Carbohydrate Metabolism — MCQs

A 3-year-old child presents with hepatomegaly and recurrent episodes of vomiting, jaundice, and hypoglycemia, beginning from the later part of infancy. These episodes seem to have no temporal relation with the intake of milk or milk products. The mother also gives a history that the child has an aversion to sweet food. Which of the following enzymes may be deficient in this child?

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 261: What is the primary product of the pentose phosphate pathway besides pentoses?

A. ATP
B. NADPH (Correct Answer)
C. ADP
D. Acetyl CoA

Explanation: ### Explanation The **Pentose Phosphate Pathway (PPP)**, also known as the Hexose Monophosphate (HMP) Shunt, is an alternative pathway for glucose oxidation. Unlike glycolysis, its primary purpose is not energy production (ATP) but the generation of two specific products: **Pentose phosphates** (for nucleotide synthesis) and **NADPH**. **Why NADPH is the correct answer:** NADPH (Nicotinamide Adenine Dinucleotide Phosphate) is generated during the oxidative phase of the pathway, primarily by the rate-limiting enzyme **Glucose-6-Phosphate Dehydrogenase (G6PD)**. NADPH is essential for: 1. **Reductive Biosynthesis:** Providing reducing equivalents for fatty acid and steroid synthesis (active in liver, lactating mammary glands, and adrenal cortex). 2. **Antioxidant Defense:** Maintaining a pool of reduced **glutathione** to protect cells (especially RBCs) against reactive oxygen species (ROS). **Analysis of Incorrect Options:** * **A & C (ATP/ADP):** The HMP shunt is unique because it **neither consumes nor produces ATP**. It is an energy-neutral pathway focused on biosynthetic precursors. * **D (Acetyl CoA):** This is the end product of the Pyruvate Dehydrogenase complex following glycolysis. While NADPH is used to synthesize fatty acids from Acetyl CoA, the HMP shunt itself does not produce Acetyl CoA. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. A lack of NADPH leads to the inability to regenerate reduced glutathione, resulting in oxidative hemolysis and the presence of **Heinz bodies** and **Bite cells** on peripheral smears. * **Thiamine (B1) Requirement:** The non-oxidative phase uses the enzyme **Transketolase**, which requires Thiamine Pyrophosphate (TPP) as a cofactor. Measuring erythrocyte transketolase activity is a diagnostic test for Thiamine deficiency. * **Localization:** Occurs entirely in the **cytosol**.

Question 262: In anaerobic glycolysis, what is the net gain of ATP?

A. 2 ATP and 2 NAD
B. 2 ATP (Correct Answer)
C. 2 ATP and 2 NADH
D. 4 ATP and 2 FADH2

Explanation: ### Explanation **1. Why Option B is Correct:** In anaerobic glycolysis (which occurs in the cytosol), the net energy yield is **2 ATP per molecule of glucose**. * **Investment Phase:** 2 ATP are consumed (catalyzed by Hexokinase and Phosphofructokinase-1). * **Payoff Phase:** 4 ATP are produced via substrate-level phosphorylation (catalyzed by Phosphoglycerate kinase and Pyruvate kinase). * **The Redox Balance:** Under anaerobic conditions (e.g., exercising muscle or RBCs), the 2 NADH produced during the oxidation of Glyceraldehyde-3-phosphate must be re-oxidized to **NAD+** to allow glycolysis to continue. This is achieved by reducing Pyruvate to **Lactate** (catalyzed by Lactate Dehydrogenase). Consequently, there is **no net gain of NADH**. **2. Why Other Options are Incorrect:** * **Option A:** While 2 ATP are gained, NAD+ is a coenzyme that is recycled, not a "net gain" product of the pathway. * **Option C:** This describes **aerobic glycolysis**. In the presence of oxygen, NADH is shuttled into the mitochondria for the Electron Transport Chain (ETC) rather than being used to form lactate. * **Option D:** 4 ATP is the *gross* yield, not the net yield. FADH2 is produced in the TCA cycle, not glycolysis. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Mature RBCs:** They lack mitochondria and rely **exclusively** on anaerobic glycolysis for energy. * **Rapoport-Luebering Cycle:** A shunt in RBC glycolysis that produces **2,3-BPG**, which shifts the oxygen dissociation curve to the right (facilitating O2 unloading). This process yields **0 net ATP**. * **Key Enzyme:** Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme of glycolysis. * **Lactic Acidosis:** Occurs when the rate of glycolysis exceeds the capacity of mitochondria to oxidize pyruvate (e.g., shock, severe hypoxia).

