Carbohydrate Metabolism Practice Questions

Q: A breast-fed infant presented with frequent vomiting and weight loss. Several days later, the infant developed jaundice, hepatomegaly, and bilateral cataracts. What is the most likely cause for these symptoms?

Galactosemia. **Explanation:** The clinical presentation of **Galactosemia** (specifically Classic Galactosemia due to **GALT deficiency**) typically manifests shortly after the introduction of milk (breast milk or formula), which contains lactose. Lactose is broken down into glucose and galactose. In GALT deficiency, the accumulation of **Galactose-1-phosphate** causes hepatotoxicity (jaundice, hepatomegaly) and renal damage. The formation of **galactitol** via aldose reductase in the lens leads to osmotic swelling and the development of **bilateral oil-drop cataracts**. **Why other options are incorrect:** * **Juvenile Diabetes Mellitus:** Presents with polyuria, polydipsia, and weight loss, but not with neonatal jaundice, hepatomegaly, or cataracts in the first few days of life. * **Hereditary Fructose Intolerance (HFI):** Symptoms (vomiting, hypoglycemia) only appear after the introduction of **fructose or sucrose** (e.g., fruit juices or honey). Since the infant is exclusively breast-fed, HFI is ruled out as breast milk contains lactose, not fructose. * **Gaucher Disease:** A lysosomal storage disorder presenting with hepatosplenomegaly and bone pain, but it does not typically cause acute neonatal jaundice or cataracts. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficit:** Most common is Galactose-1-phosphate uridyltransferase (**GALT**). * **Cataract Mechanism:** Accumulation of **Galactitol** in the lens. * **Diagnostic Clue:** Presence of **reducing sugars in urine** (Clinitest positive) but a **negative glucose oxidase test** (Dipstick). * **Risk:** Infants with Galactosemia are at a significantly increased risk of **E. coli sepsis**. * **Treatment:** Immediate withdrawal of milk; switch to soy-based or lactose-free formula.

Q: Andersen disease is due to lack of:

Branching enzyme. **Explanation:** **Andersen Disease (GSD Type IV)** is caused by a deficiency of the **Branching Enzyme** (amylo-1,4→1,6-transglucosidase). This enzyme is responsible for creating $\alpha$-1,6-glycosidic bonds, which introduce branches into the glycogen molecule. In its absence, the body produces an abnormal, long-chain, unbranched glycogen known as **amylopectin-like polysaccharide** (polyglucosan). These insoluble molecules trigger an immune response, leading to progressive liver cirrhosis and liver failure, often fatal in early childhood. **Analysis of Incorrect Options:** * **Option B: Debranching enzyme** deficiency causes **Cori disease (GSD Type III)**. This results in the accumulation of "limit dextrins" (short, branched glycogen chains) because the body cannot break down the $\alpha$-1,6 bonds. * **Option C: Acid maltase** (lysosomal $\alpha$-1,4-glucosidase) deficiency causes **Pompe disease (GSD Type II)**. It is unique because it is a lysosomal storage disorder, primarily affecting the heart (massive cardiomegaly). * **Option D: Myophosphorylase** deficiency causes **McArdle disease (GSD Type V)**. This is a muscle-specific disorder characterized by exercise intolerance, muscle cramps, and myoglobinuria. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "ABCD" – **A**ndersen = **B**ranching enzyme; **C**ori = **D**ebranching enzyme. * **Key Feature:** Andersen disease is the only GSD that typically presents with **early-onset liver cirrhosis**. * **Biopsy finding:** Presence of PAS-positive, diastase-resistant eosinophilic cytoplasmic inclusions (amylopectin-like bodies).

Q: The gene expression of which of the following enzymes is not increased by insulin?

