Carbohydrate Metabolism Practice Questions

Q: In Maroteaux-Lamy syndrome, which enzyme is deficient?

Arylsulfatase B. **Explanation:** **Maroteaux-Lamy Syndrome (Mucopolysaccharidosis Type VI)** is an autosomal recessive lysosomal storage disorder. It is caused by a deficiency of the enzyme **Arylsulfatase B** (also known as N-acetylgalactosamine-4-sulfatase). This enzyme is essential for the degradation of the glycosaminoglycan (GAG) **Dermatan sulfate**. When deficient, dermatan sulfate accumulates in lysosomes, leading to clinical features such as coarse facial features, skeletal deformities (dysostosis multiplex), and hepatosplenomegaly, notably **without** intellectual disability. **Analysis of Options:** * **Option A (Correct):** Arylsulfatase B deficiency leads to MPS VI. * **Option B (Incorrect):** Glucosidase (specifically alpha-glucosidase) deficiency causes Pompe disease (GSD Type II). * **Option C (Incorrect):** Hydroxylases are involved in various pathways (e.g., Phenylalanine hydroxylase in PKU), but are not the primary defect in Mucopolysaccharidoses. * **Option D (Incorrect):** Beta-glucuronidase deficiency causes **Sly Syndrome (MPS VII)**, which involves the accumulation of dermatan sulfate, heparan sulfate, and chondroitin sulfate. **High-Yield Clinical Pearls for NEET-PG:** * **MPS VI vs. MPS I/II:** Unlike Hurler (MPS I) or Hunter (MPS II), Maroteaux-Lamy patients typically have **normal intelligence**. * **Corneal Clouding:** Present in MPS VI (similar to Hurler, unlike Hunter which has no corneal clouding). * **Diagnosis:** Increased urinary excretion of Dermatan sulfate. * **Mnemonic:** "Six (VI) Marot-B" (Type 6 = Maroteaux = Arylsulfatase B).

Q: Dehydrogenase enzymes in the HMP shunt's oxidative phase generate which of the following?

NADPH. **Explanation:** The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), occurs in the cytosol and consists of two phases: oxidative and non-oxidative. **Why NADPH is the correct answer:** The **oxidative phase** is irreversible and involves two key dehydrogenase enzymes: 1. **Glucose-6-Phosphate Dehydrogenase (G6PD):** The rate-limiting enzyme that converts Glucose-6-P to 6-Phosphogluconolactone. 2. **6-Phosphogluconate Dehydrogenase:** Converts 6-Phosphogluconate to Ribulose-5-Phosphate. Both these enzymes utilize **NADP+ as a coenzyme** and reduce it to **NADPH**. Therefore, the primary products of the oxidative phase are NADPH (used for reductive biosynthesis) and Pentoses (used for nucleotide synthesis). **Why other options are incorrect:** * **NADP+:** This is the *oxidized* form used as a substrate/coenzyme; it is consumed, not generated, by the dehydrogenases. * **FAD+ / FADH:** These flavin nucleotides are typically involved in the TCA cycle (e.g., Succinate dehydrogenase) and the Mitochondrial Electron Transport Chain, not the HMP shunt. **High-Yield Clinical Pearls for NEET-PG:** * **Tissues involved:** The HMP shunt is highly active in tissues requiring NADPH for lipid/steroid synthesis (Adrenal cortex, Liver, Lactating mammary gland) or for maintaining reduced glutathione (Erythrocytes). * **G6PD Deficiency:** This is the most common enzymopathy. A lack of NADPH in RBCs leads to an inability to regenerate reduced glutathione, resulting in hemoglobin oxidation, **Heinz bodies**, and hemolytic anemia under oxidative stress (e.g., Fava beans, Primaquine). * **Regulation:** The pathway is regulated by the **NADPH/NADP+ ratio**; high levels of NADPH inhibit G6PD.

Q: What are the two important byproducts of the Hexose Monophosphate (HMP) shunt?

