Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 751: Fructose intolerance is due to deficiency of which enzyme?

A. Aldolase B (Correct Answer)
B. Aldolase A
C. Fructokinase
D. Triokinase

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.

Question 752: Which of the following is the major glycosaminoglycan of synovial fluid?

A. Chondroitin sulfate
B. Dermatan sulfate
C. Heparan sulfate
D. Hyaluronic acid (Correct Answer)

Explanation: ***Hyaluronic acid*** - **Hyaluronic acid** is the primary glycosaminoglycan in **synovial fluid**, providing its characteristic **viscosity** and **lubricating properties**. - It plays a crucial role in maintaining **joint health** by reducing friction and acting as a shock absorber. *Chondroitin sulfate* - **Chondroitin sulfate** is abundant in **cartilage**, contributing to its **compressive strength**. - While present in connective tissues, it is not the major glycosaminoglycan of synovial fluid. *Dermatan sulfate* - **Dermatan sulfate** is primarily found in **skin**, **blood vessels**, and **heart valves**. - Its main roles involve tissue structure and repair, not lubrication of synovial fluid. *Heparan sulfate* - **Heparan sulfate** is found on **cell surfaces** and in the **extracellular matrix**, especially in the **basement membranes**. - It regulates cell growth, adhesion, and signaling, and is not a major component of synovial fluid viscosity.

Question 753: Gluconeogenesis occurs in all except:

A. Muscle (Correct Answer)
B. Kidney
C. Gut
D. Liver

Explanation: ***Muscle*** - **Muscle tissue** lacks the enzyme **glucose-6-phosphatase**, which is essential for releasing free glucose into the bloodstream during gluconeogenesis. - While muscle can store glycogen, it primarily uses glucose for its own energy needs and does not contribute significantly to systemic glucose homeostasis through gluconeogenesis. *Liver* - The **liver** is the primary site of **gluconeogenesis**, producing glucose to maintain blood glucose levels during fasting and starvation. - It contains all the necessary enzymes, including **glucose-6-phosphatase**, to convert precursors like lactate, amino acids, and glycerol into glucose. *Kidney* - The **kidney** becomes a significant site of **gluconeogenesis** during prolonged fasting, contributing up to 10-20% of the body's glucose production. - Renal gluconeogenesis primarily utilizes **lactate** and **glutamine** as substrates. *Gut* - The **small intestine (gut)** has been identified as a site of **gluconeogenesis**, particularly following a meal rich in protein. - Its contribution is relatively smaller compared to the liver but plays a role in **postprandial glucose homeostasis**.

Question 754: The energy for glycogenesis is provided by -

A. GTP
B. GDP
C. UTP (Correct Answer)
D. AMP

Explanation: ***UTP*** - **Uridine triphosphate (UTP)** is essential for **glycogenesis** as it activates glucose by forming **UDP-glucose** from glucose-1-phosphate. - The reaction (Glucose-1-P + UTP → UDP-glucose + PPi) creates a **high-energy intermediate** that drives glycogen synthesis. - The subsequent hydrolysis of pyrophosphate (PPi) makes this activation step **irreversible**, and the energy stored in UDP-glucose is used for **glycosidic bond formation** when glucose is added to the growing glycogen chain. *GTP* - **Guanosine triphosphate (GTP)** is primarily involved in **protein synthesis**, G-protein signaling, and the citric acid cycle. - It is not used for glucose activation in glycogenesis; that role is specific to **UTP**. *GDP* - **Guanosine diphosphate (GDP)** is a product of GTP hydrolysis and functions in regulatory processes. - It does not serve as an energy donor for glycogen synthesis. *AMP* - **Adenosine monophosphate (AMP)** is a low-energy signal molecule that indicates cellular energy depletion. - High AMP levels **inhibit glycogenesis** and activate glycogenolysis through allosteric regulation of key enzymes. - It does not provide energy for anabolic pathways like glycogen synthesis.

Question 755: Which of the following statements about GLUT 2 transporters is correct?

