Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 371: A 56-year-old man with a 14-year history of diabetes mellitus presents with poor vision, peripheral vascular disease, and mild proteinuria. Which of the following best monitors the control of blood sugar levels in this patient?

A. Glycosylated hemoglobin (Correct Answer)
B. Islet cell autoantibody
C. Serum myoinositol
D. Serum sorbitol

Explanation: **Explanation:** **1. Why Glycosylated Hemoglobin (HbA1c) is correct:** HbA1c is formed by the non-enzymatic glycation of the N-terminal valine of the beta chain of hemoglobin. Since erythrocytes have a lifespan of approximately 120 days, HbA1c reflects the **average blood glucose levels over the preceding 2–3 months**. In a patient with long-standing diabetes and complications (retinopathy, nephropathy, and PVD), monitoring HbA1c is the gold standard for assessing long-term glycemic control and predicting the risk of further microvascular damage. **2. Why the other options are incorrect:** * **Islet cell autoantibody:** These are markers of autoimmune destruction of beta cells, used primarily to diagnose **Type 1 Diabetes Mellitus** or LADA, not to monitor glucose control. * **Serum myoinositol:** In diabetes, intracellular myoinositol levels decrease (especially in nerve tissues) due to competition with glucose. While it plays a role in the pathogenesis of diabetic neuropathy, it is not a clinical tool for monitoring blood sugar. * **Serum sorbitol:** Under hyperglycemic conditions, the **Polyol pathway** converts glucose to sorbitol via aldose reductase. Sorbitol accumulation causes osmotic damage (cataracts, neuropathy), but its serum level is not used for routine monitoring of glycemic control. **3. High-Yield Clinical Pearls for NEET-PG:** * **HbA1c Targets:** For most non-pregnant adults, the goal is **<7%**. * **Fructosamine (Glycated Albumin):** Reflects glycemic control over the last **2–3 weeks**. Useful when HbA1c is unreliable (e.g., hemolytic anemia, pregnancy). * **False Low HbA1c:** Seen in conditions with high RBC turnover (Hemolytic anemia, recent blood transfusion). * **False High HbA1c:** Seen in Iron deficiency anemia (due to increased lifespan of old RBCs).

Question 372: Which molecule activates the key regulatory enzyme of glycolysis?

A. ATP
B. cAMP
C. Insulin (Correct Answer)
D. Glucagon

Explanation: **Explanation:** The key regulatory (rate-limiting) enzyme of glycolysis is **Phosphofructokinase-1 (PFK-1)**. Glycolysis is an oxidative process aimed at breaking down glucose to produce energy (ATP) and intermediates for fatty acid synthesis. **1. Why Insulin is Correct:** Insulin is an anabolic hormone secreted in the "well-fed" state. It promotes glucose utilization by increasing the synthesis and activity of key glycolytic enzymes (Glucokinase, PFK-1, and Pyruvate Kinase). Specifically, insulin increases the levels of **Fructose-2,6-bisphosphate**, which is the most potent allosteric activator of PFK-1, thereby accelerating glycolysis. **2. Why the Other Options are Incorrect:** * **ATP (Option A):** ATP acts as an **allosteric inhibitor** of PFK-1. High energy levels signal the cell that further glucose oxidation is unnecessary, slowing down glycolysis. * **cAMP (Option B):** cAMP is a second messenger for glucagon. High cAMP levels activate Protein Kinase A, which leads to the inhibition of glycolysis and stimulation of gluconeogenesis. * **Glucagon (Option D):** Glucagon is a catabolic hormone secreted during fasting. It inhibits glycolysis in the liver to conserve glucose for the brain and RBCs, primarily by decreasing Fructose-2,6-bisphosphate levels. **NEET-PG High-Yield Pearls:** * **PFK-1** is the "committed step" and the most important control point of glycolysis. * **Fructose-2,6-bisphosphate** is the most potent activator of PFK-1 and a potent inhibitor of Fructose-1,6-bisphosphatase (gluconeogenesis), preventing a futile cycle. * **Citrate** and **Low pH** (H+ ions) also act as allosteric inhibitors of PFK-1.

