Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 91: A patient with hereditary fructose intolerance is deficient in which of the following enzymes?

A. Aldolase (Correct Answer)
B. Fructokinase
C. Triokinase
D. All of the above

Explanation: **Explanation:** **Hereditary Fructose Intolerance (HFI)** is an autosomal recessive disorder caused by a deficiency of **Aldolase B**. In the liver, fructose is first converted to Fructose-1-Phosphate (F1P) by fructokinase. Aldolase B is responsible for cleaving F1P into Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde. When Aldolase B is deficient, **Fructose-1-Phosphate accumulates** intracellularly. This "traps" inorganic phosphate, leading to ATP depletion. The lack of ATP inhibits gluconeogenesis and glycogenolysis, resulting in severe postprandial hypoglycemia, vomiting, and jaundice following the ingestion of fructose, sucrose, or sorbitol. **Analysis of Options:** * **Option A (Aldolase B):** Correct. Specifically, the 'B' isoform found in the liver, kidney, and small intestine is deficient. * **Option B (Fructokinase):** Deficiency of this enzyme causes **Essential Fructosuria**. This is a benign, asymptomatic condition because fructose is not "trapped" in cells and is simply excreted in the urine. * **Option C (Triokinase):** This enzyme converts glyceraldehyde to glyceraldehyde-3-phosphate. Its deficiency is not associated with HFI. **High-Yield Clinical Pearls for NEET-PG:** * **The "Trapping" Phenomenon:** The accumulation of F1P is the primary cause of hepatic and renal toxicity in HFI. * **Dietary Management:** Treatment involves the strict removal of **fructose, sucrose** (glucose + fructose), and **sorbitol** (which converts to fructose via polyol pathway) from the diet. * **Clinical Presentation:** Symptoms typically appear when an infant is weaned from breast milk and introduced to fruits or formula containing sucrose. * **Diagnosis:** Reducing sugars will be present in the urine (Clinitest positive), but a glucose oxidase dipstick will be negative.

Question 92: Which glycogen storage disease causes hypoglycemia, hepatomegaly, growth retardation, muscle weakness, and accumulation of limit dextrins?

A. Cori's disease (Correct Answer)
B. Von Gierke's disease
C. Anderson disease
D. Pompe's disease

Explanation: **Explanation:** The clinical presentation of hypoglycemia, hepatomegaly, and growth retardation is characteristic of several Glycogen Storage Diseases (GSDs). However, the presence of **limit dextrins** (abnormally short outer branches of glycogen) and **muscle weakness** specifically points to **Cori’s disease (GSD Type III)**. 1. **Why Cori’s Disease is Correct:** It is caused by a deficiency of the **Debranching enzyme** (α-1,6-glucosidase). Without this enzyme, glycogen can be broken down by phosphorylase only until it reaches a branch point, leaving behind "limit dextrins." Unlike Von Gierke’s, Cori’s involves both the liver and muscles, explaining the myopathy/muscle weakness. Gluconeogenesis remains intact, so hypoglycemia is generally milder than in Type I. 2. **Why Other Options are Incorrect:** * **Von Gierke’s disease (Type I):** Caused by Glucose-6-Phosphatase deficiency. It presents with severe hypoglycemia, lactic acidosis, and hyperuricemia, but **no muscle involvement** and no limit dextrins. * **Andersen disease (Type IV):** Caused by **Branching enzyme** deficiency. It results in long, unbranched glucose chains (amylopectin-like). It typically presents with infantile cirrhosis and liver failure, not hypoglycemia. * **Pompe’s disease (Type II):** Caused by Lysosomal acid maltase deficiency. It primarily affects the heart (cardiomegaly) and muscles. Blood glucose levels are typically **normal**. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "ABCD" – **A**ndersen is **B**ranching; **C**ori is **D**ebranching. * **Limit Dextrins:** Pathognomonic for Cori’s disease. * **Muscle Involvement:** If a GSD looks like Von Gierke’s but includes muscle weakness/wasting, think Cori’s (Type III). * **Type I vs. Type III:** Type I has elevated lactate; Type III has normal lactate levels.

Question 93: After overnight fasting, in which of the following cells are glucose transporter levels reduced?

