Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 521: Which mucopolysaccharide/proteoglycans are present in the eye?

A. Keratan sulfate & dermatan sulfate
B. Dermatan sulfate & heparan sulfate
C. Dermatan sulfate & chondroitin sulfate
D. Keratan sulfate and chondroitin sulfate (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Keratan sulfate and chondroitin sulfate**. Mucopolysaccharides (Glycosaminoglycans or GAGs) are essential components of the extracellular matrix. In the eye, specifically within the **cornea**, the transparency and structural integrity are maintained by a precise arrangement of collagen fibrils and specific GAGs. 1. **Keratan Sulfate (KS I):** This is the most abundant GAG in the cornea. It plays a critical role in maintaining corneal transparency by regulating the spacing between collagen fibrils. 2. **Chondroitin Sulfate:** Along with its isomer, Dermatan sulfate, it is found in the corneal stroma and the vitreous humor. In the cornea, Chondroitin-4-sulfate and Chondroitin-6-sulfate contribute to the hydration and osmotic pressure of the tissue. **Analysis of Incorrect Options:** * **Options A, B, & C:** While **Dermatan sulfate** is present in the cornea (often as a hybrid with chondroitin), it is primarily associated with the skin, blood vessels, and heart valves. **Heparan sulfate** is a component of cell surfaces and basement membranes (like the glomerular basement membrane) but is not a primary structural GAG of the ocular media. **NEET-PG High-Yield Pearls:** * **Keratan Sulfate I** is corneal; **Keratan Sulfate II** is found in cartilage. * **Hyaluronic acid** is the primary GAG found in the **vitreous humor** (it is the only GAG that is non-sulfated and not covalently bound to a protein). * **Macular Corneal Dystrophy** is a clinical condition caused by a defect in the sulfation of Keratan Sulfate, leading to corneal opacity. * **Heparin** has the highest negative charge density of any biological molecule.

Question 522: Pyruvate dehydrogenase complex contains all except which of the following cofactors?

A. Biotin (Correct Answer)
B. NAD
C. FAD
D. CoA

Explanation: **Explanation:** The **Pyruvate Dehydrogenase (PDH) Complex** is a multi-enzyme cluster that catalyzes the oxidative decarboxylation of pyruvate into Acetyl-CoA, serving as the critical bridge between glycolysis and the TCA cycle. **Why Biotin is the Correct Answer:** Biotin (Vitamin B7) is a cofactor involved in **carboxylation** reactions (adding CO₂), such as those catalyzed by Pyruvate Carboxylase or Acetyl-CoA Carboxylase. The PDH complex, however, performs **decarboxylation** (removing CO₂). Therefore, Biotin is not a component of this complex. **Analysis of Incorrect Options (The 5 Required Cofactors):** The PDH complex requires five specific cofactors, often remembered by the mnemonic **"Tender Loving Care For No-one"**: 1. **Thiamine Pyrophosphate (TPP/B1):** Bound to E1 (Pyruvate dehydrogenase); involved in decarboxylation. 2. **Lipoic Acid:** Bound to E2 (Dihydrolipoyl transacetylase); handles the acyl group transfer. 3. **CoA (B5):** Substrate for E2; accepts the acetyl group to form Acetyl-CoA. (**Option D**) 4. **FAD (B2):** Bound to E3 (Dihydrolipoyl dehydrogenase); accepts electrons to become FADH₂. (**Option C**) 5. **NAD+ (B3):** Final electron acceptor for E3; becomes NADH. (**Option B**) **Clinical Pearls for NEET-PG:** * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the SH-groups of **Lipoic Acid**, leading to lactic acidosis and neurological symptoms. * **Thiamine Deficiency:** Common in alcoholics (Wernicke-Korsakoff); leads to PDH failure, causing ATP depletion in highly aerobic tissues (brain/heart). * **Regulation:** PDH is inhibited by its products (Acetyl-CoA, NADH) and by phosphorylation via PDH Kinase. It is activated by Calcium and Insulin (via PDH Phosphatase).

Question 523: Substrate level phosphorylation in glycolysis is seen in which enzyme?

