Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 151: Which disaccharide is not digested in the intestine?

A. Sucrose
B. Isomaltose
C. Trehalose
D. Sucralose (Correct Answer)

Explanation: **Explanation:** The correct answer is **Sucralose**. **1. Why Sucralose is the correct answer:** Sucralose is an artificial, non-caloric sweetener. Chemically, it is a **trichlorinated derivative of sucrose** (1,6-dichloro-1,6-dideoxy-β-D-fructofuranosyl-4-chloro-4-deoxy-α-D-galactopyranoside). The substitution of three hydroxyl groups with chlorine atoms creates a molecule that the human body cannot recognize as a carbohydrate. Consequently, there are no specific enzymes (disaccharidases) in the human small intestine capable of breaking the bonds in sucralose. It passes through the digestive tract largely unchanged and is excreted, providing zero calories. **2. Why the other options are incorrect:** * **Sucrose (A):** A natural disaccharide (Glucose + Fructose) digested by the enzyme **Sucrase** located in the brush border of the small intestine. * **Isomaltose (B):** An isomer of maltose with an α(1→6) glycosidic bond. It is a product of starch digestion and is hydrolyzed by the enzyme **Isomaltase** (part of the Sucrase-Isomaltase complex). * **Trehalose (C):** A disaccharide found in mushrooms and yeast (Glucose + Glucose with an α1→1 linkage). It is digested by the brush border enzyme **Trehalase**. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Sucrase-Isomaltase Deficiency:** A congenital condition leading to osmotic diarrhea and abdominal distension upon consuming sugar or starch. * **Trehalase Deficiency:** Rare, but presents with symptoms similar to lactose intolerance after eating mushrooms. * **Lactose:** The only major dietary disaccharide with a **β(1→4) linkage**; digested by Lactase. * **Sucralose** is approximately 600 times sweeter than sucrose and is heat-stable, making it popular for baking.

Question 152: Strenuous exercise is not indicated in which of the following glycogen storage diseases?

A. McArdle disease (Correct Answer)
B. Anderson disease
C. Pompe disease
D. Von Gierke disease

Explanation: **Explanation:** **McArdle Disease (GSD Type V)** is caused by a deficiency of **muscle glycogen phosphorylase (myophosphorylase)**. This enzyme is essential for glycogenolysis in skeletal muscle, converting glycogen into glucose-1-phosphate to fuel muscle contraction. During strenuous exercise, muscles rely heavily on glycogen for rapid ATP production. In McArdle disease, the inability to mobilize glycogen leads to an energy crisis, resulting in severe muscle cramps, exercise intolerance, and **myoglobinuria** (due to rhabdomyolysis). A classic clinical feature is the **"second wind phenomenon,"** where patients improve after a brief period of exercise as the body switches to using free fatty acids and blood glucose. **Why other options are incorrect:** * **Anderson Disease (Type IV):** Caused by a branching enzyme deficiency. It primarily affects the liver, leading to cirrhosis and hepatosplenomegaly in infancy; it is not primarily characterized by exercise-induced muscle crises. * **Pompe Disease (Type II):** Caused by lysosomal acid alpha-glucosidase deficiency. It leads to generalized glycogen accumulation (especially in the heart), causing hypertrophic cardiomyopathy and respiratory failure, rather than acute exercise-induced cramps. * **Von Gierke Disease (Type I):** Caused by glucose-6-phosphatase deficiency. It affects the liver and kidneys, presenting with severe fasting hypoglycemia, lactic acidosis, and hepatomegaly, but does not directly impair muscle glycogenolysis. **High-Yield Facts for NEET-PG:** * **Ischemic Forearm Exercise Test:** In McArdle disease, there is a failure of blood lactate to rise, but a significant rise in ammonia levels. * **Biopsy finding:** Subsarcolemmal deposits of glycogen in muscle fibers. * **Management:** Patients are advised to consume simple sugars before exercise and avoid sudden, strenuous bursts of activity.

Question 153: Which enzyme is inhibited by insulin?

