Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 161: What is the source of ATP in red blood cells?

A. Beta oxidation of fatty acids
B. TCA cycle
C. Anaerobic glycolysis (Correct Answer)
D. Gluconeogenesis

Explanation: ### Explanation **Correct Answer: C. Anaerobic glycolysis** **1. Why Anaerobic Glycolysis is Correct:** Mature red blood cells (RBCs) lack **mitochondria**, which are the "powerhouses" required for aerobic respiration (TCA cycle and Electron Transport Chain). Consequently, RBCs cannot utilize oxygen for energy production, despite being the primary carriers of oxygen. They rely exclusively on **anaerobic glycolysis** (the Embden-Meyerhof pathway) in the cytosol to convert glucose into lactate, generating a net gain of **2 ATP** molecules per glucose molecule. This ATP is vital for maintaining ion gradients (Na⁺/K⁺ ATPase pump) and erythrocyte membrane integrity. **2. Why Other Options are Incorrect:** * **A. Beta-oxidation of fatty acids:** This process occurs within the mitochondrial matrix. Since RBCs lack mitochondria, they cannot oxidize fatty acids for energy. * **B. TCA cycle:** The enzymes for the Tricarboxylic Acid (TCA) cycle are located in the mitochondria. Without these organelles, RBCs cannot perform aerobic metabolism. * **D. Gluconeogenesis:** This is the synthesis of glucose from non-carbohydrate precursors (primarily occurring in the liver and kidneys). RBCs are consumers of glucose, not producers. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Rapoport-Luebering Shunt:** A side pathway of glycolysis in RBCs that produces **2,3-BPG**. This molecule is crucial as it decreases the affinity of hemoglobin for oxygen, facilitating oxygen delivery to tissues. * **Lactate Production:** The end product of glycolysis in RBCs is always lactate, which is released into the blood and taken up by the liver for gluconeogenesis (**Cori Cycle**). * **G6PD Deficiency:** The HMP shunt (Pentose Phosphate Pathway) is also vital in RBCs to produce **NADPH**, which maintains reduced glutathione to protect the cell against oxidative damage. Deficiency leads to hemolytic anemia.

Question 162: Which of the following represents the normal glucose tolerance curve?

A. Curve A
B. Curve B (Correct Answer)
C. Curve C
D. None of the above

Explanation: ***Curve B*** - Demonstrates the normal **glucose tolerance pattern** with fasting glucose around **80-90 mg/dL**, peak at **120-140 mg/dL** at 30-60 minutes, and return to near-fasting levels by **2 hours**. - Represents normal **insulin sensitivity** and **pancreatic beta-cell function** with appropriate glucose clearance from blood. *Curve A* - Shows a **diabetic pattern** with elevated fasting glucose and peak values exceeding **200 mg/dL** during oral glucose tolerance test. - Blood glucose remains **persistently elevated** at 2 hours (>200 mg/dL), indicating **impaired glucose tolerance** or diabetes mellitus. *Curve C* - Demonstrates an **abnormal flat curve** pattern, possibly indicating **renal glycosuria** or **alimentary hypoglycemia**. - May show **minimal glucose rise** despite oral glucose load, suggesting **malabsorption** or rapid glucose clearance without normal physiological response. *None of the above* - This option is incorrect as **Curve B** clearly represents the normal glucose tolerance curve pattern. - The **WHO/ADA criteria** for normal OGTT are well-established, making one of the given curves definitively correct.