NADPH and pentose sugars. The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), is an alternative pathway for glucose oxidation that occurs in the cytosol. Unlike glycolysis, its primary purpose is not the generation of ATP, but the production of two vital biosynthetic precursors: 1. **NADPH:** Generated by the oxidative phase (via Glucose-6-Phosphate Dehydrogenase), it serves as a reducing agent for fatty acid and steroid synthesis and maintains reduced glutathione to prevent oxidative damage. 2. **Pentose Sugars (Ribose-5-Phosphate):** Produced for the synthesis of nucleotides (DNA/RNA) and coenzymes. **Analysis of Options:** * **Option A (Incorrect):** NADH is primarily produced in glycolysis and the TCA cycle for ATP generation via the electron transport chain. The HMP shunt specifically produces **NADPH**. * **Option C & D (Incorrect):** While 4-membered sugars (Erythrose-4-P) and 7-membered sugars (Sedoheptulose-7-P) are intermediates in the non-oxidative phase, they are not the primary "functional byproducts" the pathway is designed to provide. They serve as links to return carbon skeletons to glycolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **G6PD Deficiency:** The most common enzymopathy. It leads to neonatal jaundice and drug-induced hemolytic anemia (e.g., after Primaquine or Fava beans) because RBCs cannot generate NADPH to combat oxidative stress. * **Tissue Distribution:** The pathway is highly active in the liver, lactating mammary glands, adrenal cortex, and RBCs. * **Transketolase:** An enzyme in the non-oxidative phase that requires **Thiamine (Vitamin B1)** as a cofactor; its activity is measured to diagnose Thiamine deficiency.

Q: Classic Galactosemia is due to deficiency of which enzyme?

Galactose-1-Phosphate-Uridyl-Transferase. **Explanation:** **Classic Galactosemia** (Type 1) is an autosomal recessive disorder caused by a deficiency of the enzyme **Galactose-1-Phosphate Uridyl Transferase (GALT)**. In the normal metabolic pathway (Leloir pathway), GALT is responsible for converting Galactose-1-phosphate and UDP-glucose into UDP-galactose and Glucose-1-phosphate. Its deficiency leads to the toxic accumulation of Galactose-1-phosphate and galactitol in tissues like the liver, brain, and lens of the eye. **Analysis of Incorrect Options:** * **Hexosaminidase A:** Deficiency leads to **Tay-Sachs Disease**, a lysosomal storage disorder characterized by GM2 ganglioside accumulation and a "cherry-red spot" on the macula. * **Glucocerebrosidase:** Deficiency causes **Gaucher Disease**, the most common lysosomal storage disease, characterized by hepatosplenomegaly and "wrinkled tissue paper" appearance of macrophages. * **Sphingomyelinase:** Deficiency results in **Niemann-Pick Disease**, leading to sphingomyelin accumulation and foam cells. **Clinical Pearls for NEET-PG:** * **Presentation:** Symptoms appear shortly after starting milk feeds (lactose = glucose + galactose). Look for jaundice, hepatomegaly, infantile cataracts (due to galactitol), and failure to thrive. * **Sepsis Risk:** These infants have a significantly increased predisposition to **E. coli neonatal sepsis**. * **Diagnosis:** Reducing substances in urine (Clinitest positive) but a negative glucose oxidase test (Dipstick). * **Treatment:** Immediate and lifelong elimination of galactose and lactose from the diet.

Q: Which enzyme of the citric acid cycle catalyzes a reaction involving substrate-level phosphorylation?

