A 45-year-old man is brought to the emergency department by ambulance after vomiting blood. The patient reports that he only ate a small snack the morning before and had not eaten anything for over 24 hours. At the hospital, the patient is stabilized. He is admitted to a surgical floor and placed on NPO with a nasogastric tube set to intermittent suction. He has been previously diagnosed with liver cirrhosis. An esophagogastroduodenoscopy (EGD) has been planned for the next afternoon. At the time of endoscopy, some pathways were generating glucose to maintain serum glucose levels. Which of the following enzymes catalyzes the irreversible biochemical reaction of this process?
Q2
To prepare for an endoscopy, a 27-year-old male was asked by the gastroenterologist to fast overnight for his 12 p.m. appointment the next day. Therefore, his last meal was dinner at 5 p.m. the day before the appointment. By 12 p.m. the day of the appointment, his primary source of glucose was being generated from gluconeogenesis, which occurs via the reversal of glycolysis with extra enzymes to bypass the irreversible steps in glycolysis. Which of the following irreversible steps of gluconeogenesis occurs in the mitochondria?
Q3
A 2-year-old boy presents to the emergency department with new onset seizures. After controlling the seizures with fosphenytoin loading, a history is obtained that reveals mild hypotonia and developmental delay since birth. There is also a history of a genetic biochemical disorder on the maternal side but the family does not know the name of the disease. Physical exam is unrevealing and initial lab testing shows a pH of 7.34 with a pCO2 of 31 (normal range 35-45) and a bicarbonate level of 17 mEq/L (normal range 22-28). Further bloodwork shows an accumulation of alanine and pyruvate. A deficiency in which of the following enzymes is most likely responsible for this patient's clinical syndrome?
Q4
A 26-year-old African American man comes to the physician because of a 3-day history of fatigue, back pain, and dark urine. One week ago, he developed a headache and was treated with aspirin. He does not smoke or use illicit drugs. Physical examination shows conjunctival pallor. A peripheral blood smear shows erythrocytes with inclusions of denatured hemoglobin. Which of the following enzymes is involved in providing precursors for nucleotide synthesis in this patient?
Q5
A 12-year-old boy and his siblings are referred to a geneticist for evaluation of a mild but chronic hemolytic anemia that has presented with fatigue, splenomegaly, and scleral icterus. Coombs test is negative and blood smear does not show any abnormal findings. An enzymatic panel is assayed, and pyruvate kinase is found to be mutated on both alleles. The geneticist explains that pyruvate kinase functions in glycolysis and is involved in a classic example of feed-forward regulation. Which of the following metabolites is able to activate pyruvate kinase?
Q6
A 55-year-old man presents to his primary care physician with a complaint of fatigue for a couple of months. He was feeling well during his last visit 6 months ago. He has a history of hypertension for the past 8 years, diabetes mellitus for the past 5 years, and chronic kidney disease (CKD) for a year. The vital signs include: blood pressure 138/84 mm Hg, pulse 81/min, temperature 36.8°C (98.2°F), and respiratory rate 9/min. On physical examination, moderate pallor is noted on the palpebral conjunctiva and nail bed.
Complete blood count results are as follows:
Hemoglobin 8.5 g/dL
RBC 4.2 million cells/µL
Hematocrit 39%
Total leukocyte count 6,500 cells/µL
Neutrophils 61%
Lymphocytes 34%
Monocytes 4%
Eosinophils 1%
Basophils 0%
Platelets 240,000 cells/µL
A basic metabolic panel shows:
Sodium 133 mEq/L
Potassium 5.8 mEq/L
Chloride 101 mEq/L
Bicarbonate 21 mEq/L
Albumin 3.1 mg/dL
Urea nitrogen 31 mg/dL
Creatinine 2.8 mg/dL
Uric acid 6.4 mg/dL
Calcium 8.1 mg/dL
Glucose 111 mg/dL
Which of the following explanations best explains the mechanism for his decreased hemoglobin?
