Carbohydrate Metabolism Practice Questions

Q: What is the cycle shown below called?

Cori cycle. ***Cori cycle*** - The diagram illustrates the **Cori cycle**, showing **lactate produced in the muscle** during anaerobic glycolysis being transported to the liver. - In the liver, this **lactate is converted back to glucose** via gluconeogenesis, which can then be returned to the muscle. *Embden-Meyerhof pathway* - The **Embden-Meyerhof pathway** is an older name for **glycolysis**, which is only one part of the cycle shown in the diagram (specifically, the conversion of glucose to lactate in the muscle). - It does not encompass the entire pathway of lactate transport to the liver and its conversion to glucose. *Pentose phosphate pathway* - The **pentose phosphate pathway** is a metabolic pathway parallel to glycolysis that generates **NADPH** and the precursor for nucleotide biosynthesis. - It is not depicted in this diagram, which focuses on glucose-lactate interconversion between muscle and liver. *Pyruvate decarboxylation* - **Pyruvate decarboxylation** is the conversion of pyruvate to acetyl-CoA, primarily occurring when oxygen is available for aerobic respiration. - The cycle shown specifically highlights the **anaerobic metabolism** in muscle producing lactate and its subsequent processing.

Q: Name the antigen marked as X determining blood group A.

N-Acetyl-Galactosamine. ***N-Acetyl-Galactosamine*** - Blood group A antigens are formed by the addition of **N-acetylgalactosamine** to the H antigen precursor molecule on the surface of red blood cells. - This sugar modification is catalyzed by the **A transferase enzyme**, which is specific for N-acetylgalactosamine. *N-Acetyl-Glucosamine* - While N-acetylglucosamine is a component of many glycans, it is not the terminal sugar that defines the **blood group A antigen**. - **N-acetylglucosamine** is a key building block for the H antigen and other blood group precursors, but not the specific modifying sugar for A. *Dermatan sulphate* - **Dermatan sulfate** is a **glycosaminoglycan** primarily found in connective tissues, skin, and blood vessels. - It plays a role in wound healing and coagulation, but is not involved in **ABO blood group determination**. *Keratan sulfate* - **Keratan sulfate** is another **glycosaminoglycan** found in cartilage, cornea, and bone. - It contributes to tissue hydration and structural integrity, but it is not part of the **ABO blood group antigens**.

Q: The enzyme catalyzing this pathway is absent in?

Muscle. The image depicts the conversion of **Glucose-6-phosphate** to **Glucose**, catalyzed by the enzyme **Glucose-6-phosphatase**. The question asks where this enzyme is *absent*. ***Correct: Muscle*** - **Muscle tissue lacks glucose-6-phosphatase**, ensuring that glucose-6-phosphate formed within muscle cells is committed to intracellular use (glycolysis or glycogen synthesis) - This prevents muscle from releasing free glucose into the bloodstream, which is physiologically appropriate since muscle primarily uses glucose for its own energy needs - The absence of this enzyme in muscle is a key concept in understanding **tissue-specific glucose metabolism** and why muscle glycogen cannot directly contribute to blood glucose levels *Incorrect: Liver* - Liver is one of the **primary sites where glucose-6-phosphatase is highly expressed** - This enzyme is essential for hepatic glucose production during **gluconeogenesis** and **glycogenolysis** - Allows the liver to release free glucose into the bloodstream to maintain blood glucose homeostasis *Incorrect: Kidney* - Kidney also **contains glucose-6-phosphatase**, particularly in the renal cortex - The kidney contributes to glucose homeostasis through **renal gluconeogenesis**, especially during prolonged fasting - Can release free glucose into the circulation *Incorrect: Intestine* - Small intestine (particularly during fed state) **expresses glucose-6-phosphatase** - Plays a role in glucose production, though to a lesser extent than liver and kidney - Can contribute to postprandial glucose homeostasis **Clinical Relevance:** Deficiency of glucose-6-phosphatase causes **Von Gierke disease (Glycogen Storage Disease Type I)**, characterized by severe hypoglycemia, hepatomegaly, and lactic acidosis.