Pyruvate Carboxylase. ### Explanation **1. Why Pyruvate Carboxylase is the Correct Answer:** Insulin is an **anabolic hormone** that promotes glucose utilization (glycolysis) and storage (glycogenesis/lipogenesis). **Pyruvate Carboxylase (PC)** is a key regulatory enzyme in **gluconeogenesis**, converting pyruvate to oxaloacetate. Since insulin aims to lower blood glucose levels, it **represses** the gene expression of gluconeogenic enzymes (PC, PEPCK, Fructose-1,6-bisphosphatase, and Glucose-6-phosphatase). Therefore, its expression is decreased, not increased, by insulin. **2. Why the Other Options are Incorrect:** * **Phosphofructokinase-1 (PFK-1):** This is the rate-limiting enzyme of glycolysis. Insulin increases its expression and activity (via Fructose-2,6-bisphosphate) to enhance glucose breakdown. * **Acetyl CoA Carboxylase (ACC):** This is the rate-limiting enzyme for fatty acid synthesis. Insulin promotes lipogenesis by inducing the gene expression of ACC to store excess energy as fat. * **Pyruvate Dehydrogenase (PDH):** PDH links glycolysis to the TCA cycle. Insulin increases its activity (via dephosphorylation) and gene expression to promote the oxidative decarboxylation of pyruvate into Acetyl-CoA for energy or fat synthesis. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Insulin-Sensitive" Enzymes:** Insulin induces enzymes of **Glycolysis** (Glucokinase, PFK-1, Pyruvate Kinase) and **Lipogenesis** (ACC, Fatty Acid Synthase). * **The "Glucagon-Sensitive" Enzymes:** Glucagon and Cortisol induce enzymes of **Gluconeogenesis** (PEPCK is the primary target for transcriptional control). * **Biotin Dependency:** Pyruvate Carboxylase requires **Biotin (B7)** as a cofactor and is allosterically **activated by Acetyl-CoA**. * **Mnemonic:** Insulin "builds" (Anabolic) and "burns" (Glycolysis); it never "creates" new glucose (Gluconeogenesis).

Q: Glucose-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). Because the liver cannot release glucose into the bloodstream, patients present with severe fasting hypoglycemia, hepatomegaly (due to glycogen accumulation), and metabolic derangements like lactic acidosis, hyperuricemia, and hyperlipidemia. **Analysis of Incorrect Options:** * **Tay-Sachs Disease:** A lysosomal storage disorder (Sphingolipidosis) caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of GM2 ganglioside. It presents with neurodegeneration and a cherry-red spot on the macula. * **Pompe’s Disease (Type II GSD):** Caused by a deficiency of **Lysosomal α-1,4-glucosidase** (Acid Maltase). It is unique because it is both a GSD and a lysosomal storage disease, primarily affecting the heart (massive cardiomegaly) and muscles. * **Anderson’s Disease (Type IV GSD):** Caused by a deficiency of the **Branching enzyme**. This leads to the accumulation of abnormal glycogen with long outer chains (amylopectin-like), resulting in infantile liver cirrhosis and failure. **High-Yield Clinical Pearls for NEET-PG:** * **Type Ia:** Deficiency of Glucose-6-phosphatase enzyme. * **Type Ib:** Deficiency of Glucose-6-phosphate **translocase** (presents with additional findings of neutropenia and recurrent infections). * **Key Biochemical Marker:** Hyperlactatemia is a hallmark of Von Gierke’s, distinguishing it from Cori’s disease (Type III), where lactate levels are typically normal. * **Management:** Frequent oral glucose/cornstarch and avoidance of fructose/galactose.

Q: A newborn baby refuses breast milk from the second day of birth, vomits on force-feeding but accepts glucose-water, develops diarrhea on the third day, and by the fifth day is jaundiced with liver enlargement and shows cataracts. Urinary reducing sugar was positive, but blood glucose estimated by the glucose oxidation method was found to be low. What is the most likely cause of these symptoms?