Q7
A 33-year-old woman, gravida 1, para 0, at 26 weeks' gestation comes to the physician for a routine prenatal examination. Her pregnancy has been uneventful. Physical examination shows a uterus consistent in size with a 26-week gestation. She is given an oral 50-g glucose load; 1 hour later, her serum glucose concentration is 116 mg/dL. Which of the following most likely occurred immediately after the entrance of glucose into the patient's pancreatic beta-cells?
Q8
A 6-month-old boy presents with decreased growth, pigmented retinopathy, hemolytic anemia, and peripheral neuropathy. You suspect that these signs are the result of a vitamin deficiency leading to increased oxidative damage to lipids. Which of the following is most likely responsible for this patient's symptoms?
Q9
A 30-year-old man presents with dark urine and fatigue. The patient states that the symptoms started 2 days ago. Since yesterday, he also noticed that his eyes look yellow. The past medical history is significant for recent right ear pain diagnosed 3 days ago as acute otitis media, which he was prescribed trimethoprim-sulfamethoxazole. He currently does not take any other medications on a daily basis. The patient was adopted and has no knowledge of his family history. The vital signs include: temperature 37.0°C (98.6°F), blood pressure 100/75 mm Hg, pulse 105/min, respiratory rate 15/min, and oxygen saturation 100% on room air. On physical exam, the patient is alert and cooperative. The cardiac exam is significant for an early systolic murmur that is best heard at the 2nd intercostal space, midclavicular line. There is scleral icterus present. The peripheral blood smear shows the presence of bite cells and Heinz bodies. Which of the following laboratory findings would most likely be present in this patient?
Q10
An 11-year-old boy is brought to the emergency room with acute abdominal pain and hematuria. Past medical history is significant for malaria. On physical examination, he has jaundice and a generalized pallor. His hemoglobin is 5 g/dL, and his peripheral blood smear reveals fragmented RBC, microspherocytes, and eccentrocytes (bite cells). Which of the following reactions catalyzed by the enzyme is most likely deficient in this patient?
Pay-off phase reactions US Medical PG Practice Questions and MCQs
Question 1: A 45-year-old man is brought to the emergency department by ambulance after vomiting blood. The patient reports that he only ate a small snack the morning before and had not eaten anything for over 24 hours. At the hospital, the patient is stabilized. He is admitted to a surgical floor and placed on NPO with a nasogastric tube set to intermittent suction. He has been previously diagnosed with liver cirrhosis. An esophagogastroduodenoscopy (EGD) has been planned for the next afternoon. At the time of endoscopy, some pathways were generating glucose to maintain serum glucose levels. Which of the following enzymes catalyzes the irreversible biochemical reaction of this process?
A. Glucose-6-phosphate dehydrogenase
B. Glycogen phosphorylase
C. Enolase
D. Glyceraldehyde-3-phosphate dehydrogenase
E. Fructose-1,6-bisphosphatase (Correct Answer)
Explanation: ***Fructose-1,6-bisphosphatase***
- The scenario describes a patient in a fasting state for over 24 hours, during which **gluconeogenesis** is crucial for maintaining blood glucose levels.
- **Fructose-1,6-bisphosphatase** is one of the key regulatory enzymes in gluconeogenesis, catalyzing an **irreversible reaction** that bypasses the phosphofructokinase-1 step of glycolysis.
*Glucose-6-phosphate dehydrogenase*
- This enzyme is involved in the **pentose phosphate pathway**, which generates NADPH and precursors for nucleotide synthesis.
- It does not directly participate in gluconeogenesis to produce glucose from non-carbohydrate sources.
*Glycogen phosphorylase*
- This enzyme is involved in **glycogenolysis**, the breakdown of glycogen into glucose-1-phosphate.
- While it releases glucose, the body's glycogen stores would likely be depleted after over 24 hours of fasting, making gluconeogenesis the primary pathway for glucose production.
*Enolase*
- Enolase is an enzyme in the glycolytic pathway, catalyzing the reversible conversion of 2-phosphoglycerate to phosphoenolpyruvate.
- It is not an enzyme of gluconeogenesis, nor does it catalyze an irreversible step in the glucose production process during fasting.
*Glyceraldehyde-3-phosphate dehydrogenase*
- This enzyme is also part of glycolysis, catalyzing the reversible oxidation and phosphorylation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate.
