Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 681: Which pathway is primarily affected in glucose-6-phosphate dehydrogenase deficiency?

A. Gluconeogenesis
B. Glycolysis
C. Pentose phosphate pathway (Correct Answer)
D. Beta-oxidation

Explanation: ***Correct: Pentose phosphate pathway*** - Glucose-6-phosphate dehydrogenase (G6PD) is the **rate-limiting enzyme** of the pentose phosphate pathway (PPP) - G6PD deficiency leads to impaired **NADPH production**, which is critical for maintaining reduced glutathione - Reduced glutathione protects red blood cells from **oxidative damage** - Deficiency results in **hemolytic anemia** when exposed to oxidative stressors (infections, certain drugs, fava beans) *Incorrect: Gluconeogenesis* - This pathway synthesizes **glucose from non-carbohydrate precursors** (primarily in liver and kidney) - G6PD deficiency does not affect the enzymes or substrates involved in glucose synthesis - Gluconeogenesis uses different enzymes (glucose-6-phosphatase, fructose-1,6-bisphosphatase, etc.) *Incorrect: Glycolysis* - Glycolysis is the **metabolic pathway that breaks down glucose** into pyruvate to generate ATP - While glucose-6-phosphate is a substrate for both glycolysis and PPP, G6PD is **not involved in glycolysis** - G6PD deficiency specifically impacts the PPP branch, not the glycolytic enzymes *Incorrect: Beta-oxidation* - This process involves the **breakdown of fatty acids** into acetyl-CoA for energy production - Beta-oxidation is a **mitochondrial process** unrelated to G6PD function - The pentose phosphate pathway occurs in the cytoplasm and involves carbohydrate metabolism

Question 682: Which of the following statements about gluconeogenesis is true?

A. Occurs only in liver
B. Uses ATP (Correct Answer)
C. Activated by insulin
D. Uses only lactate as a substrate

Explanation: ***Uses ATP*** - Gluconeogenesis is an **anabolic process** that synthesizes glucose from non-carbohydrate precursors, requiring significant energy input in the form of **6 ATP and 2 GTP molecules per glucose molecule**. - Key energy-consuming reactions include **pyruvate carboxylase** (uses ATP) and **phosphoenolpyruvate carboxykinase (PEPCK)** (uses GTP). - This high energy requirement distinguishes it from glycolysis, which produces ATP. *Occurs only in liver* - This is **incorrect** as gluconeogenesis occurs predominantly in the **liver (90%)** but also takes place in the **renal cortex (10%)** and to a minimal extent in the epithelial cells of the small intestine. - The liver's role is crucial for maintaining **blood glucose homeostasis** during fasting or starvation. *Activated by insulin* - Gluconeogenesis is **inhibited by insulin**, which signals a state of high blood glucose and promotes glucose utilization and storage. - It is primarily **activated by glucagon and cortisol**, hormones that signal low blood glucose and energy deficit states. *Uses only lactate as a substrate* - This is **incorrect** as gluconeogenesis utilizes multiple substrates, not just lactate. - Key substrates include **lactate** (via the Cori cycle), **amino acids** (especially alanine via the glucose-alanine cycle), **glycerol** (from lipolysis), and **propionate**. - This substrate diversity allows glucose production from various metabolic pathways during fasting.

Question 683: During fasting, which process provides glucose to maintain blood sugar levels?

