A 34-year-old man is brought to the emergency room by emergency medical technicians after being found unconscious near a park bench. He appears disheveled with a strong odor of alcohol. There is no known past medical history other than treatment for alcohol withdrawal in the past at this institution.The patient is laying on the stretcher with altered mental status, occasionally muttering a few words that are incomprehensible to the examiner. Physical examination reveals a heart rate of 94/min, blood pressure of 110/62 mm Hg, respiratory rate of 14/min, and temperature is 37.0°C (98.6°F). The patient’s physical exam is otherwise unremarkable with lungs clear to auscultation, a soft abdomen, and no skin rashes. Initial laboratory findings reveal:
Blood glucose 56 mg/dL
Blood alcohol level 215 mg/dL
Hemoglobin 10.9 g/dL
WBC 10,000/mm3
Platelets 145,000/mm3
Lactate level
2.2 mmol/L
Which of the following describes the most likely physiological factor underlying the patient’s hypoglycemia?
Q12
A 6-month-old boy is referred to a geneticist after he is found to have persistent hypotonia and failure to thrive. He has also had episodes of what appears to be respiratory distress and has an enlarged heart on physical exam. There is a family history of childhood onset hypertrophic cardiomyopathy, so a biopsy is performed showing electron dense granules within the lysosomes. Genetic testing is performed showing a defect in glycogen processing. A deficiency in which of the following enzymes is most likely to be responsible for this patient's symptoms?
Q13
A 2-day-old newborn boy is brought to the emergency department because of apnea, cyanosis, and seizures. He is severely hypoglycemic and does not improve with glucagon administration. His blood pressure is 100/62 mm Hg and heart rate is 75/min. Blood tests show high lactate levels. Physical examination is notable for hepatomegaly. Which of the following enzymes is most likely to be deficient in this baby?
Q14
An investigator is studying muscle tissue in high-performance athletes. He obtains blood samples from athletes before and after a workout session consisting of short, fast sprints. Which of the following findings is most likely upon evaluation of blood obtained after the workout session?
Q15
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?
Q16
A 15-year-old boy is sent from gym class with a chief complaint of severe muscle aches. In class today he was competing with his friends and therefore engaged in weightlifting for the first time. A few hours later he was extremely sore and found that his urine was red when he went to urinate. This concerned him and he was sent to the emergency department for evaluation.
Upon further questioning, you learn that since childhood he has always had muscle cramps with exercise. Physical exam was unremarkable. Upon testing, his creatine kinase level was elevated and his urinalysis was negative for blood and positive for myoglobin.
Thinking back to biochemistry you suspect that he may be suffering from a hereditary glycogen disorder. Given this suspicion, what would you expect to find upon examination of his cells?
Q17
A 25-year-old man is brought to the emergency department 6 hours after rescuing babies and puppies from a burning daycare center. He says that he has a severe headache, feels nauseous and dizzy. He is tachypneic. An arterial blood gas shows pH 7.3, PaCO2 49 mmHg, PaO2 80 mmHg. Serum lactate level is 6 mmol/L. What biochemical process explains these laboratory values?
Q18
A 21-year-old woman comes to the physician for a routine physical examination. She feels well. She is 163 cm (5 ft 4 in) tall and weighs 54 kg (120 lb); BMI is 20.3 kg/m2. Physical examination shows no abnormalities. Her fasting serum glucose concentration is 132 mg/dL. Serum insulin concentration 30 minutes after oral glucose administration is 20 mIU/L (N: 30–230). Her hemoglobin A1C concentration is 7.1%. After a thorough workup, the physician concludes that the patient has a chronic condition that can likely be managed with diet only and that she is not at a significantly increased risk of micro- or macrovascular complications. Which of the following is the most likely cause of the patient's condition?
Q19
An investigator is conducting an experiment to study different pathways of glucose metabolism. He obtains cells cultured from various tissues to study the effect of increased extracellular glucose concentration. Following the incubation of these cells in 5% dextrose, he measures the intracellular fructose concentration. The concentration of fructose is expected to be highest in cells obtained from which of the following tissues?
Q20
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?