- Like enolase, it is not an irreversible enzyme in gluconeogenesis that would be generating glucose under fasting conditions.
Question 2: To prepare for an endoscopy, a 27-year-old male was asked by the gastroenterologist to fast overnight for his 12 p.m. appointment the next day. Therefore, his last meal was dinner at 5 p.m. the day before the appointment. By 12 p.m. the day of the appointment, his primary source of glucose was being generated from gluconeogenesis, which occurs via the reversal of glycolysis with extra enzymes to bypass the irreversible steps in glycolysis. Which of the following irreversible steps of gluconeogenesis occurs in the mitochondria?
A. Glucose-6-phosphate to glucose
B. Pyruvate to oxaloacetate (Correct Answer)
C. Phosphoenolypyruvate to pyruvate
D. Glucose-6-phosphate to 6-phosphogluconolactone
E. Fructose-1,6-bisphosphate to fructose-6-phosphate
Explanation: ***Pyruvate to oxaloacetate***
- This step, catalyzed by **pyruvate carboxylase**, is the initial and irreversible step of **gluconeogenesis** that occurs within the **mitochondrial matrix**.
- **Pyruvate** is converted to **oxaloacetate**, which then either is converted to malate to exit the mitochondria or remains in the mitochondria for subsequent steps of gluconeogenesis depending on the tissue.
*Glucose-6-phosphate to glucose*
- This final dephosphorylation step of gluconeogenesis, catalyzed by **glucose-6-phosphatase**, occurs in the **endoplasmic reticulum** lumen, not the mitochondria.
- It is crucial for releasing free glucose into the bloodstream.
*Phosphoenolypyruvate to pyruvate*
- This is an irreversible step in **glycolysis**, catalyzed by **pyruvate kinase**, and it is going in the *opposite direction* to what happens in gluconeogenesis.
- In gluconeogenesis, **pyruvate** is converted back to **phosphoenolpyruvate** via oxaloacetate, involving enzymes in both the mitochondria and cytoplasm.
*Glucose-6-phosphate to 6-phosphogluconolactone*
- This reaction is the first committed step of the **pentose phosphate pathway**, catalyzed by **glucose-6-phosphate dehydrogenase** and it occurs in the cytoplasm, not mitochondria.
- It is involved in producing NADPH and ribose-5-phosphate, not directly in gluconeogenesis.
*Fructose-1,6-bisphosphate to fructose-6-phosphate*
- This irreversible dephosphorylation step in gluconeogenesis, catalyzed by **fructose-1,6-bisphosphatase**, occurs in the **cytoplasm**.
- It bypasses the phosphofructokinase-1 step of glycolysis.
Question 3: A 2-year-old boy presents to the emergency department with new onset seizures. After controlling the seizures with fosphenytoin loading, a history is obtained that reveals mild hypotonia and developmental delay since birth. There is also a history of a genetic biochemical disorder on the maternal side but the family does not know the name of the disease. Physical exam is unrevealing and initial lab testing shows a pH of 7.34 with a pCO2 of 31 (normal range 35-45) and a bicarbonate level of 17 mEq/L (normal range 22-28). Further bloodwork shows an accumulation of alanine and pyruvate. A deficiency in which of the following enzymes is most likely responsible for this patient's clinical syndrome?
A. Glucose-6-phosphatase
B. Alanine transaminase
C. Glucose-6-phosphate dehydrogenase
D. Pyruvate dehydrogenase (Correct Answer)
E. Pyruvate kinase
Explanation: ***Pyruvate dehydrogenase***
- A deficiency in pyruvate dehydrogenase complex leads to the accumulation of **pyruvate** and **alanine**, as pyruvate cannot be converted into acetyl-CoA to enter the citric acid cycle.
- This accumulation results in **lactic acidosis**, presenting with symptoms like **seizures**, **hypotonia**, and developmental delay, consistent with the patient's presentation.
*Glucose-6-phosphatase*
- Deficiency in **glucose-6-phosphatase** causes **Type I glycogen storage disease (Von Gierke disease)**, characterized by **hypoglycemia**, hepatomegaly, and lactic acidosis.
