Metabolism of Fructose and Galactose Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Metabolism of Fructose and Galactose. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Metabolism of Fructose and Galactose Indian Medical PG Question 1: A patient with hemolytic anemia has a defect in the enzyme glucose-6-phosphate dehydrogenase. Which of the following pathways is directly affected by this defect?
- A. Glycolysis
- B. Pentose phosphate pathway (Correct Answer)
- C. TCA cycle
- D. Urea cycle
Metabolism of Fructose and Galactose Explanation: ***Pentose phosphate pathway***
- **Glucose-6-phosphate dehydrogenase (G6PD)** is the **rate-limiting enzyme** in the **pentose phosphate pathway (PPP)**, initiating the oxidative phase.
- Deficiency in G6PD impairs the production of **NADPH**, which is crucial for reducing **oxidative stress** in red blood cells.
*Glycolysis*
- This pathway metabolizes glucose to pyruvate for **ATP production** and does not directly involve G6PD.
- While G6P is an intermediate in both pathways, its conversion in glycolysis is catalyzed by phosphoglucose isomerase, not G6PD.
*TCA cycle*
- The **tricarboxylic acid (TCA) cycle** is a central metabolic pathway for energy production occurring in the **mitochondria**.
- It involves the oxidation of acetyl-CoA and does not directly utilize G6PD.
*Urea cycle*
- The **urea cycle** is responsible for detoxifying ammonia by converting it into urea, primarily occurring in the **liver**.
- This pathway is unrelated to glucose metabolism or G6PD activity.
Metabolism of Fructose and Galactose Indian Medical PG Question 2: Maltase hydrolyzes maltose to form:
- A. Glucose and fructose
- B. Galactose and fructose
- C. Two glucose molecules (Correct Answer)
- D. Glucose and galactose
Metabolism of Fructose and Galactose Explanation: ***Two glucose molecules***
- Maltase is an enzyme that specifically breaks down **maltose**.
- Maltose, a disaccharide, is composed of **two glucose units** linked by an α-1,4-glycosidic bond.
*Glucose and fructose*
- This is the hydrolysis product of **sucrose**, a disaccharide broken down by the enzyme **sucrase**.
- Sucrose consists of one **glucose** and one **fructose** molecule.
*Galactose and fructose*
- This combination does not represent a common disaccharide hydrolysis product.
- While galactose and fructose are monosaccharides, they do not form a common dietary disaccharide linked together.
*Glucose and galactose*
- This is the hydrolysis product of **lactose**, a disaccharide broken down by the enzyme **lactase**.
- Lactose is composed of one **glucose** and one **galactose** molecule.
Metabolism of Fructose and Galactose Indian Medical PG Question 3: Galactosemia is due to deficiency of which enzyme?
- A. Galactose-1-phosphate uridyltransferase (Correct Answer)
- B. HGPRT
- C. Galactokinase
- D. Epimerase
Metabolism of Fructose and Galactose Explanation: ***Galactose-1-phosphate uridyltransferase***
- Deficiency of **galactose-1-phosphate uridyltransferase (GALT)** leads to the most severe form, **classic galactosemia**.
- This enzyme is crucial for converting **galactose-1-phosphate** to **glucose-1-phosphate** in the Leloir pathway.
*HGPRT*
- **HGPRT** (hypoxanthine-guanine phosphoribosyltransferase) deficiency causes **Lesch-Nyhan syndrome**, a distinct metabolic disorder.
- Lesch-Nyhan syndrome is characterized by **hyperuricemia**, neurological dysfunction, and self-mutilation, unrelated to galactose metabolism.
*Galactokinase*
- Deficiency of **galactokinase** causes Type II galactosemia, a milder form than classic galactosemia.
- This defect primarily leads to **cataracts** due to galactitol accumulation but does not result in the severe systemic issues seen in classic galactosemia.
*Epimerase*
- Deficiency of **UDP-galactose-4'-epimerase** (GALE) causes Type III galactosemia, which has a variable clinical presentation from mild to severe.
- While involved in galactose metabolism, it's not the primary enzyme deficient in the most common and severe form of **galactosemia**.
