Carbohydrate Metabolism Practice Questions

Q: Which of the following sugars is not synthesized or utilized in the human body?

L-fructose. **Explanation:** The human body primarily utilizes **D-isomers** of sugars (like D-glucose and D-fructose) for metabolism. While most sugars in our body belong to the D-series, there are specific exceptions where L-isomers are synthesized and utilized. **L-fructose**, however, is neither synthesized nor metabolized by human enzymes and has no physiological role in the body. **Analysis of Options:** * **L-fructose (Correct):** This is a synthetic sugar not found in human metabolic pathways. Human hexokinase and other glycolytic enzymes are stereospecific for D-fructose. * **L-fucose:** This is a crucial exception to the "D-sugar rule." It is an L-isomer sugar synthesized in the body and is a vital component of **glycoproteins** and **blood group substances** (H-substance). * **D-Glucose:** The primary metabolic fuel for the human body, especially for the brain and RBCs. * **D-Fructose:** A common dietary monosaccharide (found in fruits and honey) metabolized via the fructokinase pathway in the liver. **High-Yield Clinical Pearls for NEET-PG:** * **The "L-Sugar" Exceptions:** While most human sugars are D-isomers, remember these two L-isomers: **L-fucose** (found in glycoproteins) and **L-xylulose** (excreted in excess in Essential Pentosuria due to deficiency of L-xylulose reductase). * **Stereospecificity:** Enzymes are highly stereospecific; for example, glucose oxidase reacts only with D-glucose, not L-glucose. * **L-fucose Source:** It is derived from GDP-mannose. Deficiency in its transport leads to **Leukocyte Adhesion Deficiency Type II**.

Q: Which glucose transporter (GLUT) is primarily responsible for transporting glucose from the intestine to the liver?

GLUT2. **Explanation:** The correct answer is **GLUT2**. **Why GLUT2 is correct:** GLUT2 is a high-capacity, low-affinity (high $K_m$) bidirectional transporter. It is primarily located in the **basolateral membrane of intestinal mucosal cells**, the liver, pancreatic beta cells, and the renal tubular cells. After glucose is absorbed from the intestinal lumen via SGLT-1, it exits the enterocyte into the portal circulation via GLUT2. Because of its high $K_m$, GLUT2 acts as a "glucose sensor," ensuring that glucose uptake by the liver is proportional to blood glucose levels, especially in the postprandial (fed) state. **Why other options are incorrect:** * **GLUT1:** This is a high-affinity transporter found in almost all tissues, particularly **RBCs and the Blood-Brain Barrier**. It provides basal glucose uptake. * **GLUT3:** This is the primary transporter in **neurons**. It has a very low $K_m$, ensuring the brain receives glucose even during hypoglycemia. * **GLUT4:** This is the only **insulin-dependent** transporter. It is located in **skeletal muscle and adipose tissue**. In the absence of insulin, it remains sequestered in intracellular vesicles. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT-1 vs. GLUT2:** Remember that SGLT-1 (Sodium-Glucose Co-transporter 1) handles the *active* uptake from the intestinal lumen, while GLUT2 handles the *facilitated* exit into the blood. * **Fanconi-Bickel Syndrome:** A rare glycogen storage disease caused by a congenital defect in **GLUT2**, leading to hepatomegaly and glucose intolerance. * **Bidirectionality:** GLUT2 is unique because it allows glucose to enter the liver for glycogenesis and exit the liver during gluconeogenesis/glycogenolysis.

Q: Which enzyme is responsible for the postprandial utilization of glucose?

Glucokinase. **Explanation:** The correct answer is **Glucokinase (Hexokinase IV)**. **Why Glucokinase is the correct answer:** Glucokinase is primarily located in the liver and pancreatic beta cells. Its unique kinetic properties make it ideal for the **postprandial (fed) state**: 1. **High $K_m$ (Low affinity):** It only becomes significantly active when blood glucose levels are high (e.g., after a meal). 2. **High $V_{max}$ (High capacity):** It can rapidly phosphorylate large amounts of glucose, allowing the liver to "clear" glucose from the portal blood and store it as glycogen, preventing postprandial hyperglycemia. 3. **Lack of Product Inhibition:** Unlike hexokinase, it is not inhibited by its product (Glucose-6-Phosphate), allowing continuous glucose uptake even when energy levels are high. **Why other options are incorrect:** * **Hexokinase (Types I-III):** These are found in extrahepatic tissues. They have a **low $K_m$ (high affinity)**, meaning they are already saturated at fasting glucose levels. Their role is to ensure tissues like the brain get glucose even during starvation, not to manage a postprandial surplus. * **Fructokinase:** This enzyme is specific to fructose metabolism (converting fructose to fructose-1-phosphate) and does not utilize glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Molecular Sensor:** Glucokinase acts as the "glucose sensor" in the pancreas for insulin release. * **MODY Type 2:** Mutations in the glucokinase gene lead to Maturity-Onset Diabetes of the Young (MODY) type 2. * **Localization:** Glucokinase is regulated by the **Glucokinase Regulatory Protein (GKRP)**, which sequesters it in the nucleus during fasting. * **Inducibility:** Glucokinase is induced by **Insulin**, further enhancing its role in the fed state.