Glycolysis US Medical PG Practice Questions and MCQs
Question 11: A 34-year-old man is brought to the emergency room by emergency medical technicians after being found unconscious near a park bench. He appears disheveled with a strong odor of alcohol. There is no known past medical history other than treatment for alcohol withdrawal in the past at this institution.The patient is laying on the stretcher with altered mental status, occasionally muttering a few words that are incomprehensible to the examiner. Physical examination reveals a heart rate of 94/min, blood pressure of 110/62 mm Hg, respiratory rate of 14/min, and temperature is 37.0°C (98.6°F). The patient’s physical exam is otherwise unremarkable with lungs clear to auscultation, a soft abdomen, and no skin rashes. Initial laboratory findings reveal:
Blood glucose 56 mg/dL
Blood alcohol level 215 mg/dL
Hemoglobin 10.9 g/dL
WBC 10,000/mm3
Platelets 145,000/mm3
Lactate level
2.2 mmol/L
Which of the following describes the most likely physiological factor underlying the patient’s hypoglycemia?
A. Overactive pyruvate dehydrogenase
B. Increase in insulin secretion
C. An increase in the ratio of reduced form of nicotinamide adenine dinucleotide to nicotinamide adenine dinucleotide (NADH/NAD+ ratio) (Correct Answer)
D. Glycogen depletion
E. Alcohol-induced diuresis
Explanation: ***An increase in the ratio of reduced form of nicotinamide adenine dinucleotide to nicotinamide adenine dinucleotide (NADH/NAD+ ratio)***
- Alcohol metabolism by **alcohol dehydrogenase** and **aldehyde dehydrogenase** significantly increases the **NADH/NAD+ ratio**.
- This altered ratio inhibits key gluconeogenic enzymes (e.g., lactate dehydrogenase, malate dehydrogenase), diverting substrates away from **glucose production** and leading to **hypoglycemia**.
*Overactive pyruvate dehydrogenase*
- **Pyruvate dehydrogenase** converts pyruvate to acetyl-CoA, a step in glucose utilization, not production.
- An overactive enzyme would typically lead to less pyruvate available for gluconeogenesis, but it's not the primary mechanism of **alcohol-induced hypoglycemia**.
*Alcohol-induced diuresis*
- Alcohol causes **diuresis** (increased urine production) primarily through inhibition of **antidiuretic hormone (ADH/vasopressin)**.
- While diuresis can lead to dehydration and electrolyte imbalances, it does not directly cause **hypoglycemia**.
*Increase in insulin secretion*
- Alcohol can initially cause a transient increase in insulin, but sustained **hypoglycemia** in chronic alcoholics is not due to **hyperinsulinemia**.
- Moreover, the predominant mechanism involves impaired **hepatic glucose production**, not excessive glucose uptake due to insulin.
*Glycogen depletion*
- While chronic alcohol use can lead to **glycogen depletion** in the liver due to poor nutrition and altered metabolism, this patient's acute presentation points to a more immediate mechanism.
- The elevated **NADH/NAD+ ratio** directly inhibits gluconeogenesis, which is the primary contributor to acute **alcohol-induced hypoglycemia**, even before significant **glycogen depletion** occurs.
Question 12: A 6-month-old boy is referred to a geneticist after he is found to have persistent hypotonia and failure to thrive. He has also had episodes of what appears to be respiratory distress and has an enlarged heart on physical exam. There is a family history of childhood onset hypertrophic cardiomyopathy, so a biopsy is performed showing electron dense granules within the lysosomes. Genetic testing is performed showing a defect in glycogen processing. A deficiency in which of the following enzymes is most likely to be responsible for this patient's symptoms?
A. Lysosomal alpha 1,4-glucosidase (Correct Answer)
B. Branching enzyme
C. Muscle phosphorylase
D. Debranching enzyme
E. Glucose-6-phosphatase
Explanation: ***Lysosomal alpha 1,4-glucosidase***
- The constellation of **hypotonia**, **failure to thrive**, **respiratory distress**, and **cardiomegaly** in an infant, along with **electron-dense granules in lysosomes** and a defect in **glycogen processing**, is characteristic of **Pompe disease (Type II glycogen storage disease)**.
- **Pompe disease** is caused by a deficiency of **lysosomal alpha 1,4-glucosidase** (also known as acid maltase), which is responsible for breaking down glycogen in lysosomes.
*Branching enzyme*
- A deficiency in **branching enzyme (amylo-alpha-1,4-to-alpha-1,6-transglucosidase)** causes **Andersen disease (Type IV glycogen storage disease)**, which typically presents with **hepatosplenomegaly**, **cirrhosis**, and **failure to thrive**.
- While it involves glycogenopathy, the specific features of **cardiomyopathy** and **lysosomal accumulation** are not primary to this disorder.