Q: A 2-week-old neonate is brought with complaints of recurrent vomiting, prolongation of physiological jaundice. Urine tests negative for sugar by dipstick but Benedict's test is positive. Which of the following enzymatic defect is most likely responsible for this presentation?

1. ***1*** - The enzymatic defect at position 1 represents **galactokinase deficiency**. This leads to the accumulation of **galactose** in the blood and urine. - While urinary dipstick tests for glucose will be negative, the Benedict's test, which detects all reducing sugars, will be **positive due to galactose**, explaining the presentation of recurrent vomiting and prolonged jaundice. *2* - The enzyme at position 2 is **galactose-1-phosphate uridylyltransferase (GALT)**. A deficiency in GALT causes classic **galactosemia**, a more severe condition. - Patients with classic galactosemia typically present with **hepatic dysfunction**, **cataracts**, and **developmental delay**, in addition to the symptoms mentioned, which are more pronounced due to the accumulation of toxic intermediates like galactose-1-phosphate. *3* - The enzyme at position 3 is **UDP-galactose 4'-epimerase (GALE)**. A deficiency in this enzyme is known as **epimerase deficiency galactosemia**. - This condition has variable severity; milder forms might be asymptomatic, while severe forms can present similarly to classic galactosemia, including **jaundice**, **vomiting**, and **cataracts**. However, it is less common than galactokinase deficiency. *4* - The enzyme at position 4 is **UDP-glucose pyrophosphorylase**, which is involved in the synthesis of UDP-glucose. - A defect in this enzyme would primarily affect glucose metabolism and glycogen synthesis, presenting with **hypoglycemia** and related symptoms, rather than the primary accumulation of galactose or its metabolites.

Q: A floppy infant was brought with difficulty in breathing. CXR was performed. Family history was positive for death of sibling at 1-year of age. Which of the following enzyme deficiency will lead to this presentation?

Acid maltase deficiency. ***Acid maltase deficiency*** - This presentation describes **Pompe disease (Glycogen Storage Disease Type II)**, caused by a deficiency of **acid alpha-glucosidase (acid maltase)**, leading to glycogen accumulation in lysosomes. - The "floppy infant" (generalized hypotonia), difficulty breathing due to **cardiomyopathy** and **respiratory muscle weakness**, and the CXR showing **cardiomegaly** (enlarged heart) are classic findings of infantile-onset Pompe disease. The family history of sibling death supports a genetic cause. *Muscle phosphorylase deficiency* - This deficiency causes **McArdle disease (Glycogen Storage Disease Type V)**, primarily affecting **skeletal muscle**. - Symptoms usually appear in adolescence or adulthood, characterized by exercise intolerance, muscle pain, and cramps, not typically severe hypotonia and respiratory distress in infancy. *Glucose-6-phosphatase deficiency* - This is **Von Gierke disease (Glycogen Storage Disease Type I)**, affecting the **liver and kidneys**. - It presents with severe **hypoglycemia** (especially fasting), hepatomegaly, lactic acidosis, and hyperlipidemia, but not primary muscle weakness or cardiomyopathy. *Arylsulfatase B deficiency* - This deficiency causes **Maroteaux-Lamy syndrome (Mucopolysaccharidosis Type VI)**, a lysosomal storage disorder. - It primarily affects connective tissues, leading to skeletal deformities (**dysostosis multiplex**), coarse facial features, and corneal clouding, but not the severe infantile hypotonia or cardiomegaly seen in Pompe disease.

Q: Which transporter helps in the improvement of insulin resistance in type 2 diabetes mellitus (DM2) with regular exercise and physical activities?