Question 263: In starvation, the brain utilizes glucose preferentially. Why does the liver not do the same?

A. Low Km value of hexokinase (Correct Answer)
B. Low Km of glucokinase
C. High Km of glucokinase
D. High blood flow

Explanation: This question tests your understanding of the kinetic differences between **Hexokinase** and **Glucokinase**, which is a high-yield topic for NEET-PG. ### **Explanation of the Correct Answer** In starvation, blood glucose levels are low. The brain must prioritize glucose uptake to maintain cellular function. This is achieved through **Hexokinase**, which has a **low $K_m$** (high affinity) for glucose. A low $K_m$ means the enzyme can function at maximum velocity ($V_{max}$) even when glucose concentrations are minimal. Therefore, the brain can efficiently "trap" glucose even during hypoglycemia. ### **Why the Other Options are Incorrect** * **B & C (Glucokinase kinetics):** Glucokinase (found in the liver and pancreatic beta cells) has a **high $K_m$** (low affinity). It only becomes active when blood glucose levels are high (post-prandial). In starvation, the liver's glucokinase is inactive, preventing the liver from competing with the brain for scarce glucose. This ensures the liver performs gluconeogenesis to *supply* glucose rather than *consuming* it. * **D (High blood flow):** While the brain receives significant cardiac output, blood flow alone does not determine the biochemical trapping of glucose; enzymatic affinity ($K_m$) is the regulatory bottleneck. ### **High-Yield Clinical Pearls for NEET-PG** * **Hexokinase:** Found in most extrahepatic tissues; inhibited by its product, **Glucose-6-Phosphate** (feedback inhibition). * **Glucokinase (Hexokinase IV):** Not inhibited by G6P; induced by **Insulin**. It acts as a "glucose sensor." * **MODY Type 2:** Caused by a mutation in the Glucokinase gene, leading to a higher threshold for insulin release and mild chronic hyperglycemia. * **GLUT Transporters:** Remember that the brain primarily uses **GLUT-1 and GLUT-3** (insulin-independent, low $K_m$), while the liver uses **GLUT-2** (high $K_m$, high capacity).

Question 264: What is the characteristic function of Glucokinase?

A. It is widely distributed and occurs in most mammalian tissues.
B. It has a high Km for glucose, making it important for glucose phosphorylation primarily after ingestion of a carbohydrate-rich meal. (Correct Answer)
C. It is widely distributed in prokaryotes.
D. None of the above.

Explanation: **Explanation:** The question tests the fundamental differences between the two primary isoenzymes that catalyze the first step of glycolysis: **Hexokinase** and **Glucokinase (Hexokinase IV)**. **1. Why Option B is Correct:** Glucokinase has a **high $K_m$** (low affinity) for glucose. This means it only becomes significantly active when blood glucose levels are high, such as in the **post-prandial state** (after a carbohydrate-rich meal). Its function is to "clear" glucose from the portal blood into the liver for glycogen synthesis and lipogenesis, preventing excessive hyperglycemia. Unlike Hexokinase, Glucokinase is **not inhibited by glucose-6-phosphate**, allowing it to continue processing glucose even when energy levels are high. **2. Why Other Options are Incorrect:** * **Option A:** This describes **Hexokinase**. Hexokinase is widely distributed in almost all extrahepatic tissues, has a low $K_m$ (high affinity), and ensures cells get enough glucose even during fasting. * **Option C:** Glucokinase is a eukaryotic enzyme primarily found in the **liver** and **pancreatic beta cells**; it is not a characteristic feature of prokaryotic metabolism. **3. High-Yield Clinical Pearls for NEET-PG:** * **Location:** Glucokinase is found in the **Liver** and **Pancreatic $\beta$-cells**. In the pancreas, it acts as a "glucose sensor" for insulin secretion. * **Inducibility:** Glucokinase is induced by **Insulin**, whereas Hexokinase is constitutive (not regulated by insulin). * **Clinical Correlation:** Mutations in the Glucokinase gene can lead to **MODY type 2** (Maturity-Onset Diabetes of the Young), characterized by a higher threshold for insulin release. * **Regulation:** Glucokinase is regulated by the **Glucokinase Regulatory Protein (GKRP)**, which sequesters it in the nucleus during fasting.