*Muscle phosphorylase*
- A deficiency in **muscle phosphorylase** causes **McArdle disease (Type V glycogen storage disease)**, which primarily affects **skeletal muscle**.
- Symptoms include **exercise intolerance**, **muscle cramps**, and **myoglobinuria**, typically presenting later in childhood or adolescence, and does not involve cardiomyopathy or lysosomal storage.
*Debranching enzyme*
- A deficiency in **debranching enzyme (alpha-1,6-glucosidase)** causes **Cori disease (Type III glycogen storage disease)**, which presents with **hepatomegaly**, **hypoglycemia**, and **muscle weakness**.
- While it can sometimes involve a milder form of cardiomyopathy, the significant **lysosomal involvement** and severe infantile onset with respiratory distress and profound hypotonia point away from Cori disease.
*Glucose-6-phosphatase*
- A deficiency in **glucose-6-phosphatase** causes **Von Gierke disease (Type I glycogen storage disease)**, characterized by **severe fasting hypoglycemia**, **lactic acidosis**, **hepatomegaly**, and **hyperlipidemia**.
- This condition primarily affects the liver and kidneys, and typically does not present with primary cardiomyopathy, hypotonia, or lysosomal glycogen accumulation.
Question 13: A 2-day-old newborn boy is brought to the emergency department because of apnea, cyanosis, and seizures. He is severely hypoglycemic and does not improve with glucagon administration. His blood pressure is 100/62 mm Hg and heart rate is 75/min. Blood tests show high lactate levels. Physical examination is notable for hepatomegaly. Which of the following enzymes is most likely to be deficient in this baby?
A. α-ketoacid dehydrogenase
B. Phenylalanine hydroxylase
C. Glucose-6-phosphatase (Correct Answer)
D. Glucocerebrosidase
E. Sphingomyelinase
Explanation: ***Correct: Glucose-6-phosphatase***
- The presentation of severe **hypoglycemia** not responsive to glucagon, coupled with **hepatomegaly** and **lactic acidosis** in a neonate, is highly suggestive of **Type I glycogen storage disease (von Gierke disease)**.
- Deficiency of **glucose-6-phosphatase** prevents the liver from releasing glucose into the bloodstream (the final step of both gluconeogenesis and glycogenolysis), leading to profound hypoglycemia.
- **Key diagnostic clue**: Lack of response to glucagon occurs because glucagon stimulates glycogenolysis, but without functional glucose-6-phosphatase, glucose-6-phosphate cannot be converted to free glucose for release.
- Accumulated glucose-6-phosphate shunts to glycolysis, producing **lactate** (lactic acidosis), and to glycogen synthesis, causing **hepatomegaly**.
*Incorrect: α-ketoacid dehydrogenase*
- Deficiency of **branched-chain α-ketoacid dehydrogenase** causes **maple syrup urine disease (MSUD)**, which presents with poor feeding, vomiting, lethargy, and a characteristic maple syrup odor in urine.
- While MSUD can cause neurological symptoms and seizures, **severe hypoglycemia unresponsive to glucagon** and **hepatomegaly** as primary features are not typical.
*Incorrect: Phenylalanine hydroxylase*
- Deficiency in **phenylalanine hydroxylase** causes **phenylketonuria (PKU)**, which is primarily characterized by intellectual disability, seizures (if untreated), and a musty odor, usually manifesting later in infancy.
- PKU does not present with acute neonatal hypoglycemia, lactic acidosis, or hepatomegaly.
*Incorrect: Glucocerebrosidase*
- Deficiency in **glucocerebrosidase** leads to **Gaucher disease**, a lysosomal storage disorder characterized by hepatosplenomegaly, bone crises, and neurological symptoms in severe infantile forms.
- While hepatomegaly may be present, Gaucher disease does not cause acute, severe neonatal hypoglycemia, lactic acidosis, or lack of response to glucagon.
*Incorrect: Sphingomyelinase*
- Deficiency in **sphingomyelinase** causes **Niemann-Pick disease**, another lysosomal storage disorder, which typically presents with hepatosplenomegaly, neurological deterioration, and "cherry-red spots" in the retina.
- This condition does not cause acute neonatal hypoglycemia, lactic acidosis, or glucagon unresponsiveness.
Question 14: An investigator is studying muscle tissue in high-performance athletes. He obtains blood samples from athletes before and after a workout session consisting of short, fast sprints. Which of the following findings is most likely upon evaluation of blood obtained after the workout session?