A. Pyruvate kinase (Correct Answer)
B. Succinate thiokinase
C. Enolase
D. Pyruvate dehydrogenase (PDH)

Explanation: **Explanation:** **Substrate-level phosphorylation (SLP)** is the direct synthesis of ATP (or GTP) from ADP (or GDP) by the transfer of a high-energy phosphate group from a phosphorylated intermediate, without the involvement of the electron transport chain or oxygen. **1. Why Pyruvate Kinase is correct:** In the final step of glycolysis, **Pyruvate Kinase** catalyzes the conversion of Phosphoenolpyruvate (PEP) to Pyruvate. PEP contains a high-energy phosphate bond; its hydrolysis releases enough energy to drive the phosphorylation of ADP to **ATP**. This is one of the two SLP steps in glycolysis (the other being Phosphoglycerate kinase). **2. Why the other options are incorrect:** * **Succinate thiokinase (Succinyl-CoA Synthetase):** While this enzyme *does* perform substrate-level phosphorylation (converting Succinyl-CoA to Succinate and generating GTP), it occurs in the **TCA Cycle**, not glycolysis. * **Enolase:** This enzyme catalyzes the dehydration of 2-phosphoglycerate to PEP. It creates a high-energy bond but does not synthesize ATP. * **Pyruvate dehydrogenase (PDH):** This is a multi-enzyme complex that converts pyruvate to Acetyl-CoA (oxidative decarboxylation). It produces NADH but does not perform SLP. **High-Yield Clinical Pearls for NEET-PG:** * **Total SLP in Glycolysis:** 4 ATP are generated per glucose molecule (via Phosphoglycerate kinase and Pyruvate kinase). The *net* gain is 2 ATP. * **Arsenite Poisoning:** Arsenite inhibits the PDH complex. However, **Arsenate** competes with inorganic phosphate in glycolysis, bypassing the SLP step of 1,3-BPG, resulting in zero net ATP production. * **Rapoport-Luebering Cycle:** In RBCs, 1,3-BPG can be converted to 2,3-BPG. This bypasses an SLP step, meaning the RBC sacrifices ATP production to facilitate oxygen delivery to tissues.

Question 524: All of the following are causes of fasting hypoglycemia, except?

A. Glucagon Excess (Correct Answer)
B. Glucose-6-Phosphatase deficiency
C. Glycogen-synthase deficiency
D. Uremia

Explanation: **Explanation:** **1. Why Glucagon Excess is the Correct Answer:** Glucagon is a **counter-regulatory hormone** that acts as the primary defense against hypoglycemia. It stimulates hepatic glycogenolysis and gluconeogenesis, thereby **increasing blood glucose levels**. Therefore, an excess of glucagon (as seen in a Glucagonoma) would lead to **hyperglycemia**, not hypoglycemia. **2. Analysis of Incorrect Options:** * **Glucose-6-Phosphatase deficiency (Von Gierke Disease):** This enzyme is essential for the final step of both glycogenolysis and gluconeogenesis. Its absence prevents the liver from releasing free glucose into the blood, leading to **severe fasting hypoglycemia**. * **Glycogen-synthase deficiency (GSD Type 0):** Patients cannot store glycogen in the liver. During a fast, they lack glycogen stores to maintain blood glucose, resulting in **fasting hypoglycemia** and ketosis. * **Uremia:** Chronic kidney disease/uremia causes hypoglycemia through multiple mechanisms, including impaired renal gluconeogenesis (the kidney contributes ~20% of glucose production during fasting), reduced insulin clearance, and malnutrition. **Clinical Pearls for NEET-PG:** * **Hormonal Balance:** Insulin is the only hypoglycemic hormone; Glucagon, Cortisol, Epinephrine, and Growth Hormone are all **diabetogenic** (hyperglycemic). * **Alcohol & Hypoglycemia:** Alcohol inhibits gluconeogenesis by increasing the NADH/NAD+ ratio, a common cause of fasting hypoglycemia in clinical vignettes. * **Critical Enzyme:** Glucose-6-Phosphatase is absent in muscle, which is why muscle glycogen cannot contribute directly to blood glucose levels.

