Carbohydrate Metabolism Practice Questions

Q: G-6-Phosphatase deficiency is seen in which of the following conditions?

Von Gierke's disease. **Explanation:** **Von Gierke’s Disease (Type I Glycogen Storage Disease)** is the correct answer. It is caused by a deficiency of the enzyme **Glucose-6-Phosphatase**, which is responsible for the final step of both glycogenolysis and gluconeogenesis (converting Glucose-6-Phosphate to free Glucose). Since this enzyme is primarily located in the liver and kidneys, its deficiency leads to severe fasting hypoglycemia and the accumulation of glycogen in these organs, resulting in hepatomegaly. **Analysis of Incorrect Options:** * **Tay-Sachs Disease:** This is a lysosomal storage disorder (Sphingolipidosis) caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of GM2 gangliosides. It is characterized by neurodegeneration and a cherry-red spot on the macula. * **Pompe’s Disease (Type II GSD):** This is caused by a deficiency of **Lysosomal acid alpha-1,4-glucosidase** (Acid Maltase). Unlike Type I, it affects the heart and muscles, leading to massive cardiomegaly. * **Anderson’s Disease (Type IV GSD):** This results from a deficiency of the **Branching enzyme**. It leads to the accumulation of abnormal glycogen with long outer branches (amylopectin-like), causing early-onset liver cirrhosis. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Triad of Von Gierke’s:** Hyperuricemia (leading to gout), Hyperlipidemia, and Lactic Acidosis. * **Diagnostic Hallmark:** Failure of blood glucose levels to rise after the administration of glucagon or epinephrine. * **Type Ia vs. Ib:** Type Ia is a deficiency of the enzyme itself, while Type Ib is a deficiency of the **Glucose-6-Phosphate translocase** (associated with neutropenia).

Q: In glycogen, what is the type of linkage at branch points?

Alpha-1,6. **Explanation:** Glycogen is a highly branched homopolysaccharide of D-glucose. To understand its structure, one must distinguish between the linear chain and the branching points: 1. **Alpha-1,6 Linkage (Correct):** This linkage occurs at the **branch points** of glycogen. It is formed by the enzyme **Branching Enzyme** (Amylo-1,4 $\rightarrow$ 1,6 transglucosidase). Branching increases the solubility of glycogen and creates multiple non-reducing ends, allowing for rapid mobilization of glucose during glycogenolysis. 2. **Alpha-1,4 Linkage (Incorrect):** This is the primary linkage found in the **linear (straight) chains** of glycogen. It is formed by Glycogen Synthase and broken down by Glycogen Phosphorylase. 3. **Alpha-2,3 Linkage (Incorrect):** This type of linkage is not found in human carbohydrate metabolism; it is more characteristic of certain bacterial cell wall components or sialic acid attachments. 4. **Beta-1,4 Linkage (Incorrect):** This linkage is found in **Cellulose**. Humans lack the enzyme (cellulase) to break beta-1,4 bonds, which is why cellulose serves as dietary fiber rather than an energy source. **High-Yield NEET-PG Pearls:** * **Branching Frequency:** In glycogen, branches occur approximately every **8 to 12 glucose residues**. * **Debranching Enzyme:** This is a bifunctional enzyme. While it has transferase activity, it specifically uses **$\alpha$-1,6-glucosidase** activity to break the bond at the branch point, releasing one free glucose molecule. * **Clinical Correlation:** **Andersen Disease (GSD Type IV)** is caused by a deficiency in the branching enzyme, leading to the formation of long, unbranched glycogen chains (polyglucosan bodies) which trigger an immune response (cirrhosis).

Q: Which one of the following metabolites is used by all cells for glycolysis, glycogen synthesis, and the hexose monophosphate shunt pathway?

