Carbohydrate Metabolism Practice Questions

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.

Q: Amino sugars are formed from which of the following precursors?

Fructose-6-phosphate. ### Explanation The synthesis of amino sugars (hexosamines) occurs via the **Hexosamine Biosynthetic Pathway (HBP)**. **1. Why Fructose-6-phosphate is correct:** The key rate-limiting step in the formation of amino sugars is the conversion of **Fructose-6-phosphate (F6P)** into **Glucosamine-6-phosphate**. This reaction is catalyzed by the enzyme **Glutamine-fructose-6-phosphate amidotransferase (GFAT)**. In this process, an amino group is transferred from the donor amino acid, **Glutamine**, to Fructose-6-phosphate. Glucosamine-6-phosphate then serves as the precursor for N-acetylglucosamine (GlcNAc), N-acetylgalactosamine (GalNAc), and sialic acids, which are essential components of glycoproteins, glycolipids, and glycosaminoglycans (GAGs). **2. Why the other options are incorrect:** * **Glucose-6-phosphate (B):** While G6P is the starting point of glycolysis and the HMP shunt, it must first be isomerized to Fructose-6-phosphate by phosphohexose isomerase before it can enter the hexosamine pathway. * **Glucose-1-phosphate (A):** This is primarily involved in glycogenesis (glycogen synthesis) and uronic acid pathways, not direct amino sugar formation. * **Fructose-1-phosphate (C):** This is an intermediate of fructose metabolism in the liver (via fructokinase) and does not serve as a substrate for GFAT. ### High-Yield Clinical Pearls for NEET-PG: * **Nitrogen Donor:** Glutamine is the obligatory nitrogen donor for amino sugar synthesis. * **Feedback Inhibition:** The end product, UDP-GlcNAc, inhibits the rate-limiting enzyme GFAT. * **Clinical Significance:** The hexosamine pathway is linked to **insulin resistance**. High intracellular glucose levels increase flux through this pathway, leading to the modification of signaling proteins (O-GlcNAcylation), which can impair insulin sensitivity. * **Essential Amino Sugars:** Glucosamine, Galactosamine, and N-acetylneuraminic acid (NANA/Sialic acid).

Q: Which of the following does NOT allosterically inhibit phosphofructokinase-1 (PFK-1)?

AMP/ADP. **Explanation:** Phosphofructokinase-1 (PFK-1) is the **rate-limiting and key committed step** of glycolysis, converting Fructose-6-phosphate to Fructose-1,6-bisphosphate. Its regulation is crucial for balancing cellular energy needs. **Why AMP/ADP is the correct answer:** AMP and ADP are indicators of a **low-energy state** in the cell. When ATP is consumed, AMP levels rise. AMP acts as a potent **allosteric activator** of PFK-1, signaling the cell to increase glycolytic flux to generate more ATP. Therefore, it does not inhibit the enzyme; it stimulates it. **Analysis of Incorrect Options (Inhibitors):** * **ATP:** Although a substrate, high levels of ATP act as an allosteric inhibitor. It binds to a regulatory site to decrease the enzyme's affinity for Fructose-6-phosphate, signaling that the cell has sufficient energy. * **Citrate:** An intermediate of the TCA cycle. High citrate levels indicate that the cycle is "saturated" and precursors for energy production are abundant, leading to the feedback inhibition of PFK-1. * **H+ (Low pH):** Accumulation of protons (acidosis) inhibits PFK-1. This is a protective mechanism, particularly in skeletal muscle, to prevent excessive lactic acid production and subsequent tissue damage during anaerobic glycolysis. **High-Yield NEET-PG Pearls:** * **Most Potent Activator:** Fructose-2,6-bisphosphate is the most powerful allosteric activator of PFK-1 (overcomes ATP inhibition). * **Insulin vs. Glucagon:** Insulin increases PFK-1 activity (via F-2,6-BP), while glucagon decreases it. * **Reciprocal Regulation:** Factors that activate PFK-1 (like F-2,6-BP and AMP) typically inhibit Fructose-1,6-bisphosphatase (gluconeogenesis), preventing a futile cycle.

Q: All of the following are enzymes of the TCA cycle, EXCEPT?