Question 373: A positive D-xylose test indicates all of the following, except:

A. Pancreatic insufficiency (Correct Answer)
B. Small intestinal mucosal disease
C. Impaired carbohydrate absorption in the small intestine
D. Malabsorption

Explanation: **Explanation:** The **D-xylose absorption test** is a classic diagnostic tool used to differentiate between **malabsorption** caused by mucosal disease and **maldigestion** caused by pancreatic insufficiency. **1. Why Pancreatic Insufficiency is the correct answer:** D-xylose is a pentose sugar that is absorbed directly by the proximal small intestinal mucosa via passive diffusion (and some facilitated transport). Unlike complex carbohydrates, it **does not require pancreatic enzymes** (like amylase) or bile salts for digestion. Therefore, in patients with pancreatic insufficiency, D-xylose absorption remains **normal**. A "positive" test (low levels of D-xylose in urine/blood) indicates a problem with the intestinal wall itself, not the pancreas. **2. Analysis of Incorrect Options:** * **Small intestinal mucosal disease (B):** Conditions like Celiac disease or Tropical sprue damage the villi, leading to decreased surface area and impaired D-xylose absorption, resulting in a positive (abnormal) test. * **Impaired carbohydrate absorption (C):** Since D-xylose is a marker for monosaccharide absorption, a low result directly confirms that the small intestine's ability to absorb carbohydrates is compromised. * **Malabsorption (D):** D-xylose is the gold standard for identifying "intestinal malabsorption" as opposed to "maldigestion." **Clinical Pearls for NEET-PG:** * **Normal Result:** >4g excreted in urine over 5 hours after a 25g oral dose. * **False Positives:** Can occur in patients with **renal dysfunction** (impaired excretion), **ascites**, or **Small Intestinal Bacterial Overgrowth (SIBO)** (bacteria metabolize the xylose before absorption). * **Key Distinction:** If D-xylose test is abnormal, think **Celiac Disease**; if it is normal but malabsorption symptoms persist, think **Chronic Pancreatitis**.

Question 374: In Diabetes Mellitus, the activity of which of the following enzymes is increased?

A. CPT-1
B. Phosphoenol Pyruvate carboxykinase (PEPCK)
C. Glucose-6-Phosphatase
D. All (Correct Answer)

Explanation: **Explanation:** In Diabetes Mellitus (DM), there is either a deficiency of insulin or resistance to its action. Insulin is an anabolic hormone that normally suppresses gluconeogenesis and fatty acid oxidation. In its absence, the "starvation in the midst of plenty" state triggers catabolic pathways. **1. Phosphoenolpyruvate Carboxykinase (PEPCK) & Glucose-6-Phosphatase:** These are key **rate-limiting enzymes of gluconeogenesis**. Insulin normally represses the gene expression of these enzymes. In DM, the lack of insulin action leads to the induction of these enzymes in the liver and kidneys. This results in increased endogenous glucose production, contributing to fasting hyperglycemia. **2. Carnitine Palmitoyltransferase-1 (CPT-1):** CPT-1 is the rate-limiting enzyme for **Beta-oxidation of fatty acids** (the "gatekeeper" for fatty acids entering the mitochondria). In DM, low insulin and high glucagon levels decrease the concentration of Malonyl-CoA (a potent inhibitor of CPT-1). This relieves the inhibition, significantly increasing CPT-1 activity, leading to excessive fatty acid oxidation and the subsequent production of ketone bodies (Ketogenesis). **Clinical Pearls for NEET-PG:** * **Insulin-Independent Tissues:** Retina, Kidney, Adrenal medulla, and RBCs (Mnemonic: **LUCRE** - Lens, Urethra/Kidney, Cornea, RBCs, Epithelium). * **Bifunctional Enzyme:** In DM, PFK-2 is inactive and Fructose-2,6-Bisphosphatase is active, further favoring gluconeogenesis over glycolysis. * **Key Concept:** DM is biochemically characterized by a high **Glucagon:Insulin ratio**, which mimics a prolonged fasting state regardless of blood glucose levels.