- While there is lactic acidosis, the primary manifestations are related to glucose metabolism and not typically the accumulation of alanine and pyruvate to this extent.
*Alanine transaminase*
- **Alanine transaminase (ALT)** is an enzyme involved in amino acid metabolism, converting alanine and α-ketoglutarate into pyruvate and glutamate.
- A deficiency in ALT is not known to cause a distinct clinical syndrome with seizures, hypotonia, and the specific metabolic profile observed.
*Glucose-6-phosphate dehydrogenase*
- **Glucose-6-phosphate dehydrogenase (G6PD) deficiency** primarily affects **red blood cells**, leading to **hemolytic anemia** triggered by oxidative stress.
- It does not typically cause seizures, hypotonia, or the accumulation of pyruvate and alanine described in this case.
*Pyruvate kinase*
- **Pyruvate kinase deficiency** is another enzymatic defect in **glycolysis** that predominantly affects **red blood cells**, causing **hemolytic anemia**.
- While it can lead to some metabolic derangements, it is not the classic cause of central nervous system symptoms like seizures and developmental delay with the specific Lactic Acidosis described.
Question 4: A 26-year-old African American man comes to the physician because of a 3-day history of fatigue, back pain, and dark urine. One week ago, he developed a headache and was treated with aspirin. He does not smoke or use illicit drugs. Physical examination shows conjunctival pallor. A peripheral blood smear shows erythrocytes with inclusions of denatured hemoglobin. Which of the following enzymes is involved in providing precursors for nucleotide synthesis in this patient?
A. Glucose-6-phosphatase
B. Carbamoyl phosphate synthetase I
C. Pyruvate carboxylase
D. Transaldolase (Correct Answer)
E. Enolase
Explanation: ***Transaldolase***
- This patient likely has **glucose-6-phosphate dehydrogenase (G6PD) deficiency**, indicated by fatigue, dark urine (hemolysis), and **Heinz bodies** (erythrocytes with inclusions of denatured hemoglobin) after aspirin exposure, which is an **oxidative stressor**.
- **Transaldolase** is an enzyme in the **non-oxidative phase of the pentose phosphate pathway (PPP)**, which produces **ribose-5-phosphate**, a precursor for nucleotide synthesis.
*Glucose-6-phosphatase*
- **Glucose-6-phosphatase** is involved in **gluconeogenesis** and glycogenolysis, primarily in the liver and kidneys, to release free glucose into the bloodstream.
- Deficiency leads to **Von Gierke disease**, characterized by hypoglycemia, hepatomegaly, lactic acidosis, and hyperlipidemia, which are not described here.
*Carbamoyl phosphate synthetase I*
- **Carbamoyl phosphate synthetase I (CPS I)** is a mitochondrial enzyme that catalyzes the first committed step in the **urea cycle**, converting ammonia and bicarbonate into carbamoyl phosphate.
- Its deficiency causes **hyperammonemia**, not hemolytic anemia or issues with nucleotide synthesis.
*Pyruvate carboxylase*
- **Pyruvate carboxylase** is a mitochondrial enzyme that converts **pyruvate to oxaloacetate**, a crucial step in **gluconeogenesis** and replenishing intermediates of the citric acid cycle.
- Deficiency can lead to lactic acidosis and hypoglycemia, which are not the primary symptoms here.
*Enolase*
- **Enolase** is an enzyme in **glycolysis** that catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate.
- It is not directly involved in providing precursors for nucleotide synthesis.
Question 5: A 12-year-old boy and his siblings are referred to a geneticist for evaluation of a mild but chronic hemolytic anemia that has presented with fatigue, splenomegaly, and scleral icterus. Coombs test is negative and blood smear does not show any abnormal findings. An enzymatic panel is assayed, and pyruvate kinase is found to be mutated on both alleles. The geneticist explains that pyruvate kinase functions in glycolysis and is involved in a classic example of feed-forward regulation. Which of the following metabolites is able to activate pyruvate kinase?