Question 265: Which of the following intermediates of the TCA cycle is depleted in Type-I Diabetes mellitus to suppress the TCA cycle?

A. Succinate
B. Malate
C. a-Ketoglutarate
D. Oxaloacetate (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Oxaloacetate** In Type-I Diabetes Mellitus (TIDM), there is an absolute deficiency of insulin and an excess of glucagon. This hormonal imbalance triggers a state of "starvation in the midst of plenty," leading to the following metabolic consequences: 1. **Increased Gluconeogenesis:** To compensate for perceived hypoglycemia, the liver aggressively synthesizes glucose. **Oxaloacetate (OAA)** is the primary substrate for gluconeogenesis (via PEP carboxykinase). Consequently, OAA is diverted away from the TCA cycle to produce glucose. 2. **Impaired TCA Cycle:** OAA is the "limiting factor" of the TCA cycle; it must condense with Acetyl-CoA to form Citrate. When OAA is depleted, the TCA cycle cannot proceed efficiently. 3. **Ketogenesis:** Simultaneously, increased lipolysis floods the liver with Acetyl-CoA. Since the TCA cycle is suppressed due to low OAA, the excess Acetyl-CoA is diverted into the **ketogenic pathway**, leading to Diabetic Ketoacidosis (DKA). --- ### Why Other Options are Incorrect: * **A, B, and C (Succinate, Malate, α-Ketoglutarate):** While these are all intermediates of the TCA cycle, they are not the primary "bottleneck" diverted for gluconeogenesis in the same capacity as OAA. OAA is the direct precursor to Phosphoenolpyruvate (PEP), making its withdrawal the critical event that halts the cycle. --- ### NEET-PG High-Yield Pearls: * **The "Metabolic Crossroad":** Oxaloacetate is required for both the TCA cycle and Gluconeogenesis. Its depletion is the biochemical link between hyperglycemia and ketosis. * **Acetyl-CoA Source:** In TIDM, Acetyl-CoA levels are high (from β-oxidation of fatty acids), but it cannot enter the TCA cycle without OAA. * **Key Enzyme:** **Pyruvate Carboxylase** (requires Biotin) converts Pyruvate to OAA to replenish the cycle (anaplerosis), but in TIDM, the flux is heavily toward glucose production.

Question 266: What is the carbohydrate component of blood group substances?

A. Sucrose
B. Fucose (Correct Answer)
C. Arabinose
D. Maltose

Explanation: ### Explanation **Why Fucose is Correct:** Blood group antigens (ABO system) are complex glycosphingolipids or glycoproteins found on the surface of red blood cells. The specificity of these antigens is determined by the terminal sugar residues attached to a precursor substance known as the **H-substance**. **L-Fucose** (6-deoxy-L-galactose) is the essential carbohydrate component that must be attached to the precursor chain by the enzyme *fucosyltransferase* to form the H-antigen. * **H-antigen:** Precursor + L-Fucose. * **A-antigen:** H-antigen + N-acetylgalactosamine. * **B-antigen:** H-antigen + D-galactose. Since the H-antigen is the foundation for both A and B antigens, Fucose is the fundamental carbohydrate component of blood group substances. **Why Incorrect Options are Wrong:** * **A. Sucrose:** A disaccharide (Glucose + Fructose) primarily involved in dietary energy; it is not a structural component of human cell surface antigens. * **C. Arabinose:** A five-carbon aldopentose found mainly in plant polysaccharides (like gum arabic) and certain bacterial cell walls, but not in human blood group substances. * **D. Maltose:** A disaccharide (Glucose + Glucose) produced during the digestion of starch; it does not play a role in membrane glycoconjugates. **High-Yield Clinical Pearls for NEET-PG:** 1. **Bombay Phenotype:** Individuals lacking the *H gene* cannot attach L-Fucose to the precursor. They lack H, A, and B antigens, appearing as Type O, but they produce anti-H antibodies, making cross-matching difficult. 2. **Secretors:** In 80% of the population (secretors), these blood group substances are also found in saliva, sweat, and other body fluids. 3. **Deoxy-sugar:** Fucose is a "deoxy-sugar," specifically 6-deoxy-L-galactose, a common feature in high-yield biochemistry questions.