A. Glucokinase
B. PFK-1
C. Glycogen phosphorylase (Correct Answer)
D. Glycogen synthase

Explanation: **Explanation:** The regulation of carbohydrate metabolism by insulin is a high-yield topic for NEET-PG. Insulin is an **anabolic hormone** secreted by the pancreatic beta cells in response to high blood glucose levels. Its primary goal is to lower blood glucose by promoting glucose utilization (glycolysis), storage (glycogenesis), and inhibiting glucose production. **1. Why Glycogen Phosphorylase is the Correct Answer:** Glycogen phosphorylase is the rate-limiting enzyme of **glycogenolysis** (breakdown of glycogen into glucose). Insulin triggers a signaling cascade that activates **Protein Phosphatase-1 (PP1)**. This phosphatase dephosphorylates glycogen phosphorylase, converting it from its active form (Phosphorylase *a*) to its inactive form (Phosphorylase *b*). By inhibiting this enzyme, insulin prevents the mobilization of glucose stores. **2. Analysis of Incorrect Options:** * **Glucokinase (Option A):** This enzyme facilitates glucose uptake in the liver. Insulin **induces** the synthesis of Glucokinase to promote glucose utilization. * **PFK-1 (Option B):** Phosphofructokinase-1 is the rate-limiting enzyme of glycolysis. Insulin stimulates PFK-1 indirectly by increasing levels of Fructose-2,6-bisphosphate, its most potent allosteric activator. * **Glycogen Synthase (Option D):** This is the rate-limiting enzyme for glycogen synthesis. Insulin **activates** this enzyme via dephosphorylation (by PP1), promoting the storage of glucose as glycogen. **Clinical Pearls for NEET-PG:** * **Rule of Thumb:** Insulin generally **dephosphorylates** enzymes. In most cases, dephosphorylation **activates** anabolic enzymes (e.g., Glycogen synthase) and **inhibits** catabolic enzymes (e.g., Glycogen phosphorylase, Hormone-sensitive lipase). * **Glucagon/Epinephrine** act as antagonists to insulin by increasing cAMP, which activates Protein Kinase A (PKA), leading to the phosphorylation (activation) of glycogen phosphorylase.

Question 154: Which enzyme catalyzes substrate-level phosphorylation?

A. Succinate dehydrogenase
B. Pyruvate kinase (Correct Answer)
C. Malate dehydrogenase
D. Acetyl CoA carboxylase

Explanation: **Explanation:** **Substrate-level phosphorylation (SLP)** is the direct synthesis of ATP (or GTP) from ADP (or GDP) by the transfer of a high-energy phosphate group from a phosphorylated intermediate, independent of the electron transport chain and oxygen. **Why Pyruvate Kinase is Correct:** In the final step of **Glycolysis**, Pyruvate Kinase catalyzes the conversion of Phosphoenolpyruvate (PEP) to Pyruvate. PEP contains a high-energy phosphate bond; its hydrolysis releases enough energy to drive the phosphorylation of ADP to **ATP**. This is one of the two SLP steps in glycolysis (the other being Phosphoglycerate kinase). **Analysis of Incorrect Options:** * **A. Succinate dehydrogenase:** This is an enzyme of the TCA cycle (and Complex II of ETC) that catalyzes the oxidation of Succinate to Fumarate. It produces **FADH₂**, not ATP directly. * **C. Malate dehydrogenase:** This enzyme catalyzes the conversion of Malate to Oxaloacetate in the TCA cycle, generating **NADH**. * **D. Acetyl CoA carboxylase:** This is the rate-limiting enzyme for **Fatty Acid Synthesis**. It actually **consumes ATP** to convert Acetyl CoA to Malonyl CoA. **High-Yield Clinical Pearls for NEET-PG:** * **Total SLP sites to remember:** 1. **Glycolysis:** Phosphoglycerate kinase and Pyruvate kinase. 2. **TCA Cycle:** Succinate thiokinase (Succinyl CoA synthetase) – produces **GTP**. * **Clinical Correlation:** Pyruvate Kinase deficiency is the second most common cause of **enzyme-deficient hemolytic anemia** (after G6PD deficiency). Since RBCs lack mitochondria, they rely entirely on SLP for ATP; a deficiency leads to ATP depletion, causing membrane fragility and hemolysis.

