Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 101: What is the effect of oral administration of 50 gm of glucose on the body?

A. Decreased ketone body production
B. Increased lactate production upon exercise
C. Decreased gluconeogenesis (Correct Answer)
D. Increased gluconeogenesis

Explanation: ### Explanation **Core Concept: The Fed State and Insulin Action** The oral administration of 50g of glucose triggers a transition from the fasting state to the **absorptive (fed) state**. This leads to a rise in blood glucose levels, which stimulates the pancreatic $\beta$-cells to secrete **insulin** while suppressing glucagon secretion. **Why "Decreased Gluconeogenesis" is Correct:** Insulin is a potent anabolic hormone. It decreases gluconeogenesis through two primary mechanisms: 1. **Enzyme Regulation:** It transcriptionally represses key rate-limiting enzymes of gluconeogenesis, specifically **PEPCK** (Phosphoenolpyruvate carboxykinase) and **Glucose-6-phosphatase**. 2. **Substrate Availability:** Insulin inhibits lipolysis in adipose tissue, reducing the supply of glycerol and acetyl-CoA (an activator of pyruvate carboxylase), thereby starving the gluconeogenic pathway of necessary precursors and activators. **Analysis of Incorrect Options:** * **A. Decreased ketone body production:** While insulin does inhibit ketogenesis, this is a secondary effect. In a healthy individual with 50g of glucose, the primary metabolic shift is the suppression of endogenous glucose production (gluconeogenesis) to prevent hyperglycemia. * **B. Increased lactate production upon exercise:** Lactate production during exercise depends on the intensity of muscle contraction and oxygen availability (anaerobic glycolysis), not directly on the preceding oral glucose load. * **D. Increased gluconeogenesis:** This is physiologically opposite to the effect of insulin. Gluconeogenesis increases during fasting, starvation, or in states of insulin deficiency (e.g., Diabetes Mellitus). **NEET-PG High-Yield Pearls:** * **Insulin/Glucagon Ratio:** The metabolic direction of the liver is determined by this ratio. A high ratio (post-glucose) favors glycolysis and glycogenesis while inhibiting gluconeogenesis and glycogenolysis. * **Key Inhibitor:** Fructose 2,6-bisphosphate is the most potent allosteric effector; it increases after glucose intake, stimulating PFK-1 (glycolysis) and inhibiting Fructose 1,6-bisphosphatase (gluconeogenesis). * **Metformin Connection:** Remember that Metformin, the first-line drug for Type 2 Diabetes, works primarily by inhibiting hepatic gluconeogenesis.

Question 102: Which of the following is a component of a polysaccharide?

A. Synovium
B. Glucosamine (Correct Answer)
C. Glucuronic acid
D. All of the above

Explanation: ### Explanation **Correct Answer: B. Glucosamine** **Understanding the Concept:** Polysaccharides, specifically **Mucopolysaccharides** (also known as Glycosaminoglycans or GAGs), are long unbranched chains composed of repeating disaccharide units. These units typically consist of an **amino sugar** and a **uronic acid**. **Glucosamine** is an amino sugar (specifically an aldohexose where the hydroxyl group at C2 is replaced by an amino group). It is a fundamental building block of several important polysaccharides, such as **Chitin** (found in fungal cell walls and arthropod exoskeletons) and various GAGs like **Heparin** and **Hyaluronic acid**. **Analysis of Options:** * **A. Synovium:** This is an anatomical structure (the synovial membrane) that lines joints. While it *secretes* synovial fluid containing the polysaccharide hyaluronic acid, the synovium itself is a tissue, not a chemical component of a polysaccharide. * **C. Glucuronic acid:** While glucuronic acid is indeed a component of many GAGs (like Chondroitin sulfate), the question asks for "a" component. In many standardized biochemistry contexts and specific MCQ frames, Glucosamine is highlighted as the primary amino sugar precursor. However, in a strictly chemical sense, if this were a "multiple correct" format, C would also be true. In the context of NEET-PG, Glucosamine is often the preferred answer when discussing the basic monomeric amino sugar unit. * **D. All of the above:** Incorrect because "Synovium" is a tissue, not a molecule. **High-Yield Clinical Pearls for NEET-PG:** * **Hyaluronic acid** is unique among GAGs because it is **not sulfated** and is not covalently bound to a protein core. * **Heparin** is the most highly acidic (negatively charged) molecule in the human body due to its high sulfate content. * **Hurler and Hunter Syndromes** are Mucopolysaccharidoses caused by the deficiency of lysosomal enzymes required to degrade GAGs like Dermatan and Heparan sulfate. * **Glucosamine supplements** are clinically used to support cartilage repair in osteoarthritis.