Q: Which of the following hormones stimulates glucose utilization?

Insulin. **Explanation:** The regulation of blood glucose is a balance between **anabolic (hypoglycemic)** and **catabolic (hyperglycemic)** hormones. **Why Insulin is Correct:** Insulin is the only major anabolic hormone that lowers blood glucose by stimulating **glucose utilization**. It achieves this through three primary mechanisms: 1. **Increased Uptake:** It promotes the translocation of **GLUT-4** transporters to the cell membranes of skeletal muscle and adipose tissue. 2. **Glycolysis:** It activates key enzymes like Phosphofructokinase-1 (PFK-1) and Glucokinase, favoring glucose breakdown. 3. **Glycogenesis:** It stimulates Glycogen Synthase to store glucose as glycogen in the liver and muscles. **Why Other Options are Incorrect:** Options B, C, and D are all **counter-regulatory (diabetogenic) hormones** that increase blood glucose levels: * **Glucagon:** Stimulates hepatic glycogenolysis and gluconeogenesis during fasting states. * **Corticosteroids (Cortisol):** Increase gluconeogenesis and decrease peripheral glucose uptake (insulin resistance) to ensure glucose availability during stress. * **Growth Hormone:** Inhibits glucose uptake in peripheral tissues (anti-insulin effect) and stimulates lipolysis. **High-Yield NEET-PG Pearls:** * **GLUT-4** is the only insulin-dependent glucose transporter (found in heart, skeletal muscle, and adipose tissue). * **Brain and Liver** do not require insulin for glucose uptake (GLUT-1 and GLUT-2 respectively). * **Rate-limiting enzyme of Glycolysis:** Phosphofructokinase-1 (PFK-1), which is induced by insulin via Fructose 2,6-bisphosphate. * **C-peptide** levels can be used to differentiate endogenous insulin production from exogenous insulin injection.

Q: All of the following are glycogen storage disorders except?

All of the above. **Explanation:** This question tests your knowledge of **Glycogen Storage Disorders (GSDs)**, which are a group of inherited metabolic disorders caused by enzyme deficiencies affecting glycogen synthesis or breakdown. **1. Why "All of the above" is correct:** All three options listed are classic examples of Glycogen Storage Disorders. Each represents a deficiency in a specific enzyme required for glycogen metabolism: * **Pompe Disease (GSD Type II):** Caused by a deficiency of **Acid α-1,4-glucosidase** (Acid Maltase). It is unique because it is also a lysosomal storage disorder, leading to glycogen accumulation in the heart and skeletal muscles. * **Anderson Disease (GSD Type IV):** Caused by a deficiency of the **Branching enzyme**. This leads to the formation of abnormal glycogen with long outer chains (amylopectin-like), which triggers an immune response leading to liver cirrhosis. * **Hers Disease (GSD Type VI):** Caused by a deficiency of **Liver Phosphorylase**. This results in the inability to break down glycogen in the liver, leading to hepatomegaly and mild hypoglycemia. **2. Clinical Pearls for NEET-PG:** * **Von Gierke Disease (Type I):** Most common GSD; deficiency of Glucose-6-Phosphatase. Presents with severe hypoglycemia, lactic acidosis, and hyperuricemia. * **Cori Disease (Type III):** Deficiency of Debranching enzyme; presents with milder hypoglycemia than Type I. * **McArdle Disease (Type V):** Deficiency of Muscle Phosphorylase; presents with exercise-induced cramps and myoglobinuria. * **Mnemonic for Types I-VI:** "**V**on **P**ompe **C**orrects **A**nderson's **M**uscular **H**ers" (Von Gierke, Pompe, Cori, Anderson, McArdle, Hers). **Conclusion:** Since Pompe, Anderson, and Hers are all recognized types of GSDs, the correct answer is "All of the above."

Q: Which one of the following human tissues contains the greatest amount of body glycogen?