A. Brain cells
B. Hepatocytes
C. Adipocytes (Correct Answer)
D. Red blood cells

Explanation: **Explanation:** The key to this question lies in understanding the **insulin-dependency** of different glucose transporters (GLUT). **Why Adipocytes are correct:** Adipocytes and skeletal muscle cells primarily utilize **GLUT-4**, which is the only insulin-dependent glucose transporter. In a fasting state, insulin levels are low. In the absence of insulin, GLUT-4 transporters are sequestered into intracellular vesicles, reducing their expression on the cell membrane. This mechanism ensures that during fasting, glucose is diverted away from storage tissues (fat) and toward glucose-dependent vital organs. **Why other options are incorrect:** * **Brain cells:** Utilize **GLUT-1 and GLUT-3**, which are insulin-independent. This ensures the brain receives a constant glucose supply regardless of fasting status. * **Hepatocytes (Liver):** Utilize **GLUT-2**, a high-capacity, low-affinity transporter that is insulin-independent. It allows the liver to perform gluconeogenesis and glycogenolysis to export glucose into the blood during fasting. * **Red Blood Cells (RBCs):** Utilize **GLUT-1**, which is insulin-independent. Since RBCs lack mitochondria and rely solely on glycolysis, they require constant glucose uptake. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT-4:** Found in **Heart, Skeletal Muscle, and Adipose tissue**. It is the only transporter regulated by insulin. * **GLUT-2:** Found in **Liver, Pancreatic beta cells, Kidney, and Small Intestine**. It acts as a "glucose sensor." * **SGLT-1/SGLT-2:** These are active transporters (sodium-glucose co-transporters) found in the small intestine and renal tubules, unlike the GLUT family which facilitates passive diffusion. * **Exercise** can also trigger GLUT-4 translocation to the cell membrane in skeletal muscle, independent of insulin.

Question 94: Which enzyme is specific for gluconeogenesis?

A. Glucose-6-phosphatase (Correct Answer)
B. Aldolase
C. Phosphoglycerate kinase
D. Phosphoglycerate mutase

Explanation: **Explanation:** Gluconeogenesis is the metabolic pathway that results in the generation of glucose from non-carbohydrate precursors. While many steps of gluconeogenesis are simply the reverse of glycolysis, there are **three irreversible steps** in glycolysis that must be bypassed by four specific gluconeogenic enzymes. **1. Why Glucose-6-phosphatase is correct:** Glucose-6-phosphatase is one of the four key "bypass enzymes" of gluconeogenesis. It catalyzes the conversion of Glucose-6-phosphate to free Glucose, bypassing the irreversible hexokinase/glucokinase reaction of glycolysis. This enzyme is primarily located in the **lumen of the endoplasmic reticulum** of the liver and kidney, allowing these organs to release free glucose into the blood. **2. Why the other options are incorrect:** * **Aldolase (B):** This enzyme is involved in both glycolysis (cleaving Fructose-1,6-bisphosphate) and gluconeogenesis (condensing DHAP and Glyceraldehyde-3-phosphate). Because it functions in both directions, it is not "specific" to gluconeogenesis. * **Phosphoglycerate kinase (C) & Phosphoglycerate mutase (D):** These are reversible enzymes shared by both the glycolytic and gluconeogenic pathways. **High-Yield Clinical Pearls for NEET-PG:** * **The Four Unique Gluconeogenic Enzymes:** 1. Pyruvate carboxylase, 2. PEP carboxykinase (PEPCK), 3. Fructose-1,6-bisphosphatase (Rate-limiting step), and 4. Glucose-6-phosphatase. * **Von Gierke Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. It presents with severe fasting hypoglycemia, hepatomegaly, and hyperuricemia. * **Location:** Muscle lacks Glucose-6-phosphatase; therefore, muscle glycogen cannot contribute directly to blood glucose levels.

Question 95: Oxidation without oxygen leads to the formation of which product?