A. Glycolysis
B. Gluconeogenesis (Correct Answer)
C. Lipolysis
D. Ketogenesis

Explanation: ***Gluconeogenesis*** - **Gluconeogenesis** is the metabolic pathway that synthesizes glucose from non-carbohydrate precursors, such as lactate, glycerol, and amino acids, primarily in the liver and kidneys. - During **fasting**, when dietary glucose is unavailable and glycogen stores are depleted, gluconeogenesis becomes crucial for maintaining **blood glucose homeostasis** to fuel glucose-dependent tissues like the brain. *Glycolysis* - **Glycolysis** is the metabolic pathway that breaks down glucose to produce pyruvate, ATP, and NADH, releasing energy. - This process **consumes glucose** rather than producing it, making it unsuitable for maintaining blood sugar during fasting. *Lipolysis* - **Lipolysis** is the breakdown of triglycerides into fatty acids and glycerol. While glycerol can be used for gluconeogenesis, the primary products, **fatty acids**, cannot be converted to glucose in humans. - It primarily provides **energy substrates** (fatty acids and ketone bodies) for most tissues, sparing glucose for essential organs, but does not directly produce glucose in significant amounts. *Ketogenesis* - **Ketogenesis** is the process by which the liver produces **ketone bodies** from fatty acids when glucose availability is low. - Ketone bodies provide an alternative fuel source for many tissues, including the brain, but they are **not glucose** and do not directly contribute to glucose levels.

Question 684: Which enzyme deficiency is associated with hemolytic anemia due to impaired NADPH production in the pentose phosphate pathway?

A. Glucose-6-phosphate dehydrogenase (Correct Answer)
B. 6-Phosphogluconate dehydrogenase
C. Transketolase
D. Transaldolase

Explanation: ***Glucose-6-phosphate dehydrogenase*** - **Glucose-6-phosphate dehydrogenase (G6PD)** is the rate-limiting enzyme of the **pentose phosphate pathway (PPP)**, producing **NADPH**. - A deficiency in G6PD impairs **NADPH** production, leading to **oxidative stress** in red blood cells and subsequent **hemolytic anemia**. *6-Phosphogluconate dehydrogenase* - This enzyme is also part of the **oxidative phase** of the **PPP** and generates **NADPH**, but its deficiency is much **rarer** and less commonly associated with significant hemolytic anemia than G6PD deficiency. - While it contributes to NADPH production, G6PD is the **primary bottleneck** for NADPH synthesis. *Transketolase* - **Transketolase** is an enzyme in the **non-oxidative phase** of the **PPP**. - Its primary role is to interconvert sugars, and its deficiency is associated with conditions like **Wernicke-Korsakoff syndrome**, not hemolytic anemia. *Transaldolase* - **Transaldolase** is another enzyme in the **non-oxidative phase** of the **PPP**. - It also functions in interconverting sugar phosphates and its deficiency does not directly lead to impaired **NADPH** production or hemolytic anemia.

Question 685: A patient presents with hypoglycemia and lactic acidosis, and genetic testing reveals a defect in the enzyme glucose-6-phosphatase. Which metabolic pathway for releasing stored glucose is directly impaired?

A. Glycogenolysis (Correct Answer)
B. Glycolysis
C. Pentose phosphate pathway
D. Gluconeogenesis

Explanation: ***Glycogenolysis*** * **Glucose-6-phosphatase** is the crucial enzyme in the final step of glycogenolysis, responsible for converting **glucose-6-phosphate** to **free glucose** that can be released into the bloodstream. * A defect in this enzyme prevents the liver from releasing glucose from **glycogen stores** into the bloodstream, leading to **hypoglycemia** despite adequate glycogen reserves. * This is the hallmark of **Von Gierke disease (Glycogen Storage Disease Type I)**, where glycogen accumulates but cannot be mobilized effectively. *Glycolysis* * Glycolysis is the breakdown of glucose for energy production, generating pyruvate and ATP. This pathway does not require glucose-6-phosphatase. * In glucose-6-phosphatase deficiency, glycolysis may actually be *upregulated* as accumulated glucose-6-phosphate is shunted into this pathway, contributing to **lactic acidosis**. *Pentose phosphate pathway* * This pathway produces **NADPH** and **ribose-5-phosphate** for biosynthetic processes and does not require glucose-6-phosphatase. * Accumulated **glucose-6-phosphate** may actually increase flux through this pathway, but it is not directly impaired by the enzyme defect. *Gluconeogenesis* * While **glucose-6-phosphatase** is also required for the final step of gluconeogenesis (synthesizing new glucose from non-carbohydrate precursors), the question specifically asks about **releasing stored glucose**. * **Glycogenolysis** refers to the breakdown of stored glycogen, whereas gluconeogenesis synthesizes glucose de novo from precursors like lactate, amino acids, and glycerol. * Both pathways are impaired in glucose-6-phosphatase deficiency, but only glycogenolysis involves releasing **stored** glucose.

