pathway is active in tissues such as the liver, adipose tissue, adrenal cortex, thyroid, erythrocytes, testis, and lactating mammary gland. Why is this pathway less active in skeletal muscle?
A. Muscle tissue contains very small amounts of dehydrogenases
B. Muscle tissue does not require NADPH
C. Pentoses, the major product of this pathway, are not required by muscle tissue
D. Muscle tissue contains very small amounts of enzymes of the non-oxidative phase
E. Muscle tissue primarily relies on glycolysis for its energy needs
Correct Answer – A. Muscle tissue contains very small amounts of dehydrogenases: Skeletal muscle has low levels of enzymes, particularly glucose-6-phosphate dehydrogenase, required for the oxidative phase of the pentose phosphate pathway. This limits the pathway’s overall activity in muscle, reducing NADPH production from this source.
Incorrect Options:
B. Muscle tissue does not require NADPH: Although skeletal muscle’s demand for NADPH is lower than that of tissues actively involved in lipid and steroid synthesis, muscle still needs NADPH for antioxidant defense and some biosynthetic reactions.
C. Pentoses, the major product of this pathway, are not required by muscle tissue: While skeletal muscle does have a lower demand for ribose-5-phosphate compared to rapidly dividing cells, it can still produce ribose-5-phosphate via the reversal of the non-oxidative phase of the pentose phosphate pathway. This allows muscle tissue to generate ribose from glycolytic intermediates as needed.
D. Muscle tissue contains very small amounts of enzymes of the non-oxidative phase: The enzymes of the non-oxidative phase are present in muscle tissue, and it can reverse this phase to produce ribose-5-phosphate from glycolytic intermediates. The lower activity of the pathway in muscle is primarily due to limited oxidative phase activity.
E. Muscle tissue primarily relies on glycolysis for its energy needs: While muscle tissue does rely heavily on glycolysis for ATP, this does not directly explain the reduced activity of the pentose phosphate pathway, which serves specific biosynthetic and antioxidant functions rather than primary energy production.
8. A 28-year-old man of Mediterranean descent presents to the clinic with fatigue, jaundice, and dark-colored urine following a recent respiratory infection. He mentions that his symptoms worsened after he took an over-the-counter medication. Laboratory tests reveal anemia with signs of hemolysis. The physician suspects a deficiency in an enzyme involved in protecting red blood cells from oxidative damage, particularly in the pentose phosphate pathway. Which of the following is the key regulatory enzyme of this pathway?
A. Glucose-6-Phosphate Dehydrogenase
B. Transaldolase
C. Transketolase
D. 6-Phosphogluconate Dehydrogenase
E. Gluconolactone Hydrolase
Correct Answer – A. Glucose-6-Phosphate Dehydrogenase: Glucose-6-phosphate dehydrogenase (G6PD) is the key regulatory enzyme of the pentose phosphate pathway. It catalyzes the rate-limiting step, producing NADPH, which is crucial for protecting red blood cells from oxidative stress by maintaining reduced glutathione. A deficiency in G6PD can lead to hemolytic anemia, especially when triggered by infections, certain medications, or foods that increase oxidative stress.
Incorrect Options:
B. Transaldolase: This enzyme participates in the non-oxidative phase of the pathway, interconverting sugars but not regulating the pathway’s activity.
C. Transketolase: Transketolase also acts in the non-oxidative phase and does not control the pathway’s overall rate.
D. 6-Phosphogluconate Dehydrogenase: Although involved in NADPH production, this enzyme is not the primary regulatory step in the pathway.
E. Gluconolactone Hydrolase: This enzyme converts 6-phosphogluconolactone to 6-phosphogluconate in the pathway, but it is not rate-limiting or regulatory.
9. A 25-year-old man of Middle Eastern descent presents to the clinic with a history of recurrent bacterial infections, particularly affecting his skin and respiratory tract. He reports frequent bouts of pneumonia and skin abscesses. Blood tests reveal a deficiency in an enzyme that is crucial for the respiratory burst in phagocytes. Which of the following enzyme deficiencies is most likely responsible for his symptoms?
A. Glucose-6-Phosphate Dehydrogenase (G6PD)
B. Pyruvate Kinase
C. Hexokinase
D. Lactate Dehydrogenase
E. Glucokinase
Correct Answer – A. Glucose-6-Phosphate Dehydrogenase (G6PD): G6PD deficiency impairs the production of NADPH, which is essential for the respiratory burst in phagocytes. NADPH is required for generating reactive oxygen species that phagocytes use to kill pathogens. Individuals with G6PD deficiency have a reduced ability to produce these reactive oxygen species, making them more susceptible to infections.
Incorrect Options:
B. Pyruvate Kinase: This enzyme is involved in glycolysis and does not produce NADPH, so its deficiency does not impair the respiratory burst in phagocytes.