A. Pyruvate
B. Fructose
C. Lactate (Correct Answer)
D. None

Explanation: ### Explanation **Concept Overview** In the absence of oxygen (anaerobic conditions), cells must regenerate **NAD+** from NADH to allow glycolysis to continue and produce ATP. This process occurs via **Anaerobic Glycolysis**, where the end product of the pathway is shifted from pyruvate to lactate. **Why Lactate is Correct** Under anaerobic conditions, the enzyme **Lactate Dehydrogenase (LDH)** reduces pyruvate into **Lactate**. This reaction is crucial because it oxidizes NADH back to NAD+. Without this regeneration of NAD+, the enzyme Glyceraldehyde-3-phosphate dehydrogenase would lack its coenzyme, and glycolysis would come to a complete halt, leading to cellular energy failure. **Analysis of Incorrect Options** * **Pyruvate (Option A):** Pyruvate is the end product of *aerobic* glycolysis. In the presence of oxygen, pyruvate enters the mitochondria to be converted into Acetyl-CoA for the TCA cycle. * **Fructose (Option B):** Fructose is a monosaccharide and an intermediate (as Fructose-6-P or Fructose-1,6-BP) within the glycolytic pathway, not an end product of oxidation. * **None (Option D):** Incorrect, as lactate is the definitive physiological product of anaerobic oxidation in humans. **Clinical Pearls for NEET-PG** * **Site of Anaerobic Glycolysis:** Occurs in **Erythrocytes** (RBCs) because they lack mitochondria, and in **exercising skeletal muscle** when oxygen demand exceeds supply. * **Net ATP Yield:** Anaerobic glycolysis yields only **2 ATP** per glucose molecule (compared to 30-32 ATP in aerobic conditions). * **Cori Cycle:** The lactate produced in muscles is transported to the liver, where it is converted back to glucose via gluconeogenesis. * **Lactic Acidosis:** A common clinical condition where tissue hypoxia (e.g., shock, severe anemia) leads to excessive lactate buildup and a drop in blood pH.

Question 96: Which enzyme is inhibited by sodium fluoride?

A. Enolase (Correct Answer)
B. Aconitase
C. Glyceraldehyde 3-phosphate dehydrogenase
D. Pyruvate dehydrogenase

Explanation: **Explanation:** **Correct Answer: A. Enolase** Sodium fluoride (NaF) is a potent inhibitor of **Enolase**, the enzyme responsible for the penultimate step of glycolysis (converting 2-phosphoglycerate to phosphoenolpyruvate). The inhibition occurs because fluoride ions form a complex with magnesium ($Mg^{2+}$) and phosphate, which then binds to the active site of the enzyme, displacing the essential $Mg^{2+}$ cofactor required for its activity. **Analysis of Incorrect Options:** * **B. Aconitase:** This enzyme of the TCA cycle is inhibited by **Fluoroacetate** (via conversion to fluorocitrate), not sodium fluoride. * **C. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH):** This enzyme is inhibited by **Iodoacetate** and **Arsenite**. * **D. Pyruvate dehydrogenase (PDH):** This complex is primarily inhibited by **Arsenite**, which binds to the -SH groups of lipoic acid, a crucial coenzyme for PDH. **Clinical Pearls for NEET-PG:** 1. **Blood Glucose Estimation:** In clinical practice, blood is collected in **grey-top vials** containing sodium fluoride and potassium oxalate. NaF prevents "in vitro glycolysis" by RBCs, ensuring that the measured glucose level reflects the patient's actual blood sugar at the time of collection. 2. **Anticoagulant vs. Preservative:** While Potassium Oxalate acts as the anticoagulant (by chelating calcium), Sodium Fluoride acts specifically as the **preservative** for glucose. 3. **Fluoride and Teeth:** In low concentrations, fluoride prevents dental caries by inhibiting bacterial enolase and forming acid-resistant fluorapatite in tooth enamel.

Question 97: Enantiomers are isomers that differ in structure at which carbon?

