Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 441: Beta-glucosidase is defective in which disease?

A. Gaucher's disease (Correct Answer)
B. Tay-Sachs disease
C. Galactosaemia
D. Diabetes Mellitus

Explanation: **Explanation:** **1. Why Gaucher's Disease is Correct:** Gaucher’s disease is the most common **Lysosomal Storage Disorder (LSD)**. It is caused by a deficiency of the enzyme **Acid Beta-glucosidase** (also known as **Glucocerebrosidase**). Under normal conditions, this enzyme breaks down glucocerebroside into glucose and ceramide. When defective, glucocerebroside accumulates within the lysosomes of macrophages, transforming them into characteristic **"Gaucher cells"** (described as having a "wrinkled tissue paper" appearance). **2. Why the Other Options are Incorrect:** * **Tay-Sachs Disease:** This is caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of GM2 gangliosides. It is clinically characterized by a cherry-red spot on the macula and progressive neurodegeneration. * **Galactosaemia:** This is a disorder of carbohydrate metabolism, most commonly due to a deficiency of **Galactose-1-phosphate uridyltransferase (GALT)**. It is not a lysosomal storage disease. * **Diabetes Mellitus:** This is a metabolic disorder characterized by hyperglycemia due to insulin deficiency or resistance, unrelated to lysosomal enzyme defects. **3. High-Yield Clinical Pearls for NEET-PG:** * **Gaucher Cells:** Pathognomonic macrophages with fibrillary cytoplasm (wrinkled tissue paper appearance) found in the bone marrow, liver, and spleen. * **Clinical Triad:** Hepatosplenomegaly, bone involvement (Erlenmeyer flask deformity of the femur, bone crises), and pancytopenia. * **Biochemical Marker:** Elevated levels of **Serum Acid Phosphatase** (Tartrate-resistant) are often seen. * **Treatment:** Enzyme Replacement Therapy (ERT) with recombinant glucocerebrosidase (Imiglucerase) is the gold standard.

Question 442: Which of the following dietary components determines the glycemic index of food?

A. Protein
B. Fat
C. Fibre (Correct Answer)
D. None of the above

Explanation: ### Explanation **1. Why Fibre is the Correct Answer:** The **Glycemic Index (GI)** is a ranking of carbohydrates (0–100) based on how quickly they raise blood glucose levels after consumption. Dietary **fibre**, particularly soluble fibre (e.g., pectin, gums), is the primary determinant that lowers the GI of a food. It functions by increasing the viscosity of the intestinal contents, which slows down gastric emptying and delays the enzymatic digestion of starch. This results in a slower, more gradual absorption of glucose into the bloodstream, preventing rapid postprandial "spikes." **2. Why Other Options are Incorrect:** * **Protein (A) and Fat (B):** While protein and fat can influence the overall glycemic *load* of a meal by slowing digestion, they are not the primary dietary components used to define or determine the GI of a specific carbohydrate-rich food. GI is specifically a property of the carbohydrates present in the food. * **None of the above (D):** Incorrect, as fibre is a well-established physiological modulator of the glycemic response. **3. NEET-PG High-Yield Pearls:** * **Glycemic Load (GL):** Unlike GI, GL accounts for the **quantity** of carbohydrates in a typical serving (GL = GI × grams of carbohydrate / 100). * **Clinical Significance:** Low-GI diets (rich in fibre) are crucial in managing **Diabetes Mellitus**, metabolic syndrome, and PCOS, as they improve insulin sensitivity. * **Processing Factor:** Highly processed or overcooked foods generally have a higher GI because the physical barriers to digestion are removed, allowing for rapid glucose release.

Question 443: Which of the following is not an aldose sugar?

