Carbohydrate Metabolism — MCQs

A 3 months old child was started on supplemental foods along with breastmilk. The child was fed with fruit pulp and sweetened cereals. Soon the child developed bloating of abdomen, vomiting, lethargy, and irritability. On investigation, there was hyperbilirubinemia and elevated transaminase levels. The child is suffering from which of the following enzyme deficiencies?

Q709

All of the following are true about lactate utilization in the liver except -

Q710

In glycogen synthesis the active form of glucose used is-

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 701: Which enzyme is deficient in McArdle's disease, resulting in muscle pain and weakness after exercise?

A. Glucose-6-phosphatase
B. Liver phosphorylase
C. Muscle phosphorylase (Correct Answer)
D. Debranching enzyme

Explanation: ***Muscle phosphorylase*** - **McArdle's disease** (Glycogen Storage Disease Type V) is characterized by a deficiency in **muscle glycogen phosphorylase**, also known as **myophosphorylase**. - This enzyme is crucial for breaking down **glycogen in muscle cells** to release glucose for energy during exercise, leading to **muscle pain and weakness** when deficient. *Glucose-6-phosphatase* - A deficiency in **glucose-6-phosphatase** causes **Von Gierke's disease** (Glycogen Storage Disease Type I), which primarily affects the liver and kidneys. - This deficiency results in **hepatomegaly**, **hypoglycemia**, and **lactic acidosis**, not predominantly muscle pain and weakness after exercise. *Liver phosphorylase* - A deficiency in **liver phosphorylase** (or phosphorylase kinase) causes **Hers' disease** (Glycogen Storage Disease Type VI). - This condition mainly affects the **liver**, leading to **hepatomegaly** and mild **hypoglycemia**, with less significant muscle symptoms. *Debranching enzyme* - A deficiency in the **debranching enzyme** causes **Cori's disease** (Glycogen Storage Disease Type III). - This leads to **abnormal glycogen structure**, affecting both **liver and muscle**, causing **hepatomegaly**, **hypoglycemia**, and **muscle weakness**, but typically presenting differently than McArdle's.

Question 702: An individual with a rare metabolic disorder exhibits elevated levels of pyruvate and alanine in the blood. This suggests a defect in which enzyme involved in gluconeogenesis?

A. Pyruvate carboxylase (Correct Answer)
B. Phosphoenolpyruvate carboxykinase
C. Fructose-1,6-bisphosphatase
D. Glucose-6-phosphatase

Explanation: ***Pyruvate carboxylase*** - **Pyruvate carboxylase** converts **pyruvate** to **oxaloacetate** in gluconeogenesis. A defect will cause a buildup of its substrate, **pyruvate**, and its transamination product, **alanine**. - This enzyme is crucial for diverting pyruvate from **glycolysis** towards glucose synthesis in the liver and kidneys. *Phosphoenolpyruvate carboxykinase* - This enzyme converts **oxaloacetate** to **phosphoenolpyruvate (PEP)**. A defect would lead to elevated oxaloacetate, not pyruvate or alanine. - While essential for gluconeogenesis, its malfunction would manifest differently from the given elevated substrate levels. *Fructose-1,6-bisphosphatase* - This enzyme catalyzes the dephosphorylation of **fructose-1,6-bisphosphate** to **fructose-6-phosphate**. A defect would cause an accumulation of fructose-1,6-bisphosphate. - This step occurs further down the gluconeogenic pathway and would not directly lead to elevated pyruvate or alanine. *Glucose-6-phosphatase* - **Glucose-6-phosphatase** converts **glucose-6-phosphate** to **free glucose**, the final step in both gluconeogenesis and glycogenolysis. - A defect would result in the accumulation of **glucose-6-phosphate** and **hypoglycemia**, not elevated pyruvate or alanine.

Question 703: Which enzyme deficiency is associated with a history of vomiting, irritability, and jaundice in infants?