GLUT 4. ***GLUT 4*** - **GLUT 4** is the **insulin-sensitive glucose transporter** predominantly found in **skeletal muscle** and **adipose tissue**. Regular exercise and physical activity increase the expression and translocation of GLUT 4 to the cell membrane, enhancing glucose uptake independent of insulin, thereby improving **insulin sensitivity** and reducing insulin resistance in DM2. - Exercise also stimulates **AMPK (AMP-activated protein kinase)**, which promotes GLUT 4 translocation to the cell surface, facilitating glucose uptake and utilization in active muscles. *GLUT 1* - **GLUT 1** is a basal glucose transporter found in nearly all cells, responsible for **basal glucose uptake** to meet basic metabolic needs, especially in **red blood cells** and the **blood-brain barrier**. - Its activity is largely **insulin-independent** and does not significantly contribute to the exercise-induced improvement in insulin sensitivity in skeletal muscle. *GLUT 2* - **GLUT 2** is primarily found in the **liver, pancreatic beta cells, kidneys, and small intestine**. It has a **low affinity** but **high capacity** for glucose transport, serving as a glucose sensor in beta cells and allowing efficient glucose uptake/release in the liver. - It plays a role in glucose homeostasis but is **not directly involved in the exercise-mediated improvement of insulin resistance** in peripheral tissues like muscle. *GLUT 3* - **GLUT 3** is a **high-affinity glucose transporter** primarily expressed in **neurons** and the **placenta**. It is crucial for providing a constant supply of glucose to the brain, even at low glucose concentrations. - Like GLUT 1 and GLUT 2, its activity is largely **insulin-independent** and does not play a significant role in improving insulin resistance through exercise in DM2.

Q: Corneal Transparency is maintained by which of the following GAGs?

Keratan Sulphate. ***Keratan Sulphate*** - **Keratan sulfate** is a major glycosaminoglycan (GAG) found in the **cornea**, where its specific highly hydrated structure and arrangement help maintain corneal transparency. - The uniform spacing of collagen fibrils, maintained by keratan sulfate, is crucial for minimizing light scattering and allowing light to pass through the cornea. *Dermatan Sulphate* - **Dermatan sulfate** is primarily found in **skin, blood vessels, and heart valves**, contributing to tissue strength and elasticity. - It plays a significant role in wound healing and cardiovascular function, but not directly in maintaining corneal transparency. *Heparan Sulphate* - **Heparan sulfate** is ubiquitously found on **cell surfaces and in the extracellular matrix**, particularly in the basement membranes. - It is involved in cell adhesion, growth factor binding, and anticoagulant activity, but is not the primary GAG responsible for corneal transparency. *Chondroitin Sulphate* - **Chondroitin sulfate** is abundant in **cartilage, bone, and connective tissues**, providing compressive strength and elasticity. - While present in some ocular tissues, it is not the dominant GAG responsible for the unique transparent properties of the cornea.

Q: Pompe disease is seen due to deficiency of?

Lysosomal acid alpha glucosidase. ***Lysosomal acid alpha glucosidase*** - **Pompe disease**, also known as **glycogen storage disease type II**, is caused by the deficiency of **acid alpha-glucosidase (GAA)**, a lysosomal enzyme. - This deficiency leads to the accumulation of **glycogen** within lysosomes in various tissues, particularly muscle, liver, and heart, causing muscle weakness and cardiomegaly. *β-galactocerebrosidase* - Deficiency of **β-galactocerebrosidase** is associated with **Krabbe disease** (globoid cell leukodystrophy). - Krabbe disease involves the buildup of **galactocerebroside**, leading to progressive destruction of the myelin sheath in the central and peripheral nervous systems. *β-glucocerebrosidase* - Deficiency in **β-glucocerebrosidase** causes **Gaucher disease**, the most common lysosomal storage disorder. - This results in the accumulation of **glucocerebroside** in macrophages, leading to hepatosplenomegaly, bone pain, and neurologic symptoms in some forms. *β-hexosaminidase-A* - Deficiency of **β-hexosaminidase-A** is the underlying cause of **Tay-Sachs disease**. - Tay-Sachs disease is characterized by the accumulation of **GM2 gangliosides** in neurons, leading to progressive neurodegeneration, developmental delay, and a characteristic cherry-red spot in the retina.

Q: Which of the following is not a substrate for glucose formation?