Metabolism of Fructose and Galactose Indian Medical PG Question 4: History of dislike for sweet food items is typically present in:
- A. Glycogen storage disease
- B. Diabetes mellitus
- C. Galactosemia
- D. Hereditary fructose intolerance (Correct Answer)
Metabolism of Fructose and Galactose Explanation: ***Hereditary fructose intolerance***
- Patients with hereditary fructose intolerance develop severe symptoms like **nausea, vomiting, abdominal pain, and hypoglycemia** after ingesting fructose, leading to an aversive response and **dislike for sweet food items**.
- This aversion is a protective mechanism, as avoiding fructose-containing foods (including many sweets) prevents the accumulation of toxic metabolites due to a deficiency in **hepatic aldolase B**.
*Glycogen storage disease*
- While glycogen storage diseases can cause hypoglycemia, they typically do not lead to a specific **aversion to sweet foods**.
- The primary defect is in **glycogen synthesis or breakdown**, leading to symptoms like hepatomegaly, muscle weakness, and exercise intolerance.
*Diabetes mellitus*
- Patients with diabetes mellitus often have a **craving for sweet foods** due to uncontrolled blood glucose levels and insulin resistance, rather than a dislike.
- The condition is characterized by **hyperglycemia** and may involve polydipsia, polyuria, and polyphagia.
*Galactosemia*
- Galactosemia involves an inability to metabolize galactose, leading to symptoms such as **vomiting, lethargy, and jaundice** upon milk ingestion [1].
- While patients will avoid milk, their aversion is not generally to all sweet foods, as sweet foods do not always contain galactose [1].
Metabolism of Fructose and Galactose Indian Medical PG Question 5: Which enzyme is deficient in Galactosemia?
- A. Hexosaminidase B
- B. Hexosaminidase A
- C. Galactose 1-phosphate uridyltransferase (Correct Answer)
- D. Glucocerebrosidase
Metabolism of Fructose and Galactose Explanation: ***Galactose 1-phosphate uridyltransferase***
- **Classical galactosemia** (Type I) is caused by a deficiency in this enzyme, which converts **galactose-1-phosphate** and UDP-glucose into UDP-galactose and glucose-1-phosphate.
- This deficiency leads to the accumulation of toxic galactose metabolites, such as **galactitol** and galactose-1-phosphate.
*Hexosaminidase B*
- Deficiency of this enzyme is seen in **Sandhoff disease**, a lysosomal storage disorder, which leads to the accumulation of **GM2 gangliosides** in neurons.
- This enzyme is less commonly associated with the primary defect in **Tay-Sachs disease**, which is predominantly due to hexosaminidase A deficiency.
*Hexosaminidase A*
- A deficiency in **hexosaminidase A** causes **Tay-Sachs disease**, another lysosomal storage disorder, leading to the accumulation of **GM2 gangliosides** primarily in nerve cells.
- This enzyme is not involved in the metabolism of galactose.
*Glucocerebrosidase*
- A deficiency in **glucocerebrosidase** causes **Gaucher disease**, which leads to the accumulation of **glucocerebroside** in macrophages and other cells.
- This enzyme is not involved in the metabolic pathway of galactose.
Metabolism of Fructose and Galactose Indian Medical PG Question 6: A cataract formation in both eyes was discovered in a 1-year-old child during a routine well-child visit, with blood tests showing elevated galactose and galactitol levels. To determine which enzyme might be defective in the child, which intracellular metabolite should be measured?
- A. Galactose
- B. Fructose
- C. Glucose
- D. Galactose-1-phosphate (Correct Answer)
Metabolism of Fructose and Galactose Explanation: ***Galactose-1-phosphate***
- An elevation of **galactose-1-phosphate** in a patient with cataracts and elevated galactose and galactitol levels points to a deficiency in **galactose-1-phosphate uridyltransferase (GALT)**, indicating **classic galactosemia**.
- The accumulation of **galactose-1-phosphate** is toxic and underlies the severe symptoms of classic galactosemia, including cataracts, liver damage, and intellectual disability.