C. Hexokinase: Hexokinase catalyzes the first step in glycolysis, unrelated to NADPH production.
D. Lactate Dehydrogenase: Lactate dehydrogenase is part of anaerobic glycolysis and does not affect NADPH or the respiratory burst.
E. Glucokinase: Glucokinase is involved in glucose metabolism in the liver and pancreas and does not play a role in NADPH production or immune function.
10. In a patient with glucose-6-phosphate dehydrogenase (G6PD) deficiency, red blood cells are lysed, while other cells of the body remain intact. Which of the following explains this phenomenon?
A. Red blood cells lack mitochondria and are fully dependent on the pentose phosphate pathway for NADPH production.
B. Red blood cells have an increased demand for glucose compared to other cell types.
C. G6PD deficiency only affects red blood cells due to their unique cell membrane structure.
D. Other cells compensate for the lack of NADPH by increasing glycolysis.
Correct Answer – A. Red blood cells lack mitochondria and are fully dependent on the pentose phosphate pathway for NADPH production:
Red blood cells lack mitochondria, so they cannot produce NADPH through other pathways, such as the malic enzyme or mitochondrial electron transport chain. Instead, they rely entirely on the pentose phosphate pathway, where G6PD is the key enzyme for NADPH production. NADPH is essential for maintaining reduced glutathione, which protects red blood cells from oxidative damage. In G6PD deficiency, the lack of NADPH leaves red blood cells vulnerable to oxidative stress, leading to hemolysis.
Incorrect Options:
B. Red blood cells have an increased demand for glucose compared to other cell types: While red blood cells do consume glucose, the key issue in G6PD deficiency is the lack of NADPH production rather than glucose demand.
C. G6PD deficiency only affects red blood cells due to their unique cell membrane structure: The vulnerability of red blood cells is due to their reliance on the pentose phosphate pathway for NADPH, not their membrane structure.
D. Other cells compensate for the lack of NADPH by increasing glycolysis: Glycolysis does not produce NADPH; other cells can produce NADPH through alternate pathways, but glycolysis itself does not compensate for NADPH loss.
12. Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an example of hemolytic anemia that results from the interaction between intracorpuscular and extracorpuscular factors. Which of the following best explains this interaction in the context of G6PD deficiency?
A. G6PD deficiency leads to intrinsic membrane defects in red blood cells that make them fragile.
B. Hemolysis occurs due to the intrinsic deficiency of NADPH in red blood cells combined with exposure to oxidative stress from external sources.
C. The deficiency affects the liver and spleen, leading to systemic oxidative damage in red blood cells.
D. G6PD deficiency is purely intracorpuscular, with hemolysis occurring without external factors.
E. Red blood cells produce excess reactive oxygen species internally, causing hemolysis without any external trigger.
Correct Answer – B. Hemolysis occurs due to the intrinsic deficiency of NADPH in red blood cells combined with exposure to oxidative stress from external sources:
In G6PD deficiency, red blood cells have an intrinsic (intracorpuscular) defect in their ability to produce NADPH, making them susceptible to oxidative damage. When exposed to extracorpuscular oxidative stressors, such as infections, certain drugs (e.g., primaquine), or fava beans, these cells are unable to neutralize reactive oxygen species effectively, leading to hemolysis.
Incorrect Options:
A. G6PD deficiency leads to intrinsic membrane defects in red blood cells that make them fragile: G6PD deficiency does not directly cause structural membrane defects; it primarily affects the cell’s oxidative defense mechanism.
C. The deficiency affects the liver and spleen, leading to systemic oxidative damage in red blood cells: G6PD deficiency is specific to red blood cells and does not directly involve the liver or spleen in causing hemolysis.
D. G6PD deficiency is purely intracorpuscular, with hemolysis occurring without external factors: Although G6PD deficiency is an intracorpuscular defect, hemolysis often requires an extracorpuscular oxidative trigger.
E. Red blood cells produce excess reactive oxygen species internally, causing hemolysis without any external trigger: Red blood cells are not the primary source of reactive oxygen species; oxidative stress usually comes from external factors or metabolic processes exacerbated by G6PD deficiency.
13. A 34-year-old African-American man was admitted with fever and shortness of breath, later developing pancreatitis. He was treated with clindamycin and primaquine. Four days into therapy, he developed hematuria; his hemoglobin dropped from 11.0 g/dl to 7.4 g/dl, and his total bilirubin increased from 1.2 mg/dl to 4.3 mg/dl. Which of the following best
explains the primaquine-induced hemolytic anemia?
A. G6PD deficiency leading to decreased NADPH production
B. Increased oxidative stress from clindamycin
C. Intrinsic red blood cell membrane defect
D. Immune-mediated hemolysis from antibiotic use
E. Inhibition of erythropoiesis by primaquine
Correct Answer – A. G6PD deficiency leading to decreased NADPH production:
G6PD deficiency impairs the production of NADPH in red blood cells, making them susceptible to oxidative damage from primaquine. This results in hemolysis, as the red blood cells lack sufficient NADPH to combat oxidative stress.