A. Insulin dependent
B. Insulin independent (Correct Answer)
C. Found in cardiac muscle
D. Found in brain

Explanation: ***Insulin independent*** - GLUT2 transporters facilitate glucose transport into cells **regardless of insulin levels**, making them crucial for basal glucose sensing and transport functions. - This **insulin independence** is vital for organs like the liver and pancreatic beta cells to respond to varying glucose concentrations. *Insulin dependent* - **Insulin-dependent** transporters, such as **GLUT4**, respond to insulin by relocating to the cell membrane to increase glucose uptake. - This characteristic applies to tissues like **skeletal muscle** and **adipose tissue**, not GLUT2. *Found in cardiac muscle* - **Cardiac muscle** primarily utilizes **GLUT4** for glucose uptake, which is insulin-dependent. - While other GLUT transporters might be present in cardiac tissue, **GLUT2** is not the primary mechanism for glucose transport here. *Found in brain* - The **brain** predominantly uses **GLUT1** and **GLUT3** for glucose transport, which have **high affinity** for glucose to ensure constant supply. - **GLUT2** is not a primary transporter of glucose in the brain.

Question 756: Inhibition of glycolysis by increased supply of O2 is called ?

A. Pasteur effect (Correct Answer)
B. Crabtree phenomenon
C. Lewis phenomenon
D. None of the options

Explanation: ***Pasteur effect*** - The **Pasteur effect** describes the phenomenon where the rate of **glycolysis** is inhibited when **oxygen** is available (aerobic conditions). - This inhibition occurs because **oxidative phosphorylation** is more efficient at generating ATP, leading to reduced reliance on glycolysis for energy production. *Crabtree phenomenon* - The **Crabtree phenomenon** is the opposite of the Pasteur effect, where high concentrations of **glucose** inhibit oxygen consumption in the presence of oxygen. - This is primarily observed in some **cancer cells** and yeast, leading to increased glycolysis even under aerobic conditions. *Lewis phenomenon* - The **Lewis phenomenon** (also known as the hunting reaction) refers to the cyclical vasodilation and constriction of peripheral blood vessels in response to **cold exposure**. - It's a physiological response to protect tissues from **frostbite** and is not related to glycolysis or oxygen supply. *None of the options* - This option is incorrect as the phenomenon described, inhibition of glycolysis by increased O2, is a well-established biochemical process known as the **Pasteur effect**.

Question 757: Which of the following tests is most commonly used to detect glucose in urine?

A. a) Benedicts test
B. c) Glucose-oxidase test (Correct Answer)
C. b) Fehling solution
D. d) None of the above

Explanation: ***Glucose-oxidase test*** - The **glucose-oxidase test** is a highly specific and sensitive enzymatic test used to detect **glucose** in urine. - It uses the enzyme glucose oxidase which specifically catalyzes the oxidation of glucose to gluconic acid and hydrogen peroxide, which then produces a color change. - This is the **most commonly used method** in modern clinical practice for detecting glucosuria due to its **high specificity for glucose** and ease of use (dipstick method). - It is the preferred test for **monitoring diabetes** and screening for hyperglycemia. *Benedict's test* - **Benedict's test** is a general chemical test for **all reducing sugars** (glucose, fructose, galactose, lactose, maltose), not specifically glucose. - It works by reducing copper sulfate (Cu²⁺) to copper oxide (Cu⁺) in an alkaline solution, forming a colored precipitate (green, yellow, orange, or brick-red depending on sugar concentration). - While it can detect glucose, it **lacks specificity** and can give false positives with other reducing substances (vitamin C, certain drugs), making it less suitable for routine clinical testing. *Fehling's solution* - **Fehling's solution** is also a general chemical test for **reducing sugars** based on copper reduction, similar to Benedict's test. - It consists of two solutions mixed before use and detects various reducing sugars, not just glucose. - It is **not commonly used in clinical urine analysis** due to lack of specificity and the need for heating and mixing two solutions, making it impractical compared to the simple glucose-oxidase dipstick. *None of the above* - This option is incorrect because the **glucose-oxidase test** is indeed the most commonly used test for detecting glucose in urine in modern clinical practice.