- Measuring this metabolite specifically identifies GALT deficiency and distinguishes it from other enzyme defects in galactose metabolism.
*Galactose*
- Elevated **galactose** is observed in **galactosemia**, but measuring galactose itself doesn't differentiate between the different enzyme deficiencies (e.g., GALT vs. GALK deficiency).
- While elevated, it's the downstream metabolites like **galactose-1-phosphate** that are more specific for diagnosing the enzyme defect in classic galactosemia.
*Fructose*
- **Fructose** metabolism is distinct from galactose metabolism, and its levels would not be directly affected by defects in galactose-metabolizing enzymes.
- Elevated fructose would suggest a different metabolic disorder, such as **hereditary fructose intolerance**, which has different clinical presentations.
*Glucose*
- **Glucose** levels are not specific for diagnosing enzyme defects in galactose metabolism.
- While hypoglycemia can occur in severe galactosemia, measuring glucose doesn't identify which specific enzyme is deficient and is not the primary diagnostic metabolite.
Metabolism of Fructose and Galactose Indian Medical PG Question 7: Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following are the allosteric activators of phosphofructokinase-1?
- A. 2,3-Bisphosphoglycerate (2,3-BPG)
- B. Fructose 2,6-bisphosphate (Correct Answer)
- C. Glucokinase
- D. Phosphoenolpyruvate (PEP)
Metabolism of Fructose and Galactose Explanation: ***Fructose 2,6-bisphosphate***
- **Fructose 2,6-bisphosphate** is a potent **allosteric activator** of **phosphofructokinase-1 (PFK-1)**, increasing its affinity for fructose 6-phosphate and overcoming ATP inhibition.
- Its synthesis is regulated by **insulin** (stimulating) and **glucagon** (inhibiting), linking glucose availability to glycolytic flux.
*2,3-Bisphosphoglycerate (2,3-BPG)*
- **2,3-BPG** is an important regulator of **hemoglobin oxygen affinity** in red blood cells.
- It is not an allosteric activator of **PFK-1**; its primary role is in oxygen delivery.
*Glucokinase*
- **Glucokinase** is an **enzyme** in glycolysis, specifically catalyzing the phosphorylation of glucose to glucose 6-phosphate in the liver and pancreatic beta cells.
- It is not an allosteric activator of **PFK-1** but rather an upstream enzyme in the pathway.
*Phosphoenolpyruvate (PEP)*
- **PEP** is an intermediate in glycolysis, formed from 2-phosphoglycerate and converted to pyruvate by pyruvate kinase.
- It acts as an **allosteric inhibitor** of phosphofructokinase-1, signaling high energy status and slowing down glycolysis.
Metabolism of Fructose and Galactose Indian Medical PG Question 8: What are the immediate metabolic products formed during the conversion of Fructose 1,6-bisphosphate to two molecules of pyruvate?
- A. Glyceraldehyde-3-phosphate and 1,3-bisphosphoglycerate
- B. Dihydroxyacetone phosphate and 1,3 bisphosphoglycerate
- C. Glyceraldehyde-3-phosphate and dihydroxy-acetone phosphate (Correct Answer)
- D. 3-phosphoglycerate and 1,3 bisphosphoglycerate
Metabolism of Fructose and Galactose Explanation: ***Glyceraldehyde-3-phosphate and dihydroxy-acetone phosphate***
- Fructose-1,6-bisphosphate is cleaved by the enzyme **aldolase** into two 3-carbon isomers: **dihydroxyacetone phosphate (DHAP)** and **glyceraldehyde-3-phosphate (GAP)**.
- This reaction is a key step in **glycolysis**, occurring after the phosphorylation of fructose-6-phosphate.
*Glyceraldehyde-3-phosphate and 1,3-bisphosphoglycerate*
- **1,3-bisphosphoglycerate** is formed later in glycolysis from glyceraldehyde-3-phosphate, catalyzed by **glyceraldehyde-3-phosphate dehydrogenase**, not directly from Fructose-1,6-bisphosphate cleavage.