A. Fructose-1,6-bisphosphate (Correct Answer)
B. Alanine
C. ATP
D. Glucose-6-phosphate
E. Glyceraldehyde-3-phosphate
Explanation: ***Fructose-1,6-bisphosphate***
- **Fructose-1,6-bisphosphate** is a potent **allosteric activator** of pyruvate kinase. This is an example of **feed-forward activation**, where a product of an early irreversible step in glycolysis (catalyzed by phosphofructokinase-1) activates a later enzyme (pyruvate kinase) in the pathway.
- This activation ensures that substrates for the later steps of glycolysis are rapidly utilized when earlier steps are highly active, matching the rate of metabolite flow and increasing the overall efficiency of glycolysis for energy production.
*Alanine*
- **Alanine** is an **inhibitor** of pyruvate kinase, not an activator. It serves as an indicator of a high cellular energy state and ample amino acid supply.
- High levels of alanine signal the cell that there is sufficient energy and building blocks, thus **shutting down** glycolysis at the pyruvate kinase step to conserve glucose for other needs like glycogen synthesis.
*ATP*
- **ATP** (adenosine triphosphate) is an **allosteric inhibitor** of pyruvate kinase. High ATP levels signal a high energy state in the cell.
- When the cell has sufficient energy, ATP binds to a regulatory site on pyruvate kinase, reducing its activity and **slowing down glycolysis** to prevent overproduction of ATP.
*Glucose-6-phosphate*
- **Glucose-6-phosphate** is an intermediate in glycolysis but does not directly activate pyruvate kinase. It can act as an allosteric inhibitor of hexokinase, the first enzyme in glycolysis, but not pyruvate kinase.
- Its accumulation typically signifies a **backup** in the glycolytic pathway (e.g., due to downstream inhibition), leading to a *reduction* in overall glucose flux rather than a direct activation of pyruvate kinase.
*Glyceraldehyde-3-phosphate*
- **Glyceraldehyde-3-phosphate** is an intermediate in glycolysis, but it does not directly activate pyruvate kinase. It is a substrate for glyceraldehyde-3-phosphate dehydrogenase.
- While its presence indicates active glycolysis, it does not exert a specific allosteric regulatory effect on pyruvate kinase in the way fructose-1,6-bisphosphate does.
Question 6: A 55-year-old man presents to his primary care physician with a complaint of fatigue for a couple of months. He was feeling well during his last visit 6 months ago. He has a history of hypertension for the past 8 years, diabetes mellitus for the past 5 years, and chronic kidney disease (CKD) for a year. The vital signs include: blood pressure 138/84 mm Hg, pulse 81/min, temperature 36.8°C (98.2°F), and respiratory rate 9/min. On physical examination, moderate pallor is noted on the palpebral conjunctiva and nail bed.
Complete blood count results are as follows:
Hemoglobin 8.5 g/dL
RBC 4.2 million cells/µL
Hematocrit 39%
Total leukocyte count 6,500 cells/µL
Neutrophils 61%
Lymphocytes 34%
Monocytes 4%
Eosinophils 1%
Basophils 0%
Platelets 240,000 cells/µL
A basic metabolic panel shows:
Sodium 133 mEq/L
Potassium 5.8 mEq/L
Chloride 101 mEq/L
Bicarbonate 21 mEq/L
Albumin 3.1 mg/dL
Urea nitrogen 31 mg/dL
Creatinine 2.8 mg/dL
Uric acid 6.4 mg/dL
Calcium 8.1 mg/dL
Glucose 111 mg/dL
Which of the following explanations best explains the mechanism for his decreased hemoglobin?
A. Progressive metabolic acidosis
B. Side effect of his medication
C. Failure of adequate erythropoietin production (Correct Answer)
D. Failure of 1-alpha-hydroxylation of 25-hydroxycholecalciferol
E. Increased retention of uremic products
Explanation: ***Failure of adequate erythropoietin production***
- The patient's history of **chronic kidney disease (CKD)** is the key factor. As kidney function declines, the peritubular interstitial cells in the renal cortex, which produce **erythropoietin (EPO)**, are damaged, leading to inadequate EPO synthesis.