A. Last Carbon
B. First Carbon
C. Penultimate Carbon (Correct Answer)
D. Carbonyl Carbon

Explanation: ### Explanation **Concept Overview:** Enantiomers are a specific type of stereoisomer that are **non-superimposable mirror images** of each other. In carbohydrate chemistry, the classification of a sugar as a **D-isomer** or **L-isomer** is determined by the orientation of the hydroxyl (-OH) group on the **penultimate carbon** (the chiral carbon furthest from the carbonyl group). **Why Option C is Correct:** The penultimate carbon (the "next-to-last" carbon) is the reference point for stereoisomerism in sugars. For example, in a 6-carbon glucose molecule, this is **C-5**. If the -OH group on this carbon is on the right, it is a D-isomer; if on the left, it is an L-isomer. Since D and L forms are mirror images of each other, they are enantiomers. **Analysis of Incorrect Options:** * **A. Last Carbon:** The last carbon (e.g., C-6 in glucose) is usually a primary alcohol group (CH₂OH) and is **achiral** (not bonded to four different groups), so it cannot determine isomerism. * **B. First Carbon:** In aldoses, the first carbon is the aldehyde group. While it is functional, it is not the reference for D/L nomenclature. * **D. Carbonyl Carbon:** This is the functional group (C-1 in aldoses, C-2 in ketoses). When sugars cyclize, this becomes the **anomeric carbon**, giving rise to **anomers** (alpha and beta forms), not enantiomers. **NEET-PG High-Yield Pearls:** * **Most abundant form:** Most naturally occurring sugars in the human body are **D-isomers** (enzymes are stereospecific for D-sugars). * **Amino Acids:** Unlike sugars, naturally occurring amino acids are primarily **L-isomers**. * **Epimers:** Isomers differing at a single asymmetric carbon *other* than the penultimate carbon (e.g., Glucose and Galactose are C-4 epimers). * **Racemic Mixture:** An equimolar mixture of D and L enantiomers that shows no optical activity.

Question 98: Which of the following statements is TRUE regarding glycogenolysis?

A. Protein Phosphatase 1 (PP1) is a dephosphorylating enzyme involved in glycogenolysis.
B. Vasopressin increases glycogenolysis.
C. Oxytocin decreases glycogenolysis. (Correct Answer)
D. Calcium (Ca²⁺) acts as a synchronizer and allosteric activator in glycogenolysis.

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** **Oxytocin** is traditionally known for its roles in parturition and lactation; however, recent metabolic studies indicate it has an **insulin-like effect** on the liver. It promotes glycogen synthesis and inhibits glycogenolysis, thereby decreasing blood glucose levels. This makes Option C the correct statement regarding its metabolic influence. **2. Analysis of Incorrect Options:** * **Option A:** While **Protein Phosphatase 1 (PP1)** is a dephosphorylating enzyme, it actually **inhibits glycogenolysis**. It dephosphorylates (inactivates) Glycogen Phosphorylase and dephosphorylates (activates) Glycogen Synthase. Thus, it promotes glycogenesis, not glycogenolysis. * **Option B:** **Vasopressin (ADH)** actually **increases glycogenolysis**. In the liver, vasopressin binds to V1 receptors, triggering the IP3/DAG pathway. This increases intracellular calcium, which activates Phosphorylase Kinase, leading to the breakdown of glycogen to glucose. * **Option D:** This statement is technically a "distractor" because of the terminology. While **Calcium (Ca²⁺)** is indeed a synchronizer of muscle contraction and glycogenolysis, it acts as a **functional activator** by binding to the **calmodulin subunit** of Phosphorylase Kinase. It is not a classic "allosteric" activator in the same sense as AMP; rather, it is a regulatory subunit activator. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glycogen Phosphorylase (requires Pyridoxal Phosphate/B6 as a cofactor). * **Key Synchronizer:** In skeletal muscle, Ca²⁺ ensures that energy production (glycogenolysis) matches the demand of muscle contraction. * **Hormonal Control:** Glucagon and Epinephrine increase cAMP, activating Protein Kinase A (PKA), which phosphorylates and activates Glycogen Phosphorylase. Insulin reverses this via PP1. * **Von Gierke’s Disease:** Deficiency of Glucose-6-Phosphatase; the most common glycogen storage disease presenting with severe hypoglycemia and hepatomegaly.

Question 99: Which of the following statements is false regarding glycolysis?

A. The net production of ATP from anaerobic glycolysis is 2 ATP. (Correct Answer)
B. Glycolysis occurs in the cytosol of all cells.
C. The net production of ATP from aerobic glycolysis is approximately 7 ATP.
D. None of the above statements are false.

