Carbohydrate Metabolism Practice Questions

Q: In which condition does hemolysis occur on oxidation?

G6PD deficiency. **Explanation:** **G6PD Deficiency (Correct Answer):** Glucose-6-Phosphate Dehydrogenase (G6PD) is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. This pathway is the sole source of **NADPH** in mature erythrocytes. NADPH is essential for maintaining a pool of **reduced glutathione**, which acts as a cellular antioxidant by neutralizing reactive oxygen species (ROS) like hydrogen peroxide. In G6PD deficiency, oxidative stress (triggered by infections, fava beans, or drugs like Primaquine and Sulfonamides) leads to the oxidation of hemoglobin. This results in the formation of **Heinz bodies** (denatured hemoglobin), which damage the RBC membrane, leading to acute hemolysis. **Why other options are incorrect:** * **Hereditary Spherocytosis:** This is a **cytoskeletal defect** (deficiency of Spectrin or Ankyrin) causing RBCs to become spherical and trapped in the spleen. It is not primarily driven by oxidative stress. * **Sickle Cell Anemia:** This is a **qualitative hemoglobinopathy** (Glu → Val at 6th position of β-chain). Hemolysis occurs due to polymerization of HbS under hypoxic conditions, not oxidative stress. * **Hemophilia:** This is a **coagulation disorder** (deficiency of Factor VIII or IX) leading to bleeding tendencies; it does not involve intrinsic RBC defects or hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** G6PD deficiency is **X-linked Recessive**. * **Morphology:** Look for **"Bite cells"** (Degmacytes) and **"Blister cells"** on a peripheral smear. * **Staining:** Heinz bodies are visualized using supravital stains like **Crystal Violet**. * **Protection:** G6PD deficiency provides a selective advantage against *Plasmodium falciparum* malaria.

Q: What is the primary purpose of carbohydrate loading practiced by endurance athletes?

To form glycogen with fewer branch points than normal. **Explanation:** **1. Why Option A is Correct:** Carbohydrate loading (glycogen supercompensation) involves depleting glycogen stores through intense exercise followed by a high-carbohydrate diet. During the rapid resynthesis phase, the activity of **Glycogen Synthase** (responsible for linear α-1,4 linkages) increases significantly more than the activity of the **Branching Enzyme** (amylo-1,4→1,6-transglucosidase). This results in the formation of glycogen molecules that are larger and have **fewer branch points** than normal. These "less branched" molecules are more compact, allowing the muscle to store a higher total mass of glucose per unit volume, providing a larger energy reservoir for endurance activities. **2. Why Other Options are Incorrect:** * **Option B:** More branch points would increase the solubility and rate of degradation (glycogenolysis) but would occupy more space, preventing the "packing" of extra glucose required for endurance. * **Option C:** Carbohydrate loading is specifically designed to maximize polysaccharide storage, not to facilitate gluconeogenesis or protein synthesis. * **Option D:** While blood glucose maintenance is a general physiological function of *liver* glycogen, the primary goal of carb-loading is to maximize *muscle* glycogen for local energy use during prolonged exertion. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of Glycogenesis:** Glycogen Synthase (activated by Insulin/Glucose-6-P). * **Branching Enzyme:** Also known as *Glucosyl 4:6 transferase*. Deficiency leads to **Andersen’s Disease (GSD Type IV)**, characterized by long, unbranched glucose chains (amylopectin-like) that trigger an immune response. * **Muscle vs. Liver:** Muscle glycogen lacks **Glucose-6-Phosphatase**; therefore, it cannot contribute to blood glucose and is used exclusively for muscle contraction.

Q: Essential fructosuria is due to deficiency of which enzyme?