Question 163: In the TCA cycle, which two carbon atoms leave in the form of CO2 are derived from:

A. Acetyl CoA
B. Oxaloacetate (Correct Answer)
C. CO2
D. Citrate

Explanation: ### Explanation The Citric Acid Cycle (TCA cycle) is a series of reactions used by all aerobic organisms to generate energy. Understanding the carbon tracking within this cycle is a high-yield concept for NEET-PG. **Why Oxaloacetate is Correct:** When Acetyl CoA (2C) condenses with Oxaloacetate (4C) to form Citrate (6C), the two carbons that are lost as $CO_2$ during the oxidative decarboxylation steps (catalyzed by Isocitrate Dehydrogenase and $\alpha$-Ketoglutarate Dehydrogenase) are **not** the same carbons that just entered via Acetyl CoA. Isotopic labeling studies show that the two carbons released as $CO_2$ in any single turn of the cycle are derived from the **oxaloacetate** molecule used in that specific turn. The carbons from Acetyl CoA remain in the cycle intermediates (as part of Succinate, Fumarate, etc.) and are only eligible for release in subsequent turns of the cycle. **Analysis of Incorrect Options:** * **A. Acetyl CoA:** While Acetyl CoA provides the two carbons that enter the cycle, they are incorporated into the backbone of the intermediates and are released as $CO_2$ only in later rounds. * **C. $CO_2$:** This is a byproduct of the cycle, not the source of the carbon atoms being released. * **D. Citrate:** Citrate is the first intermediate formed, but it contains carbons from both precursors. The question specifically asks for the *origin* of the released carbons. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Isocitrate Dehydrogenase (the first $CO_2$ releasing step). * **Energy Yield:** One turn produces 3 NADH, 1 $FADH_2$, and 1 GTP (Total ~10 ATP). * **Thiamine (B1) Dependency:** $\alpha$-Ketoglutarate Dehydrogenase requires Thiamine pyrophosphate. Deficiency (Wernicke-Korsakoff) impairs the TCA cycle, severely affecting ATP-dependent organs like the brain. * **Amphibolic Nature:** The TCA cycle is both catabolic (energy production) and anabolic (provides precursors for gluconeogenesis and amino acid synthesis).

Question 164: Where is Keratan sulfate 1 primarily located?

A. Skin
B. Bone
C. Cornea (Correct Answer)
D. Lung

Explanation: **Explanation:** Keratan sulfate (KS) is a unique glycosaminoglycan (GAG) because it contains galactose instead of the usual uronic acid. It is classified into two types based on its location and linkage: * **Keratan Sulfate I (KS I):** This is primarily found in the **cornea**. It is essential for corneal transparency; the precise spacing of collagen fibrils maintained by KS I allows light to pass through without scattering. * **Keratan Sulfate II (KS II):** This is found in skeletal tissues like cartilage and bone. **Analysis of Options:** * **Option C (Cornea):** Correct. KS I is the specific isomer found here. Clinical deficiency or sulfation defects lead to **Macular Corneal Dystrophy**, characterized by corneal opacity. * **Option A (Skin):** Incorrect. The predominant GAG in the skin is **Dermatan sulfate** (also found in blood vessels and heart valves). * **Option B (Bone):** Incorrect. While Keratan sulfate II is found in bone and cartilage, KS I is specific to the cornea. The main GAG in bone/cartilage is **Chondroitin sulfate**. * **Option D (Lung):** Incorrect. Heparan sulfate and Heparin are more relevant in pulmonary and vascular tissues. **High-Yield NEET-PG Pearls:** 1. **Unique Structure:** Keratan sulfate is the only GAG that **lacks uronic acid** (it has Galactose instead). 2. **Linkage:** KS I is **N-linked** (to asparagine), whereas KS II is **O-linked** (to serine/threonine). 3. **Morquio Syndrome (MPS IV):** This is the specific Mucopolysaccharidosis associated with the inability to degrade Keratan sulfate, leading to severe skeletal dysplasia but (notably) no mental retardation.

Question 165: Which of the following glycosaminoglycans does not contain a sulfur-containing bond?