Question 267: Pick the odd one out?

A. Lactose
B. Maltose
C. Sucrose
D. Cellulose (Correct Answer)

Explanation: ### Explanation The correct answer is **Cellulose**. The primary classification criterion used here is the complexity of the carbohydrate structure (Oligosaccharides vs. Polysaccharides). **1. Why Cellulose is the odd one out:** Cellulose is a **Polysaccharide**. It is a high-molecular-weight polymer consisting of thousands of glucose units linked by **$\beta(1\to4)$ glycosidic bonds**. Unlike the other options, it is a structural carbohydrate found in plant cell walls. In humans, it remains undigested due to the lack of the enzyme cellulase, contributing to dietary fiber. **2. Why the other options are incorrect:** * **Lactose:** It is a **Disaccharide** (reducing sugar) composed of Glucose and Galactose linked by a $\beta(1\to4)$ bond. It is the primary sugar in milk. * **Maltose:** It is a **Disaccharide** (reducing sugar) composed of two Glucose units linked by an $\alpha(1\to4)$ bond. It is an intermediate product of starch digestion. * **Sucrose:** It is a **Disaccharide** (non-reducing sugar) composed of Glucose and Fructose linked by an $\alpha1\to\beta2$ bond. It is common table sugar. **3. NEET-PG High-Yield Clinical Pearls:** * **Reducing vs. Non-reducing:** All common monosaccharides and disaccharides (Maltose, Lactose) are reducing sugars **except Sucrose**, because its functional groups are involved in the glycosidic bond. * **Invert Sugar:** Sucrose is called "invert sugar" because its optical rotation changes from dextrorotatory to levorotatory upon hydrolysis into glucose and fructose. * **Lactose Intolerance:** Caused by a deficiency of the enzyme lactase (brush border disaccharidase), leading to osmotic diarrhea and abdominal bloating. * **Cellulose in Diet:** While indigestible, it is essential for increasing fecal bulk and preventing constipation and diverticulosis.

Question 268: In one cycle of glycolysis under aerobic conditions, how many molecules of ATP and NADPH are formed?

A. 4 ATP, 2 NADPH
B. 2 ATP, 2 NADPH (Correct Answer)
C. 4 ATP, 4 NADPH
D. 2 ATP, 4 NADPH

Explanation: ### Explanation **1. Why the Correct Answer (B) is Right:** Glycolysis is the metabolic pathway that converts one molecule of glucose into two molecules of pyruvate. The net yield is determined by the balance between the energy investment and payoff phases: * **ATP Yield:** 2 ATP molecules are consumed (Hexokinase and Phosphofructokinase-1 steps), and 4 ATP molecules are produced via **substrate-level phosphorylation** (Phosphoglycerate kinase and Pyruvate kinase steps). This results in a **net gain of 2 ATP**. * **Reducing Equivalents:** 2 molecules of **NADH** (not NADPH) are produced at the Glyceraldehyde-3-phosphate dehydrogenase step. * *Note on the Question:* In many medical exams, including NEET-PG, the terms NADH and NADPH are occasionally used interchangeably in options, or the question specifically tests the numerical yield. Under aerobic conditions, these 2 NADH molecules enter the Electron Transport Chain (ETC) via shuttles to produce more ATP, but the immediate glycolytic yield remains 2 ATP and 2 NADH. **2. Why Other Options are Wrong:** * **Option A & C (4 ATP):** These represent the *gross* ATP production. They are incorrect because they fail to account for the 2 ATP molecules "invested" at the start of the pathway. * **Option D (4 NADPH):** There is no physiological stage in glycolysis that produces 4 reducing equivalents from a single glucose molecule. 4 NADH/NADPH would imply two cycles or a different pathway (like the HMP shunt). **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Rate-Limiting Enzyme:** Phosphofructokinase-1 (PFK-1) is the key regulatory enzyme of glycolysis. * **Rapoport-Luebering Cycle:** In RBCs, a bypass occurs producing 2,3-BPG. This results in **zero net ATP** gain from glycolysis because the ATP-producing phosphoglycerate kinase step is skipped. * **Aerobic vs. Anaerobic:** In anaerobic conditions (e.g., exercising muscle), NADH is re-oxidized to NAD+ by converting pyruvate to lactate, yielding only 2 ATP net. * **Arsenic Poisoning:** Arsenate competes with inorganic phosphate in the G3P dehydrogenase step, resulting in zero net ATP production.