Fructokinase. **Explanation:** **Essential Fructosuria** is a benign, autosomal recessive metabolic disorder caused by a deficiency of the enzyme **Fructokinase** (also known as Ketohexokinase). 1. **Why Fructokinase is correct:** In the normal metabolic pathway, fructokinase converts fructose into fructose-1-phosphate in the liver. When this enzyme is deficient, fructose cannot be trapped inside the cell. Instead, it accumulates in the blood (fructosemia) and is excreted in the urine (fructosuria). Because fructose is a reducing sugar, it will give a positive result on a Benedict’s test but a negative result on a glucose oxidase dipstick. 2. **Why other options are incorrect:** * **Aldolase B:** Deficiency leads to **Hereditary Fructose Intolerance (HFI)**. This is a severe condition where fructose-1-phosphate accumulates, depleting intracellular phosphate and causing hypoglycemia and liver failure. * **Beta-galactosidase:** Deficiency is associated with **GM1 gangliosidosis** or Morquio syndrome Type B, involving the breakdown of keratan sulfate and gangliosides, not fructose. * **Aldose reductase:** This enzyme converts glucose to sorbitol. It is implicated in diabetic complications (cataracts, neuropathy) but its deficiency does not cause fructosuria. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Essential fructosuria is **asymptomatic**. It is often discovered incidentally during routine urine screening for reducing sugars. * **Alternative Pathway:** In the absence of fructokinase, **hexokinase** becomes the primary enzyme for fructose metabolism, converting it to fructose-6-phosphate (though this is a slow, minor pathway). * **Mnemonic:** "Fructokinase deficiency is **Friendly** (asymptomatic), Aldolase B deficiency is **Bad** (symptomatic/HFI)."

Q: A 3-year-old boy presented with several episodes of vomiting and lethargy, accompanied by hypoglycemia. His pediatrician suspected possible hepatic failure given the recurrent vomiting and lethargy. It was noted that these episodes occur after the ingestion of sweets or fruits. What is the most likely diagnosis?

Hereditary Fructose Intolerance. **Explanation:** The clinical presentation of vomiting, lethargy, and hypoglycemia triggered specifically by the ingestion of **fruits or sweets** is a classic hallmark of **Hereditary Fructose Intolerance (HFI)**. **1. Why Option A is Correct:** HFI is caused by a deficiency of **Aldolase B**. When fructose is ingested (found in fruits, honey, and sucrose/sweets), it is phosphorylated to **Fructose-1-Phosphate (F1P)** by fructokinase. Due to the absence of Aldolase B, F1P accumulates in the hepatocytes. This "trapping" of inorganic phosphate leads to: * **Inhibition of Glycogenolysis:** Phosphorylase enzyme is inhibited. * **Inhibition of Gluconeogenesis:** Due to lack of ATP and substrate availability. The result is profound **postprandial hypoglycemia** and liver dysfunction (vomiting/jaundice). **2. Why Other Options are Incorrect:** * **B. Glucose Homeostasis:** This is a physiological process, not a disease entity. * **C. Glycogen Storage Disease Type III (Cori Disease):** While it causes hypoglycemia and hepatomegaly, symptoms are typically triggered by fasting, not specifically by fructose/fruit ingestion. It is a deficiency of the debranching enzyme. * **D. Galactosemia:** Symptoms (cataracts, hepatomegaly, sepsis) usually appear in the neonatal period upon starting **milk** (lactose), not later with fruits/sweets. **NEET-PG High-Yield Pearls:** * **Enzyme Deficient:** Aldolase B (Chromosome 9). * **Key Trigger:** Introduction of weaning (fruit juices/sucrose). * **Biochemical Hallmark:** Intracellular depletion of inorganic phosphate ($P_i$) and ATP. * **Essential Fructosuria:** A benign condition caused by **Fructokinase** deficiency; unlike HFI, it does not cause hypoglycemia or liver damage as F1P does not accumulate.

Q: Non-ketotic hypoglycemia is seen in all of the following conditions except?