Question 525: The uronic acid pathway is important for the formation of?

A. GAG (glycosaminoglycans)
B. Glycoproteins
C. Conjugation of bilirubin
D. All of the above (Correct Answer)

Explanation: The **Uronic Acid Pathway** is an alternative pathway for glucose metabolism that does not generate ATP but produces essential metabolites, primarily **UDP-glucuronate** (active glucuronic acid). ### **Why "All of the Above" is Correct:** The primary product of this pathway, UDP-glucuronate, serves three critical physiological functions: 1. **Synthesis of GAGs (Option A):** Glucuronic acid is a major structural component of Glycosaminoglycans (GAGs) like hyaluronic acid, chondroitin sulfate, and heparin. These are essential for the extracellular matrix and connective tissue. 2. **Synthesis of Glycoproteins (Option B):** Glucuronic acid is incorporated into various glycoproteins and proteoglycans, which are vital for cell signaling and membrane structure. 3. **Conjugation Reactions (Option C):** In the liver, UDP-glucuronate is used by the enzyme *UDP-glucuronyltransferase* to conjugate hydrophobic substances, making them water-soluble for excretion. This includes **bilirubin** (forming bilirubin diglucuronide), steroid hormones, and various drugs (e.g., morphine). ### **Clinical Pearls for NEET-PG:** * **Vitamin C Synthesis:** In most animals, this pathway leads to the synthesis of Ascorbic acid. However, **humans lack the enzyme L-gulonolactone oxidase**, making Vitamin C an essential dietary requirement. * **Essential Pentosuria:** A rare, benign genetic deficiency of **Xylitol dehydrogenase** leads to the excretion of L-xylulose in urine. This can give a false-positive Benedict's test (reducing sugar) but is clinically asymptomatic. * **Drug Induction:** Drugs like Phenobarbital can induce the enzymes of the uronic acid pathway, increasing the rate of glucuronidation.

Question 526: Which glucose transporter is stimulated by insulin?

A. Brain and retina
B. Liver and beta cells of pancreas
C. Skeletal muscle and adipose tissue (Correct Answer)
D. RBCs and intestine

Explanation: **Explanation:** The correct answer is **Skeletal muscle and adipose tissue** because these tissues express **GLUT-4**, the only insulin-dependent glucose transporter. ### 1. Why the Correct Answer is Right In the resting state, GLUT-4 transporters are sequestered in intracellular vesicles. When insulin binds to its receptor, it triggers a signaling cascade (via PI3-kinase) that causes these vesicles to fuse with the plasma membrane. This increases the number of transporters on the cell surface, facilitating glucose uptake. This mechanism is crucial for lowering postprandial blood glucose levels. ### 2. Analysis of Incorrect Options * **A. Brain and retina:** These tissues primarily use **GLUT-1** and **GLUT-3**. They require a constant supply of glucose regardless of insulin levels to maintain metabolic activity. * **B. Liver and beta cells of pancreas:** These tissues express **GLUT-2**. GLUT-2 is a high-capacity, low-affinity transporter that is insulin-independent. In the liver, it allows for bidirectional glucose flux; in beta cells, it acts as a glucose sensor. * **D. RBCs and intestine:** RBCs use **GLUT-1** (insulin-independent). The intestine uses **SGLT-1** (active transport) for glucose absorption from the lumen and **GLUT-2** for transport into the blood. ### 3. High-Yield Clinical Pearls for NEET-PG * **GLUT-4** is also stimulated by **exercise** in skeletal muscle via an insulin-independent pathway (AMPK activation). This is why exercise helps manage Type 2 Diabetes. * **GLUT-2** has the highest $K_m$ (lowest affinity), ensuring it only transports large amounts of glucose when blood levels are high. * **GLUT-5** is unique as it is a primary transporter for **fructose**, located in the small intestine and spermatozoa. * **SGLT-1/2** are symporters (secondary active transport) using sodium gradients, unlike the GLUT family which uses facilitated diffusion.