Question 155: All of the following substrates are glucogenic except:

A. Acetyl CoA (Correct Answer)
B. Pyruvate
C. Glycerol
D. Lactate

Explanation: **Explanation:** The core concept tested here is the **irreversibility of the Pyruvate Dehydrogenase (PDH) complex**. In humans, the conversion of Pyruvate to Acetyl CoA is a one-way reaction. **1. Why Acetyl CoA is the Correct Answer:** Acetyl CoA cannot be used for the net synthesis of glucose (gluconeogenesis). When Acetyl CoA enters the TCA cycle, it condenses with Oxaloacetate (OAA) to form Citrate. During the cycle, two carbons are lost as $CO_2$ before OAA is regenerated. Therefore, there is **no net gain of carbon atoms** to be diverted toward glucose synthesis. Furthermore, the PDH reaction is irreversible, meaning Acetyl CoA cannot be converted back into Pyruvate. **2. Why the other options are Glucogenic:** * **Pyruvate:** It is the primary substrate for gluconeogenesis. It is converted to OAA by *Pyruvate Carboxylase*, bypassing the irreversible step of glycolysis. * **Glycerol:** Derived from triacylglycerol breakdown in adipose tissue, it is phosphorylated to Glycerol-3-Phosphate and then oxidized to Dihydroxyacetone phosphate (DHAP), an intermediate of gluconeogenesis. * **Lactate:** Produced via anaerobic glycolysis (e.g., in RBCs or exercising muscle), it is converted back to Pyruvate by *Lactate Dehydrogenase* in the liver (**Cori Cycle**) to form glucose. **High-Yield NEET-PG Pearls:** * **Odd-chain fatty acids** are glucogenic because their terminal product, **Propionyl CoA**, enters the TCA cycle as Succinyl CoA. * **Even-chain fatty acids** are strictly non-glucogenic (they produce only Acetyl CoA). * **Leucine and Lysine** are the only two amino acids that are purely ketogenic (non-glucogenic). * **Key Regulatory Enzyme:** Pyruvate Carboxylase (requires Biotin and is activated by Acetyl CoA).

Question 156: Which of the following metabolic pathways occurs in two cellular compartments?

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

Explanation: **Explanation:** The correct answer is **Gluconeogenesis**. In biochemistry, metabolic pathways are often compartmentalized to ensure efficient regulation. While many pathways occur entirely in the cytoplasm, gluconeogenesis is a classic example of a "dual-compartment" pathway. **1. Why Gluconeogenesis is correct:** Gluconeogenesis begins in the **mitochondria** and ends in the **cytosol**. * **Mitochondria:** Pyruvate is converted to Oxaloacetate (OAA) by *Pyruvate Carboxylase*. Since OAA cannot cross the mitochondrial membrane, it is reduced to Malate, exported to the cytosol, and then re-oxidized back to OAA. * **Cytosol:** The remaining steps, starting from the conversion of OAA to Phosphoenolpyruvate (PEP) by *PEPCK*, occur in the cytoplasm. * *Note:* The final step (Glucose-6-Phosphate to Glucose) occurs in the **Endoplasmic Reticulum** of liver and kidney cells. **2. Why other options are incorrect:** * **Glycolysis:** Occurs entirely in the **cytosol** of all cells. * **Glycogenesis (Glycogen synthesis):** Occurs entirely in the **cytosol**, primarily in the liver and skeletal muscle. * **Glycogenolysis (Glycogen breakdown):** Occurs in the **cytosol**. (Small amounts of glycogen are degraded in lysosomes by acid maltase, but the primary metabolic pathway is cytosolic). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Dual-Compartment Pathways:** Remember **"HUG"** — **H**eme synthesis, **U**rea cycle, and **G**luconeogenesis. * **Rate-limiting enzyme of Gluconeogenesis:** Fructose-1,6-bisphosphatase. * **Key Organelle:** The enzyme **Glucose-6-Phosphatase** is located on the luminal surface of the **Smooth ER**. Its deficiency leads to Von Gierke’s Disease (GSD Type I).