Glycogen storage disorders. **Explanation:** The hallmark of **non-ketotic hypoglycemia** is the absence of ketone bodies despite low blood glucose levels. This occurs when there is either an inhibition of fatty acid oxidation or a suppression of lipolysis. **1. Why Glycogen Storage Disorders (GSDs) are the exception:** In most GSDs (specifically Type I - von Gierke disease), hypoglycemia occurs due to the inability to release glucose from glycogen. However, the counter-regulatory hormones (glucagon and epinephrine) are activated. This triggers **intact fatty acid oxidation**, leading to the production of acetyl-CoA and subsequently **ketone bodies**. Therefore, GSDs typically present with **ketotic hypoglycemia**. **2. Analysis of Incorrect Options:** * **Hyperinsulinism:** Insulin is a potent inhibitor of lipolysis and fatty acid oxidation. High insulin levels prevent the formation of ketones, leading to profound non-ketotic hypoglycemia. * **Galactosemia & Hereditary Fructose Intolerance (HFI):** In these conditions, the accumulation of phosphorylated sugars (Galactose-1-P or Fructose-1-P) depletes intracellular inorganic phosphate. This inhibits both glycogenolysis and gluconeogenesis. The acute metabolic derangement often interferes with the transition to ketosis, typically presenting as non-ketotic hypoglycemia in the acute phase. **3. NEET-PG High-Yield Pearls:** * **Ketotic Hypoglycemia:** Seen in GSD Type I, Ketotic hypoglycemia of childhood (most common cause in toddlers), and Maple Syrup Urine Disease (MSUD). * **Non-Ketotic Hypoglycemia:** Think of **MCAD deficiency** (impaired beta-oxidation), **Hyperinsulinism**, and **Systemic Carnitine deficiency**. * **Key Diagnostic:** If a question mentions "hypoglycemia with absent ketones," always prioritize Fatty Acid Oxidation Disorders or Insulin excess.

Q: All of the following are true regarding proteoglycans, EXCEPT?

They hold a large amount of water.. **Explanation:** Proteoglycans are complex macromolecules consisting of a core protein covalently attached to long, unbranched chains of **Glycosaminoglycans (GAGs)**. **Why Option B is the "Except" (Correct Answer):** In the context of this specific question format, Option B is often marked as the "incorrect statement" if the examiner defines proteoglycans strictly by their chemical composition rather than their physical properties. However, physiologically, proteoglycans **do** hold large amounts of water. If this is the designated answer, it implies a technicality: Proteoglycans *attract* water due to their osmotic pressure, but the water is not a structural component of the molecule itself. *Note: In many standard textbooks, B is actually a true statement, making this a controversial "recall" question. If B is the keyed answer, it is likely because the examiner considers "holding water" a function of the GAG component, not the proteoglycan as a whole.* **Analysis of Other Options:** * **Option A:** **True.** Chondroitin sulfate is the most abundant GAG in the body, found in cartilage and bone. * **Option C:** **True.** They consist of a protein core (amino acids) and GAGs (long sugar chains). * **Option D:** **True.** GAGs contain sulfate and carboxyl groups, giving them a high **negative charge**. This causes the chains to repel each other, creating the "bottle-brush" appearance and allowing them to act as biological lubricants. **High-Yield NEET-PG Pearls:** * **Hyaluronic Acid:** The only GAG that is **not sulfated** and not covalently bound to a protein core. * **Heparin:** The GAG with the highest negative charge density; acts as an intracellular anticoagulant. * **Mucopolysaccharidoses (MPS):** Genetic disorders (e.g., Hurler, Hunter syndromes) caused by the deficiency of lysosomal enzymes that degrade GAGs. * **Aggrecan:** The major proteoglycan in cartilage.

Q: Which of the following is a homopolysaccharide of fructose?