Incorrect Options:
B. Increased oxidative stress from clindamycin: While clindamycin has antibiotic properties, it is not typically associated with significant oxidative stress leading to hemolysis.
C. Intrinsic red blood cell membrane defect: G6PD deficiency affects the oxidative pathway rather than the red blood cell membrane structure directly.
D. Immune-mediated hemolysis from antibiotic use: The hemolysis in this case is due to oxidative stress in G6PD-deficient cells, not immune mechanisms.
E. Inhibition of erythropoiesis by primaquine: Primaquine does not inhibit red blood cell production; rather, it increases oxidative stress, which leads to hemolysis in G6PD-deficient individuals.
14. A 22-year-old man of Mediterranean descent presents to the emergency room with fatigue, dark urine, and jaundice. He reports that he felt fine until recently when he started taking an antibiotic prescribed for a respiratory infection. He denies any history of similar symptoms during childhood. Laboratory tests reveal low hemoglobin, elevated bilirubin, and signs of hemolysis. The physician suspects glucose-6-phosphate dehydrogenase (G6PD) deficiency.
Why did this patient’s G6PD deficiency only manifest now rather than earlier in life? Which of the following best explains this question?
A. Higher baseline G6PD activity in newborns
B. Symptoms are triggered by exposure to oxidative stress
C. G6PD deficiency worsens with age
D. Only mature red blood cells are affected
E. Infants have naturally higher antioxidant levels
Correct Answer – B. Symptoms are triggered by exposure to oxidative stress. In G6PD deficiency, symptoms are often latent until the individual is exposed to oxidative stress, such as certain medications, infections, or foods. This patient’s symptoms were triggered by the antibiotic, which induced oxidative stress in his G6PD-deficient red blood cells, leading to hemolysis.
Incorrect options
A. Higher baseline G6PD activity in newborns: While G6PD levels can vary, there is no significant increase in baseline G6PD activity in newborns that would prevent symptoms from occurring. G6PD deficiency can affect individuals of any age when exposed to oxidative stress.
C. G6PD deficiency worsens with age: G6PD deficiency does not progress or worsen with age; it remains a consistent genetic trait. The enzyme deficiency itself does not become more severe over time, but symptoms can appear at any age if oxidative stressors are introduced.
D. Only mature red blood cells are affected: G6PD deficiency affects all red blood cells, regardless of maturity. However, younger red blood cells (reticulocytes) may have slightly higher residual G6PD activity, which provides temporary protection. Still, both mature and immature red blood cells are susceptible to hemolysis under oxidative stress in G6PD deficiency.
E. Infants have naturally higher antioxidant levels: There is no evidence to suggest that infants have significantly higher antioxidant levels than adults. Symptoms of G6PD deficiency are dependent on exposure to oxidative stress rather than innate antioxidant levels.
15. A 5-year-old boy presents with jaundice and fatigue after recovering from a recent viral infection. Laboratory tests reveal hemolytic anemia, and further investigation confirms glucose-6-phosphate dehydrogenase (G6PD) deficiency. His mother mentions that her brother (the boy’s maternal uncle) also has G6PD deficiency, but no other family members are known to have the condition. The mother herself has never shown symptoms.
Which of the following best describes the inheritance pattern of G6PD deficiency?
A. Autosomal dominant
B. Autosomal recessive
C. X-linked dominant
D. X-linked recessive
E. Mitochondrial inheritance
Correct Answer – D. X-linked recessive. G6PD deficiency follows an X-linked recessive inheritance pattern. The gene responsible for this deficiency is located on the X chromosome. Males, like the boy and his maternal uncle, are more likely to show symptoms if they inherit the mutated gene from their mother, as they have only one X chromosome. Females, such as the boy’s mother, typically carry the gene on one of their two X chromosomes but do not show symptoms unless they inherit two copies of the mutated gene or experience X-inactivation in one of the chromosomes. The mother, in this case, is likely a carrier, explaining why her son and her brother are affected while she is asymptomatic.
Incorrect Options:
A. Autosomal dominant: This pattern would require only one mutated copy of a gene on a non-sex chromosome (autosome) to cause symptoms in both males and females, which is not observed here.
B. Autosomal recessive: If G6PD deficiency were autosomal recessive, both sexes would be equally affected and would need two copies of the mutated gene on autosomes to exhibit symptoms.
C. X-linked dominant: With an X-linked dominant inheritance pattern, both males and females would exhibit symptoms if they inherit the mutated gene, which is inconsistent with G6PD deficiency, as it primarily affects males in this family.
E. Mitochondrial inheritance: Mitochondrial inheritance is passed down only from the mother and involves mitochondrial DNA, not the X or Y chromosomes, making this option incorrect for G6PD deficiency.