Succinate thiokinase. **Explanation:** **1. Why Succinate Thiokinase is Correct:** Succinate thiokinase (also known as **Succinyl-CoA synthetase**) is the only enzyme in the Citric Acid Cycle (TCA cycle) that performs **substrate-level phosphorylation (SLP)**. In this reaction, the high-energy thioester bond of Succinyl-CoA is cleaved to form Succinate. The energy released is used to phosphorylate GDP to **GTP** (which is later converted to ATP). This is a crucial step because it generates high-energy phosphate directly without the requirement of the Electron Transport Chain or Oxygen. **2. Why the Other Options are Incorrect:** * **Pyruvate Kinase (Option A):** While this enzyme does perform substrate-level phosphorylation (converting Phosphoenolpyruvate to Pyruvate), it is a part of **Glycolysis**, not the Citric Acid Cycle. * **Phosphoglycerate Kinase (Option C):** Similarly, this enzyme performs SLP (converting 1,3-bisphosphoglycerate to 3-phosphoglycerate), but it is also an enzyme of the **Glycolytic pathway**. * **Option D:** Incorrect because only Succinate thiokinase fits both criteria: being an SLP enzyme and belonging to the TCA cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Definition of SLP:** The synthesis of ATP/GTP from ADP/GDP coupled directly to the dephosphorylation of a high-energy intermediate. * **Total SLP sites:** In one complete turn of the aerobic oxidation of glucose, there are **3 SLP sites**: two in Glycolysis (Phosphoglycerate kinase, Pyruvate kinase) and one in the TCA cycle (Succinate thiokinase). * **TCA Yield:** One turn of the TCA cycle produces **1 GTP (via SLP)**, 3 NADH, and 1 FADH2, totaling 10 ATP equivalents. * **Isoenzymes:** Succinate thiokinase has two isoforms; one is specific for GDP (liver/kidney) and one for ADP (skeletal/heart muscle).

Q: Forbes' disease is due to deficiency of which enzyme?

Debranching enzyme. **Explanation:** **Forbes' disease**, also known as **Cori’s disease (GSD Type III)**, is caused by a deficiency of the **Debranching enzyme** (Amylo-1,6-glucosidase). In this condition, glycogen can be partially broken down by phosphorylase, but the process stops at the branch points, leading to the accumulation of abnormal glycogen with short outer chains known as **limit dextrin**. * **Why Option B is correct:** The debranching enzyme has two activities: transferase and glucosidase. Its deficiency prevents the complete mobilization of glucose from glycogen stores, leading to hepatomegaly, growth retardation, and fasting hypoglycemia (though milder than von Gierke’s). **Analysis of Incorrect Options:** * **Option A (Branching enzyme):** Deficiency causes **Andersen’s disease (GSD Type IV)**. This leads to the accumulation of long, unbranched glucose chains (amylopectin-like) which trigger an immune response, causing progressive liver cirrhosis. * **Option C (Myophosphorylase):** Deficiency causes **McArdle’s disease (GSD Type V)**. This affects only skeletal muscle, presenting with exercise intolerance, muscle cramps, and myoglobinuria. * **Option D (Hepatic phosphorylase):** Deficiency causes **Hers’ disease (GSD Type VI)**. It presents with hepatomegaly and mild hypoglycemia, but unlike Forbes' disease, the glycogen structure remains normal. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "ABCD" — **A**ndersen = **B**ranching; **C**ori = **D**ebranching. * Forbes' disease involves both **liver and muscle** (Type IIIa) or just the liver (Type IIIb). * Unlike Type I (von Gierke), blood lactate and uric acid levels are usually **normal** in Forbes' disease. * Limit dextrinosis is the hallmark histological finding.

Q: An infant presents with hepatomegaly, hypoglycemia, hyperlipidemia, and acidosis. What is the diagnosis?