A. Fructokinase
B. Aldolase B
C. Galactose-1-phosphate uridyl transferase (Correct Answer)
D. Alpha glucosidase

Explanation: ***Galactose-1-phosphate uridyl transferase*** - Deficiency in **galactose-1-phosphate uridyl transferase** (GALT) causes **classic galactosemia**, leading to the accumulation of toxic galactose metabolites. - This accumulation results in symptoms such as **vomiting, irritability, jaundice, hepatomegaly, cataracts**, and poor feeding in infants once they start consuming milk. *Fructokinase* - Deficiency of **fructokinase** causes **essential fructosuria**, a benign condition where fructose is excreted in the urine. - It is typically **asymptomatic** and does not lead to severe symptoms like vomiting or jaundice. *Aldolase B* - Deficiency of **aldolase B** causes **hereditary fructose intolerance**, leading to severe symptoms upon ingestion of fructose, sucrose, or sorbitol. - While it can manifest with vomiting and jaundice, the clinical picture usually develops **after initial exposure to fructose-containing foods**, which might not be immediate in infants (e.g., when complementary feeding starts). *Alpha glucosidase* - Deficiency of **alpha glucosidase** (also known as acid maltase) causes **Pompe disease** (Type II glycogen storage disease). - This lysosomal storage disorder primarily affects muscle function, leading to **cardiomegaly**, **hypotonia**, and muscle weakness, not typically early-onset vomiting and jaundice without other prominent muscular symptoms.

Question 704: In the liver, what is the primary function of the enzyme glucose-6-phosphatase?

A. Converts glucose-6-phosphate into glucose (Correct Answer)
B. Phosphorylates glucose to glucose-6-phosphate
C. Catalyzes the first step of glycolysis
D. Generates ATP from glucose

Explanation: ***Converts glucose-6-phosphate into glucose*** - **Glucose-6-phosphatase** is a key enzyme in **gluconeogenesis** and **glycogenolysis**, removing the phosphate group from glucose-6-phosphate. - This dephosphorylation allows **free glucose** to be released into the bloodstream, maintaining blood glucose homeostasis. *Phosphorylates glucose to glucose-6-phosphate* - The phosphorylation of glucose to glucose-6-phosphate is catalyzed by **hexokinase** (in most tissues) or **glucokinase** (primarily in the liver). - This reaction traps glucose within the cell and is the initial step for both glycolysis and glycogen synthesis. *Catalyzes the first step of glycolysis* - The first committed step of glycolysis is the phosphorylation of glucose to glucose-6-phosphate, regulated by **hexokinase** or **glucokinase**. - Glucose-6-phosphatase performs the reverse reaction (dephosphorylation) and is active when glucose is being released from the liver. *Generates ATP from glucose* - The generation of ATP from glucose primarily occurs through **glycolysis** and **oxidative phosphorylation** in the mitochondria. - Glucose-6-phosphatase is involved in glucose release, not direct ATP generation from glucose.

Question 705: A patient with hypoglycemia and hepatomegaly is found to have a defect in the enzyme glucose-6-phosphatase. What is the metabolic impact of this deficiency?

A. Impaired gluconeogenesis and glycogenolysis (Correct Answer)
B. Increased glycogen synthesis
C. Enhanced glycolysis
D. Decreased lipid metabolism

Explanation: ***Impaired gluconeogenesis and glycogenolysis*** - Glucose-6-phosphatase is essential for the final step in both **gluconeogenesis** and **glycogenolysis**, converting glucose-6-phosphate to free glucose for release into the bloodstream. - A deficiency in this enzyme, characteristic of **Von Gierke disease (Type I glycogen storage disease)**, prevents the liver from producing and releasing sufficient glucose, leading to **hypoglycemia** and **hepatomegaly** due to accumulated glycogen. *Increased glycogen synthesis* - While glycogen accumulates in the liver due to the inability to break it down, the primary defect isn't an *increase* in synthesis but rather a block in the **breakdown and release** of glucose. - Glycogen synthase activity might even be indirectly affected by the buildup of glucose-6-phosphate, but the core metabolic impact is impaired release. *Enhanced glycolysis* - Glycolysis is the breakdown of glucose, and while some extra glucose-6-phosphate might be shunted towards glycolysis, the overall metabolic picture is dominated by the inability to *produce* glucose from stores or other precursors. - The liver's main role in maintaining blood glucose means impaired glucose release has a far greater systemic impact. *Decreased lipid metabolism* - This deficiency actually leads to **increased lipid synthesis** and **hyperlipidemia**, not decreased lipid metabolism. - The accumulation of glucose-6-phosphate promotes divergent pathways like the **pentose phosphate pathway** and subsequent increase in acetyl-CoA, which serves as a precursor for fatty acid synthesis.