A. Erythrose
B. Glucose
C. Fructose (Correct Answer)
D. Galactose

Explanation: ### Explanation **Core Concept: Aldose vs. Ketose Sugars** Monosaccharides are classified based on the type of carbonyl group they contain. * **Aldoses:** Contain an **aldehyde group (-CHO)**, typically located at the C1 position. * **Ketoses:** Contain a **keto group (>C=O)**, typically located at the C2 position. **Why Fructose is the Correct Answer:** Fructose is a **ketohexose**. It contains six carbon atoms but features a functional keto group at the second carbon (C2). Therefore, it is not an aldose. **Analysis of Incorrect Options:** * **A. Erythrose:** This is a four-carbon sugar (**aldotetrose**) used in the Pentose Phosphate Pathway. It contains an aldehyde group. * **B. Glucose:** The most common **aldohexose**. It serves as the primary metabolic fuel and contains an aldehyde group at C1. * **D. Galactose:** An **aldohexose** and a C4-epimer of glucose. It is a constituent of lactose (milk sugar). --- ### NEET-PG High-Yield Clinical Pearls 1. **Functional Isomers:** Glucose and Fructose are functional isomers because they have the same molecular formula ($C_6H_{12}O_6$) but different functional groups (aldehyde vs. ketone). 2. **Reducing Sugars:** Both aldoses and ketoses are reducing sugars. Fructose (a ketose) can reduce Benedict’s reagent because it undergoes **tautomerization** (enediol formation) in alkaline conditions to convert into glucose and mannose. 3. **Seliwanoff’s Test:** This biochemical test is specifically used to distinguish ketoses from aldoses. Ketoses (like Fructose) give a **cherry-red color** rapidly. 4. **Sorbitol Pathway:** In the polyol pathway, Glucose (aldose) is reduced to Sorbitol, which is then oxidized to Fructose (ketose) by sorbitol dehydrogenase. This is the primary source of energy for sperm in seminal fluid.

Question 444: Mutarotation refers to a change in which of the following properties?

A. pH
B. Optical rotation (Correct Answer)
C. Conductance
D. Chemical properties

Explanation: **Explanation:** **Mutarotation** is a fundamental biochemical phenomenon observed in reducing sugars like glucose and fructose. It refers to the spontaneous change in the **specific optical rotation** of a freshly prepared solution of an optically active carbohydrate until it reaches a stable equilibrium value. **Why Optical Rotation is Correct:** When D-glucose is dissolved in water, it exists in two cyclic anomeric forms: **α-D-glucopyranose** (optical rotation +112.2°) and **β-D-glucopyranose** (optical rotation +18.7°). Over time, these forms interconvert through an open-chain intermediate until an equilibrium mixture is reached (approximately 36% alpha and 64% beta), resulting in a final stable optical rotation of **+52.7°**. This change in the ability to rotate plane-polarized light is the definition of mutarotation. **Why Other Options are Incorrect:** * **pH:** Mutarotation involves the rearrangement of atoms around the anomeric carbon; it does not involve the release or uptake of protons ($H^+$), thus the pH remains unchanged. * **Conductance:** Conductance depends on the concentration of ions in a solution. Since sugars are non-electrolytes and do not ionize during this process, conductance is unaffected. * **Chemical Properties:** The chemical identity of the sugar remains the same (it is still glucose). While the physical property of rotation changes, the fundamental chemical reactivity (like being a reducing sugar) remains constant. **High-Yield Clinical Pearls for NEET-PG:** * **Requirement:** Mutarotation occurs only in sugars with a **free anomeric carbon** (reducing sugars). * **Sucrose Exception:** Sucrose does **not** show mutarotation because its anomeric carbons are locked in a glycosidic bond. * **Enzyme:** In the body, the enzyme **mutarotase** accelerates this interconversion, which is essential because certain metabolic pathways preferentially use one specific anomer (e.g., glucose oxidase is specific for β-D-glucose).

Question 445: Which of the following are sources of NADPH?