Inulin. ### Explanation **Correct Answer: C. Inulin** **Why Inulin is correct:** Inulin is a **homopolysaccharide** (a polymer consisting of only one type of monosaccharide unit) composed of **D-fructose** units. These fructose units are linked by **β(2→1) glycosidic bonds**. It typically ends with a terminal glucose residue. Inulin is found in plants like chicory, dahlia bulbs, and garlic. Because it is not digested by human enzymes, it serves as a soluble fiber. **Why other options are incorrect:** * **A. Chitin:** This is a homopolysaccharide of **N-acetyl-D-glucosamine** (NAG) linked by β(1→4) bonds. It is a structural component found in the exoskeletons of arthropods and fungal cell walls. * **B. Dextran:** This is a branched homopolysaccharide of **D-glucose**. It is produced by bacteria and yeast, characterized by α(1→6) main chains and α(1→3) branches. (Note: Do not confuse Dextran with Dextrin, which is an intermediate product of starch hydrolysis). **High-Yield Clinical Pearls for NEET-PG:** 1. **Renal Function:** Inulin is the "Gold Standard" for measuring **Glomerular Filtration Rate (GFR)** because it is freely filtered by the glomeruli but is neither reabsorbed nor secreted by the renal tubules. 2. **Fructans:** Inulin belongs to a class of carbohydrates called fructans. 3. **Other Homopolysaccharides:** Remember that Starch, Glycogen, and Cellulose are all homopolysaccharides of **Glucose**, differing only in their linkages and branching patterns. 4. **Diagnostic Use:** While Inulin is the gold standard for GFR, **Creatinine clearance** is more commonly used in clinical practice as it is endogenous and does not require a continuous intravenous infusion.

Q: After strenuous exercise, an alkaline pH of skeletal muscle is characteristic of which glycogen storage disease?

McArdle's disease. **Explanation:** **McArdle’s Disease (GSD Type V)** is caused by a deficiency in **muscle glycogen phosphorylase (myophosphorylase)**. Under normal physiological conditions, strenuous exercise triggers glycogenolysis, converting muscle glycogen into glucose-1-phosphate, which eventually enters glycolysis to produce **lactic acid**. The accumulation of lactic acid typically causes the intramuscular pH to drop (become acidic). In McArdle’s disease, the inability to break down glycogen means no lactic acid is produced during exercise. Instead, there is a compensatory increase in blood flow and the breakdown of creatine phosphate and proteins, leading to an accumulation of **ammonia**. This lack of lactate combined with increased ammonia results in an **alkaline pH** (or a failure of the pH to fall) during an ischemic exercise test—a classic diagnostic hallmark. **Why other options are incorrect:** * **Von Gierke’s Disease (Type I):** Caused by Glucose-6-Phosphatase deficiency. It primarily affects the liver, leading to severe fasting hypoglycemia and **lactic acidosis**, not alkalinity. * **Hers’ Disease (Type VI):** Caused by liver phosphorylase deficiency. It presents with hepatomegaly and mild hypoglycemia; it does not affect muscle metabolism or muscle pH. * **Pompe’s Disease (Type II):** Caused by lysosomal α-1,4-glucosidase deficiency. It affects the heart and muscles via lysosomal accumulation but does not specifically cause an alkaline pH during exercise. **High-Yield Clinical Pearls for NEET-PG:** * **Ischemic Exercise Test:** Failure of blood lactate to rise with a concomitant rise in ammonia is diagnostic for McArdle’s. * **Second Wind Phenomenon:** Patients often experience relief after 10–15 minutes of exercise as the body switches to using fatty acids and blood glucose. * **Clinical Triad:** Exercise intolerance, muscle cramps, and **myoglobinuria** (burgundy-colored urine) after exertion.

Q: Gluconeogenesis is inhibited by:

Insulin. **Explanation:** **Gluconeogenesis** is the metabolic pathway that results in the generation of glucose from non-carbohydrate precursors (like lactate, glycerol, and glucogenic amino acids). It occurs primarily in the liver and kidneys during periods of fasting to maintain blood glucose levels. **1. Why Insulin is the correct answer:** Insulin is an **anabolic hormone** secreted by the beta cells of the pancreas in the fed state. Its primary goal is to lower blood glucose. It inhibits gluconeogenesis by: * **Transcriptional Regulation:** Decreasing the expression of key rate-limiting enzymes, specifically **Phosphoenolpyruvate carboxykinase (PEPCK)** and **Glucose-6-phosphatase**. * **Allosteric Regulation:** Increasing the levels of Fructose-2,6-bisphosphate, which stimulates glycolysis and inhibits gluconeogenesis. **2. Why the other options are incorrect:** * **Glucagon:** This is the primary stimulator of gluconeogenesis. It increases cAMP levels, activating Protein Kinase A, which induces the transcription of the PEPCK gene. * **Glucocorticoids (e.g., Cortisol):** These are "diabetogenic" hormones. They stimulate gluconeogenesis by increasing the breakdown of muscle proteins into amino acids (substrate supply) and inducing the synthesis of gluconeogenic enzymes. * **Gonadotropin-releasing hormone (GnRH):** This hormone regulates the reproductive axis (FSH/LH secretion) and has no direct regulatory role in carbohydrate metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step of Gluconeogenesis:** Conversion of Fructose-1,6-bisphosphate to Fructose-6-phosphate by **Fructose-1,6-bisphosphatase**. * **Key Stimulators:** Glucagon, Epinephrine, Cortisol, and Acetyl-CoA (obligatory activator of Pyruvate Carboxylase). * **Key Inhibitor:** Insulin and Alcohol (NADH/NAD+ ratio imbalance inhibits gluconeogenesis, leading to fasting hypoglycemia).

Question 1

In which condition does hemolysis occur on oxidation?

Accepted Answer

G6PD deficiency

Answer

Hereditary spherocytosis

Answer

Sickle cell anemia

Answer

Hemophilia

Question 2

What is the primary purpose of carbohydrate loading practiced by endurance athletes?

Accepted Answer

To form glycogen with fewer branch points than normal

Answer

To form glycogen with more branch points than normal

Answer

To convert glucose to proteins

Answer

To maintain optimal blood glucose concentration

Question 3

Essential fructosuria is due to deficiency of which enzyme?

Accepted Answer

Fructokinase

Answer

beta-galactosidase

Answer

Aldolase B

Answer

Aldose reductase

Question 4

A 3-year-old boy presented with several episodes of vomiting and lethargy, accompanied by hypoglycemia. His pediatrician suspected possible hepatic failure given the recurrent vomiting and lethargy. It was noted that these episodes occur after the ingestion of sweets or fruits. What is the most likely diagnosis?

Accepted Answer

Hereditary Fructose Intolerance

Answer

Glucose Homeostasis

Answer

Glycogen storage disease type III

Answer

Galactosemia

Question 5

Non-ketotic hypoglycemia is seen in all of the following conditions except?

Accepted Answer

Glycogen storage disorders

Answer

Galactosemia

Answer

Hereditary fructose intolerance

Answer

Hyperinsulinism

Question 6

The immediate degradation of glycogen under normal conditions gives rise to which one of the following?

Accepted Answer

More glucose-1-phosphate than glucose

Answer

More glucose than glucose-1-phosphate

Answer

Equal amounts of glucose and glucose-1-phosphate

Answer

Neither glucose nor glucose-1-phosphate

Question 7

All of the following are true regarding proteoglycans, EXCEPT?

Accepted Answer

They hold a large amount of water.

Answer

Chondroitin sulfate is a glycosaminoglycan.

Answer

They are made up of sugar and amino acids.

Answer

They carry a negative charge.

Question 8

Which of the following is a homopolysaccharide of fructose?

Accepted Answer

Inulin

Answer

Chitin

Answer

Dextran

Answer

All of the above

Question 9

After strenuous exercise, an alkaline pH of skeletal muscle is characteristic of which glycogen storage disease?

Accepted Answer

McArdle's disease

Answer

Von Gierke's disease

Answer

Her's disease

Answer

Pompe's disease

Question 10

Gluconeogenesis is inhibited by:

Accepted Answer

Insulin

Answer

Glucagon

Answer

Glucocorticoids

Answer

Gonadotropin-releasing hormone

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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