Glucose-6-phosphate. **Explanation:** **Glucose-6-phosphate (G6P)** is the central hub of carbohydrate metabolism. Once glucose enters a cell, it is immediately phosphorylated by Hexokinase (or Glucokinase in the liver) to G6P. This "traps" the glucose inside the cell and serves as the common starting point for multiple metabolic fates: 1. **Glycolysis:** G6P is isomerized to Fructose-6-phosphate to proceed toward ATP production. 2. **Glycogenesis:** G6P is converted to Glucose-1-phosphate (by phosphoglucomutase) to initiate glycogen synthesis. 3. **HMP Shunt (Pentose Phosphate Pathway):** G6P is the substrate for G6P Dehydrogenase (G6PD), the rate-limiting enzyme of this pathway, which produces NADPH and ribose-5-phosphate. **Analysis of Incorrect Options:** * **A. Glucose-1-phosphate:** This is an intermediate primarily involved in glycogen synthesis and breakdown. It is not a direct substrate for glycolysis or the HMP shunt. * **C. UDP-glucose:** This is the "activated" form of glucose used specifically for glycogen synthesis and galactose metabolism; it does not enter glycolysis or the HMP shunt directly. * **D. Fructose-6-phosphate:** While an intermediate in glycolysis, it is downstream of the branch points for the HMP shunt and glycogen synthesis. **NEET-PG High-Yield Pearls:** * **G6P Dehydrogenase Deficiency:** The most common enzyme deficiency worldwide, leading to hemolytic anemia due to the inability of the HMP shunt to maintain reduced glutathione in RBCs. * **Von Gierke Disease (GSD Type I):** Caused by a deficiency of **Glucose-6-phosphatase**. This prevents the conversion of G6P back to free glucose, leading to severe fasting hypoglycemia and hepatomegaly. * **The "Trapping" Step:** Phosphorylation of glucose to G6P is irreversible in most peripheral tissues, ensuring a concentration gradient for continued glucose uptake.

Q: What is the major metabolic pathway in erythrocytes?

Pentose phosphate pathway. **Explanation:** The correct answer is **D. Pentose Phosphate Pathway (PPP)**, also known as the Hexose Monophosphate (HMP) Shunt. **Why it is correct:** Mature erythrocytes (RBCs) lack mitochondria. Therefore, they rely entirely on anaerobic glycolysis for energy (ATP) and the PPP for generating **NADPH**. In RBCs, NADPH is critical because it acts as a co-factor for the enzyme **Glutathione Reductase**. This enzyme maintains a pool of reduced glutathione, which neutralizes reactive oxygen species (ROS) like hydrogen peroxide. Without the PPP, oxidative stress would lead to hemoglobin denaturation (Heinz bodies) and hemolysis. Approximately 10% of glucose in RBCs is metabolized via this pathway. **Why incorrect options are wrong:** * **A. Beta-oxidation:** This process occurs in the **mitochondria**. Since RBCs lack mitochondria, they cannot oxidize fatty acids for energy. * **B. Citric acid cycle (TCA):** Like beta-oxidation, the TCA cycle occurs within the **mitochondrial matrix**. RBCs cannot perform aerobic respiration. * **C. Gluconeogenesis:** This is the synthesis of glucose from non-carbohydrate precursors, occurring primarily in the **liver and kidneys**. RBCs are consumers of glucose, not producers. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzyme deficiency in the PPP. It leads to episodic hemolytic anemia when RBCs are exposed to oxidative stress (e.g., Fava beans, Primaquine, or infections). * **Rapoport-Luebering Shunt:** Another unique RBC pathway that produces **2,3-BPG**, which decreases hemoglobin's affinity for oxygen, facilitating oxygen delivery to tissues. * **Key Enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD) is the rate-limiting enzyme of the PPP.

Q: A four-year-old child presents with exercise intolerance. Investigations reveal blood pH 7.3, fasting blood sugar 60 mg%, hypertriglyceridemia, ketosis, and lactic acidosis. The child also has hepatomegaly and renomegaly. Biopsy of the liver and kidney shows increased glycogen content. What is the diagnosis?

Von Gierke's disease. **Explanation:** The clinical presentation of **fasting hypoglycemia, lactic acidosis, hypertriglyceridemia, and hepatorenal enlargement** is classic for **Von Gierke’s Disease (GSD Type I)**. **1. Why Von Gierke’s Disease is Correct:** This condition is caused by a deficiency of **Glucose-6-Phosphatase**. This enzyme is essential for the final step of both glycogenolysis and gluconeogenesis. Its absence leads to: * **Hypoglycemia:** Inability to release glucose into the blood during fasting. * **Lactic Acidosis:** Excess Glucose-6-Phosphate is diverted into the glycolytic pathway, producing lactate. * **Hepatomegaly & Renomegaly:** Massive accumulation of glycogen (normal structure) in the liver and kidneys. * **Hyperlipidemia & Ketosis:** Impaired glucose metabolism shifts the body toward fat mobilization. **2. Why Other Options are Incorrect:** * **McArdle’s Disease (Type V):** Muscle phosphorylase deficiency. It presents with muscle cramps and myoglobinuria after exercise, but **no hypoglycemia or hepatomegaly**, as the liver enzyme is normal. * **Cori’s Disease (Type III):** Debranching enzyme deficiency. While it presents with hepatomegaly and hypoglycemia, **lactic acid levels are typically normal**, and there is no renomegaly. * **Pompe’s Disease (Type II):** Lysosomal acid maltase deficiency. It primarily affects the heart (**cardiomegaly**) and muscles. Blood glucose levels are usually normal. **High-Yield Clinical Pearls for NEET-PG:** * **"Doll-like facies"** (fatty cheeks) is a characteristic physical feature of Von Gierke’s. * **Hyperuricemia** is a common finding (due to increased PPP pathway activity leading to purine synthesis). * **Treatment:** Frequent oral cornstarch (slow-release glucose) and avoidance of fructose/galactose.