Galactose 1-phosphate uridyl transferase deficiency. ### Explanation The clinical presentation describes **Classic Galactosemia**, caused by a deficiency of **Galactose 1-phosphate uridyl transferase (GALT)**. **1. Why Option A is Correct:** When the infant consumes breast milk (which contains lactose = glucose + galactose), galactose cannot be converted to glucose. This leads to an accumulation of **Galactose 1-phosphate**, which is toxic to the liver (jaundice, hepatomegaly) and kidneys. Excess galactose is diverted to the polyol pathway, where **aldose reductase** converts it to **galactitol**, causing osmotic damage and **cataracts**. * **The Diagnostic Clue:** The "Glucose Oxidase" method specifically measures glucose. In GALT deficiency, blood glucose is low (hypoglycemia), but the urine is positive for **reducing sugars** (due to galactose), creating a classic diagnostic mismatch. **2. Why Other Options are Incorrect:** * **B. Beta-galactosidase deficiency:** This causes Lactose Intolerance. It presents with diarrhea and bloating but **never** with jaundice, hepatomegaly, or cataracts, as galactose is not absorbed. * **C. G6PD deficiency:** This causes neonatal jaundice due to hemolysis, but it does not cause cataracts, reducing sugars in urine, or hepatomegaly triggered by milk. * **D. Galactokinase deficiency:** This is a milder form of galactosemia. It presents **only with cataracts**; it does not cause liver failure, jaundice, or systemic illness. **Clinical Pearls for NEET-PG:** * **Classic Galactosemia:** Deficient GALT; symptoms start as soon as milk feeding begins. * **Cataract Mechanism:** Accumulation of **Galactitol** in the lens. * **Infection Risk:** These infants are at high risk for **E. coli sepsis**. * **Treatment:** Immediate withdrawal of milk; switch to soy-based or lactose-free formula.

Q: Increase in cAMP releases glycogen from the muscle due to:

Epinephrine. ### Explanation **Why Epinephrine is Correct:** The regulation of glycogenolysis in skeletal muscle is primarily driven by **Epinephrine** (Adrenaline). When epinephrine binds to **$\beta_2$-adrenergic receptors** on the muscle cell membrane, it activates the enzyme Adenylate Cyclase. This leads to an increase in intracellular **cAMP** (cyclic Adenosine Monophosphate). cAMP then activates **Protein Kinase A (PKA)**, which phosphorylates and activates **Phosphorylase Kinase**. This enzyme, in turn, converts inactive Glycogen Phosphorylase *b* into active **Glycogen Phosphorylase *a***, triggering the breakdown of glycogen into glucose-1-phosphate to provide immediate energy for muscle contraction ("Fight or Flight" response). **Why Other Options are Incorrect:** * **Glucagon:** While glucagon also increases cAMP to trigger glycogenolysis, it acts **exclusively on the liver**. Skeletal muscle lacks glucagon receptors; therefore, glucagon cannot release glycogen from muscle. * **Thyroxine (T4):** Thyroxine increases the overall metabolic rate and sensitizes cells to catecholamines, but it does not directly trigger the cAMP-mediated glycogenolytic cascade in muscle. * **Growth Hormone:** This is a counter-regulatory hormone that generally promotes gluconeogenesis and lipolysis but does not acutely stimulate muscle glycogenolysis via the cAMP pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Muscle vs. Liver:** Muscle glycogen serves as a local fuel source (it lacks **Glucose-6-Phosphatase**, so it cannot release free glucose into the blood). Liver glycogen maintains blood glucose levels. * **Dual Regulation in Muscle:** Besides cAMP, muscle glycogenolysis is also triggered by **$\text{Ca}^{2+}$ ions** (via Calmodulin) during muscle contraction, bypassing the need for cAMP. * **Key Enzyme:** Glycogen Phosphorylase is the rate-limiting enzyme of glycogenolysis.

Q: Snow flake cataract is produced because of which enzyme?