A. Chondroitin
B. Keratan
C. Dermatan
D. Hyaluronic acid (Correct Answer)

Explanation: **Explanation:** **Hyaluronic acid** (Hyaluronan) is the correct answer because it is the only Glycosaminoglycan (GAG) that is **non-sulfated**. Unlike other GAGs, it is not synthesized in the Golgi apparatus by sulfotransferases; instead, it is synthesized by integral membrane proteins called hyaluronan synthases directly into the extracellular space. Additionally, it is the only GAG that is not covalently attached to a protein core (i.e., it does not form proteoglycans). **Analysis of Incorrect Options:** * **Chondroitin (Chondroitin Sulfate):** The most abundant GAG in the body, found in cartilage and bone. It is heavily sulfated at the carbon-4 or carbon-6 positions. * **Keratan (Keratan Sulfate):** Found in the cornea and loose connective tissue. It contains sulfate groups and is unique because its repeating disaccharide unit contains galactose instead of uronic acid. * **Dermatan (Dermatan Sulfate):** Found in skin, blood vessels, and heart valves. It is a sulfated GAG formed by the epimerization of glucuronic acid to iduronic acid. **High-Yield Clinical Pearls for NEET-PG:** * **Hyaluronic Acid:** Acts as a lubricant and shock absorber in synovial fluid and the vitreous humor. It plays a vital role in cell migration during embryogenesis and wound healing. * **Heparin:** The GAG with the **highest negative charge** and highest degree of sulfation; it acts as an intracellular anticoagulant. * **Mucopolysaccharidoses (MPS):** Genetic deficiencies in lysosomal enzymes that degrade sulfated GAGs (e.g., Hurler and Hunter syndromes). Note that Hyaluronic acid is typically not involved in these storage disorders. * **Unique Feature:** Hyaluronic acid is the largest GAG by molecular weight, often consisting of thousands of sugar residues.

Question 166: Glycogenolysis is best described by which of the following statements?

A. It involves enzymes cleaving beta(1-4) glycosidic linkages.
B. It requires activation of glycogen synthase.
C. It requires a bifunctional enzyme (debranching and transferase). (Correct Answer)
D. It requires inactivation of phosphorylase kinase.

Explanation: **Explanation:** Glycogenolysis is the biochemical breakdown of glycogen into glucose-1-phosphate and glucose. This process requires two primary enzymes: **Glycogen Phosphorylase** and the **Debranching Enzyme**. **Why Option C is Correct:** Glycogen phosphorylase can only cleave $\alpha(1-4)$ linkages and stops 4 glucose residues away from a branch point (limit dextrin). To continue breakdown, a **bifunctional debranching enzyme** is required. It possesses two distinct activities: 1. **4- $\alpha$-glucanotransferase:** Transfers a trisaccharide unit from the branch to a nearby straight chain. 2. **$\alpha(1-6)$ glucosidase:** Hydrolytically removes the remaining single glucose residue at the branch point, releasing free glucose. **Why Other Options are Incorrect:** * **Option A:** Glycogen consists of **$\alpha(1-4)$** and **$\alpha(1-6)$** glycosidic linkages. $\beta(1-4)$ linkages are found in cellulose, which humans cannot digest. * **Option B:** Glycogen synthase is the rate-limiting enzyme for **glycogenesis** (synthesis). During glycogenolysis, glycogen synthase is phosphorylated and **inactivated**. * **Option D:** Phosphorylase kinase is the enzyme that activates glycogen phosphorylase. Therefore, glycogenolysis requires the **activation** (not inactivation) of phosphorylase kinase via cAMP-mediated phosphorylation. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** Glycogen Phosphorylase (requires Pyridoxal Phosphate/Vitamin B6 as a cofactor). * **End products:** Glycogenolysis in the liver yields ~90% Glucose-1-Phosphate and ~10% free Glucose (from branch points). * **Clinical Correlation:** Deficiency of the debranching enzyme leads to **Cori’s Disease (GSD Type III)**, characterized by the accumulation of limit dextrins. * **Muscle vs. Liver:** Muscle glycogenolysis lacks Glucose-6-Phosphatase; hence, it provides energy for local contraction but cannot contribute to blood glucose levels.

Question 167: Which hormone stimulates gluconeogenesis?