Question 269: Which of the following is the insulin-dependent glucose transporter?

A. GLUT-1
B. GLUT-5
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, cardiac muscle, 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 that causes these vesicles to fuse with the plasma membrane, increasing glucose uptake. This mechanism is crucial for lowering postprandial blood glucose levels. **Analysis of Incorrect Options:** * **GLUT-1 (Option A):** This is an insulin-independent transporter found in **RBCs**, the blood-brain barrier, and kidneys. It provides a basal level of glucose uptake. * **GLUT-5 (Option B):** This is a unique transporter primarily located in the small intestine and spermatozoa. It is specific for **fructose** transport, not glucose. * **GLUT-3 (Option C):** This is an insulin-independent transporter with a very high affinity for glucose. It is the primary transporter in **neurons**, ensuring the brain receives glucose even during hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT-2:** A bidirectional transporter found in the **Liver, Pancreas (B-cells), and Kidney**. It acts as a "glucose sensor." * **Exercise & GLUT-4:** Muscle contraction can also trigger GLUT-4 translocation to the cell membrane independent of insulin, which is why exercise helps manage Type 2 Diabetes. * **SGLT-1/2:** Unlike GLUTs (facilitated diffusion), SGLTs are **Secondary Active Transporters** (Sodium-Glucose co-transporters) found in the intestine and renal tubules. * **Mnemonic:** "BRICK L" for insulin-independent tissues: **B**rain, **R**BCs, **I**ntestine, **C**ornea, **K**idney, **L**iver.

Question 270: A 3-year-old child presents with hepatomegaly and recurrent episodes of vomiting, jaundice, and hypoglycemia, beginning from the later part of infancy. These episodes seem to have no temporal relation with the intake of milk or milk products. The mother also gives a history that the child has an aversion to sweet food. Which of the following enzymes may be deficient in this child?

A. Glucose-6-phosphatase
B. Aldolase B (Correct Answer)
C. Branching enzyme
D. Galactose 1 phosphate uridyl transferase

Explanation: ### Explanation The clinical presentation of hepatomegaly, jaundice, and hypoglycemia triggered by weaning (introduction of fruit juices/sucrose) rather than milk, combined with a characteristic **aversion to sweets**, points toward **Hereditary Fructose Intolerance (HFI)**. #### Why Aldolase B is Correct HFI is caused by a deficiency of **Aldolase B**. This enzyme is responsible for cleaving Fructose-1-Phosphate (F1P) into DHAP and Glyceraldehyde. When deficient, **F1P accumulates** in hepatocytes. This accumulation traps intracellular inorganic phosphate, leading to: 1. **Inhibition of Glycogenolysis:** High F1P inhibits Glycogen Phosphorylase. 2. **Inhibition of Gluconeogenesis:** Depletion of ATP and phosphate prevents glucose synthesis. The result is profound postprandial hypoglycemia and liver damage (jaundice/hepatomegaly) following sucrose or fructose ingestion. #### Why Other Options are Incorrect * **A. Glucose-6-phosphatase:** Deficiency causes Von Gierke Disease. While it presents with hepatomegaly and hypoglycemia, symptoms typically appear earlier and are not specifically triggered by fructose or associated with an aversion to sweets. * **C. Branching enzyme:** Deficiency causes Andersen Disease (GSD IV), characterized by cirrhosis and failure to thrive, but not the acute metabolic crises triggered by specific sugars. * **D. Galactose 1-phosphate uridyl transferase:** Deficiency causes Classic Galactosemia. Symptoms appear in early infancy **immediately after milk intake** (lactose), unlike this case where milk is tolerated. #### High-Yield Clinical Pearls for NEET-PG * **The "Aversion" Clue:** A child refusing sweets/fruits is a classic "buzzword" for Aldolase B deficiency. * **Essential Fructosuria:** Caused by Fructokinase deficiency; it is a benign condition (asymptomatic) because fructose is not trapped in cells. * **Diagnostic Test:** Reducing sugars in urine (Clinitest positive) but a negative glucose oxidase test (Dipstick) during an acute episode.

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

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Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

Practice Questions

Lactose Intolerance and Galactosemia

Practice Questions

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