Q: Which is the most abundant glucose transporter in Red Blood Cells (RBCs)?

GLUT-1. **Explanation:** **1. Why GLUT-1 is the Correct Answer:** GLUT-1 is the primary glucose transporter found in **Red Blood Cells (RBCs)** and the **Blood-Brain Barrier**. RBCs lack mitochondria and are entirely dependent on anaerobic glycolysis for energy; therefore, they require a constant, insulin-independent supply of glucose. GLUT-1 has a high affinity (low $K_m$) for glucose, ensuring a steady basal uptake even during fasting states. In RBCs, GLUT-1 constitutes about 5% of the total membrane protein. **2. Why Other Options are Incorrect:** * **GLUT-3:** While it also has a high affinity for glucose, it is the primary transporter for **neurons** (Brain). It ensures glucose delivery to the brain even at low blood sugar levels. * **GLUT-4:** This is the only **insulin-dependent** glucose transporter. It is primarily located in **skeletal muscle and adipose tissue**. It is sequestered in intracellular vesicles and moves to the cell membrane only in the presence of insulin. * **GLUT-5:** This is a specialized transporter primarily responsible for the absorption of **fructose**, located mainly in the small intestine and spermatozoa. **3. High-Yield Clinical Pearls for NEET-PG:** * **GLUT-2:** A high-capacity, low-affinity (high $K_m$) transporter found in the **Liver, Pancreatic beta-cells, and Kidney**. It acts as a "glucose sensor." * **SGLT-1/SGLT-2:** These are sodium-dependent active transporters (Secondary active transport) found in the small intestine and renal tubules, unlike the GLUT family which facilitates **passive diffusion**. * **GLUT-1 Deficiency Syndrome:** Can lead to infantile seizures and developmental delay because glucose cannot cross the blood-brain barrier efficiently.

Q: Gluconeogenesis occurs in all except:

Palmitate. **Explanation:** The core concept tested here is the **irreversibility of the Pyruvate Dehydrogenase (PDH) complex**. Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. To form glucose, a substrate must be capable of being converted into Pyruvate or an intermediate of the TCA cycle (like Oxaloacetate). **Why Palmitate is the correct answer:** Palmitate is a 16-carbon saturated fatty acid. Through beta-oxidation, even-chain fatty acids are broken down into **Acetyl-CoA**. In humans, Acetyl-CoA cannot be converted back into Pyruvate because the PDH reaction is irreversible. Furthermore, for every two carbons of Acetyl-CoA that enter the TCA cycle, two carbons are lost as $CO_2$. Therefore, there is no net synthesis of glucose from even-chain fatty acids like palmitate. **Why the other options are incorrect:** * **Glycerol:** Derived from triacylglycerol breakdown, it is phosphorylated to glycerol-3-phosphate and converted to **Dihydroxyacetone phosphate (DHAP)**, a direct intermediate of glycolysis/gluconeogenesis. * **Amino acids:** Glucogenic amino acids (e.g., Alanine) are deaminated to form Pyruvate or TCA cycle intermediates (alpha-ketoglutarate, succinyl-CoA), which can then enter the gluconeogenic pathway. * **Lactic acid:** Via the **Cori Cycle**, lactate produced by anaerobic glycolysis in muscles/RBCs is transported to the liver and converted back to Pyruvate by Lactate Dehydrogenase (LDH) to enter gluconeogenesis. **High-Yield Clinical Pearls for NEET-PG:** 1. **Odd-chain fatty acids** ARE gluconeogenic because their final breakdown product is **Propionyl-CoA**, which converts to Succinyl-CoA. 2. **Leucine and Lysine** are the only purely ketogenic amino acids (cannot form glucose). 3. The major site of gluconeogenesis is the **Liver**, followed by the **Kidney** (especially during prolonged starvation). 4. The rate-limiting enzyme of gluconeogenesis is **Fructose-1,6-bisphosphatase**.

Q: Cataract in diabetic patients is due to the accumulation of sorbitol in the lens. Which enzyme is responsible for this conversion?

