Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 131: Alcohol causes hypoglycemia due to:

A. Decreased gluconeogenesis (Correct Answer)
B. Increased NADH
C. Decreased lipogenesis
D. Decreased glycogenesis

Explanation: **Explanation:** The primary mechanism behind alcohol-induced hypoglycemia is the **inhibition of gluconeogenesis**. **1. Why Decreased Gluconeogenesis is Correct:** Alcohol is metabolized in the liver by the enzymes *Alcohol Dehydrogenase* and *Aldehyde Dehydrogenase*. Both reactions reduce $NAD^+$ to **NADH**, leading to a high **NADH/NAD+ ratio**. This excess NADH shifts the equilibrium of reversible reactions away from gluconeogenesis: * **Pyruvate → Lactate:** Pyruvate is diverted to lactate, causing lactic acidosis and depriving the liver of a key glucose precursor. * **Oxaloacetate (OAA) → Malate:** OAA is reduced to malate, preventing it from entering the PEP carboxykinase step of gluconeogenesis. * **DHAP → Glycerol-3-Phosphate:** This prevents glycerol from entering the gluconeogenic pathway. **2. Analysis of Other Options:** * **Increased NADH:** While this is the *biochemical cause*, the physiological *result* that directly leads to hypoglycemia is the decreased gluconeogenesis. In NEET-PG, always prioritize the functional outcome (hypoglycemia = lack of glucose production). * **Decreased Lipogenesis:** High NADH actually **increases** lipogenesis (fatty acid synthesis) by providing reducing equivalents, contributing to the "fatty liver" seen in alcoholics. * **Decreased Glycogenesis:** Alcohol does not primarily decrease glycogenesis; however, it causes hypoglycemia specifically once liver glycogen stores are depleted (fasting state). **High-Yield Clinical Pearls for NEET-PG:** * **The "Fasting" Factor:** Alcohol-induced hypoglycemia typically occurs in individuals who have not eaten, as they rely solely on gluconeogenesis once glycogen is exhausted. * **Lactic Acidosis:** The shift of pyruvate to lactate explains why chronic alcoholics often present with metabolic acidosis. * **Treatment Caution:** Always administer **Thiamine** before Glucose in alcoholic patients to prevent Wernicke’s Encephalopathy.

Question 132: Keratan sulphate is found in abundance in which of the following locations?

A. Heart muscle
B. Liver
C. Adrenal cortex
D. Cornea (Correct Answer)

Explanation: **Explanation:** **Keratan Sulphate (KS)** is a unique Glycosaminoglycan (GAG) because it is the only one that does not contain uronic acid (it contains galactose instead). It exists in two forms: KS I and KS II. **Why Cornea is the Correct Answer:** **Keratan Sulphate I** is found predominantly in the **cornea**. Its primary physiological role is to maintain corneal transparency. The specific arrangement and hydration levels of KS within the corneal stroma minimize light scattering, ensuring a clear path for vision. Deficiency or abnormalities in KS metabolism (as seen in Macular Corneal Dystrophy) lead to corneal opacification. **Analysis of Incorrect Options:** * **Heart Muscle:** While the heart valves contain GAGs like Dermatan sulphate and Hyaluronic acid to maintain structural integrity, Keratan sulphate is not a major constituent of the myocardium (heart muscle). * **Liver:** The liver is the site of GAG degradation, but it does not store Keratan sulphate. Heparan sulphate is the more relevant GAG associated with cell surfaces and basement membranes in systemic organs. * **Adrenal Cortex:** This is an endocrine tissue primarily involved in steroidogenesis; it does not contain significant amounts of specialized structural GAGs like Keratan sulphate. **High-Yield NEET-PG Pearls:** * **KS I vs. KS II:** KS I is found in the **cornea**, while KS II is found in **cartilage and bone**. * **Unique Structure:** Remember, KS is the "odd one out" among GAGs—it contains **Galactose** instead of Uronic acid. * **Clinical Correlation:** **Morquio Syndrome (MPS IV)** is characterized by the inability to degrade Keratan sulphate, leading to skeletal deformities and corneal clouding. * **Most Abundant GAG:** Chondroitin sulphate (found in cartilage/bone). * **Only Non-Sulphated GAG:** Hyaluronic acid.

Question 133: During a hunger strike with a liquid diet and minimal calories, which of the following hormonal and metabolic changes would occur after 4 to 5 hours?