A. Glucagon
B. Epinephrine
C. None of the above
D. Both of the above (Correct Answer)

Explanation: **Explanation:** Gluconeogenesis is the metabolic pathway that results in the generation of glucose from non-carbohydrate precursors (such as lactate, glycerol, and glucogenic amino acids). This process is primarily stimulated by **counter-regulatory hormones** that act to increase blood glucose levels during fasting or stress. **Why the correct answer is "Both of the above":** 1. **Glucagon:** Secreted by the alpha cells of the pancreas during fasting. It binds to G-protein coupled receptors (GPCR), increasing cAMP levels. This activates Protein Kinase A, which leads to the induction of key gluconeogenic enzymes like **PEPCK** and **Fructose-1,6-bisphosphatase**, while inhibiting glycolysis. 2. **Epinephrine:** Released by the adrenal medulla during "fight or flight" situations. It stimulates gluconeogenesis in the liver to ensure a steady supply of glucose for the brain and muscles during acute stress. It acts via similar signaling pathways (cAMP) to enhance glucose production. **Analysis of Options:** * **Option A & B:** While both are correct individually, selecting only one would be incomplete as both hormones synergistically promote glucose synthesis. * **Insulin (Contrast):** It is important to remember that Insulin is the primary **inhibitor** of gluconeogenesis, as it promotes glucose utilization and storage. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Fructose-1,6-bisphosphatase is the key regulatory enzyme of gluconeogenesis. * **Glucocorticoids (Cortisol):** Also stimulate gluconeogenesis by increasing the synthesis of gluconeogenic enzymes (transcriptional regulation). * **Substrates:** Acetyl-CoA is an **obligatory activator** of Pyruvate Carboxylase, the first enzyme in the gluconeogenic pathway. * **Site:** Occurs primarily in the Liver (90%) and Kidney (10%).

Question 168: Familial fructokinase deficiency causes no symptoms because:

A. Hexokinase can phosphorylate fructose (Correct Answer)
B. Liver aldolase can metabolize it
C. Excess fructose does not escape into the urine
D. Excess fructose is excreted through feces

Explanation: **Explanation:** **1. Why Option A is Correct:** Familial Fructokinase Deficiency (also known as **Essential Fructosuria**) is a benign, autosomal recessive condition caused by a deficiency of the enzyme **fructokinase**. Normally, fructokinase converts fructose to fructose-1-phosphate in the liver. When this primary pathway is blocked, **hexokinase** (which is present in most tissues) acts as a compensatory bypass. Hexokinase can phosphorylate fructose to **fructose-6-phosphate**, which then directly enters the glycolytic pathway. Because this alternative route prevents the toxic accumulation of fructose metabolites, the condition remains asymptomatic. **2. Why the Other Options are Incorrect:** * **Option B:** Liver aldolase (specifically Aldolase B) acts on fructose-1-phosphate. If fructokinase is deficient, fructose-1-phosphate is never formed, so Aldolase B has no substrate to act upon in this context. * **Option C:** This is factually incorrect. In fructokinase deficiency, blood fructose levels rise (fructosemia), exceeding the renal threshold. This leads to **fructosuria** (fructose in the urine), which is the hallmark diagnostic finding. * **Option D:** Fructose is a water-soluble sugar; excess amounts are cleared by the kidneys into the urine, not excreted through feces. **3. High-Yield Clinical Pearls for NEET-PG:** * **Essential Fructosuria:** Benign, asymptomatic, and often discovered incidentally when a urine dipstick is positive for "reducing sugars" but negative for glucose (glucose oxidase test). * **Hereditary Fructose Intolerance (HFI):** Caused by **Aldolase B deficiency**. Unlike fructokinase deficiency, HFI is **severe and symptomatic** (hypoglycemia, jaundice, cirrhosis) because fructose-1-phosphate accumulates and traps intracellular phosphate. * **Key Distinction:** Fructokinase deficiency = "Fructose in urine, but the patient is fine." Aldolase B deficiency = "Fructose intake leads to severe illness."