A. Decreased concentration of NADH
B. Increased concentration of H+ (Correct Answer)
C. Decreased concentration of lactate
D. Increased concentration of insulin
E. Increased concentration of ATP
Explanation: ***Increased concentration of H+***
- During **anaerobic metabolism** in high-intensity exercise like sprints, pyruvate is converted to **lactate** by **lactate dehydrogenase** to regenerate NAD+. This process produces H+, leading to a decrease in pH and an increase in H+ concentration in the blood.
- The accumulation of **hydrogen ions (H+)** contributes to metabolic acidosis, muscle fatigue, and the burning sensation experienced during intense exertion.
- Blood gas analysis would show **decreased pH** and **increased H+ concentration**.
*Decreased concentration of NADH*
- NADH is primarily an **intracellular metabolite** and is not typically measured in blood samples as it does not circulate freely in significant concentrations.
- Within muscle cells during anaerobic glycolysis, NADH is consumed by lactate dehydrogenase to convert pyruvate to lactate, regenerating NAD+ for continued glycolysis.
- This option is not a realistic blood finding from a clinical laboratory perspective.
*Decreased concentration of lactate*
- **High-intensity sprints** primarily rely on **anaerobic metabolism**, which rapidly produces **lactate** from pyruvate.
- Therefore, the concentration of lactate in the blood would significantly **increase** after such a workout, not decrease.
- Elevated blood lactate is a hallmark finding after intense anaerobic exercise.
*Increased concentration of insulin*
- **Insulin** levels typically **decrease** during exercise, especially high-intensity exercise, due to **sympathetic nervous system activation** and the body's need to mobilize glucose from liver glycogen and fatty acids.
- Exercise promotes glucose uptake through **insulin-independent mechanisms** (GLUT4 translocation via AMP-activated protein kinase).
- Increased insulin would be counterproductive during intense exercise when glucose mobilization is needed.
*Increased concentration of ATP*
- ATP does not circulate in blood in measurable concentrations as a typical laboratory finding.
- Within muscle cells, ATP is rapidly **consumed** during intense exercise to fuel muscle contraction.
- While cells work to maintain ATP levels through anaerobic glycolysis and the creatine phosphate system, net ATP does not accumulate in the blood.
Question 15: 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.
Question 16: A 15-year-old boy is sent from gym class with a chief complaint of severe muscle aches. In class today he was competing with his friends and therefore engaged in weightlifting for the first time. A few hours later he was extremely sore and found that his urine was red when he went to urinate. This concerned him and he was sent to the emergency department for evaluation.
Upon further questioning, you learn that since childhood he has always had muscle cramps with exercise. Physical exam was unremarkable. Upon testing, his creatine kinase level was elevated and his urinalysis was negative for blood and positive for myoglobin.
Thinking back to biochemistry you suspect that he may be suffering from a hereditary glycogen disorder. Given this suspicion, what would you expect to find upon examination of his cells?
A. Normal glycogen structure (Correct Answer)
B. Short outer glycogen chains
C. Accumulation of glycogen in lysosomes forming dense granules
D. Glycogen without normal branching pattern
E. Absence of glycogen in muscles
Explanation: ***Normal glycogen structure***
- The patient's symptoms (exercise-induced muscle cramps, myoglobinuria, and elevated CK) are classic for **McArdle disease** (Glycogen Storage Disease Type V), caused by a deficiency in **muscle glycogen phosphorylase**.
- In McArdle disease, the enzyme responsible for breaking down glycogen (glycogen phosphorylase) is deficient, but the enzymes involved in synthesizing glycogen are normal. Therefore, the **structure of glycogen is normal**, but it accumulates in muscle cells because it cannot be catabolized.
*Short outer glycogen chains*
- **Short outer glycogen chains** are characteristic of **Cori disease** (Glycogen Storage Disease Type III), caused by a deficiency in **debranching enzyme**.
- This condition also presents with hypoglycemia and hepatomegaly, which are not described in the patient's presentation.
*Accumulation of glycogen in lysosomes forming dense granules*
- **Accumulation of glycogen in lysosomes** and the formation of **dense granules** is characteristic of **Pompe disease** (Glycogen Storage Disease Type II), caused by a deficiency in **lysosomal alpha-glucosidase (acid maltase)**.
- Pompe disease typically presents as a severe infantile form with cardiomegaly and hypotonia, or a later-onset form with proximal muscle weakness, which differs from the patient's primary complaint of exercise intolerance and myoglobinuria.