Question 706: Which of the following is not a type of dietary fiber?

A. Starch (Correct Answer)
B. Lignin
C. Cellulose
D. Pectin

Explanation: ***Starch*** - **Starch** is a **complex carbohydrate** that serves as a storage form of glucose in plants and is readily digestible by human enzymes (amylase) into monosaccharides. - While it is a carbohydrate, its ability to be enzymatically broken down and absorbed means it does not meet the definition of dietary fiber. *Pectin* - **Pectin** is a type of **soluble dietary fiber** found in fruits, particularly apples and citrus, and is known for its gelling properties. - It is not digested or absorbed in the small intestine but is fermented by bacteria in the large intestine. *Lignin* - **Lignin** is a **non-carbohydrate dietary fiber**, a complex polymer that provides structural support to plants. - It is generally considered an **insoluble fiber** and passes largely unchanged through the human digestive tract. *Cellulose* - **Cellulose** is a major component of plant cell walls and is a type of **insoluble dietary fiber**. - Humans lack the enzymes to digest cellulose, so it passes through the digestive system largely intact, aiding in bowel regularity.

Question 707: Galactosemia is due to deficiency of which enzyme?

A. Galactose-1-phosphate uridyltransferase (Correct Answer)
B. HGPRT
C. Galactokinase
D. Epimerase

Explanation: ***Galactose-1-phosphate uridyltransferase*** - Deficiency of **galactose-1-phosphate uridyltransferase (GALT)** leads to the most severe form, **classic galactosemia**. - This enzyme is crucial for converting **galactose-1-phosphate** to **glucose-1-phosphate** in the Leloir pathway. *HGPRT* - **HGPRT** (hypoxanthine-guanine phosphoribosyltransferase) deficiency causes **Lesch-Nyhan syndrome**, a distinct metabolic disorder. - Lesch-Nyhan syndrome is characterized by **hyperuricemia**, neurological dysfunction, and self-mutilation, unrelated to galactose metabolism. *Galactokinase* - Deficiency of **galactokinase** causes Type II galactosemia, a milder form than classic galactosemia. - This defect primarily leads to **cataracts** due to galactitol accumulation but does not result in the severe systemic issues seen in classic galactosemia. *Epimerase* - Deficiency of **UDP-galactose-4'-epimerase** (GALE) causes Type III galactosemia, which has a variable clinical presentation from mild to severe. - While involved in galactose metabolism, it's not the primary enzyme deficient in the most common and severe form of **galactosemia**.

Question 708: A 3 months old child was started on supplemental foods along with breastmilk. The child was fed with fruit pulp and sweetened cereals. Soon the child developed bloating of abdomen, vomiting, lethargy, and irritability. On investigation, there was hyperbilirubinemia and elevated transaminase levels. The child is suffering from which of the following enzyme deficiencies?

A. Fructokinase
B. Aldolase B (Correct Answer)
C. Galactose - 1 - phosphate uridyl transferase
D. Galactokinase

Explanation: ***Aldolase B*** - This presentation is characteristic of **hereditary fructose intolerance**, an autosomal recessive disorder caused by a deficiency of **aldolase B**. - Infants typically appear normal until fructose or sucrose (hydrolyzed to glucose and fructose) is introduced into their diet, leading to symptoms like **vomiting**, **bloating**, **lethargy**, and **liver and kidney dysfunction** (hyperbilirubinemia, elevated transaminases). ***Fructokinase*** - Deficiency in fructokinase causes **essential fructosuria**, a benign condition where fructose accumulates in the blood and urine. - It does not lead to the severe gastrointestinal or hepatic symptoms described, as fructose metabolism is not completely blocked. ***Galactokinase*** - Deficiency of galactokinase results in **Type II galactosemia**, primarily causing **cataracts** due to galactitol accumulation. - While galactosemia can present with liver dysfunction, it typically involves lactose intolerance from breastmilk or formula and doesn't align with the introduction of fruit pulp and sweetened cereals as the trigger. ***Galactose-1-phosphate uridyl transferase*** - Deficiency of this enzyme causes **Classic Galactosemia (Type I)**, a severe genetic disorder often diagnosed early due to intolerance to lactose in breast milk or formula. - Symptoms include **vomiting**, **jaundice**, **hepatomegaly**, and **failure to thrive**, with potential for severe complications if untreated. However, the trigger of fruit pulp and sweetened cereals (sources of fructose/sucrose) more strongly points away from galactosemia and towards fructose intolerance.