Explanation: **Explanation** In the context of this question, the correct answer is **D (None of the above statements are false)**, as all statements A, B, and C are biochemically accurate. **1. Why the statements are correct:** * **Anaerobic Glycolysis (Option A):** In the absence of oxygen, 1 molecule of glucose is converted into 2 molecules of lactate. While 4 ATPs are produced via substrate-level phosphorylation, 2 ATPs are consumed in the preparatory phase (Hexokinase and PFK-1 steps), resulting in a **net gain of 2 ATP**. * **Location (Option B):** Glycolysis is a universal pathway occurring exclusively in the **cytosol** of all human cells. It is the only pathway for ATP production in cells lacking mitochondria, such as mature erythrocytes. * **Aerobic Glycolysis (Option C):** In aerobic conditions, the 2 NADH produced during the glyceraldehyde-3-phosphate dehydrogenase step enter the electron transport chain. Depending on the shuttle used (Malate-aspartate vs. Glycerol-3-phosphate), these 2 NADH yield approximately 3 or 5 ATP. Adding the 2 net ATP from substrate-level phosphorylation, the total is **5 to 7 ATP** (averaging 7 in many standard textbooks). **2. High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Enzyme:** Phosphofructokinase-1 (PFK-1), which is allosterically inhibited by ATP and Citrate, and activated by AMP and Fructose-2,6-bisphosphate. * **Rapoport-Luebering Cycle:** In RBCs, a bypass of glycolysis produces 2,3-BPG, which shifts the oxygen dissociation curve to the right (decreasing O2 affinity). * **Arsenic Poisoning:** Arsenate competes with inorganic phosphate in the GAPDH reaction, resulting in zero net ATP production during glycolysis. * **Essential Fructosuria:** Caused by Fructokinase deficiency; it is a benign condition compared to Hereditary Fructose Intolerance (Aldolase B deficiency).

Question 100: What are the steps involved in the production of fructose in seminal fluid?

A. Glucose-6-phosphate fructose-6-phosphate fructose
B. Glucose-6-phosphate sorbitol phosphate fructose phosphate fructose
C. Glucose-6-phosphate glucose sorbitol fructose (Correct Answer)
D. Glucose-6-phosphate fructose-1-phosphate fructose

Explanation: **Explanation:** The production of fructose in the seminal vesicles occurs via the **Polyol Pathway** (also known as the Sorbitol Pathway). This pathway is essential because fructose is the primary energy source for sperm motility. **Why Option C is Correct:** The synthesis follows a specific sequence starting from blood glucose: 1. **Glucose-6-phosphate** is converted back to **Glucose** by Glucose-6-phosphatase. 2. **Glucose** is reduced to **Sorbitol** (a polyol) by the enzyme **Aldose Reductase**, using NADPH as a cofactor. 3. **Sorbitol** is then oxidized to **Fructose** by the enzyme **Sorbitol Dehydrogenase**, using NAD+ as a cofactor. This bypasses the glycolytic hexokinase step, allowing for efficient fructose production in the seminal vesicles. **Why Other Options are Incorrect:** * **Option A:** Describes a simple isomerization (like in glycolysis), but the seminal vesicles utilize the polyol pathway to ensure high concentrations of fructose specifically. * **Option B:** "Sorbitol phosphate" is not a physiological intermediate in this pathway; the pathway involves free sugars, not phosphorylated intermediates. * **Option D:** Fructose-1-phosphate is an intermediate of fructose metabolism in the liver (fructolysis), not its synthesis in seminal fluid. **Clinical Pearls for NEET-PG:** * **Enzymes:** Aldose Reductase (Glucose → Sorbitol) and Sorbitol Dehydrogenase (Sorbitol → Fructose). * **Tissue Distribution:** The seminal vesicles and lens have both enzymes. However, tissues like the **retina, kidneys, and nerves** lack Sorbitol Dehydrogenase. * **Pathology:** In **Diabetes Mellitus**, hyperglycemia leads to excessive sorbitol accumulation in tissues lacking Sorbitol Dehydrogenase. Since sorbitol is osmotically active, it causes water influx, leading to **cataracts, retinopathy, and neuropathy**.

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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