A. Malic enzyme
B. Cytoplasmic isocitrate dehydrogenase
C. Pentose phosphate pathway
D. All of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **D (All of the above)** because NADPH is primarily generated through specific enzymatic reactions in the cytoplasm to support reductive biosynthesis (like fatty acid and steroid synthesis) and to maintain antioxidant defenses. 1. **Pentose Phosphate Pathway (PPP/HMP Shunt):** This is the **major source** of NADPH in most tissues. The rate-limiting enzyme, **Glucose-6-Phosphate Dehydrogenase (G6PD)**, along with 6-phosphogluconate dehydrogenase, reduces $NADP^+$ to $NADPH$ during the oxidative phase. 2. **Malic Enzyme:** This enzyme converts Malate to Pyruvate in the cytoplasm. It plays a crucial role in the "Citrate-Malate Shuttle," providing the NADPH necessary for fatty acid synthesis in the liver and adipose tissue. 3. **Cytoplasmic Isocitrate Dehydrogenase (IDH1):** While the mitochondrial version (IDH3) produces NADH for the TCA cycle, the cytosolic isoform (NADP-dependent IDH) produces NADPH. It is particularly important in the brain and for maintaining the cytosolic pool of reduced glutathione. **Why other options are not "the only" answer:** Options A, B, and C are all individual, valid sources of NADPH. Therefore, selecting any single one would be incomplete, making "All of the above" the most accurate choice. **NEET-PG High-Yield Pearls:** * **G6PD Deficiency:** The most common enzymopathy worldwide; it leads to hemolytic anemia because RBCs lack mitochondria and rely *exclusively* on the PPP for NADPH to keep glutathione reduced. * **NADPH vs. NADH:** Remember: **NADH** is for **ATP** production (catabolic), while **NADPH** is for **B**iosynthesis (anabolic) and **B**reaking down free radicals. * **Tissues rich in PPP:** Adrenal cortex, liver, mammary glands, and testes (sites of active steroid or fatty acid synthesis).

Question 446: What are the enzymes required for the formation of Phosphoenolpyruvate from Pyruvate?

A. Pyruvate dehydrogenase and Pyruvate carboxylase
B. Lactate Dehydrogenase and Pyruvate Dehydrogenase
C. Pyruvate carboxylase and Phosphoenolpyruvate Carboxykinase (Correct Answer)
D. Phosphoenolpyruvate carboxykinase and Pyruvate Dehydrogenase

Explanation: ### Explanation The conversion of Pyruvate to Phosphoenolpyruvate (PEP) is the first major hurdle in **Gluconeogenesis**. In Glycolysis, the conversion of PEP to Pyruvate by *Pyruvate Kinase* is irreversible. To bypass this, the body employs a two-step "bypass" mechanism: 1. **Pyruvate Carboxylase (PC):** Located in the mitochondria, this enzyme converts Pyruvate into Oxaloacetate (OAA). This reaction requires **Biotin** as a cofactor and ATP. 2. **Phosphoenolpyruvate Carboxykinase (PEPCK):** This enzyme converts OAA into PEP. This step occurs in the cytosol (or mitochondria) and requires **GTP**. **Analysis of Incorrect Options:** * **Pyruvate Dehydrogenase (PDH):** This enzyme converts Pyruvate to Acetyl-CoA. This is an irreversible step that leads into the TCA cycle, not gluconeogenesis. "Acetyl-CoA cannot be converted back to glucose." * **Lactate Dehydrogenase (LDH):** This catalyzes the reversible conversion of Pyruvate to Lactate (anaerobic glycolysis). It does not participate in the bypass of Pyruvate Kinase. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Pyruvate Carboxylase is a **mitochondrial** enzyme, while PEPCK is found in both the mitochondria and cytosol. * **Obligatory Activator:** Pyruvate Carboxylase is allosterically activated by **Acetyl-CoA**. High levels of Acetyl-CoA signal that the cell has enough energy, shunting pyruvate toward gluconeogenesis. * **Cofactor:** Remember the mnemonic **ABC** for carboxylases: **A**TP, **B**iotin, and **C**O₂ are required. * **Rate-limiting step:** PEPCK is often considered a key regulatory point in gluconeogenesis.