Acetyl coenzyme A. ***Acetyl coenzyme A*** - **Acetyl CoA** cannot be converted to glucose because the two carbons from the acetyl group are lost as carbon dioxide in the **Krebs cycle**, making a net synthesis of glucose impossible. - The irreversible nature of the **pyruvate dehydrogenase complex** prevents the conversion of Acetyl CoA back to **pyruvate**, which is a crucial step for gluconeogenesis. *Lactate* - **Lactate** is a major substrate for gluconeogenesis, particularly during exercise and fasting, via the **Cori cycle**. - **Lactate dehydrogenase** converts lactate to **pyruvate**, which can then enter the gluconeogenic pathway. *Glycerol* - **Glycerol**, derived from triglyceride breakdown, enters gluconeogenesis by being converted to **glycerol-3-phosphate** and then to **dihydroxyacetone phosphate (DHAP)**. - DHAP is an intermediate in glycolysis and gluconeogenesis, allowing for its conversion to glucose. *Alanine* - **Alanine** is a **glucogenic amino acid** that can be transaminated to **pyruvate**. - **Pyruvate** can then proceed through the gluconeogenic pathway to synthesize glucose, especially during prolonged fasting.

Q: What is the primary mechanism for glucose uptake in neurons?

GLUT3. ***GLUT3*** - **GLUT3** is the primary glucose transporter in **neurons** and has a **high affinity** for glucose. - This high affinity ensures that neurons can continuously take up glucose, even when blood glucose levels are relatively low, to meet their significant energy demands. *GLUT1* - **GLUT1** is abundant in **red blood cells** and at the **blood-brain barrier**, where it provides basal glucose transport to many cell types. - While present in the brain, it is primarily responsible for glucose transport across the **blood-brain barrier** into the interstitial fluid, not directly into neurons as the main mechanism. *GLUT2* - **GLUT2** has a **low affinity** and **high capacity** for glucose, primarily found in the **liver, pancreatic beta cells, kidney, and intestine**. - Its role is to sense high glucose levels and transport large amounts of glucose accordingly, which is not characteristic of neuronal glucose uptake. *GLUT4* - **GLUT4** is the **insulin-sensitive** glucose transporter, predominantly found in **adipose tissue** and **skeletal muscle**. - Its translocation to the cell membrane is stimulated by insulin, a mechanism not central to neuronal glucose uptake.

Question 1

What is the cycle shown below called?

Accepted Answer

Cori cycle

Answer

Embden-Meyerhof pathway

Answer

Pentose phosphate pathway

Answer

Pyruvate decarboxylation

Question 2

Name the antigen marked as X determining blood group A.

Accepted Answer

N-Acetyl-Galactosamine

Answer

N-Acetyl-Glucosamine

Answer

Dermatan sulphate

Answer

Keratan sulfate

Question 3

The enzyme catalyzing this pathway is absent in?

Accepted Answer

Muscle

Answer

Liver

Answer

Kidney

Answer

Intestine

Question 4

A 2-week-old neonate is brought with complaints of recurrent vomiting, prolongation of physiological jaundice. Urine tests negative for sugar by dipstick but Benedict's test is positive. Which of the following enzymatic defect is most likely responsible for this presentation?

Accepted Answer

1

Answer

2

Answer

3

Answer

4

Question 5

A floppy infant was brought with difficulty in breathing. CXR was performed. Family history was positive for death of sibling at 1-year of age. Which of the following enzyme deficiency will lead to this presentation?

Accepted Answer

Acid maltase deficiency

Answer

Muscle phosphorylase deficiency

Answer

Glucose-6-phosphatase deficiency

Answer

Arylsulfatase B deficiency

Question 6

Which transporter helps in the improvement of insulin resistance in type 2 diabetes mellitus (DM2) with regular exercise and physical activities?

Accepted Answer

GLUT 4

Answer

GLUT 1

Answer

GLUT 2

Answer

GLUT 3

Question 7

Corneal Transparency is maintained by which of the following GAGs?

Accepted Answer

Keratan Sulphate

Answer

Dermatan Sulphate

Answer

Heparan Sulphate

Answer

Chondroitin Sulphate

Question 8

Pompe disease is seen due to deficiency of?

Accepted Answer

Lysosomal acid alpha glucosidase

Answer

β-galactocerebrosidase

Answer

β-glucocerebrosidase

Answer

β-hexosaminidase-A

Question 9

Which of the following is not a substrate for glucose formation?

Accepted Answer

Acetyl coenzyme A

Answer

Lactate

Answer

Glycerol

Answer

Alanine

Question 10

What is the primary mechanism for glucose uptake in neurons?

Accepted Answer

GLUT3

Answer

GLUT1

Answer

GLUT2

Answer

GLUT4

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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