Von Gierke's disease. **Explanation:** The clinical presentation of hepatomegaly, hypoglycemia, hyperlipidemia, and acidosis is the classic "quadrad" of **Von Gierke’s Disease (GSD Type I)**. **1. Why Option A is Correct:** Von Gierke’s disease 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: * **Severe Fasting Hypoglycemia:** The liver cannot release free glucose into the blood. * **Hepatomegaly:** Excessive accumulation of glycogen (normal structure) in the liver. * **Lactic Acidosis:** Glucose-6-phosphate shunts into the glycolytic pathway, increasing lactate. * **Hyperlipidemia & Hyperuricemia:** Diverted metabolites lead to increased VLDL/triglycerides and purine degradation (Gout). **2. Why Other Options are Incorrect:** * **B. Cori’s Disease (GSD Type III):** Caused by **Debranching enzyme** deficiency. While it presents with hepatomegaly and hypoglycemia, the symptoms are generally milder, and **lactate levels are typically normal** because gluconeogenesis remains intact. * **C. Pompe’s Disease (GSD Type II):** Caused by **Lysosomal α-1,4-glucosidase** deficiency. It primarily affects the heart and muscles (**Cardiomegaly**), and notably, it does **not** present with hypoglycemia as glycogen mobilization in the liver is unaffected. **Clinical Pearls for NEET-PG:** * **"Doll-like facies"** (fatty cheeks) is a characteristic physical finding in Von Gierke’s. * **Hyperuricemia** in GSD Type I can lead to "Gouty arthritis" later in life. * **Treatment:** Frequent oral cornstarch (slow-release glucose) and avoidance of fructose/galactose.

Q: What is the end product of glycolysis under anaerobic conditions?

Lactic acid. **Explanation:** In glycolysis, glucose is converted into two molecules of pyruvate. The fate of this pyruvate depends on the availability of oxygen and the presence of mitochondria. **1. Why Lactic Acid is Correct:** Under **anaerobic conditions** (lack of oxygen), the electron transport chain cannot function. To allow glycolysis to continue and generate ATP, the cell must regenerate **NAD+** from NADH. The enzyme **Lactate Dehydrogenase (LDH)** reduces pyruvate to **Lactic acid**, simultaneously oxidizing NADH back to NAD+. This is the primary pathway in mature erythrocytes (which lack mitochondria) and exercising skeletal muscle. **2. Why the other options are incorrect:** * **Pyruvic acid:** This is the end product of **aerobic** glycolysis. In the presence of oxygen, pyruvate enters the mitochondria to be converted into Acetyl-CoA. * **Acetoacetic acid:** This is a **ketone body** produced during ketogenesis (primarily in the liver during starvation or uncontrolled diabetes), not a product of glycolysis. * **Oxaloacetic acid:** This is an intermediate of the TCA cycle and gluconeogenesis. While pyruvate can be converted to oxaloacetate via *pyruvate carboxylase*, it is not the end product of the glycolytic pathway. **Clinical Pearls & High-Yield Facts:** * **Net ATP Yield:** Anaerobic glycolysis yields only **2 ATP** per glucose molecule, whereas aerobic respiration yields 30-32 ATP. * **Cori Cycle:** The lactic acid produced in muscles travels to the liver, where it is converted back to glucose via gluconeogenesis. * **Lactic Acidosis:** This occurs when there is excessive anaerobic metabolism (e.g., septic shock, severe hypoxia), leading to a drop in blood pH. * **Key Enzyme:** Lactate Dehydrogenase (LDH) is a tetramer; LDH-1 is prominent in the heart, while LDH-5 is prominent in the liver and skeletal muscle.

Q: A 6-month-old baby presents to the emergency department with features of cardiac failure. On examination, developmental delay and a large tongue are noted. Echocardiography reveals severe concentric left ventricular hypertrophy. What is the most probable diagnosis?