Question 375: McArdle's disease is due to the deficiency of:

A. Glucose 1 phosphatase
B. Glucose 1, 6 diphosphatase
C. Glucose 6 phosphatase
D. Myophosphorylase (Correct Answer)

Explanation: **Explanation:** **McArdle’s Disease (GSD Type V)** is a glycogen storage disease caused by a deficiency of **Myophosphorylase**, the muscle-specific isoform of glycogen phosphorylase. This enzyme is responsible for the rate-limiting step of glycogenolysis—breaking down glycogen into glucose-1-phosphate in skeletal muscle. Without it, muscles cannot mobilize glucose during exercise, leading to ATP depletion. **Analysis of Options:** * **D. Myophosphorylase (Correct):** Its deficiency prevents glycogen breakdown in muscles. Clinically, this manifests as exercise intolerance, muscle cramps, and "second wind" phenomenon (where switching to fatty acid metabolism improves symptoms). * **C. Glucose-6-Phosphatase:** Deficiency causes **Von Gierke’s Disease (GSD Type I)**. This enzyme is primarily in the liver; its absence leads to severe fasting hypoglycemia and hepatomegaly. * **A & B. Glucose-1-Phosphatase / Glucose-1,6-Diphosphatase:** These are not primary enzymes involved in the major glycogen storage diseases. Glucose-1-phosphate is an intermediate, but its phosphatase is not a recognized cause of a classic GSD. **High-Yield Clinical Pearls for NEET-PG:** * **Second Wind Phenomenon:** A hallmark of McArdle’s where symptoms improve after a few minutes of exercise as the body switches to using blood glucose and free fatty acids. * **Burgundy-colored urine:** Due to **myoglobinuria** following strenuous exercise, which can lead to acute renal failure. * **Ischemic Exercise Test:** Patients show a **failure of blood lactate to rise** (since they cannot break down glycogen to lactate) but a significant rise in ammonia levels. * **Biopsy:** Shows subsarcolemmal deposits of glycogen.

Question 376: Fatty acids are not utilized by which of the following?

A. Red blood cells (Correct Answer)
B. Skeletal muscle
C. Liver
D. Heart

Explanation: **Explanation:** The correct answer is **Red Blood Cells (RBCs)**. The utilization of fatty acids for energy occurs via **Beta-oxidation**, a metabolic pathway that takes place exclusively within the **mitochondria**. **1. Why Red Blood Cells (RBCs) cannot utilize fatty acids:** Mature RBCs lack mitochondria. Consequently, they are incapable of performing beta-oxidation or the TCA cycle. RBCs are entirely dependent on **anaerobic glycolysis** in the cytosol for their ATP requirements, converting glucose to lactate. **2. Analysis of Incorrect Options:** * **Skeletal Muscle:** These cells contain abundant mitochondria. During rest and low-intensity exercise, fatty acids are the preferred fuel source for skeletal muscle. * **Liver:** The liver is the primary site for fatty acid metabolism. It oxidizes fatty acids to generate ATP and produces ketone bodies (ketogenesis) during fasting states. * **Heart:** The myocardium is highly aerobic and has a very high mitochondrial density. Under normal physiological conditions, **60–80% of the heart's energy** is derived from the oxidation of long-chain fatty acids. **High-Yield Clinical Pearls for NEET-PG:** * **Brain Paradox:** Although the brain has mitochondria, it cannot utilize fatty acids because they are bound to albumin and cannot cross the **Blood-Brain Barrier (BBB)**. The brain uses glucose or ketone bodies (during starvation). * **Essentiality of Glucose:** Because RBCs and the Brain cannot use fatty acids, the body must maintain blood glucose levels via gluconeogenesis during fasting. * **Key Enzyme:** Carnitine Palmitoyltransferase-1 (CPT-1) is the rate-limiting enzyme for fatty acid entry into the mitochondria.

Question 377: Which of the following statements accurately describes the function of the malate shuttle?