Aldose reductase. **Explanation:** **Snowflake cataract** is a classic ocular manifestation of **Type 1 Diabetes Mellitus** (juvenile diabetes). It is characterized by subcapsular, white, needle-like opacities that resemble snowflakes. **Why Aldose Reductase is correct:** The underlying mechanism is the **Polyol Pathway**. In states of hyperglycemia, the enzyme **Aldose Reductase** converts excess glucose into **Sorbitol**. Sorbitol is a sugar alcohol that cannot easily cross the cell membrane and is metabolized very slowly. This leads to an intracellular accumulation of sorbitol within the lens fibers, creating a high osmotic gradient. Water is drawn into the lens to compensate, leading to lens swelling, fiber disruption, and the formation of the characteristic "snowflake" opacities. **Analysis of Incorrect Options:** * **B. Galactose reductase:** While galactose is converted to galactitol (dulcitol) by aldose reductase in galactosemia (leading to "Oil droplet" cataracts), there is no specific enzyme commonly referred to as galactose reductase in this clinical context. * **C & D. Fructose/Alcohol dehydrogenase:** These enzymes are involved in different metabolic pathways. While Sorbitol is eventually converted to fructose by *Sorbitol dehydrogenase*, it is the initial accumulation of sorbitol via Aldose Reductase that triggers the osmotic cataract. **High-Yield Clinical Pearls for NEET-PG:** * **Snowflake Cataract:** Associated with Type 1 Diabetes (Rapid onset). * **Senile Cataract (Cuneiform/Nuclear):** Most common type in Type 2 Diabetes (Occurs earlier than in non-diabetics). * **Oil Droplet Cataract:** Seen in Galactosemia (due to Dulcitol accumulation). * **Sunflower Cataract:** Seen in Wilson’s Disease (Copper deposition). * **Christmas Tree Cataract:** Seen in Myotonic Dystrophy.

Q: What is the net generation of ATP in glycolysis?

7. **Explanation:** In the context of NEET-PG and modern biochemistry (based on Harper’s Illustrated Biochemistry), the net ATP yield of glycolysis is calculated based on the **Malate-Aspartate Shuttle**, which is the predominant shuttle in the liver and heart. **Why 7 is the correct answer:** The net yield is calculated by subtracting the ATP consumed from the total ATP produced: 1. **ATP Consumed:** 2 ATP (at the Hexokinase and Phosphofructokinase-1 steps). 2. **ATP Produced (Substrate-level phosphorylation):** 4 ATP (at the Phosphoglycerate kinase and Pyruvate kinase steps). 3. **ATP from NADH (Oxidative phosphorylation):** 2 NADH are produced. Using the current oxidative phosphorylation ratios (1 NADH = 2.5 ATP), 2 NADH yield **5 ATP**. * **Calculation:** (4 + 5) - 2 = **7 ATP**. **Analysis of Incorrect Options:** * **Option A (5):** This would be the net yield if the **Glycerol-3-Phosphate Shuttle** (common in muscle/brain) were used, where 1 NADH yields only 1.5 ATP (Total: 4 + 3 - 2 = 5). However, unless specified, the higher yield is generally considered the standard "net" for the pathway. * **Option C (15) & D (20):** These values are incorrect for glycolysis alone. 15 ATP is the yield for one turn of the TCA cycle (including the PDH reaction), and 30-32 ATP is the total yield for the complete aerobic oxidation of one glucose molecule. **High-Yield Clinical Pearls for NEET-PG:** * **Anaerobic Glycolysis:** The net yield is always **2 ATP** because NADH is consumed to reduce pyruvate to lactate. * **Rate-limiting step:** Phosphofructokinase-1 (PFK-1). * **Rapoport-Luebering Cycle:** In RBCs, 2,3-BPG is produced, bypassing the first substrate-level phosphorylation, resulting in a net yield of **0 ATP**. * **Arsenic Poisoning:** Inhibits ATP production in glycolysis by competing with inorganic phosphate at the Glyceraldehyde-3-phosphate dehydrogenase step.

Q: Cori's cycle is a term used for which of the following pathways?