Question 103: Why is fructose not used in intravenous infusions?

A. It can cause irritability.
B. It can cause mental retardation.
C. It can increase erythrocyte protoporphyrin.
D. None of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **D (None of the above)** because the primary medical reason for avoiding fructose in intravenous (IV) infusions is its potential to cause **acute depletion of intracellular ATP** and subsequent metabolic derangements, particularly in patients with undiagnosed Hereditary Fructose Intolerance (HFI). **Why the correct answer is right:** Fructose is metabolized in the liver by **Fructokinase**, which rapidly phosphorylates fructose to Fructose-1-Phosphate. Unlike glucose metabolism, this step is not rate-limited by insulin or feedback inhibition. Rapid IV administration leads to "phosphate trapping," where inorganic phosphate is consumed to form Fructose-1-Phosphate. This results in: 1. **ATP Depletion:** Lack of Pi prevents ATP regeneration. 2. **Hyperuricemia:** Increased breakdown of adenine nucleotides (due to low ATP) leads to uric acid production. 3. **Lactic Acidosis:** Rapid glycolysis increases lactate levels. In patients with HFI (Aldolase B deficiency), this can lead to acute liver failure and hypoglycemia. **Why other options are wrong:** * **Options A & B (Irritability and Mental Retardation):** These are classic features of **Galactosemia** (due to Galactose-1-phosphate uridyltransferase deficiency), not fructose infusion. * **Option C (Erythrocyte Protoporphyrin):** Increased levels of free erythrocyte protoporphyrin are a hallmark of **Iron Deficiency Anemia** or **Lead Poisoning**, unrelated to fructose metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Essential Fructosuria:** Deficiency of Fructokinase; a benign condition where fructose appears in urine (reducing sugar positive, glucose oxidase negative). * **Hereditary Fructose Intolerance (HFI):** Deficiency of **Aldolase B**. Symptoms appear when a baby is weaned from breast milk and introduced to fruit juices/sucrose. * **Key Enzyme:** Fructokinase has a much higher affinity for fructose than Hexokinase, explaining why fructose is primarily metabolized in the liver.

Question 104: Inulin, like other fructans, is used as a prebiotic because it is non-digestible. What causes its resistance to digestion in the upper gastrointestinal tract?

A. Absence of digestive enzymes in the upper gastrointestinal tract
B. Beta configuration of the anomeric carbon (C2) (Correct Answer)
C. Low pH of the stomach
D. Presence of alpha-glycosidic linkages

Explanation: ### Explanation **1. Why Option B is Correct:** Inulin is a polymer of fructose molecules (fructan) linked by **$\beta(2 \to 1)$ glycosidic bonds**. Human digestive enzymes, such as salivary and pancreatic amylases, are stereospecific; they are designed to hydrolyze **alpha-glycosidic linkages** (like those in starch and glycogen). The **beta configuration** of the anomeric carbon (C2) in inulin makes it structurally resistant to these enzymes. Consequently, inulin passes through the upper gastrointestinal tract intact and reaches the colon, where it is fermented by gut microbiota, serving as a **prebiotic**. **2. Why Other Options are Incorrect:** * **Option A:** The upper GI tract contains numerous digestive enzymes (amylase, maltase, sucrase, etc.). The resistance is not due to a lack of enzymes in general, but the lack of a *specific* enzyme (inulinase) capable of breaking the $\beta(2 \to 1)$ bond. * **Option C:** Low stomach pH can denature proteins and aid in some hydrolysis, but it is not the primary reason for the indigestibility of complex dietary fibers like inulin. * **Option D:** This is factually incorrect. Inulin contains **beta-linkages**. Alpha-glycosidic linkages (found in starch) are easily digested by human enzymes. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **GFR Measurement:** Inulin is the **gold standard** for measuring Glomerular Filtration Rate (GFR) because it is freely filtered by the glomerulus and is neither reabsorbed nor secreted by the renal tubules. * **Prebiotic vs. Probiotic:** Inulin is a *prebiotic* (food for bacteria), whereas *probiotics* are the live beneficial bacteria themselves (e.g., *Lactobacillus*). * **Structure:** Inulin consists of a chain of fructose units typically ending in a terminal glucose unit. * **Diagnostic Use:** Inulin clearance is used in research, though **Creatinine clearance** is more common in clinical practice due to ease of use.