Question 686: What is the primary biochemical effect of insulin on carbohydrate metabolism?

A. It enhances glycogen synthesis (Correct Answer)
B. It increases glucose production from non-carbohydrate sources
C. It promotes fat breakdown
D. It stimulates the breakdown of glycogen

Explanation: ***It enhances glycogen synthesis*** - Insulin's primary role in carbohydrate metabolism is to lower blood glucose by promoting its uptake and storage. - It stimulates the activity of **glycogen synthase**, an enzyme crucial for converting glucose into **glycogen** for storage in the liver and muscles. *It increases glucose production from non-carbohydrate sources* - This process, known as **gluconeogenesis**, is primarily inhibited by insulin. - Hormones like **glucagon** and **cortisol** are responsible for increasing glucose production from non-carbohydrate sources, especially during periods of low blood sugar. *It promotes fat breakdown* - Insulin is an **anabolic hormone** that promotes energy storage, including fat synthesis, and inhibits fat breakdown (lipolysis). - **Glucagon** and **catecholamines** are the hormones that stimulate fat breakdown to provide energy. *It stimulates the breakdown of glycogen* - The breakdown of glycogen into glucose (**glycogenolysis**) is primarily stimulated by **glucagon** and **epinephrine**. - Insulin, conversely, inhibits glycogenolysis to prevent an increase in blood glucose levels.

Question 687: Which of the following pathways is primarily activated in the fed state to store excess glucose?

A. Glycogenolysis
B. Glycogenesis (Correct Answer)
C. Gluconeogenesis
D. Ketogenesis

Explanation: ***Glycogenesis*** - Primarily activated in the **fed state** to convert excess glucose into glycogen for storage, mainly in the **liver and muscle tissues** [1]. - Plays a crucial role in maintaining blood glucose levels by storing glucose when it is abundant and releasing it during fasting. *Ketogenesis* - Activated primarily during **fasting** or low-carbohydrate intake when **acetyl-CoA** is elevated, leading to ketone body production, not glucose storage. - Not relevant for glucose storage, as it focuses on fat metabolism for energy during low glucose availability. *Gluconeogenesis* - Occurs primarily in the **liver** to generate glucose from non-carbohydrate sources, mainly during **fasting** or starvation, rather than the fed state. - This pathway is contraindicated in glucose storage as it works to increase blood sugar levels when they are low. *Glycogenolysis* - Involves the breakdown of glycogen to release glucose [1], typically activated during the **fasting state** or between meals. - Opposite to glycogenesis, as it serves to increase blood glucose levels when they are needed, rather than storing them. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 164-165.

Question 688: In a patient with diabetic ketoacidosis, which of the following biochemical markers is most significantly elevated?

A. Blood urea nitrogen
B. Serum potassium
C. Blood glucose (Correct Answer)
D. Serum sodium

Explanation: ***Blood glucose*** - **Diabetic ketoacidosis (DKA)** is characterized by **severe hyperglycemia** due to insufficient insulin, leading to glucose accumulation in the blood. - Blood glucose levels are typically **above 250 mg/dL** in DKA, often much higher (300-800 mg/dL), making it the most significantly elevated marker **among the given options**. - Note: While **beta-hydroxybutyrate (ketones)** is the hallmark marker of DKA and would be the most significantly elevated if included, glucose is the highest among the options presented. *Blood urea nitrogen* - **Blood urea nitrogen (BUN)** may be elevated in DKA due to **dehydration** and impaired renal perfusion, but it is not the primary or most significantly elevated marker compared to glucose. - The elevation is usually moderate and secondary to the **fluid imbalance** rather than a direct consequence of ketoacidosis itself. *Serum potassium* - **Serum potassium levels** can be variable in DKA; initially, they may appear normal or even elevated due to **extracellular shift** from acidosis, but total body potassium is typically depleted. - During treatment, potassium levels typically **fall** as insulin drives potassium back into cells, requiring careful monitoring and replacement. - While important for monitoring, **hyperkalemia** is not the defining or most significant biochemical elevation in DKA. *Serum sodium* - **Serum sodium levels** are often **artificially lowered** (pseudohyponatremia) in DKA due to the osmotic effect of severe hyperglycemia, drawing water from the intracellular to extracellular space. - Corrected sodium may be normal or elevated, but significant elevation of serum sodium is not a characteristic feature of DKA.