*Glycogen without normal branching pattern*
- **Glycogen without a normal branching pattern** (very long unbranched chains) is characteristic of **Andersen disease** (Glycogen Storage Disease Type IV), caused by a deficiency in **branching enzyme**.
- This condition typically leads to cirrhosis and liver failure in infancy, which is not consistent with the patient's presentation.
*Absence of glycogen in muscles*
- While McArdle disease involves an inability to break down muscle glycogen, it does not result in the **absence of glycogen** in muscles; rather, there is an **over-accumulation** of normal-structured glycogen because it cannot be utilized.
- The defect is in **glycogenolysis**, not glycogen synthesis, so glycogen is formed but not broken down.
Question 17: A 25-year-old man is brought to the emergency department 6 hours after rescuing babies and puppies from a burning daycare center. He says that he has a severe headache, feels nauseous and dizzy. He is tachypneic. An arterial blood gas shows pH 7.3, PaCO2 49 mmHg, PaO2 80 mmHg. Serum lactate level is 6 mmol/L. What biochemical process explains these laboratory values?
A. Increased oxidation of NADH
B. Low lactate dehydrogenase activity
C. Low pyruvate dehydrogenase activity (Correct Answer)
D. High pyruvate dehydrogenase activity
E. Increased decarboxylation of pyruvate
Explanation: ***Low pyruvate dehydrogenase activity***
- The patient's symptoms (headache, nausea, dizziness, tachypnea) after smoke exposure, combined with a **high serum lactate (6 mmol/L)** and signs of acidosis (pH 7.3, PaCO2 49 mmHg indicating respiratory compensation for metabolic acidosis), are highly suggestive of **carbon monoxide (CO) poisoning**.
- **CO binds to cytochrome C oxidase**, inhibiting the electron transport chain and **oxidative phosphorylation**. This leads to a buildup of NADH and a shift to **anaerobic metabolism**, where pyruvate is converted to **lactate** instead of entering the **Krebs cycle** via **pyruvate dehydrogenase (PDH)**. Therefore, the effective activity of PDH is reduced because its substrate (pyruvate) is shunted to lactate production.
*Increased oxidation of NADH*
- In CO poisoning, **oxidative phosphorylation is inhibited**, leading to a *decrease* in NADH oxidation as the electron transport chain cannot efficiently accept electrons from NADH.
- Instead, NADH *accumulates*, favoring the conversion of pyruvate to lactate to regenerate NAD+ for glycolysis.
*Low lactate dehydrogenase activity*
- **Lactate dehydrogenase (LDH)** activity would likely be *increased* or normal in this scenario, as it is responsible for converting pyruvate to lactate during anaerobic metabolism, which is elevated as indicated by the high serum lactate.
- Low LDH activity would *reduce* lactate production, counteracting the observed elevated lactate levels.
*High pyruvate dehydrogenase activity*
- **Pyruvate dehydrogenase (PDH)** converts pyruvate to acetyl-CoA for entry into the Krebs cycle, a key step in *aerobic* metabolism.
- In the setting of **CO poisoning**, aerobic metabolism is compromised, leading to a *reduction* in the flow through PDH as pyruvate is shunted towards **lactate production**.
*Increased decarboxylation of pyruvate*
- **Decarboxylation of pyruvate** is catalyzed by **pyruvate dehydrogenase (PDH)**, converting pyruvate to acetyl-CoA and releasing CO2.
- With compromised oxidative phosphorylation due to CO poisoning, the cell shifts to anaerobic metabolism, which *reduces* the processing of pyruvate through PDH, thus *decreasing* its decarboxylation, leading to lactate build-up.
Question 18: A 21-year-old woman comes to the physician for a routine physical examination. She feels well. She is 163 cm (5 ft 4 in) tall and weighs 54 kg (120 lb); BMI is 20.3 kg/m2. Physical examination shows no abnormalities. Her fasting serum glucose concentration is 132 mg/dL. Serum insulin concentration 30 minutes after oral glucose administration is 20 mIU/L (N: 30–230). Her hemoglobin A1C concentration is 7.1%. After a thorough workup, the physician concludes that the patient has a chronic condition that can likely be managed with diet only and that she is not at a significantly increased risk of micro- or macrovascular complications. Which of the following is the most likely cause of the patient's condition?