Question 105: Under anaerobic conditions, the glycolysis of one mole of glucose yields how many moles of ATP?

A. One
B. Two (Correct Answer)
C. Eight
D. Thirty

Explanation: **Explanation:** In anaerobic glycolysis (occurring in the absence of oxygen or in cells lacking mitochondria like RBCs), the metabolic pathway converts one mole of glucose into two moles of **lactate**. 1. **Why Option B is Correct:** The process involves two phases: * **Investment Phase:** 2 ATP molecules are consumed (at the Hexokinase and Phosphofructokinase-1 steps). * **Payoff Phase:** 4 ATP molecules are generated via substrate-level phosphorylation (2 ATP from 1,3-bisphosphoglycerate and 2 ATP from Phosphoenolpyruvate). * **Net Yield:** 4 (Produced) – 2 (Invested) = **2 ATP**. Crucially, the NADH produced during the glyceraldehyde-3-phosphate dehydrogenase step is re-oxidized to NAD+ by converting pyruvate to lactate (catalyzed by Lactate Dehydrogenase). This ensures the cycle continues but prevents the NADH from entering the electron transport chain, resulting in no additional ATP. 2. **Why Other Options are Incorrect:** * **Option A:** Incorrect; the payoff phase generates more than is invested. * **Option C:** 8 ATP is the net yield of **aerobic glycolysis** in certain tissues (using the Malate-Aspartate shuttle) where NADH is oxidized in the mitochondria. * **Option D:** 30 (or 32) ATP is the total yield from the **complete oxidation** of glucose through glycolysis, the TCA cycle, and oxidative phosphorylation. **High-Yield Clinical Pearls for NEET-PG:** * **Mature RBCs** rely exclusively on anaerobic glycolysis for energy because they lack mitochondria. * **Lactic Acidosis:** Occurs when there is excessive anaerobic glycolysis (e.g., during shock or severe hypoxia), leading to lactate accumulation. * **Rapoport-Luebering Cycle:** A shunt in RBC glycolysis that produces 2,3-BPG, which decreases hemoglobin's affinity for oxygen, shifting the dissociation curve to the right. This process yields **zero net ATP**.

Question 106: Which mucopolysaccharide does not contain uronic acid?

A. Heparin
B. Chondroitin sulfate
C. Dermatan sulfate
D. Keratan sulfate (Correct Answer)

Explanation: **Explanation:** Mucopolysaccharides, also known as **Glycosaminoglycans (GAGs)**, are long, unbranched polysaccharides consisting of repeating disaccharide units. Typically, these units consist of an **amino sugar** (D-glucosamine or D-galactosamine) and a **uronic acid** (D-glucuronic acid or L-iduronic acid). **Keratan sulfate** is the unique exception to this rule. Instead of uronic acid, it contains **Galactose** linked to N-acetylglucosamine. Because it lacks uronic acid, it is the most distinct member of the GAG family in terms of chemical composition. **Analysis of Options:** * **Heparin:** Contains D-glucuronic acid or L-iduronic acid. It is the most highly anionic (acidic) GAG and acts as a natural anticoagulant. * **Chondroitin sulfate:** Contains D-glucuronic acid. It is the most abundant GAG in the body, found prominently in cartilage and bone. * **Dermatan sulfate:** Contains L-iduronic acid (formed by the epimerization of glucuronic acid). It is found mainly in the skin, blood vessels, and heart valves. **High-Yield Clinical Pearls for NEET-PG:** * **Hyaluronic Acid:** The only GAG that is **not sulfated** and not covalently bound to a protein (does not form proteoglycans). * **Heparin vs. Heparan Sulfate:** Heparin is intracellular (mast cells), while Heparan sulfate is extracellular (basement membranes). * **Hurler & Hunter Syndromes:** These are Mucopolysaccharidoses (MPS) caused by the inability to degrade GAGs (specifically Dermatan and Heparan sulfate), leading to skeletal deformities and mental retardation. * **Keratan Sulfate Location:** Primarily found in the **cornea** (maintains transparency) and cartilage.