Question 447: All of the following are true about changes in brain metabolism after traumatic brain injury, except:

A. Shut down of pyruvate dehydrogenase activity
B. Accumulation of lactate in brain
C. Increased lactate uptake from circulation
D. Increased CSF lactate is associated with good prognosis (Correct Answer)

Explanation: ### Explanation In Traumatic Brain Injury (TBI), the brain undergoes a profound metabolic crisis characterized by a shift from aerobic to anaerobic metabolism. **1. Why Option D is the Correct Answer (The "Except" Statement):** Increased CSF lactate is a marker of **secondary brain injury** and metabolic distress. Elevated lactate levels indicate tissue hypoxia and mitochondrial dysfunction. Clinically, higher levels of CSF lactate are associated with **poor neurological outcomes** and increased mortality, not a good prognosis. **2. Analysis of Other Options:** * **Option A (Shut down of PDH):** Following TBI, there is a significant inhibition of the **Pyruvate Dehydrogenase (PDH) complex**. This prevents pyruvate from entering the TCA cycle, leading to a "metabolic bottleneck" where glucose cannot be efficiently oxidized for energy. * **Option B (Accumulation of lactate):** Due to the PDH shutdown and impaired oxygen delivery, pyruvate is preferentially shunted to lactate via **Lactate Dehydrogenase (LDH)**. This leads to localized lactic acidosis within the brain parenchyma and CSF. * **Option C (Increased lactate uptake):** While the brain normally exports lactate, post-TBI research suggests the brain can upregulate monocarboxylate transporters (MCTs) to **sequester lactate from the circulation** as an alternative fuel source (the "Astrocyte-Neuron Lactate Shuttle" theory) to compensate for the inability to use glucose effectively. ### Clinical Pearls for NEET-PG: * **Lactate/Pyruvate (L/P) Ratio:** An elevated L/P ratio in the brain is a sensitive indicator of **mitochondrial failure** and is used in neuro-critical care monitoring. * **Hyperglycemia & TBI:** Systemic hyperglycemia post-injury worsens outcomes because it provides more substrate for anaerobic glycolysis, further increasing lactate production and cerebral acidosis. * **Gold Standard Monitoring:** Cerebral microdialysis is the technique used to measure these metabolic changes (lactate, glucose, pyruvate) in real-time.

Question 448: Which of the following compounds is characterized by a large amount of carbohydrate and a small amount of protein?

A. Glycoprotein
B. Glycosaminoglycan
C. Proteoglycan (Correct Answer)
D. Glycocalyx

Explanation: **Explanation:** The distinction between these compounds lies in the **ratio of carbohydrate to protein** and the nature of the carbohydrate chains. **1. Why Proteoglycan is Correct:** Proteoglycans consist of a core protein to which long, unbranched polysaccharide chains called **Glycosaminoglycans (GAGs)** are covalently attached. In a proteoglycan molecule, the carbohydrate content is dominant, typically accounting for **90-95% of the weight**, while the protein core makes up only 5-10%. This makes them "carbohydrate-heavy" molecules. **2. Why other options are incorrect:** * **Glycoprotein (A):** These are "protein-heavy" molecules. They consist primarily of protein with short, often branched oligosaccharide chains. The carbohydrate content is usually small (1-20%). * **Glycosaminoglycan (B):** These are the carbohydrate components themselves (e.g., Heparin, Hyaluronic acid). They are pure polysaccharides and do not contain a protein core unless they are part of a proteoglycan. * **Glycocalyx (D):** This is a descriptive term for the "sugar coat" on the outer surface of the plasma membrane, composed of both glycoproteins and glycolipids. It is a structural feature, not a specific biochemical compound defined by a fixed carbohydrate-protein ratio. **Clinical Pearls for NEET-PG:** * **Aggrecan** is the major proteoglycan found in cartilage. * **Hyaluronic Acid** is the only GAG that is **not sulfated** and **not covalently attached** to a protein core. * **Mucopolysaccharidoses (MPS):** These are lysosomal storage disorders caused by the deficiency of enzymes required to degrade GAGs (e.g., Hurler Syndrome, Hunter Syndrome). * **Heparin** is the GAG with the highest negative charge density in the body.

Question 449: Dehydrogenases of the HMP shunt are specific for which coenzyme?