- **Erythropoietin** is essential for stimulating red blood cell production in the bone marrow, so its deficiency directly causes **normocytic, normochromic anemia**, consistent with the patient's low hemoglobin (8.5 g/dL) and pallor.
*Progressive metabolic acidosis*
- While metabolic acidosis can occur in CKD, it primarily impacts overall metabolic function and can mildly suppress bone marrow, but it is not the **primary mechanism** for severe anemia in CKD.
- The patient's bicarbonate of 21 mEq/L indicates mild acidosis, not severe enough to be the dominant cause of his profound anemia.
*Side effect of his medication*
- Although some medications can cause anemia (e.g., ACE inhibitors or angiotensin receptor blockers can rarely worsen renal anemia), there is no information provided about specific medications that would directly cause this degree of **hemoglobin reduction** as their primary side effect in this context.
- His complex medical history and lab findings point more directly to a CKD-related etiology for anemia rather than an unmentioned medication side effect.
*Failure of 1-alpha-hydroxylation of 25-hydroxycholecalciferol*
- This process is crucial for the production of **calcitriol (active vitamin D)** in the kidneys, and its failure primarily leads to **hypocalcemia** and **renal osteodystrophy**.
- While related to CKD, impairment of vitamin D activation does not directly explain **decreased hemoglobin production** or significant anemia.
*Increased retention of uremic products*
- The accumulation of **uremic toxins** in CKD can indeed suppress bone marrow, shorten red blood cell survival, and impair iron utilization, contributing to anemia.
- However, the most significant and direct mechanism for anemia in CKD, especially at this stage, is the **lack of erythropoietin production**, which is a hormonal deficiency rather than a toxic effect.
Question 7: A 33-year-old woman, gravida 1, para 0, at 26 weeks' gestation comes to the physician for a routine prenatal examination. Her pregnancy has been uneventful. Physical examination shows a uterus consistent in size with a 26-week gestation. She is given an oral 50-g glucose load; 1 hour later, her serum glucose concentration is 116 mg/dL. Which of the following most likely occurred immediately after the entrance of glucose into the patient's pancreatic beta-cells?
A. Closure of membranous potassium channels
B. Generation of adenosine triphosphate (Correct Answer)
C. Increased expression of hexokinase I mRNA
D. Exocytosis of insulin granules
E. Depolarization of beta-cell membrane
Explanation: ***Generation of adenosine triphosphate***
- Immediately after glucose enters pancreatic beta-cells via **GLUT2 transporters**, it is phosphorylated by **glucokinase (hexokinase IV)** to glucose-6-phosphate.
- This glucose is then metabolized through **glycolysis** and the **Krebs cycle**, leading to the generation of **ATP**.
- This increase in intracellular **ATP/ADP ratio** is the **primary signal** that links glucose metabolism to insulin secretion.
- Among the listed options, ATP generation is the **earliest event** that occurs.
*Closure of membranous potassium channels*
- The elevated **ATP** levels from glucose metabolism lead to the closure of **ATP-sensitive potassium (K-ATP) channels**.
- This closure is a subsequent event that depends on the increased ATP/ADP ratio, not an immediate consequence of glucose entry.
*Increased expression of hexokinase I mRNA*
- While **glucokinase (hexokinase IV)** activity is crucial for glucose phosphorylation in beta-cells, increased mRNA expression is a **long-term adaptive response** requiring transcription and translation.
- The immediate response involves the existing enzyme converting glucose to **glucose-6-phosphate**, followed by ATP generation.
*Exocytosis of insulin granules*
- **Insulin granule exocytosis** is the final step in insulin release, occurring after a cascade of events: ATP generation → K-ATP channel closure → membrane depolarization → calcium influx.
- This event is a *downstream consequence*, not an immediate result of glucose entering the cell.
*Depolarization of beta-cell membrane*
- **Membrane depolarization** follows the closure of ATP-sensitive potassium channels, which then leads to the opening of **voltage-gated calcium channels**.
- This is a subsequent event that depends on the initial ATP generation and K-ATP channel closure.