Question 157: Reducing sugar in urine is seen in which of the following conditions?

A. Fanconi anemia
B. Lactose intolerance
C. Galactosemia (Correct Answer)
D. Phenylketonuria

Explanation: **Explanation:** **Why Galactosemia is correct:** Galactosemia is a disorder of galactose metabolism, most commonly due to a deficiency of **Galactose-1-phosphate uridyltransferase (GALT)**. This leads to an accumulation of galactose in the blood (galactosemia) and its subsequent excretion in the urine (**galactosuria**). Since galactose is a **reducing sugar** (containing a free aldehyde group), it reacts positively with Benedict’s test or Fehling’s solution, resulting in a positive test for reducing substances in the urine. **Analysis of Incorrect Options:** * **Fanconi Anemia:** This is a DNA repair defect characterized by bone marrow failure and physical anomalies. It should not be confused with *Fanconi Syndrome* (a renal proximal tubule defect), which can cause glucosuria. * **Lactose Intolerance:** This is a deficiency of the enzyme lactase in the brush border of the small intestine. It leads to the malabsorption of lactose in the gut, causing osmotic diarrhea. Lactose does not enter the bloodstream in significant amounts and is therefore not typically found in the urine. * **Phenylketonuria (PKU):** This is a disorder of amino acid metabolism (deficiency of Phenylalanine hydroxylase). It results in the excretion of phenylketones (like phenylpyruvate), which are not reducing sugars. **High-Yield Clinical Pearls for NEET-PG:** * **Benedict’s Test vs. Dipstick:** Benedict’s test detects all reducing sugars (glucose, galactose, fructose, lactose). The standard urine dipstick uses the **glucose oxidase** method, which is specific for glucose only. A "positive Benedict’s and negative Dipstick" is a classic clue for Galactosemia or Essential Fructosuria. * **Common Reducing Sugars in Urine:** Glucose (Diabetes), Galactose (Galactosemia), Fructose (Essential Fructosuria), and Pentose (Essential Pentosuria). * **Galactosemia Triad:** Cataracts (due to galactitol accumulation), Hepatomegaly, and Intellectual disability.

Question 158: In which organ is insulin-dependent glucose entry observed?

A. Liver
B. Brain
C. Adipose tissue (Correct Answer)
D. Kidney

Explanation: **Explanation:** The entry of glucose into cells is mediated by a family of glucose transporters known as **GLUT**. The correct answer is **Adipose tissue** because it primarily utilizes **GLUT-4**, which is the only insulin-dependent glucose transporter. 1. **Why Adipose Tissue is Correct:** In the resting state, GLUT-4 transporters are sequestered in intracellular vesicles. When insulin binds to its receptor, it triggers a signaling cascade that causes these vesicles to fuse with the plasma membrane, allowing glucose entry. This mechanism is exclusive to **skeletal muscle, cardiac muscle, and adipose tissue.** 2. **Why Other Options are Incorrect:** * **Liver (GLUT-2):** The liver uses GLUT-2, which is insulin-independent. It has a high $K_m$ (low affinity), allowing the liver to sense and uptake glucose only when blood levels are high (postprandial). * **Brain (GLUT-1 & GLUT-3):** The brain requires a constant supply of glucose regardless of insulin levels. It uses GLUT-1 and GLUT-3, which have a low $K_m$ (high affinity) to ensure glucose uptake even during hypoglycemia. * **Kidney (GLUT-2 & SGLT):** Glucose reabsorption in the kidney tubules occurs via SGLT (Sodium-Glucose Linked Transporters) and GLUT-2, neither of which require insulin. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT-1:** Found in RBCs and the Blood-Brain Barrier. * **GLUT-2:** Bidirectional transporter found in Liver, Pancreatic beta cells, and Kidney. * **GLUT-4:** The only insulin-responsive transporter; its translocation is also stimulated by **exercise** in skeletal muscles. * **GLUT-5:** Specifically functions as a **fructose** transporter (found in small intestine and spermatozoa).