Question 527: Fructokinase is necessary for the production of which of the following?

A. Fructose-1-PO4 (Correct Answer)
B. Fructose 1,6-diphosphate
C. Fructose 6-phosphate
D. Glyceraldehyde

Explanation: ### Explanation **Correct Answer: A. Fructose-1-PO4** The metabolism of fructose occurs primarily in the liver through the **fructose-1-phosphate pathway**. The enzyme **Fructokinase** (also known as ketohexokinase) catalyzes the transfer of a phosphate group from ATP to the C1 position of fructose. This reaction converts fructose into **Fructose-1-phosphate (F-1-P)**. This is the first committed step of fructose metabolism and is insulin-independent. #### Why the other options are incorrect: * **B. Fructose 1,6-diphosphate:** This is formed by the action of *Phosphofructokinase-1 (PFK-1)* on Fructose-6-phosphate during glycolysis, not by fructokinase. * **C. Fructose 6-phosphate:** This is produced when fructose is phosphorylated by *Hexokinase* (a minor pathway in non-hepatic tissues like muscle). Fructokinase specifically phosphorylates the 1st carbon, not the 6th. * **D. Glyceraldehyde:** This is a downstream product. Fructose-1-phosphate is subsequently cleaved by **Aldolase B** into Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde. #### NEET-PG High-Yield Clinical Pearls: 1. **Essential Fructosuria:** Caused by a deficiency of **Fructokinase**. It is a benign, asymptomatic condition where fructose is excreted in the urine (reducing sugar positive, but glucose oxidase test negative). 2. **Hereditary Fructose Intolerance (HFI):** Caused by a deficiency of **Aldolase B**. This leads to the toxic accumulation of **Fructose-1-phosphate**, which depletes intracellular inorganic phosphate, inhibiting glycogenolysis and gluconeogenesis, resulting in severe postprandial hypoglycemia and liver damage. 3. **Speed of Metabolism:** Fructose is metabolized faster than glucose because it bypasses the rate-limiting step of glycolysis (PFK-1).

Question 528: A child presents with hepatomegaly and hypoglycemia. There is no improvement in blood sugar even after administration of epinephrine. What is the likely diagnosis?

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

Explanation: ### Explanation **1. Why Von Gierke’s Disease (Type I GSD) is correct:** The clinical presentation of **hepatomegaly** and **fasting hypoglycemia** indicates a defect in the liver's ability to release glucose into the bloodstream. In Von Gierke’s disease, there is a deficiency of **Glucose-6-Phosphatase**. This enzyme is the final common step for both **Glycogenolysis** and **Gluconeogenesis**. When epinephrine is administered, it stimulates glycogen breakdown into Glucose-1-Phosphate, which is then converted to Glucose-6-Phosphate (G6P). However, because G6P cannot be converted to free glucose due to the enzyme deficiency, the blood sugar levels fail to rise. This "failure to respond to epinephrine/glucagon" is a classic diagnostic hallmark of Type I GSD. **2. Why the other options are incorrect:** * **Anderson’s Disease (Type IV):** Caused by a branching enzyme deficiency. It typically presents with infantile liver cirrhosis and failure to thrive; hypoglycemia is not the primary feature. * **Pompe’s Disease (Type II):** Caused by lysosomal acid maltase deficiency. It primarily affects the heart (cardiomegaly) and muscles. Since it does not involve the main metabolic pathways of the liver, blood glucose levels remain normal. * **McArdle’s Disease (Type V):** Caused by muscle phosphorylase deficiency. It affects skeletal muscle, leading to exercise-induced cramps and myoglobinuria, but does not cause hepatomegaly or hypoglycemia. **3. High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Triad of Von Gierke’s:** Hyperuricemia (Gout), Hyperlipidemia, and Lactic Acidosis. * **Appearance:** Children often have a "doll-like face" due to fat deposition and stunted growth. * **Key Diagnostic Test:** Ischemic forearm exercise test is used for McArdle’s (shows no rise in lactate), whereas the Epinephrine/Glucagon challenge test is used for Von Gierke’s (shows no rise in blood glucose).