Lactic acid cycle. **Explanation:** **Cori’s Cycle (Lactic Acid Cycle)** is a metabolic pathway that describes the interplay between the skeletal muscle and the liver. 1. **Why Option A is correct:** During vigorous exercise, muscular demand for ATP exceeds the oxygen supply, leading to **anaerobic glycolysis**. In this process, pyruvate is converted to **lactate** by the enzyme Lactate Dehydrogenase (LDH). This lactate is released into the bloodstream and taken up by the **liver**, where it is converted back into glucose via **gluconeogenesis**. This glucose is then returned to the muscle to be used as energy. This recycling of lactate to glucose is why it is synonymous with the Lactic acid cycle. 2. **Why other options are incorrect:** * **Option B (Citric acid cycle):** Also known as the Krebs cycle or TCA cycle, this occurs in the mitochondria and is the final common pathway for the oxidation of carbohydrates, fats, and proteins. * **Option C (Pentose phosphate pathway):** Also known as the Hexose Monophosphate (HMP) Shunt, this pathway generates NADPH and ribose-5-phosphate; it does not involve lactate recycling. **High-Yield Clinical Pearls for NEET-PG:** * **Net Energy Cost:** The Cori cycle is energy-consuming. It costs **6 ATP** in the liver to synthesize glucose, while only **2 ATP** are produced during anaerobic glycolysis in the muscle (Net loss of 4 ATP). * **Purpose:** Its primary role is to prevent **lactic acidosis** in the muscle and provide a continuous glucose supply during fasting or intense exercise. * **Glucose-Alanine Cycle (Cahill Cycle):** Often confused with Cori’s cycle, the Cahill cycle involves the transport of amino groups from muscle to liver via **Alanine** instead of lactate.

Question 1

A breast-fed infant presented with frequent vomiting and weight loss. Several days later, the infant developed jaundice, hepatomegaly, and bilateral cataracts. What is the most likely cause for these symptoms?

Accepted Answer

Galactosemia

Answer

Juvenile Diabetes Mellitus

Answer

Hereditary Fructose Intolerance

Answer

Gaucher Disease

Question 2

Andersen disease is due to lack of:

Accepted Answer

Branching enzyme

Answer

Debranching enzyme

Answer

Acid maltase

Answer

Myophosphorylase

Question 3

The gene expression of which of the following enzymes is not increased by insulin?

Accepted Answer

Pyruvate Carboxylase

Answer

Acetyl CoA Carboxylase

Answer

Phosphofructokinase-1

Answer

Pyruvate Dehydrogenase

Question 4

Glucose-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 5

A newborn baby refuses breast milk from the second day of birth, vomits on force-feeding but accepts glucose-water, develops diarrhea on the third day, and by the fifth day is jaundiced with liver enlargement and shows cataracts. Urinary reducing sugar was positive, but blood glucose estimated by the glucose oxidation method was found to be low. What is the most likely cause of these symptoms?

Accepted Answer

Galactose 1-phosphate uridyl transferase deficiency

Answer

Beta galactosidase deficiency

Answer

Glucose 6-phosphate dehydrogenase deficiency

Answer

Galactokinase deficiency

Question 6

Increase in cAMP releases glycogen from the muscle due to:

Accepted Answer

Epinephrine

Answer

Thyroxine

Answer

Glucagon

Answer

Growth hormone

Question 7

Snow flake cataract is produced because of which enzyme?

Accepted Answer

Aldose reductase

Answer

Galactose reductase

Answer

Fructose dehydrogenase

Answer

Alcohol dehydrogenase

Question 8

What is the net generation of ATP in glycolysis?

Accepted Answer

7

Answer

5

Answer

15

Answer

20

Question 9

Cori's cycle is a term used for which of the following pathways?

Accepted Answer

Lactic acid cycle

Answer

Citric acid cycle

Answer

Pentose phosphate pathway

Answer

None of the above

Question 10

Within the red blood cell, hypoxia stimulates glycolysis via which of the following regulatory mechanisms?

Accepted Answer

Hypoxia stimulates the release of all glycolytic enzymes from band 3 or RBC membranes

Answer

Hypoxia stimulates pyruvate dehydrogenase by increased 2,3-DPG

Answer

Hypoxia inhibits hexokinase

Answer

Activation of regulatory enzymes by high pH

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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