- The initial products of Fructose-1,6-bisphosphate cleavage are two distinct three-carbon compounds, not one three-carbon compound and one already modified intermediate.
*Dihydroxyacetone phosphate and 1,3 bisphosphoglycerate*
- Similar to the above, **1,3-bisphosphoglycerate** is a product of an oxidation and phosphorylation step following the initial cleavage, not an immediate product of Fructose-1,6-bisphosphate breakdown.
- The enzyme **triose phosphate isomerase** quickly interconverts dihydroxyacetone phosphate to glyceraldehyde-3-phosphate, ensuring that both can proceed through the subsequent steps of glycolysis.
*3-phosphoglycerate and 1,3 bisphosphoglycerate*
- **3-phosphoglycerate** is formed from 1,3-bisphosphoglycerate by the enzyme **phosphoglycerate kinase**, a later step in glycolysis where ATP is generated.
- Neither of these molecules are the direct products of the enzymatic cleavage of Fructose-1,6-bisphosphate by aldolase.
Metabolism of Fructose and Galactose Indian Medical PG Question 9: Most Common enzyme deficient in galactosemics:
- A. Galactosidase
- B. UDP galactose epimerase
- C. Galactokinase
- D. Galactose-1-phosphate uridyl transferase/GALT (Correct Answer)
Metabolism of Fructose and Galactose Explanation: ***Galactose-1-phosphate uridyl transferase/GALT***
- **GALT deficiency** is the most common cause of **classic galactosemia** (Type I), a severe inherited metabolic disorder.
- This enzyme is crucial for converting **galactose-1-phosphate** to **glucose-1-phosphate** in the main pathway of galactose metabolism.
- Accounts for approximately **95%** of all galactosemia cases.
*Galactosidase*
- **Galactosidase** enzymes are involved in the hydrolysis of galactose-containing oligosaccharides or glycoconjugates but are not the primary enzymes deficient in classic galactosemia.
- This enzyme is not part of the Leloir pathway of galactose metabolism, which is the pathway affected in galactosemia.
*UDP galactose epimerase*
- Deficiency of **UDP galactose epimerase** (GALE) causes a milder form of galactosemia (Type III), but it is much less common than GALT deficiency.
- GALE is involved in the interconversion of UDP-galactose and UDP-glucose.
- This is the rarest form of galactosemia.
*Galactokinase*
- **Galactokinase deficiency** (GALK) causes a different, milder form of galactosemia (Type II), characterized by **cataracts** as the primary symptom.
- It prevents the initial phosphorylation of galactose to galactose-1-phosphate.
- This accounts for less than 5% of galactosemia cases.
Metabolism of Fructose and Galactose Indian Medical PG Question 10: Fructose intolerance is due to deficiency of which enzyme?
- A. Aldolase B (Correct Answer)
- B. Aldolase A
- C. Fructokinase
- D. Triokinase
Metabolism of Fructose and Galactose Explanation: ***Aldolase B***
- **Hereditary fructose intolerance** is a genetic disorder caused by a deficiency in the enzyme **aldolase B**.
- This deficiency leads to an accumulation of **fructose-1-phosphate** in the liver, kidneys, and small intestine, causing **hypoglycemia**, **vomiting**, and **liver damage** upon exposure to fructose.
*Fructokinase*
- A deficiency in **fructokinase** causes **essential fructosuria**, a benign metabolic disorder.
- This condition is asymptomatic because **fructose** simply accumulates in the blood and urine without causing significant clinical problems.
*Triokinase*
- **Triokinase**, also known as **glycerol kinase**, is involved in glycerol metabolism, converting glycerol to **glycerol-3-phosphate**.
- Its deficiency is not directly linked to fructose intolerance and typically presents with **hyperglycerolemia**.
*Aldolase A*
- **Aldolase A** is one of the three aldolase isoenzymes (A, B, and C) and is primarily involved in **glycolysis**, specifically in the breakdown of **fructose-1,6-bisphosphate**.
- A deficiency in aldolase A can lead to **hemolytic anemia** and **myopathy**, not directly fructose intolerance.
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