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

Which among the following glucose transporters is present in beta cells?

Q710

Which glucose transporter is primarily affected in diabetes mellitus?

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 701: What is the primary metabolic consequence of pyruvate carboxylase deficiency?

A. Increased glycolysis due to energy demand
B. Accumulation of pyruvate due to blocked conversion
C. Decreased gluconeogenesis leading to hypoglycemia (Correct Answer)
D. Increased lactic acid production due to impaired gluconeogenesis

Explanation: ***Decreased gluconeogenesis leading to hypoglycemia*** - Pyruvate carboxylase catalyzes the conversion of **pyruvate to oxaloacetate**, the first committed step of **gluconeogenesis**. - A deficiency impairs glucose synthesis from non-carbohydrate sources, leading to severe **fasting hypoglycemia**, especially in neonates and infants. - This represents the PRIMARY defect in the **metabolic pathway** for glucose homeostasis. *Increased glycolysis due to energy demand* - Pyruvate carboxylase deficiency does not directly increase glycolysis. - In fact, **hypoglycemia** would limit glucose availability for glycolysis, making this option incorrect. - The deficiency affects gluconeogenesis (glucose synthesis), not glycolysis (glucose breakdown). *Accumulation of pyruvate due to blocked conversion* - While pyruvate does accumulate when its conversion to oxaloacetate is blocked, this is an **intermediate biochemical finding** rather than the primary metabolic consequence. - The clinically significant outcomes are what happens **downstream**: impaired glucose production and increased lactate formation. *Increased lactic acid production due to impaired gluconeogenesis* - This is indeed a **major clinical consequence** of pyruvate carboxylase deficiency, causing severe **lactic acidosis**. - Accumulated pyruvate is shunted to lactate via lactate dehydrogenase. - However, when specifically asking for the PRIMARY **metabolic consequence** of the enzyme deficiency itself, it is the **impaired gluconeogenesis** (the direct pathway affected) that is most primary, with lactic acidosis being a consequent metabolic derangement from the accumulated substrate.

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: 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 705: 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 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: Which among the following glucose transporters is present in beta cells?

A. GLUT1
B. GLUT2 (Correct Answer)
C. GLUT3
D. GLUT4

Explanation: ***GLUT2*** - **GLUT2** is a **low-affinity** glucose transporter predominantly found in pancreatic **beta cells**, liver, kidneys, and intestines. - Its low affinity allows beta cells to accurately sense high blood glucose levels, triggering **insulin release**. *GLUT1* - **GLUT1** is a widely distributed glucose transporter found in nearly all mammalian cells, including **red blood cells** and cells of the **blood-brain barrier**. - It exhibits **high affinity** for glucose, responsible for basal glucose uptake. *GLUT3* - **GLUT3** is a high-affinity glucose transporter primarily found in **neurons** and the **placenta**. - Its high affinity ensures a constant glucose supply to these metabolically demanding tissues, even at low blood glucose concentrations. *GLUT4* - **GLUT4** is an **insulin-sensitive** glucose transporter found in **adipose tissue** and **striated muscle** (skeletal and cardiac). - Its translocation to the cell surface from intracellular vesicles is stimulated by insulin, promoting glucose uptake into these tissues.

Question 710: Which glucose transporter is primarily affected in diabetes mellitus?

A. GLUT-2
B. GLUT-5
C. GLUT-4 (Correct Answer)
D. SGLT-2

Explanation: ***GLUT-4*** - **GLUT-4** is the primary glucose transporter in **insulin-sensitive** tissues such as muscle and adipose tissue. - In **diabetes mellitus**, impaired insulin signaling leads to reduced translocation of GLUT-4 to the cell membrane, resulting in decreased glucose uptake by these tissues and subsequently **hyperglycemia**. *GLUT-2* - **GLUT-2** is found in the **liver**, **pancreatic beta cells**, kidneys, and small intestine. - It has a low affinity for glucose and is primarily involved in **high-capacity glucose transport**, serving as a glucose sensor in beta cells and allowing efficient glucose uptake/release in the liver. *GLUT-5* - **GLUT-5** is a fructose transporter predominantly found in the **small intestine** and testes. - It is responsible for the absorption of **fructose** from the diet and is not directly involved in glucose regulation relevant to diabetes mellitus. *SGLT-2* - **SGLT-2** (Sodium-Glucose Co-transporter 2) is found in the **proximal tubules of the kidneys**. - It is responsible for reabsorbing approximately 90% of the **filtered glucose** from the renal filtrate back into the bloodstream, and its inhibition is a therapeutic target in diabetes.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

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

Practice Questions

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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