A. It transports malate from the cytoplasm to the mitochondria.
B. It transports malate from the mitochondria to the cytoplasm. (Correct Answer)
C. It facilitates the bidirectional transport of malate between the cytoplasm and mitochondria.
D. It is not involved in the transport of malate.

Explanation: The **Malate-Aspartate Shuttle** is a crucial biochemical mechanism used to transport reducing equivalents (NADH) from the cytosol into the mitochondrial matrix for the Electron Transport Chain, as the inner mitochondrial membrane is impermeable to NADH. ### **Explanation of the Correct Answer (B)** In the context of **Gluconeogenesis**, oxaloacetate (OAA) is formed in the mitochondria but must be transported to the cytosol to continue the pathway. Since OAA cannot cross the mitochondrial membrane directly, it is reduced to **Malate** by mitochondrial Malate Dehydrogenase. Malate then exits the mitochondria into the cytoplasm via a specific transporter. Once in the cytosol, it is re-oxidized back to OAA, providing the carbon skeleton needed for glucose synthesis. Thus, the shuttle effectively transports malate from the **mitochondria to the cytoplasm**. ### **Analysis of Incorrect Options** * **Option A:** While malate can enter the mitochondria during the NADH shuttle process, the primary "shuttle" function highlighted in metabolic regulation (especially gluconeogenesis) focuses on the export of malate to bypass the membrane barrier for OAA. * **Option C:** While the components of the shuttle exist in both compartments, the movement of malate is part of a coordinated cycle involving the exchange of other metabolites (like alpha-ketoglutarate); it is not a simple "bidirectional" free-flow of malate alone. * **Option D:** This is factually incorrect as malate is the central transport molecule of this system. ### **High-Yield Clinical Pearls for NEET-PG** * **NADH Yield:** The Malate-Aspartate shuttle is more efficient than the Glycerol-3-Phosphate shuttle, yielding **2.5 ATP** per NADH (compared to 1.5 ATP). * **Tissue Specificity:** It is predominantly active in the **heart, liver, and kidneys**. * **Key Enzymes:** Requires Malate Dehydrogenase and Aspartate Aminotransferase (AST). * **Gluconeogenesis Link:** This shuttle is essential when **Pyruvate** is the substrate for gluconeogenesis. If Lactate is the substrate, the transport mechanism differs.

Question 378: Which of the following is a component of chitin polysaccharide?

A. Ascorbic acid
B. Glucosamine (Correct Answer)
C. Synovium
D. Glucuronic acid

Explanation: ### Explanation **Correct Answer: B. Glucosamine** **Concept:** Chitin is a structural homopolysaccharide found in the exoskeletons of arthropods (insects, crustaceans) and the cell walls of fungi. It is a linear polymer composed of **N-acetyl-D-glucosamine** units linked by **β(1→4) glycosidic bonds**. While the question lists "Glucosamine," it refers to the amino sugar derivative that forms the backbone of this polymer. Structurally, chitin is very similar to cellulose, with the hydroxyl group at the C-2 position replaced by an acetamido group. **Analysis of Incorrect Options:** * **A. Ascorbic Acid:** This is Vitamin C, a water-soluble vitamin derived from glucose in most animals (except humans/primates). It serves as a cofactor for prolyl hydroxylase in collagen synthesis, not a structural component of polysaccharides. * **C. Synovium:** This is an anatomical term referring to the soft tissue (synovial membrane) that lines the joints. It secretes synovial fluid, which contains hyaluronic acid, but is not a chemical component itself. * **D. Glucuronic Acid:** This is an uronic acid formed by the oxidation of glucose. It is a key component of Glycosaminoglycans (GAGs) like Heparin and Hyaluronic acid and plays a vital role in detoxification (conjugation) in the liver. **High-Yield NEET-PG Pearls:** * **Chitin vs. Cellulose:** Both have β(1→4) linkages, but Chitin is a polymer of N-acetylglucosamine, while Cellulose is a polymer of D-glucose. * **Heteropolysaccharides:** Most GAGs (like Chondroitin sulfate) are heteropolysaccharides (repeating disaccharide units), whereas Chitin is a **homopolysaccharide**. * **Clinical Relevance:** Glucosamine supplements are frequently used in clinical practice to support cartilage repair in osteoarthritis.