Question 159: In humans, carbohydrates are stored primarily as which of the following?

A. Glucose
B. Glycogen (Correct Answer)
C. Starch
D. Cellulose

Explanation: **Explanation:** In humans, **Glycogen** is the primary storage form of carbohydrates. It is a highly branched homopolymer of D-glucose, featuring $\alpha(1\to4)$ glycosidic bonds in the chains and $\alpha(1\to6)$ bonds at branch points. This branched structure is physiologically significant because it increases solubility and allows for the rapid mobilization of glucose from multiple non-reducing ends during periods of metabolic demand. Glycogen is stored mainly in the **liver** (to maintain blood glucose levels) and **skeletal muscle** (to provide energy for contraction). **Analysis of Incorrect Options:** * **A. Glucose:** While glucose is the primary metabolic fuel, it cannot be stored in its free form. High intracellular glucose concentrations would create an osmotic pressure so high it would cause cells to swell and burst. * **C. Starch:** This is the primary storage polysaccharide in **plants**, not humans. It consists of amylose and amylopectin. * **D. Cellulose:** This is a structural polysaccharide found in plant cell walls. Humans lack the enzyme (cellulase) to break its $\beta(1\to4)$ linkages, making it an indigestible dietary fiber. **High-Yield Clinical Pearls for NEET-PG:** * **Storage Capacity:** The liver has a higher *concentration* of glycogen per gram of tissue, but the skeletal muscle contains the largest *total amount* due to its greater overall mass. * **Muscle vs. Liver:** Muscle glycogen cannot contribute to blood glucose because muscles lack the enzyme **Glucose-6-Phosphatase**. * **Key Enzyme:** **Glycogen synthase** is the rate-limiting enzyme for glycogenesis, while **Glycogen phosphorylase** is the rate-limiting enzyme for glycogenolysis. * **Clinical Correlation:** Deficiencies in enzymes involved in glycogen metabolism lead to **Glycogen Storage Diseases (GSDs)**, such as Von Gierke’s (Type I) and Pompe’s (Type II).

Question 160: Which glucose transporter in myocytes is stimulated by insulin?

A. GLUT 1
B. GLUT 2
C. GLUT 3
D. GLUT 4 (Correct Answer)

Explanation: ### Explanation **Correct Option: D (GLUT 4)** GLUT 4 is the only **insulin-dependent** glucose transporter. It is primarily expressed in **skeletal muscle (myocytes)** and **adipose tissue**. In the resting state, GLUT 4 is sequestered in intracellular vesicles. When insulin binds to its receptor, it triggers a signaling cascade (via PI3-kinase) that causes these vesicles to fuse with the plasma membrane, increasing glucose uptake. In myocytes, GLUT 4 translocation is also stimulated by **exercise/muscle contraction** (AMPK pathway), which is why exercise helps manage blood glucose in diabetics. **Incorrect Options:** * **GLUT 1:** This is a basal glucose transporter found in almost all tissues, especially **RBCs** and the **Blood-Brain Barrier**. It is insulin-independent and ensures a constant baseline glucose supply. * **GLUT 2:** A high-capacity, low-affinity transporter found in the **Liver, Pancreatic beta-cells, and Kidney**. It acts as a "glucose sensor" in the pancreas and allows bidirectional transport in the liver. It is insulin-independent. * **GLUT 3:** A high-affinity transporter found primarily in **Neurons**. It ensures glucose uptake even during hypoglycemia, independent of insulin. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT 5:** Unique because it is a **Fructose** transporter (found in the small intestine and spermatozoa). * **SGLT 1/2:** These are active transporters (Sodium-Glucose Co-transporters). SGLT-2 inhibitors (e.g., Dapagliflozin) are modern drugs used in Diabetes to promote glucose excretion in urine. * **Mnemonics:** Remember "**4** is for the **Door**" (Insulin opens the door for glucose in muscle/fat).

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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