Malic enzyme. ### Explanation The **TCA cycle (Krebs cycle)** occurs in the mitochondrial matrix and consists of eight primary enzymatic steps. **Why Malic Enzyme is the correct answer:** **Malic enzyme** is not a component of the TCA cycle. It is an enzyme that catalyzes the oxidative decarboxylation of **Malate to Pyruvate**, simultaneously reducing NADP⁺ to **NADPH**. This reaction occurs in the cytosol and is considered an **anaplerotic** or shunt reaction. Its primary roles are providing NADPH for fatty acid synthesis and participating in the pyruvate-malate shuttle. It should not be confused with *Malate Dehydrogenase*, which converts Malate to Oxaloacetate within the TCA cycle. **Analysis of Incorrect Options:** * **A. Aconitase:** Catalyzes the isomerization of Citrate to Isocitrate via the intermediate *cis*-aconitate. It requires $Fe^{2+}$ as a cofactor. * **B. Fumarase (Fumarate Hydratase):** Catalyzes the hydration of Fumarate to L-Malate. * **D. Citrate Synthase:** The "pacemaker" or rate-limiting enzyme that condenses Acetyl-CoA and Oxaloacetate to form Citrate. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** All TCA enzymes are in the mitochondrial matrix except **Succinate Dehydrogenase**, which is located on the inner mitochondrial membrane (also acts as Complex II of the ETC). * **Energy Yield:** One turn of the TCA cycle produces **10 ATP** (3 NADH = 7.5, 1 $FADH_2$ = 1.5, 1 GTP = 1). * **Inhibitors:** Fluoroacetate inhibits Aconitase; Arsenite inhibits $\alpha$-Ketoglutarate Dehydrogenase. * **Malic Enzyme vs. Malate Dehydrogenase:** Remember, Malic enzyme produces **NADPH**, while Malate Dehydrogenase produces **NADH**.

Q: A 15-year-old type I diabetic patient faints after injecting himself with insulin. He is administered Glucagon and rapidly recovers consciousness. Glucagon induces activity of which enzyme?

Glycogen phosphorylase. ### Explanation **1. Why Glycogen Phosphorylase is Correct:** The patient is experiencing **hypoglycemia** due to insulin overdose. Glucagon is the counter-regulatory hormone secreted in response to low blood glucose. It acts primarily on the liver by binding to G-protein coupled receptors (GPCR), increasing intracellular **cAMP**, and activating **Protein Kinase A (PKA)**. PKA phosphorylates **Phosphorylase Kinase**, which in turn phosphorylates and activates **Glycogen Phosphorylase** (converting it from the inactive 'b' form to the active 'a' form). This triggers **glycogenolysis**, rapidly releasing glucose into the bloodstream to restore consciousness. **2. Why the Other Options are Incorrect:** * **A. Glycogen Synthase:** This enzyme is responsible for glycogenesis (glycogen synthesis). Glucagon-induced phosphorylation **inactivates** glycogen synthase to prevent a futile cycle during hypoglycemia. * **C & D. Glucokinase/Hexokinase:** These enzymes catalyze the first step of glycolysis (glucose to glucose-6-phosphate). Glucagon inhibits glycolysis in the liver (via inhibition of PFK-1 and Pyruvate Kinase) to conserve glucose for systemic circulation. Glucokinase expression is actually induced by insulin, not glucagon. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Glucagon $\rightarrow$ cAMP $\uparrow$ $\rightarrow$ PKA $\rightarrow$ Phosphorylase Kinase $\rightarrow$ Glycogen Phosphorylase. * **Key Regulatory Step:** Glycogen phosphorylase is the **rate-limiting enzyme** of glycogenolysis. * **Tissue Specificity:** Glucagon acts on the **liver** but has **no effect on muscle glycogen**, as muscle cells lack glucagon receptors. Muscle glycogen is used only for local energy during contraction. * **Clinical Use:** Glucagon is the emergency treatment of choice for severe hypoglycemia in diabetics when intravenous glucose access is unavailable.

Question 1

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 2

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 3

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 4

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

Question 5

Amino sugars are formed from which of the following precursors?

Accepted Answer

Fructose-6-phosphate

Answer

Glucose-1-phosphate

Answer

Glucose-6-phosphate

Answer

Fructose-1-phosphate

Question 6

An 8-day-old child presents with yellow sclera, whitish stool, and turmeric-colored urine on the 3rd day of septicemia treated with broad-spectrum antibiotics. What is the likely diagnosis?

Accepted Answer

Galactose-1-phosphate uridyltransferase deficiency

Answer

Galactosidase deficiency

Answer

Ammonia toxicity

Answer

All of the above

Question 7

What are the final products of the Hexose Monophosphate Pathway?

Accepted Answer

6 NADPH

Answer

2 NADPH

Answer

3 NADPH

Answer

Variable

Question 8

Which of the following does NOT allosterically inhibit phosphofructokinase-1 (PFK-1)?

Accepted Answer

AMP/ADP

Answer

Citrate

Answer

ATP

Answer

H+

Question 9

All of the following are enzymes of the TCA cycle, EXCEPT?

Accepted Answer

Malic enzyme

Answer

Aconitase

Answer

Fumarase

Answer

Citrate synthase

Question 10

A 15-year-old type I diabetic patient faints after injecting himself with insulin. He is administered Glucagon and rapidly recovers consciousness. Glucagon induces activity of which enzyme?

Accepted Answer

Glycogen phosphorylase

Answer

Glycogen synthase

Answer

Glucokinase

Answer

Hexokinase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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