Question 709: All of the following are true about lactate utilization in the liver except -

A. Cori's cycle shifts the metabolic burden from muscle to liver
B. Cori's cycle can not be sustained indefinitely because it is energetically unfavourable
C. Cori's cycle is linked to glycogen synthesis in muscle
D. Total net number of ATP formed because of Cori's cycle is 4 (Correct Answer)

Explanation: ***Total net number of ATP formed because of cori's cycle is 4*** - This statement is incorrect. The **Cori cycle (lactic acid cycle)** is an energy-consuming process overall, as **6 ATP** molecules are consumed in the liver for gluconeogenesis to resynthesize glucose from lactate, while only a total of **2 ATP** are gained from glycolysis in the muscle. - The primary purpose of the Cori cycle is not net ATP production, but rather to shift the metabolic burden and regenerate glucose for tissues that rely on glycolysis (e.g., muscle, red blood cells). *Cori's cycle shifts the metabolic burden from muscle to liver* - This is true because **lactate produced in muscle** (during anaerobic conditions) is transported to the liver, where it is converted back to glucose. - The liver then bears the metabolic cost of **gluconeogenesis**, allowing the muscle to continue glycolysis and ATP production. *Cori's cycle can not be sustained indefinitely because it is energetically unfavourable* - This is true because the cycle involves a net consumption of ATP. **Six ATP equivalents** are used in gluconeogenesis in the liver to convert two molecules of lactate to one molecule of glucose. - In contrast, the glycolysis that produces the two lactate molecules in muscle yields only **two net ATP**. This energy deficit makes prolonged reliance on the Cori cycle unsustainable. *Cori's cycle is linked to glycogen synthesis in muscle* - This is true because the **glucose produced by the liver** via gluconeogenesis (from lactate) is released into the bloodstream. - This glucose can then be taken up by muscles and other tissues to **replenish glycogen stores** or be used for energy.

Question 710: In glycogen synthesis the active form of glucose used is-

A. Glucose 6 phosphate
B. UDP glucose (Correct Answer)
C. GTP glucose
D. Glucose 1 phosphate

Explanation: **UDP glucose** - **UDP-glucose** (uridine diphosphate glucose) is the activated form of glucose that donates glucose units for the elongation of the **glycogen chain** during glycogen synthesis. - The formation of UDP-glucose from **glucose-1-phosphate** and **UTP** (uridine triphosphate) is catalyzed by UDP-glucose pyrophosphorylase, making glucose-1-phosphate a precursor, not the active form. *Glucose 6 phosphate* - **Glucose 6-phosphate** is an important intermediate in glycolysis and gluconeogenesis, and it can be isomerized to glucose 1-phosphate, but it is not the direct substrate for glycogen synthase. - Its formation is the first committed step in glucose metabolism within the cell, trapping glucose inside. *Glucose I phosphate* - **Glucose 1-phosphate** is a precursor to UDP-glucose, formed from glucose 6-phosphate by **phosphoglucomutase**. - While essential for glycogen synthesis, it is not the directly active form that donates glucose to the glycogen chain itself. *GTP glucose* - **GTP glucose** is not a known active form of glucose involved in glycogen synthesis. - **GTP** (guanosine triphosphate) is primarily involved in other metabolic processes, such as protein synthesis and signal transduction.

Practice by Chapter

Carbohydrate Chemistry and Classification

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Glycolysis: Reactions and Regulation

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Gluconeogenesis: Reactions and Regulation

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Glycogen Metabolism: Synthesis and Breakdown

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Glycogen Storage Diseases

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Pentose Phosphate Pathway

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Metabolism of Fructose and Galactose

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Disorders of Fructose and Galactose Metabolism

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

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Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

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