Question 379: What is the most common enzyme deficiency causing hemolytic anemia?

A. Pyruvate kinase
B. Hexokinase
C. Glucose-6-phosphate dehydrogenase (Correct Answer)
D. Gucosephosphate isomerase

Explanation: **Explanation:** **Glucose-6-phosphate dehydrogenase (G6PD)** deficiency is the most common enzyme deficiency causing hemolytic anemia worldwide, affecting over 400 million people. **Why G6PD is the Correct Answer:** G6PD is the rate-limiting enzyme of the **Pentose Phosphate Pathway (HMP Shunt)**. Its primary role in mature red blood cells (RBCs) is to generate **NADPH**. NADPH is essential for maintaining a pool of **reduced glutathione**, which acts as a scavenger for reactive oxygen species (ROS) like hydrogen peroxide. Since RBCs lack mitochondria, they depend solely on the HMP shunt for NADPH. In G6PD deficiency, oxidative stress (triggered by fava beans, infections, or drugs like Primaquine) leads to the oxidation of hemoglobin, forming **Heinz bodies**. These are removed by splenic macrophages, creating **"Bite cells,"** ultimately leading to hemolysis. **Why Other Options are Incorrect:** * **Pyruvate Kinase (PK):** While PK deficiency is the most common enzyme deficiency in the **Glycolytic pathway** causing hemolytic anemia, it is significantly less common than G6PD deficiency globally. * **Hexokinase:** Deficiency is extremely rare. As the first step of glycolysis, its absence would severely compromise the cell's energy production. * **Glucosephosphate Isomerase:** A very rare cause of non-spherocytic hemolytic anemia. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** G6PD deficiency is **X-linked Recessive**. * **Morphology:** Look for **Heinz bodies** (supravital stain) and **Bite cells** (peripheral smear). * **Protective Effect:** G6PD deficiency provides a selective advantage against *Plasmodium falciparum* malaria. * **Timing:** Do not test G6PD levels during an acute hemolytic episode, as young reticulocytes have normal enzyme levels, potentially yielding a false-negative result.

Question 380: Where in the cell does glycolysis occur?

A. Cytosol (Correct Answer)
B. Mitochondria
C. Nucleus
D. Lysosome

Explanation: **Explanation:** **1. Why Cytosol is Correct:** Glycolysis (the Embden-Meyerhof pathway) is the sequence of reactions that converts glucose into pyruvate (aerobic) or lactate (anaerobic). All the enzymes required for this pathway are located in the **cytosol** (cytoplasm). This allows the cell to generate ATP and NADH even in the absence of oxygen or specialized organelles like mitochondria (e.g., in Mature Red Blood Cells). **2. Why Other Options are Incorrect:** * **Mitochondria:** This is the site for the **TCA cycle (Krebs cycle)**, Electron Transport Chain (ETC), and Beta-oxidation of fatty acids. While pyruvate (the product of glycolysis) enters the mitochondria for further oxidation, the glycolytic process itself does not occur here. * **Nucleus:** The nucleus is primarily responsible for DNA replication and transcription (RNA synthesis); it does not house the metabolic machinery for glucose breakdown. * **Lysosome:** These are "suicide bags" containing hydrolytic enzymes for intracellular digestion and degradation of macromolecules, not for energy-producing metabolic pathways. **3. NEET-PG High-Yield Pearls:** * **Mature RBCs:** Since they lack mitochondria, they depend **entirely** on anaerobic glycolysis in the cytosol for their energy needs. * **Rate-limiting step:** The conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate by the enzyme **Phosphofructokinase-1 (PFK-1)**. * **Site of Gluconeogenesis:** Unlike glycolysis, gluconeogenesis occurs in both the **mitochondria and cytosol**. * **Rapoport-Luebering Cycle:** A shunt of glycolysis occurring in RBCs that produces 2,3-BPG, which decreases hemoglobin's affinity for oxygen.

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

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