Question 758: Which metabolic pathway provides instant energy to muscles?

A. Embden-Meyerhof pathway (Correct Answer)
B. HMP shunt
C. Cori cycle
D. TCA cycle

Explanation: ***Embden-Meyerhof pathway*** - This pathway, also known as **glycolysis**, rapidly breaks down glucose into pyruvate to produce **ATP without oxygen**, providing instant energy to muscles during high-intensity activity. - Generates a net of **two ATP molecules** per glucose molecule, which is crucial for quick bursts of energy. *HMP shunt* - The **hexose monophosphate shunt** primarily produces **NADPH** for reductive biosynthesis and **ribose-5-phosphate** for nucleotide synthesis, not immediate large-scale ATP for muscle contraction. - Plays a role in protecting cells from **oxidative stress** and synthesizing precursors for DNA and RNA. *Cori cycle* - The **Cori cycle** involves the recycling of **lactate** produced in muscles back to glucose in the liver, which is a slower process for maintaining glucose homeostasis rather than providing instant muscle energy. - It helps prevent **lactic acidosis** during strenuous activity but is not a primary pathway for rapid ATP generation. *TCA cycle* - The **TCA cycle (Krebs cycle)** is part of **aerobic respiration** and produces a significant amount of ATP, but it is a slower, more sustained energy production pathway that requires oxygen. - Primarily active during **lower-intensity**, longer-duration activities when oxygen supply is adequate.

Question 759: What is the maximum value on the glycemic index scale that classifies a food as low glycemic index?

A. 25
B. 45
C. 55 (Correct Answer)
D. 65

Explanation: ***55*** - A food is classified as having a **low glycemic index (GI)** if its GI value is **55 or less**. - The GI scale classifies foods as: **low GI (≤55), medium GI (56-69), and high GI (≥70)**. - This classification indicates that the food causes a slower and lower rise in blood glucose levels compared to high or medium GI foods. *25* - This value is well below the threshold for a low GI food and is not the maximum value for this classification. - While a food with a GI of 25 would indeed be considered low GI, the question asks for the **maximum value** that still falls within this category. *45* - This value is still within the low GI range, but it is not the maximum value for this classification. - Foods with a GI up to 55 are considered low GI. *65* - A GI value of 65 falls into the **medium glycemic index** category (GI 56-69). - Therefore, this value classifies a food as medium GI, not low GI.

Question 760: Which glycogen storage disease also presents as a lysosomal storage disease?

A. Von Gierke's disease
B. McArdle's disease
C. Andersen's disease
D. Pompe's disease (Correct Answer)

Explanation: ***Pompe's disease*** - Also known as **glycogen storage disease type II**, it is caused by a deficiency of **acid alpha-glucosidase (GAA)**, a *lysosomal enzyme*. - This deficiency leads to the accumulation of **glycogen in lysosomes**, particularly affecting muscle tissue, thereby earning its classification as both a glycogen storage disease and a lysosomal storage disease. *Von Gierke's disease* - This is **glycogen storage disease type I** and is due to a deficiency in **glucose-6-phosphatase**. - It primarily affects the **liver and kidneys**, causing severe **hypoglycemia** and **lactic acidosis**, but it is not classified as a lysosomal storage disease. *McArdle's disease* - This is **glycogen storage disease type V**, caused by a deficiency in **muscle glycogen phosphorylase (myophosphorylase)**. - It manifests as **exercise intolerance** and muscle pain, but it does not involve lysosomal enzyme defects or glycogen accumulation in lysosomes. *Andersen's disease* - This is **glycogen storage disease type IV**, caused by a deficiency in the **glycogen branching enzyme**. - It leads to the formation of **abnormal glycogen structures**, primarily affecting the liver and causing early liver failure, but it is not a lysosomal storage disorder.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

Practice Questions

Blood Glucose Regulation

Practice Questions

Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

Practice Questions

Lactose Intolerance and Galactosemia

Practice Questions

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