Skeletal muscle. **Explanation:** The correct answer is **Skeletal muscle**. This question hinges on the distinction between **concentration** (percentage per gram of tissue) and **total content** (total mass in the body). 1. **Why Skeletal Muscle is Correct:** While the liver has a higher *concentration* of glycogen (approx. 5–8% of its weight), the total mass of skeletal muscle in the human body is significantly larger (approx. 40% of body weight). Consequently, skeletal muscle stores about **three-quarters (approx. 400g)** of the body's total glycogen, whereas the liver stores only about **one-quarter (approx. 100g)**. 2. **Why Other Options are Incorrect:** * **Liver:** It has the highest *density/concentration* of glycogen, but its smaller total organ mass means it holds less total glycogen than the muscular system. * **Kidney & Cardiac Muscle:** These tissues store only negligible amounts of glycogen for local, emergency metabolic needs. They do not serve as systemic reservoirs. **High-Yield NEET-PG Pearls:** * **Function:** Liver glycogen maintains **blood glucose levels** during fasting (via Glucose-6-Phosphatase). Muscle glycogen is used **locally** for ATP production during contraction because muscles lack Glucose-6-Phosphatase and cannot release glucose into the blood. * **Regulation:** Muscle glycogen is stimulated by **epinephrine** and calcium ions, while liver glycogen is primarily regulated by **glucagon** and insulin. * **Glycogen Storage Diseases (GSD):** Type I (Von Gierke’s) affects the liver; Type II (Pompe) and Type V (McArdle) primarily affect the muscles.

Q: Which one of the following helps in generating oxygen burst in the neutrophils?

NADPH oxidase. ### Explanation **Correct Option: A. NADPH oxidase** The "Oxygen Burst" (or Respiratory Burst) is a critical process in neutrophils and macrophages used to kill phagocytosed bacteria. When a pathogen is ingested, there is a rapid increase in oxygen consumption. The enzyme **NADPH oxidase**, located in the phagosomal membrane, catalyzes the conversion of molecular oxygen ($O_2$) into **superoxide radicals** ($O_2^-$) by transferring an electron from NADPH. This superoxide is the precursor for other potent reactive oxygen species (ROS) like hydrogen peroxide and hypochlorite, which destroy the microbe. **Analysis of Incorrect Options:** * **B. Superoxide dismutase (SOD):** This enzyme actually acts as an antioxidant. It converts the superoxide radical ($O_2^-$) into hydrogen peroxide ($H_2O_2$). While it is part of the pathway, it is not the *generator* of the burst; it processes the product of NADPH oxidase. * **C. Catalase:** This is a protective enzyme that breaks down hydrogen peroxide into water and oxygen. It prevents cellular damage from ROS rather than generating the burst. * **D. Glutathione peroxidase:** This enzyme reduces hydrogen peroxide to water using reduced glutathione. It is a key component of the cellular antioxidant defense system, not the pro-oxidant burst mechanism. **Clinical Pearls for NEET-PG:** * **Chronic Granulomatous Disease (CGD):** A high-yield clinical correlation. It is caused by a genetic deficiency in **NADPH oxidase**. Patients suffer from recurrent infections with **catalase-positive** organisms (e.g., *S. aureus*, *Aspergillus*) because they cannot generate their own oxygen burst. * **MPO (Myeloperoxidase):** This enzyme uses the $H_2O_2$ produced during the burst to create **HOCl (Hypochlorous acid/Bleach)**, which is the most potent bactericidal agent in neutrophils. * **Nitroblue Tetrazolium (NBT) Test:** Used to diagnose CGD; a positive result (blue color) indicates functional NADPH oxidase.

Q: What syndrome is associated with the deficiency of Dermatan sulfate, heparan sulfate, chondroitin 4-sulfate, and chondroitin 6-sulfate?

Sly syndrome. ### Explanation **Correct Answer: C. Sly syndrome** **Underlying Medical Concept:** Sly syndrome (Mucopolysaccharidosis type VII) is caused by a deficiency of the enzyme **β-glucuronidase**. This specific enzyme is required for the degradation of glucuronic acid residues found in multiple glycosaminoglycans (GAGs). Because glucuronic acid is a common constituent of **Dermatan sulfate, Heparan sulfate, and Chondroitin 4/6-sulfates**, a deficiency in β-glucuronidase leads to the systemic accumulation of all four substances. This distinguishes it from other MPS types which typically involve only one or two GAGs. **Analysis of Incorrect Options:** * **A. Hunter syndrome (MPS II):** Caused by a deficiency of **Iduronate sulfatase**. It leads to the accumulation of **Dermatan sulfate and Heparan sulfate** only. (Key NEET-PG fact: It is X-linked recessive and lacks corneal clouding). * **B. Morquio syndrome B (MPS IVB):** Caused by a deficiency of **β-galactosidase**. It primarily results in the accumulation of **Keratan sulfate**. * **D. Hurler syndrome (MPS IH):** Caused by a deficiency of **α-L-iduronidase**. Like Hunter syndrome, it leads to the accumulation of **Dermatan sulfate and Heparan sulfate**, but presents with more severe features and corneal clouding. **High-Yield Clinical Pearls for NEET-PG:** * **Sly Syndrome Unique Feature:** It is the only MPS that can present as **Hydrops fetalis** in utero. * **GAG Triad:** If a question mentions the accumulation of **Chondroitin sulfates** alongside Dermatan and Heparan, always think of **Sly Syndrome**. * **Enzyme Mnemonic:** "Sly (7) Glucuronidase" — MPS **VII** is **β-glucuronidase** deficiency.