A. Mutation in hepatocyte nuclear factor 1
B. Defect in expression of glucokinase gene (Correct Answer)
C. Resistance to insulin-mediated glucose uptake
D. Increased endogenous cortisol production
E. Autoantibodies to pancreatic beta cells
Explanation: ***Defect in expression of glucokinase gene***
- The patient's presentation with **mild, stable hyperglycemia** (fasting glucose 132 mg/dL, HbA1c 7.1%) since a young age, without features of typical type 1 or type 2 diabetes, is highly suggestive of **Maturity-Onset Diabetes of the Young (MODY)**.
- Specifically, **MODY2**, caused by a defect in the **glucokinase gene (GCK-MODY)**, is characterized by a slightly elevated fasting glucose that is often present from birth, minimally progressive, and typically managed with diet due to the negligible risk of complications.
*Mutation in hepatocyte nuclear factor 1*
- Mutations in **hepatocyte nuclear factor 1-alpha (HNF1A-MODY or MODY3)** and **HNF4A (HNF4A-MODY or MODY1)** are also common forms of MODY, but they usually lead to more progressive hyperglycemia and often require sulfonylurea treatment due to impaired insulin secretion.
- These forms of MODY are associated with a higher risk of diabetic complications compared to GCK-MODY, which contradicts the physician's conclusion of a low complication risk.
*Resistance to insulin-mediated glucose uptake*
- This describes **insulin resistance**, a hallmark of **Type 2 Diabetes Mellitus**.
- However, the patient's normal BMI (20.3 kg/m2) and relatively low insulin level after glucose challenge (20 mIU/L, within range of "normal" for some labs, but significantly lower than the expected robust response in early T2DM) make significant insulin resistance unlikely as the primary cause.
*Increased endogenous cortisol production*
- Increased cortisol, as seen in **Cushing's syndrome**, can cause hyperglycemia due to increased gluconeogenesis and insulin resistance.
- However, the patient presents with no other signs or symptoms of Cushing's syndrome (e.g., central obesity, moon facies, striae, hypertension), making this diagnosis improbable.
*Autoantibodies to pancreatic beta cells*
- The presence of autoantibodies (e.g., anti-GAD, islet cell antibodies) is characteristic of **Type 1 Diabetes Mellitus**, an autoimmune condition leading to destruction of pancreatic beta cells and usually presents with more severe hyperglycemia and insulin dependency.
- The patient's mild, stable hyperglycemia and the physician's conclusion of a low complication risk and diet-only management contradict the typical progression and management of Type 1 Diabetes.
Question 19: An investigator is conducting an experiment to study different pathways of glucose metabolism. He obtains cells cultured from various tissues to study the effect of increased extracellular glucose concentration. Following the incubation of these cells in 5% dextrose, he measures the intracellular fructose concentration. The concentration of fructose is expected to be highest in cells obtained from which of the following tissues?
A. Ovary
B. Retina
C. Myelin sheath
D. Kidney
E. Lens (Correct Answer)
Explanation: ***Lens***
- The **lens** is rich in the enzyme **aldose reductase**, which converts glucose to sorbitol, and then **sorbitol dehydrogenase** converts sorbitol to fructose via the **polyol pathway**.
- In a high-glucose environment, this pathway becomes highly active in the lens, leading to an increased production and accumulation of **fructose**, which can contribute to osmotic stress and cataract formation.
*Ovary*
- While other reproductive tissues can metabolize glucose, the **ovary** is not a primary site for significant fructose accumulation through the **polyol pathway** in response to elevated glucose.
- Its metabolic activity is more geared towards steroidogenesis and oocyte development rather than high fructose production from glucose.
*Retina*
- The **retina** contains some aldose reductase activity, and increased glucose can activate the **polyol pathway**, leading to sorbitol and fructose accumulation.
- However, the lens typically shows a more pronounced increase in fructose concentration due to its higher metabolic flux through this pathway and its susceptibility to osmotic damage.
*Myelin sheath*
- The **myelin sheath**, primarily composed of lipids, is part of the nervous system and is not known for significant **fructose production** via the **polyol pathway** in response to high glucose.
- Damage to myelin in diabetic conditions is often linked to other mechanisms like glycation and oxidative stress rather than direct fructose accumulation.
*Kidney*
- The **kidney** can utilize the **polyol pathway**, and conditions like **diabetic nephropathy** involve increased sorbitol and fructose production.
- However, the magnitude of fructose accumulation and its direct pathogenic role differ from that in the lens, where osmotic effects of polyols are particularly critical.
Question 20: 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.