Question 107: Cerebroside contains which sugar?

A. Ribose
B. Fructose
C. Galactose (Correct Answer)
D. Pentose

Explanation: **Explanation:** **Cerebrosides** are a type of **glycosphingolipid** (neutral glycolipids) that are essential components of nerve cell membranes, particularly the myelin sheath. 1. **Why Galactose is Correct:** A cerebroside consists of a **ceramide** unit (sphingosine + fatty acid) linked to a single sugar residue via a glycosidic bond. In the nervous system, the most common sugar found in cerebrosides is **Galactose**, forming **Galactosylceramide (Galactocerebroside)**. While Glucocerebrosides exist (found primarily in non-neural tissues), Galactose is the classic and most characteristic sugar associated with cerebrosides in medical biochemistry. 2. **Why Other Options are Incorrect:** * **Ribose:** This is a pentose sugar found in RNA and nucleotides (ATP/GTP), not in structural glycolipids. * **Fructose:** This is a ketohexose involved in the glycolytic pathway and seminal fluid; it does not form part of sphingolipids. * **Pentose:** This is a general category of 5-carbon sugars (like Ribose or Xylose). Cerebrosides specifically require hexose sugars (6-carbon). **NEET-PG High-Yield Clinical Pearls:** * **Krabbe’s Disease:** Caused by a deficiency of the enzyme **Galactocerebrosidase**, leading to the accumulation of galactocerebroside (presents with globoid cells and demyelination). * **Gaucher’s Disease:** The most common lysosomal storage disorder, caused by a deficiency of **Glucocerebrosidase**, leading to the accumulation of glucocerebroside (presents with "wrinkled paper" cytoplasm in macrophages). * **Gangliosides vs. Cerebrosides:** While cerebrosides contain a single sugar, gangliosides are complex sphingolipids containing oligosaccharides and at least one **Sialic acid (NANA)** residue.

Question 108: Which of the following is true for phosphofructokinase?

A. It uses Fructose 1,6 bisphosphate as substrate.
B. It generates ATP
C. It catalyzes an irreversible reaction in glycolysis (Correct Answer)
D. It is activated by increased levels of ATP and citrate.

Explanation: **Phosphofructokinase-1 (PFK-1)** is the most important regulatory enzyme and the **rate-limiting step** of glycolysis. ### **Explanation of the Correct Answer** **Option C** is correct because PFK-1 catalyzes the phosphorylation of Fructose-6-Phosphate to Fructose-1,6-Bisphosphate. This reaction is highly exergonic ($\Delta G$ is strongly negative), making it **physiologically irreversible**. In the cell, this step serves as the "committed step"; once glucose is converted to Fructose-1,6-bisphosphate, it is destined to complete glycolysis. ### **Analysis of Incorrect Options** * **Option A:** PFK-1 uses **Fructose-6-Phosphate** and ATP as substrates. Fructose-1,6-bisphosphate is the *product* of the reaction. * **Option B:** PFK-1 **consumes ATP** rather than generating it. It transfers a phosphate group from ATP to the substrate. ATP generation in glycolysis occurs later via Phosphoglycerate Kinase and Pyruvate Kinase (substrate-level phosphorylation). * **Option D:** PFK-1 is **inhibited** by high levels of ATP and Citrate (signals of high energy status). It is **activated** by AMP and **Fructose-2,6-bisphosphate** (the most potent allosteric activator). ### **High-Yield Clinical Pearls for NEET-PG** * **Rate-Limiting Step:** PFK-1 is the "Pacemaker" of glycolysis. * **Hormonal Regulation:** Insulin increases PFK-1 activity (via Fructose-2,6-BP), while Glucagon decreases it. * **Tissues:** In the liver, citrate inhibition of PFK-1 helps prioritize glucose for glycogen synthesis when energy is abundant. * **Comparison:** Do not confuse PFK-1 with **PFK-2**, which synthesizes Fructose-2,6-bisphosphate, the regulator that turns on PFK-1.

