Carbohydrate Metabolism Practice Questions

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

Aldolase. **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.

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

Cori's disease. **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.

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

Adipocytes. **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.

Q: Which enzyme is specific for gluconeogenesis?

Glucose-6-phosphatase. **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.

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

Lactate. ### 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.

Q: Which enzyme is inhibited by sodium fluoride?

Enolase. **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.

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

Penultimate Carbon. ### 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.

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

Glucose-6-phosphate glucose sorbitol fructose. **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**.

Question 1

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

Accepted Answer

Aldolase

Answer

Fructokinase

Answer

Triokinase

Answer

All of the above

Question 2

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

Accepted Answer

Cori's disease

Answer

Von Gierke's disease

Answer

Anderson disease

Answer

Pompe's disease

Question 3

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

Accepted Answer

Adipocytes

Answer

Brain cells

Answer

Hepatocytes

Answer

Red blood cells

Question 4

Which enzyme is specific for gluconeogenesis?

Accepted Answer

Glucose-6-phosphatase

Answer

Aldolase

Answer

Phosphoglycerate kinase

Answer

Phosphoglycerate mutase

Question 5

Oxidation without oxygen leads to the formation of which product?

Accepted Answer

Lactate

Answer

Pyruvate

Answer

Fructose

Answer

None

Question 6

Which enzyme is inhibited by sodium fluoride?

Accepted Answer

Enolase

Answer

Aconitase

Answer

Glyceraldehyde 3-phosphate dehydrogenase

Answer

Pyruvate dehydrogenase

Question 7

Enantiomers are isomers that differ in structure at which carbon?

Accepted Answer

Penultimate Carbon

Answer

Last Carbon

Answer

First Carbon

Answer

Carbonyl Carbon

Question 8

Which of the following statements is TRUE regarding glycogenolysis?

Accepted Answer

Oxytocin decreases glycogenolysis.

Answer

Protein Phosphatase 1 (PP1) is a dephosphorylating enzyme involved in glycogenolysis.

Answer

Vasopressin increases glycogenolysis.

Answer

Calcium (Ca²⁺) acts as a synchronizer and allosteric activator in glycogenolysis.

Question 9

Which of the following statements is false regarding glycolysis?

Accepted Answer

The net production of ATP from anaerobic glycolysis is 2 ATP.

Answer

Glycolysis occurs in the cytosol of all cells.

Answer

The net production of ATP from aerobic glycolysis is approximately 7 ATP.

Answer

None of the above statements are false.

Question 10

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

Accepted Answer

Glucose-6-phosphate 	 glucose 	 sorbitol 	 fructose

Answer

Glucose-6-phosphate 	 fructose-6-phosphate 	 fructose

Answer

Glucose-6-phosphate 	 sorbitol phosphate 	 fructose phosphate 	 fructose

Answer

Glucose-6-phosphate 	 fructose-1-phosphate 	 fructose

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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