NADPH+ dependent aldose reductase. ### Explanation **1. Why Option B is Correct:** The formation of cataracts in diabetic patients is explained by the **Polyol Pathway** (Sorbitol Pathway). In states of hyperglycemia, the enzyme **Aldose Reductase** reduces excess glucose into **Sorbitol** (a sugar alcohol or polyol). This reaction requires **NADPH** as a cofactor. Sorbitol is osmotically active; it draws water into the lens, leading to swelling, crystalline precipitation, and eventual opacity (cataract). While some tissues can convert sorbitol to fructose via sorbitol dehydrogenase, the lens has very low levels of this enzyme, leading to toxic sorbitol accumulation. **2. Why Other Options are Incorrect:** * **Option A (Hexokinase):** This enzyme phosphorylates glucose to Glucose-6-Phosphate in most tissues. It has a low Km (high affinity) for glucose and is involved in glycolysis, not the polyol pathway. * **Option C (Glucokinase):** This is the liver and pancreatic B-cell specific isoenzyme of hexokinase. It has a high Km and functions primarily when glucose levels are high to initiate glycogen synthesis or insulin release. * **Option D (Phosphofructokinase-1):** This is the rate-limiting enzyme of **Glycolysis**. It converts Fructose-6-Phosphate to Fructose-1,6-Bisphosphate and does not interact with free glucose or sorbitol. **3. High-Yield Clinical Pearls for NEET-PG:** * **Tissues involved:** "LUKE" – **L**ens, **U**rethra (Kidney), **K**idney, and **E**pinerium (Peripheral nerves) are susceptible to sorbitol damage because they lack **Sorbitol Dehydrogenase**. * **Cofactor Switch:** Aldose reductase uses **NADPH** (consuming it and causing oxidative stress), while Sorbitol Dehydrogenase uses **NAD+**. * **Galactosemia Connection:** Aldose reductase also converts Galactose to **Galactitol** (Dulcitol), which causes "Oil-drop cataracts" in infants with galactosemia.

Question 1

G-6-Phosphatase deficiency is seen in which of the following conditions?

Accepted Answer

Von Gierke's disease

Answer

Tay-Sachs disease

Answer

Pompe's disease

Answer

Anderson's disease

Question 2

In glycogen, what is the type of linkage at branch points?

Accepted Answer

Alpha-1,6

Answer

Alpha-1,4

Answer

Alpha-2,3

Answer

Beta-1,4

Question 3

Which one of the following metabolites is used by all cells for glycolysis, glycogen synthesis, and the hexose monophosphate shunt pathway?

Accepted Answer

Glucose-6-phosphate

Answer

Glucose-1-phosphate

Answer

UDP-glucose

Answer

Fructose-6-phosphate

Question 4

What is the major metabolic pathway in erythrocytes?

Accepted Answer

Pentose phosphate pathway

Answer

Beta-oxidation

Answer

Citric acid cycle

Answer

Gluconeogenesis

Question 5

Which of the following is used for the assessment of carbohydrate malabsorption?

Accepted Answer

D-xylose test

Answer

Schilling test

Answer

Steatorrhoea

Answer

Glucose tolerance test

Question 6

A four-year-old child presents with exercise intolerance. Investigations reveal blood pH 7.3, fasting blood sugar 60 mg%, hypertriglyceridemia, ketosis, and lactic acidosis. The child also has hepatomegaly and renomegaly. Biopsy of the liver and kidney shows increased glycogen content. What is the diagnosis?

Accepted Answer

Von Gierke's disease

Answer

McArdle's disease

Answer

Cori's disease

Answer

Pompe's disease

Question 7

Which is the most abundant glucose transporter in Red Blood Cells (RBCs)?

Accepted Answer

GLUT-1

Answer

GLUT-3

Answer

GLUT-4

Answer

GLUT-5

Question 8

Gluconeogenesis occurs in all except:

Accepted Answer

Palmitate

Answer

Glycerol

Answer

Amino acid

Answer

Lactic acid

Question 9

Cataract in diabetic patients is due to the accumulation of sorbitol in the lens. Which enzyme is responsible for this conversion?

Accepted Answer

NADPH+ dependent aldose reductase

Answer

Hexokinase

Answer

Glucokinase

Answer

Phosphofructokinase

Question 10

A patient with type 1 diabetes self-injected insulin prior to their evening meal, but then was distracted and forgot to eat. A few hours later, the individual fainted, and after the paramedics arrived, they did a STAT blood glucose level and found it to be 45 mg/dL. The blood glucose level was so low because which one of the following tissues assimilated most of it under these conditions?

Accepted Answer

Adipose tissue

Answer

Brain

Answer

Liver

Answer

Red blood cells

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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