Question 529: In the fasting state, from where is glucose primarily obtained?

A. Liver glycogen (Correct Answer)
B. Muscle glycogen
C. Both liver and muscle glycogen
D. None of the above

Explanation: **Explanation:** The primary source of blood glucose during the early fasting state (post-absorptive phase) is **Liver Glycogen**. This is due to the presence of the enzyme **Glucose-6-Phosphatase** in the liver, which removes the phosphate group from Glucose-6-Phosphate, allowing free glucose to be released into the bloodstream to maintain normoglycemia. **Why other options are incorrect:** * **Muscle Glycogen:** While muscles store significant amounts of glycogen, they **lack Glucose-6-Phosphatase**. Consequently, muscle glycogen can only be broken down into Glucose-6-Phosphate to enter glycolysis for local ATP production. It cannot contribute to blood glucose levels. * **Both Liver and Muscle Glycogen:** This is incorrect because of the tissue-specific expression of the phosphatase enzyme mentioned above. Only the liver (and to a minor extent, the kidneys) can export glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Timeframe:** Liver glycogen stores are typically exhausted after **12–18 hours** of fasting. Beyond this point, **Gluconeogenesis** (primarily in the liver) becomes the dominant source of blood glucose. * **Key Enzyme:** Glucose-6-Phosphatase is the "common" enzyme for both Glycogenolysis and Gluconeogenesis. Its deficiency leads to **Von Gierke’s Disease (GSD Type I)**, characterized by severe fasting hypoglycemia. * **Hormonal Control:** Glucagon and Epinephrine stimulate liver glycogenolysis, whereas Insulin inhibits it. Muscle glycogenolysis is primarily stimulated by Epinephrine and Calcium ions during exercise, not by Glucagon.

Question 530: During prolonged fasting, what determines the rate of gluconeogenesis?

A. Essential fatty acid levels in the liver
B. Alanine levels in the liver (Correct Answer)
C. Decreased cGMP levels
D. ADP levels in the liver

Explanation: **Explanation** In the context of prolonged fasting, the rate-limiting step for gluconeogenesis is the **availability of substrate**, specifically glucogenic amino acids. **Why Alanine is the Correct Answer:** Alanine is the primary glucogenic amino acid released from skeletal muscle during fasting (via the **Cahill cycle** or Glucose-Alanine cycle). In the liver, alanine is transaminated to pyruvate, which serves as a direct precursor for glucose synthesis. During prolonged fasting, the liver's capacity to process substrates is high, but the supply of these substrates—primarily alanine—becomes the determining factor for the overall rate of gluconeogenesis. **Analysis of Incorrect Options:** * **A. Essential fatty acid levels:** While fatty acid oxidation provides the necessary **ATP and NADH** to drive gluconeogenesis, essential fatty acids themselves are not the rate-limiting substrates. * **C. Decreased cGMP levels:** Gluconeogenesis is regulated by **cAMP** (via Glucagon), not cGMP. Increased cAMP activates Protein Kinase A, leading to the inhibition of glycolysis and stimulation of gluconeogenic enzymes. * **D. ADP levels:** High ADP levels actually **inhibit** gluconeogenesis (specifically inhibiting Pyruvate Carboxylase and Phosphoenolpyruvate Carboxykinase) because the process is energy-expensive and requires high ATP levels. **High-Yield NEET-PG Pearls:** * **Key Regulatory Enzyme:** Fructose-1,6-bisphosphatase (inhibited by Fructose-2,6-bisphosphate). * **Obligatory Activator:** Acetyl-CoA is an absolute requirement for **Pyruvate Carboxylase** activity. * **Major Substrates:** Lactate (Cori Cycle), Alanine (Cahill Cycle), and Glycerol (from lipolysis). * **Location:** Occurs in both the Mitochondria and Cytosol. The final step (Glucose-6-Phosphatase) occurs in the **Endoplasmic Reticulum**.

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

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Glycogen Storage Diseases

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Pentose Phosphate Pathway

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Metabolism of Fructose and Galactose

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Disorders of Fructose and Galactose Metabolism

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

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Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

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