Question 169: All the following metabolic cycles operate in the mitochondria except:

A. Ketogenesis
B. Beta oxidation
C. TCA cycle
D. Embden-Meyerhof-Parnas (EMP) pathway (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Embden-Meyerhof-Parnas (EMP) pathway**. The EMP pathway is the synonym for **Glycolysis**. This metabolic sequence involves the breakdown of glucose into pyruvate (or lactate) and occurs exclusively in the **cytosol** of the cell. All the enzymes required for glycolysis are located in the extra-mitochondrial fraction. **Analysis of Options:** * **Ketogenesis (Option A):** This process involves the synthesis of ketone bodies (Acetoacetate, $\beta$-hydroxybutyrate) from Acetyl-CoA. It occurs primarily in the **mitochondria of hepatocytes**. * **Beta-oxidation (Option B):** This is the primary pathway for fatty acid catabolism. Long-chain fatty acids are transported into the **mitochondrial matrix** via the carnitine shuttle to undergo oxidation. * **TCA Cycle (Option C):** Also known as the Krebs cycle, it is the "final common pathway" for the oxidation of carbohydrates, lipids, and proteins. It takes place entirely within the **mitochondrial matrix**. **High-Yield NEET-PG Clinical Pearls:** 1. **Purely Cytosolic Pathways:** Glycolysis, HMP Shunt, Fatty acid synthesis, and Cholesterol synthesis. 2. **Purely Mitochondrial Pathways:** TCA cycle, Beta-oxidation, Ketogenesis, and Electron Transport Chain (ETC). 3. **Dual Compartment Pathways (Both Cytosol & Mitochondria):** Remember the mnemonic **"HUG"** — **H**eme synthesis, **U**rea cycle, and **G**luconeogenesis. 4. **RBC Exception:** Since Mature RBCs lack mitochondria, they depend entirely on the EMP pathway (Glycolysis) for energy.

Question 170: In starvation, there is ketosis due to which of the following?

A. Decreased acetyl CoA
B. Increased beta-oxidation (Correct Answer)
C. Decreased lipolysis
D. Decreased fatty acid synthesis

Explanation: **Explanation:** The primary driver of ketosis during starvation is the massive mobilization of fatty acids from adipose tissue to provide an alternative energy source. **Why "Increased beta-oxidation" is correct:** In starvation, the insulin-to-glucagon ratio falls, activating **Hormone-Sensitive Lipase (HSL)**. This leads to increased lipolysis, releasing free fatty acids (FFAs) into the blood. These FFAs enter the liver and undergo **increased beta-oxidation**, producing a surplus of **Acetyl CoA**. Under these conditions, the Citric Acid Cycle (TCA) cannot process all the Acetyl CoA because oxaloacetate is diverted toward gluconeogenesis. This excess Acetyl CoA is then channeled into the **ketogenesis** pathway to produce ketone bodies (acetoacetate and beta-hydroxybutyrate). **Why other options are incorrect:** * **A. Decreased acetyl CoA:** Incorrect. Acetyl CoA levels are actually **increased** due to rapid beta-oxidation; this surplus is the direct precursor for ketone bodies. * **C. Decreased lipolysis:** Incorrect. Lipolysis is significantly **increased** in starvation to provide the fatty acids required for energy and ketogenesis. * **D. Decreased fatty acid synthesis:** While fatty acid synthesis *is* decreased in starvation (due to inhibition of Acetyl CoA Carboxylase), this is a regulatory consequence, not the direct cause of ketosis. The active production of ketones is driven by the *breakdown* of fats, not just the lack of synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of ketogenesis:** HMG-CoA Synthase (mitochondrial). * **Ketone bodies:** Acetoacetate, $\beta$-hydroxybutyrate, and Acetone (non-metabolizable, causes "fruity breath"). * **Organ utilization:** The liver *produces* ketones but cannot *use* them because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). * **Brain adaptation:** After prolonged starvation (>3 days), the brain adapts to use ketone bodies for up to 75% of its energy requirements.

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

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

Practice Questions

Blood Glucose Regulation

Practice Questions

Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

Practice Questions

Lactose Intolerance and Galactosemia

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free