Question 109: High-energy phosphate is not produced in which of the following metabolic pathways?

A. TCA cycle
B. Hexose monophosphate pathway (Correct Answer)
C. Glycolysis
D. Beta oxidation of fatty acid

Explanation: The correct answer is **B. Hexose monophosphate (HMP) pathway**, also known as the Pentose Phosphate Pathway (PPP). ### **Why HMP Pathway is Correct** The HMP pathway is a unique metabolic shunt that does **not** involve the production or consumption of ATP (high-energy phosphate). Instead, its primary objectives are: 1. **Production of NADPH:** Used for reductive biosynthesis (fatty acids, steroids) and maintaining reduced glutathione to prevent oxidative stress. 2. **Production of Ribose-5-phosphate:** Essential for nucleotide and nucleic acid synthesis. Because it bypasses the ATP-generating steps of glycolysis, it is considered a non-energetic pathway. ### **Why Other Options are Incorrect** * **TCA Cycle:** Produces high-energy phosphate directly via **Substrate Level Phosphorylation** (Succinyl CoA to Succinate produces **GTP**, which is energetically equivalent to ATP). * **Glycolysis:** Produces ATP via Substrate Level Phosphorylation at two steps: Phosphoglycerate kinase and Pyruvate kinase. * **Beta Oxidation:** While the pathway itself produces FADH₂ and NADH, these enter the Electron Transport Chain to generate large amounts of ATP via oxidative phosphorylation. ### **NEET-PG High-Yield Pearls** * **Rate-limiting enzyme of HMP:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **G6PD Deficiency:** Leads to hemolytic anemia due to the inability to produce NADPH, which is required to keep glutathione reduced in RBCs. * **Tissue Distribution:** HMP pathway is most active in tissues requiring NADPH (Adrenal cortex, Liver, Lactating mammary glands, and RBCs). * **Thiamine (B1) Connection:** Transketolase, an enzyme in the non-oxidative phase of HMP, requires Thiamine pyrophosphate as a cofactor. Measuring its activity is used to diagnose Thiamine deficiency.

Question 110: Glycogenin is a protein with self-glucosylation capacity. To which amino acid of glycogenin are glucose molecules attached?

A. Serine
B. Threonine
C. Tyrosine (Correct Answer)
D. Hydroxylysine

Explanation: **Explanation:** **Glycogenin** is a specialized protein that acts as a primer for glycogen synthesis. Since **Glycogen Synthase** can only add glucose units to an existing chain of at least four glucose residues, glycogenin initiates the process through **autocatalysis**. 1. **Why Tyrosine is Correct:** Glycogenin possesses glucosyltransferase activity. It attaches the first glucose molecule from UDP-glucose to the **hydroxyl (-OH) group** of a specific **Tyrosine residue (Tyr-194)** within its own structure. This serves as the foundation upon which a short chain of about 8 glucose units is built, providing the necessary primer for Glycogen Synthase to take over. 2. **Why Other Options are Incorrect:** * **Serine & Threonine:** While these amino acids also have hydroxyl groups and are common sites for O-linked glycosylation in many glycoproteins (and phosphorylation sites for enzyme regulation), they are not the attachment sites for the glycogen primer. * **Hydroxylysine:** This is a modified amino acid primarily found in collagen, where it serves as a site for glycosylation (attachment of glucose and galactose), but it plays no role in glycogen metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Glycogenin** remains buried at the core of every mature glycogen granule. * **UDP-Glucose** is the active donor of glucose units for both glycogenin and glycogen synthase. * **Branching Enzyme (4:6 transferase)** is required to create $\alpha$1-6 linkages, while **Glycogen Synthase** only creates $\alpha$1-4 linkages. * In the absence of glycogenin, glycogen synthesis cannot be initiated, a concept relevant to understanding certain rare Glycogen Storage Diseases (GSD Type 0).

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

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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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