Q: Muscle cannot participate in gluconeogenesis due to the absence of which enzyme?

Glucose 6 phosphatase. **Explanation:** The primary goal of gluconeogenesis is to maintain blood glucose levels during fasting. While the liver (and to a lesser extent, the kidney) can perform this task, skeletal muscle cannot. **Why Glucose-6-Phosphatase is the Correct Answer:** Gluconeogenesis involves the synthesis of glucose from non-carbohydrate precursors. The final step of this pathway is the conversion of **Glucose-6-Phosphate to free Glucose**. This reaction is catalyzed by the enzyme **Glucose-6-Phosphatase**. Skeletal muscle lacks this enzyme; therefore, even though muscle can break down glycogen to Glucose-6-Phosphate, it cannot dephosphorylate it to release free glucose into the bloodstream. Instead, the Glucose-6-Phosphate enters the glycolytic pathway to provide energy (ATP) for muscle contraction. **Why the Other Options are Incorrect:** * **Enolase:** This is a reversible enzyme used in both glycolysis and gluconeogenesis. It is present in almost all tissues, including muscle. * **Pyruvate Kinase:** This is a key regulatory enzyme of glycolysis. While it is bypassed during gluconeogenesis (by Pyruvate Carboxylase and PEP Carboxykinase), its presence or absence does not define a tissue's ability to perform gluconeogenesis. **High-Yield Clinical Pearls for NEET-PG:** * **The Cori Cycle:** Since muscle cannot release glucose, it exports **Lactate** to the liver. The liver then converts lactate back to glucose via gluconeogenesis and sends it back to the muscle. * **Von Gierke’s Disease (GSD Type I):** This condition is caused by a deficiency of Glucose-6-Phosphatase. It leads to severe fasting hypoglycemia because neither glycogenolysis nor gluconeogenesis can release glucose from the liver. * **Key Gluconeogenic Organs:** Liver (90%) and Kidney (10%). During prolonged starvation, the kidney's contribution increases significantly.

Question 1

Which of the following sugars is not synthesized or utilized in the human body?

Accepted Answer

L-fructose

Answer

L-fucose

Answer

D-Glucose

Answer

D-Fructose

Question 2

Which glucose transporter (GLUT) is primarily responsible for transporting glucose from the intestine to the liver?

Accepted Answer

GLUT2

Answer

GLUT1

Answer

GLUT3

Answer

GLUT4

Question 3

Which enzyme is responsible for the postprandial utilization of glucose?

Accepted Answer

Glucokinase

Answer

Fructokinase

Answer

Hexokinase

Answer

All of the above

Question 4

ATP is an allosteric regulator of which enzymes?

Accepted Answer

PFK 1 and Pyruvate kinase

Answer

Hexokinase and PFK 1

Answer

PFK 1 and PFK 2

Answer

Glyceraldehyde 3 phosphate dehydrogenase and PFK 1

Question 5

Which of the following hormones stimulates glucose utilization?

Accepted Answer

Insulin

Answer

Growth hormone

Answer

Corticosteroid

Answer

Glucagon

Question 6

All of the following are glycogen storage disorders except?

Accepted Answer

All of the above

Answer

Pompe disease

Answer

Hers disease

Answer

Anderson disease

Question 7

Which one of the following human tissues contains the greatest amount of body glycogen?

Accepted Answer

Skeletal muscle

Answer

Liver

Answer

Kidney

Answer

Cardiac muscle

Question 8

Which one of the following helps in generating oxygen burst in the neutrophils?

Accepted Answer

NADPH oxidase

Answer

Superoxide dismutase

Answer

Catalase

Answer

Glutathione peroxidase

Question 9

What syndrome is associated with the deficiency of Dermatan sulfate, heparan sulfate, chondroitin 4-sulfate, and chondroitin 6-sulfate?

Accepted Answer

Sly syndrome

Answer

Hunter syndrome

Answer

Morquio syndrome B

Answer

Hurler syndrome

Question 10

Muscle cannot participate in gluconeogenesis due to the absence of which enzyme?

Accepted Answer

Glucose 6 phosphatase

Answer

Enolase

Answer

Pyruvate kinase

Answer

All of the above

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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