Question 8: A 6-month-old boy presents with decreased growth, pigmented retinopathy, hemolytic anemia, and peripheral neuropathy. You suspect that these signs are the result of a vitamin deficiency leading to increased oxidative damage to lipids. Which of the following is most likely responsible for this patient's symptoms?
A. Excessive boiling of formula
B. Goat milk ingestion
C. Abetalipoproteinemia (Correct Answer)
D. Pernicious anemia
E. Hartnup disease
Explanation: ***Abetalipoproteinemia***
- This condition is characterized by a defect in **microsomal triglyceride transfer protein (MTP)**, leading to an inability to synthesize ApoB-containing lipoproteins.
- The resulting **malabsorption of fat and fat-soluble vitamins (especially vitamin E)** leads to the neurological symptoms (retinopathy, neuropathy) and hemolytic anemia due to increased oxidative stress on red blood cell membranes.
*Excessive boiling of formula*
- Excessive boiling of formula could potentially degrade some **heat-sensitive vitamins**, such as vitamin C or thiamine (B1).
- However, this is unlikely to cause a severe, combined deficiency leading to the specific constellation of symptoms seen, particularly the ophthalmologic and neurological signs related to **fat-soluble vitamin malabsorption**.
*Goat milk ingestion*
- Goat milk is naturally low in **folate** and **vitamin D** and can cause **folate deficiency anemia** and rickets if it's the sole source of nutrition for an infant.
- However, it does not typically cause the entire spectrum of symptoms described, particularly **pigmented retinopathy** and **peripheral neuropathy**, which are more indicative of **vitamin E deficiency**.
*Pernicious anemia*
- Pernicious anemia is caused by a deficiency in **intrinsic factor**, leading to **vitamin B12 malabsorption**.
- Symptoms primarily include **megaloblastic anemia**, **glossitis**, and **neurological deficits** (subacute combined degeneration of the spinal cord), but not pigmented retinopathy or hemolytic anemia due to increased fatty acid oxidation via vitamin E malabsorption.
*Hartnup disease*
- Hartnup disease is an inherited disorder of **amino acid transport**, specifically affecting the absorption of **neutral amino acids** like tryptophan.
- It leads to **niacin deficiency** (pellagra-like symptoms) and can cause skin rashes, ataxia, and psychiatric symptoms, but not hemolytic anemia or pigmented retinopathy.
Question 9: A 30-year-old man presents with dark urine and fatigue. The patient states that the symptoms started 2 days ago. Since yesterday, he also noticed that his eyes look yellow. The past medical history is significant for recent right ear pain diagnosed 3 days ago as acute otitis media, which he was prescribed trimethoprim-sulfamethoxazole. He currently does not take any other medications on a daily basis. The patient was adopted and has no knowledge of his family history. The vital signs include: temperature 37.0°C (98.6°F), blood pressure 100/75 mm Hg, pulse 105/min, respiratory rate 15/min, and oxygen saturation 100% on room air. On physical exam, the patient is alert and cooperative. The cardiac exam is significant for an early systolic murmur that is best heard at the 2nd intercostal space, midclavicular line. There is scleral icterus present. The peripheral blood smear shows the presence of bite cells and Heinz bodies. Which of the following laboratory findings would most likely be present in this patient?
A. Decreased indirect bilirubin levels
B. Decreased reticulocyte count
C. Increased serum lactate dehydrogenase (LDH) (Correct Answer)
D. Increased serum haptoglobin
E. Decreased mean corpuscular volume
Explanation: ***Increased serum lactate dehydrogenase (LDH)***
- The patient's symptoms (dark urine, fatigue, jaundice), recent trimethoprim-sulfamethoxazole use, and peripheral blood smear findings (bite cells, Heinz bodies) are classic for **G6PD deficiency** with acute **hemolytic anemia**.
- **LDH** is an intracellular enzyme found in red blood cells, and its release into the bloodstream is a marker of **cell lysis**, which is elevated in hemolytic anemia due to the breakdown of red blood cells.
*Decreased indirect bilirubin levels*
- In **hemolytic anemia**, there is an increased breakdown of red blood cells, leading to a surplus of **unconjugated (indirect) bilirubin** that overwhelms the liver's capacity for conjugation.