Pompe disease. **Explanation:** The clinical triad of **infantile-onset heart failure**, **severe concentric left ventricular hypertrophy (LVH)**, and **macroglossia** (large tongue) in a 6-month-old is classic for **Pompe Disease (Glycogen Storage Disease Type II)**. **Why Pompe Disease is Correct:** Pompe disease is caused by a deficiency of the lysosomal enzyme **acid alpha-glucosidase (acid maltase)**. Unlike other GSDs, glycogen accumulates within lysosomes in various tissues, most notably the **cardiac and skeletal muscles**. This leads to massive cardiomegaly (often described as a "globular heart" on X-ray), hypertrophic cardiomyopathy, and profound hypotonia ("floppy baby"). The macroglossia and developmental delay further support this diagnosis. **Why Other Options are Incorrect:** * **Von Gierke’s Disease (GSD Type I):** Primarily affects the liver and kidneys. It presents with severe hypoglycemia, hepatomegaly, and "doll-like" facies, but **never involves the heart**. * **Forbes/Cori Disease (GSD Type III):** Caused by debranching enzyme deficiency. While it can occasionally involve the heart, it typically presents with hepatomegaly and growth retardation; the cardiac involvement is never as severe or early-onset as in Pompe. * **McArdle’s Disease (GSD Type V):** A muscle phosphorylase deficiency that presents in **adolescence or adulthood** with exercise-induced cramps and myoglobinuria. It does not cause infantile heart failure. **High-Yield Clinical Pearls for NEET-PG:** * **ECG Finding:** Pompe disease characteristically shows **short PR intervals** and **giant QRS complexes** (due to massive hypertrophy). * **Enzyme:** Acid Alpha-Glucosidase (Acid Maltase). * **Stain:** Periodic Acid-Schiff (PAS) positive vacuoles in muscle biopsy. * **Treatment:** Enzyme Replacement Therapy (ERT) with Alglucosidase alfa.

Question 1

In Maroteaux-Lamy syndrome, which enzyme is deficient?

Accepted Answer

Arylsulfatase B

Answer

Glucosidase

Answer

Hydroxylase

Answer

beta-glucuronidase

Question 2

Dehydrogenase enzymes in the HMP shunt's oxidative phase generate which of the following?

Accepted Answer

NADPH

Answer

NADP+

Answer

FAD+

Answer

FADH

Question 3

What are the two important byproducts of the Hexose Monophosphate (HMP) shunt?

Accepted Answer

NADPH and pentose sugars

Answer

NADH and pentose sugars

Answer

Pentose sugars and 4-membered sugars

Answer

Pentose sugars and sedoheptulose

Question 4

Patients with diabetes frequently report changing visual acuities when their glucose levels are chronically high. Which of the following could explain the fluctuating acuity with high blood glucose levels?

Accepted Answer

Increased sorbitol in the lens

Answer

Decreased fructose in the lens

Answer

Increased oxidative phosphorylation in the lens

Answer

Macular degeneration

Question 5

Classic Galactosemia is due to deficiency of which enzyme?

Accepted Answer

Galactose-1-Phosphate-Uridyl-Transferase

Answer

Hexosaminidase

Answer

Glucocerebroside

Answer

Sphingomyelinase

Question 6

Which enzyme of the citric acid cycle catalyzes a reaction involving substrate-level phosphorylation?

Accepted Answer

Succinate thiokinase

Answer

Pyruvate kinase

Answer

Phosphoglycerate kinase

Answer

All of the above

Question 7

Forbes' disease is due to deficiency of which enzyme?

Accepted Answer

Debranching enzyme

Answer

Branching enzyme

Answer

Myophosphorylase

Answer

Hepatic phosphorylase

Question 8

An infant presents with hepatomegaly, hypoglycemia, hyperlipidemia, and acidosis. What is the diagnosis?

Accepted Answer

Von Gierke's disease

Answer

Cori's disease

Answer

Pompe's disease

Answer

All of the above

Question 9

What is the end product of glycolysis under anaerobic conditions?

Accepted Answer

Lactic acid

Answer

Pyruvic acid

Answer

Acetoacetic acid

Answer

Oxaloacetic acid

Question 10

A 6-month-old baby presents to the emergency department with features of cardiac failure. On examination, developmental delay and a large tongue are noted. Echocardiography reveals severe concentric left ventricular hypertrophy. What is the most probable diagnosis?

Accepted Answer

Pompe disease

Answer

Von Gierke's disease

Answer

Forbes disease

Answer

McArdle's disease

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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