A. Thiamine pyrophosphate (TPP)
B. Nicotinamide adenine dinucleotide phosphate (NADP+) (Correct Answer)
C. Flavin mononucleotide (FMN)
D. Flavin adenine dinucleotide (FAD)

Explanation: **Explanation:** The Hexose Monophosphate (HMP) Shunt, also known as the Pentose Phosphate Pathway, occurs in the cytosol and is primarily an anabolic pathway. The rate-limiting step and the subsequent oxidative step are catalyzed by **Glucose-6-Phosphate Dehydrogenase (G6PD)** and **6-Phosphogluconate Dehydrogenase**, respectively. Both of these enzymes are strictly specific for **NADP+** as their coenzyme, reducing it to **NADPH**. Unlike NADH, which is used in the electron transport chain for ATP production, NADPH is essential for: 1. **Reductive Biosynthesis:** Synthesis of fatty acids and steroids. 2. **Antioxidant Defense:** Maintaining glutathione in its reduced state to protect cells against reactive oxygen species (ROS), particularly in RBCs. **Analysis of Incorrect Options:** * **A. Thiamine pyrophosphate (TPP):** This is a derivative of Vitamin B1. While it is a coenzyme in the HMP shunt, it is used by **Transketolase** (non-oxidative phase), not by dehydrogenases. * **C & D. FMN and FAD:** These are flavin-derived coenzymes (Vitamin B2) typically involved in redox reactions within the mitochondria (e.g., TCA cycle or Complex I/II of ETC). They are not utilized by the oxidative enzymes of the HMP shunt. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzyme deficiency worldwide. It leads to hemolytic anemia because RBCs cannot generate NADPH, leaving them vulnerable to oxidative stress (e.g., fava beans, primaquine, infections) and resulting in **Heinz bodies** and **Bite cells**. * **Tissue Distribution:** The HMP shunt is most active in tissues requiring high NADPH, such as the adrenal cortex (steroidogenesis), liver (fatty acid synthesis), and lactating mammary glands. * **Transketolase Activity:** Measuring erythrocyte transketolase activity is the gold standard for diagnosing **Thiamine deficiency**.

Question 450: Glucagon receptors are NOT found in which organ?

A. Cornea (Correct Answer)
B. Kidney
C. Stomach
D. Adrenal gland

Explanation: **Explanation:** Glucagon is a peptide hormone primarily known for its role in glucose homeostasis. It acts via **G-protein coupled receptors (GPCRs)** that activate the Adenylate Cyclase-cAMP pathway. **Why Cornea is the Correct Answer:** Glucagon receptors are primarily localized in tissues involved in fuel mobilization and metabolic regulation. The **cornea** is an avascular, specialized ocular tissue that relies on atmospheric oxygen and glucose from the aqueous humor. It does not possess glucagon receptors because it does not participate in systemic glucose regulation or glycogenolysis. **Analysis of Other Options:** * **Kidney:** Glucagon receptors are present in the renal cortex and distal tubules. Glucagon plays a role in stimulating gluconeogenesis in the kidney and influencing electrolyte excretion (e.g., increasing glomerular filtration rate). * **Stomach:** Receptors are found in the gastric mucosa. Glucagon inhibits gastric acid secretion and reduces gastrointestinal motility. * **Adrenal Gland:** Glucagon receptors are present in the adrenal medulla, where glucagon can stimulate the release of catecholamines (epinephrine and norepinephrine). **High-Yield NEET-PG Pearls:** 1. **Primary Site:** The **Liver** has the highest density of glucagon receptors (stimulating glycogenolysis and gluconeogenesis). 2. **Muscle Paradox:** Glucagon receptors are **NOT** found in **Skeletal Muscle**. This is a frequent exam trap; muscle glycogen is used only for local contraction and cannot be released into the blood because muscle lacks Glucose-6-Phosphatase. 3. **Adipose Tissue:** Glucagon receptors are present here to stimulate lipolysis via Hormone-Sensitive Lipase (HSL). 4. **Clinical Correlation:** Glucagon is used as an antidote for **Beta-blocker overdose** because it bypasses the beta-receptor to increase cAMP in cardiac myocytes, improving heart rate and contractility.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

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Gluconeogenesis: Reactions and Regulation

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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

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

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