- Therefore, **indirect bilirubin levels** would be **increased**, not decreased, contributing to the observed jaundice.
*Decreased reticulocyte count*
- The body compensates for acute hemolytic anemia by increasing red blood cell production in the bone marrow, which is reflected by an **increased reticulocyte count**.
- A **decreased reticulocyte count** would suggest bone marrow suppression or aplastic anemia, which is not indicated here.
*Increased serum haptoglobin*
- **Haptoglobin** is a protein that binds to free hemoglobin released during red blood cell destruction. In hemolytic anemia, haptoglobin is rapidly consumed and cleared from the blood.
- Thus, **serum haptoglobin levels** would be **decreased**, not increased, as it is used used up in an attempt to clear the free hemoglobin.
*Decreased mean corpuscular volume*
- **Mean corpuscular volume (MCV)** measures the average size of red blood cells. G6PD deficiency with hemolytic anemia is typically a **normocytic anemia**, meaning the red blood cells are of normal size.
- A **decreased MCV** would suggest microcytic anemia, such as iron deficiency or thalassemias, which is not consistent with the clinical picture or blood smear findings.
Question 10: An 11-year-old boy is brought to the emergency room with acute abdominal pain and hematuria. Past medical history is significant for malaria. On physical examination, he has jaundice and a generalized pallor. His hemoglobin is 5 g/dL, and his peripheral blood smear reveals fragmented RBC, microspherocytes, and eccentrocytes (bite cells). Which of the following reactions catalyzed by the enzyme is most likely deficient in this patient?
A. Glucose-1-phosphate + UTP → UDP-glucose + pyrophosphate
Explanation: ***D-glucose-6-phosphate + NADP+ → 6-phospho-D-glucono-1,5-lactone + NADPH + H+***
- This reaction is catalyzed by **glucose-6-phosphate dehydrogenase (G6PD)**, an enzyme critical for the production of **NADPH** in the **pentose phosphate pathway**.
- **NADPH** is essential for reducing **oxidative stress** in red blood cells. A deficiency in G6PD leads to increased susceptibility to hemolysis, especially under oxidative triggers like malaria, resulting in symptoms such as **acute hemolytic anemia**, jaundice, and specific morphological changes (e.g., **fragmented RBCs**, **microspherocytes**, and **eccentrocytes**, also known as **bite cells**).
*Glucose-1-phosphate + UTP → UDP-glucose + pyrophosphate*
- This reaction is catalyzed by **UDP-glucose pyrophosphorylase** and is important for **glycogen synthesis**.
- A deficiency in this enzyme would primarily affect glycogen metabolism and would not explain the **hemolytic anemia** or the characteristic red blood cell morphology seen in the patient.
*Glucose + ATP → Glucose-6-phosphate + ADP + H+*
- This reaction is catalyzed by **hexokinase**, the first committed step in **glycolysis**.
- While hexokinase deficiency can cause **hemolytic anemia**, it generally presents with chronic, moderate anemia and does not typically involve the specific red blood cell morphology (eccentrocytes/bite cells) associated with oxidative damage found in G6PD deficiency.
*D-glucose 6-phosphate → D-fructose-6-phosphate*
- This reaction is catalyzed by **phosphoglucose isomerase** (also known as phosphohexose isomerase) and is part of **glycolysis**.
- A deficiency in this enzyme would impair glycolysis and lead to **hemolytic anemia**, but its clinical presentation and RBC morphology differ from what is typically seen in G6PD deficiency, particularly the absence of oxidative stress markers like bite cells.
*Glucose-6-phosphate + H2O → glucose + Pi*
- This reaction is catalyzed by **glucose-6-phosphatase**, an enzyme found primarily in the liver and kidney, responsible for the final step in **gluconeogenesis** and glycogenolysis to release free glucose into the bloodstream.
- A deficiency in glucose-6-phosphatase leads to **glycogen storage disease type I (Von Gierke's disease)**, characterized by **hypoglycemia**, **lactic acidosis**, and hepatomegaly, not hemolytic anemia.
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