Question 689: What is the primary mechanism by which the body maintains blood glucose levels during prolonged fasting?

A. Gluconeogenesis from amino acids and glycerol (Correct Answer)
B. Glycogenolysis from liver glycogen
C. Glycolysis in muscle tissue
D. Lipolysis in adipose tissue

Explanation: ***Gluconeogenesis from amino acids and glycerol*** - During **prolonged fasting**, liver and kidney produce **glucose** from non-carbohydrate precursors like **amino acids** (protein breakdown) and **glycerol** (triglyceride breakdown) to maintain blood glucose. - This process is crucial as **glycogen stores** are depleted after a few hours of fasting. *Glycogenolysis from liver glycogen* - **Glycogenolysis** (breakdown of glycogen) is the primary mechanism in the initial stages of fasting (first 12-24 hours). - However, **liver glycogen stores** are finite and are typically depleted after about 24 hours, making it insufficient for *prolonged* fasting. *Glycolysis in muscle tissue* - **Glycolysis** is the breakdown of glucose for energy, primarily in muscle tissue. - While muscles do break down their own glycogen, the glucose-6-phosphate produced cannot be released into the bloodstream to maintain blood glucose levels because muscle cells lack **glucose-6-phosphatase**. *Lipolysis in adipose tissue* - **Lipolysis** is the breakdown of triglycerides in adipose tissue into **fatty acids** and **glycerol**. - While **glycerol** can be used for gluconeogenesis, the **fatty acids** themselves cannot be directly converted to glucose in humans, though they can be metabolized to **ketone bodies** which serve as an alternative fuel for some tissues.

Question 690: During prolonged fasting, how does the liver maintain blood glucose levels?

A. Converts non-carbohydrate substrates to glucose (Correct Answer)
B. Breaks down stored hepatic glycogen to glucose
C. Increases FA oxidation to glucose
D. Promotes lipogenesis to store glucose

Explanation: ***Converts non-carbohydrate substrates to glucose*** - During **prolonged fasting**, the liver maintains glucose homeostasis primarily through **gluconeogenesis**. - **Non-carbohydrate precursors** such as amino acids (e.g., alanine), lactate, and glycerol are converted to glucose to meet the body's energy demands, especially for the brain. - After **24-48 hours of fasting**, hepatic glycogen stores are depleted, making gluconeogenesis the predominant mechanism. *Breaks down stored hepatic glycogen to glucose* - **Hepatic glycogenolysis** does contribute to maintaining blood glucose, but only in the **early phases of fasting** (first 24-48 hours). - During **prolonged fasting**, liver glycogen stores become **depleted**, and this mechanism can no longer sustain blood glucose levels. - Gluconeogenesis becomes the primary pathway after glycogen depletion. *Increases FA oxidation to glucose* - **Fatty acid oxidation (beta-oxidation)** generates acetyl-CoA, which can enter the citric acid cycle for energy production. - However, there is **no net conversion of fatty acids to glucose** in humans because acetyl-CoA cannot be converted back to pyruvate for gluconeogenesis (pyruvate dehydrogenase is irreversible). - Only the **glycerol backbone** from triglycerides can be used for gluconeogenesis, not the fatty acid chains. *Promotes lipogenesis to store glucose* - **Lipogenesis** is the process of synthesizing fatty acids and triglycerides from glucose and other precursors for energy storage. - This process occurs primarily in the **fed state** when there is an abundance of glucose, not during prolonged fasting when glucose is scarce and must be produced, not stored.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

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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