A. Decreased cyclic AMP and increased liver glycogen synthesis
B. Increased cyclic AMP and increased liver glycogenolysis (Correct Answer)
C. Decreased epinephrine levels and increased liver glycogenolysis
D. Increased Ca2+ in muscle and decreased glycogenolysis

Explanation: **Explanation:** The core concept here is the body's response to the **post-absorptive state** (early fasting). Approximately 4–5 hours after a meal, blood glucose levels begin to decline. This triggers the pancreas to secrete **glucagon** and the adrenal medulla to release **epinephrine**. **1. Why Option B is Correct:** Glucagon and epinephrine bind to G-protein coupled receptors (GPCRs) on hepatocytes. This activates **adenylyl cyclase**, which converts ATP into **cyclic AMP (cAMP)**. Elevated cAMP activates Protein Kinase A (PKA), which phosphorylates and activates **Glycogen Phosphorylase**. This leads to **liver glycogenolysis**, the primary mechanism for maintaining blood glucose levels during short-term fasting. **2. Why Other Options are Incorrect:** * **Option A:** Decreased cAMP and increased glycogen synthesis occur in the **fed state** under the influence of insulin. * **Option C:** Epinephrine levels **increase** (not decrease) during fasting or stress to stimulate glucose mobilization. * **Option D:** While Ca²⁺ does increase in muscle during contraction to stimulate glycogenolysis, the question focuses on the systemic hormonal response to a "hunger strike" (fasting), where liver glycogenolysis is the priority for blood glucose maintenance. Furthermore, increased Ca²⁺ would **increase**, not decrease, glycogenolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glycogen Phosphorylase is the rate-limiting enzyme for glycogenolysis; it is activated by phosphorylation. * **Liver vs. Muscle:** Liver glycogen maintains **blood glucose**; muscle glycogen is used only for **local energy** (muscle lacks Glucose-6-Phosphatase). * **Timeline:** Liver glycogen stores are typically exhausted after **12–24 hours** of fasting, after which gluconeogenesis becomes the sole source of blood glucose.

Question 134: What is the primary storage form of energy in the liver?

A. Glycogen (Correct Answer)
B. Triacylglycerol
C. Cholesterol ester
D. Protein

Explanation: **Explanation:** The primary storage form of energy in the liver is **Glycogen**. Glycogen is a highly branched polymer of glucose that serves as a readily available source of energy. In the liver, glycogenolysis (the breakdown of glycogen) is crucial for maintaining blood glucose levels during fasting states, as the liver possesses the enzyme **Glucose-6-Phosphatase**, which allows glucose to be released into the bloodstream. **Analysis of Options:** * **A. Glycogen (Correct):** It is the specific carbohydrate storage molecule in animals. The liver stores approximately 75–100g of glycogen (about 10% of liver weight). * **B. Triacylglycerol (TAG):** While TAGs are the body's primary long-term energy reserve, they are predominantly stored in **Adipose Tissue**, not the liver. Accumulation of TAGs in the liver is pathological (Steatosis/Fatty Liver). * **C. Cholesterol ester:** These are structural components of lipoproteins and precursors for bile acids/steroid hormones, but they are not used as an energy fuel. * **D. Protein:** Proteins serve structural and functional roles (enzymes/muscles). While they can be broken down for gluconeogenesis during prolonged starvation, they are never considered a "storage form" of energy. **High-Yield NEET-PG Pearls:** * **Glycogen Synthase** is the rate-limiting enzyme for glycogenesis. * **Von Gierke’s Disease (GSD Type I):** Deficiency of Glucose-6-Phosphatase leads to massive hepatomegaly and severe fasting hypoglycemia. * **Muscle vs. Liver:** Muscle glycogen is used only for local muscular contraction (lacks Glucose-6-Phosphatase), whereas liver glycogen maintains systemic blood glucose. * Liver glycogen is typically depleted after **12–18 hours** of fasting.

Question 135: Which enzyme catalyzes the irreversible step in the citric acid cycle?

A. Aconitase
B. Succinate thiokinase
C. α-ketoglutarate dehydrogenase (Correct Answer)
D. Isocitrate dehydrogenase

Explanation: ### Explanation The Citric Acid Cycle (TCA cycle) is regulated primarily at its **irreversible steps**, which have a large negative Gibbs free energy ($\Delta G$). **Why α-ketoglutarate dehydrogenase is correct:** The conversion of α-ketoglutarate to Succinyl-CoA by the **α-ketoglutarate dehydrogenase complex** is a key regulatory, irreversible oxidative decarboxylation step. This multienzyme complex requires five cofactors (Thiamine, Lipoic acid, CoA, FAD, and NAD+). It is inhibited by its products (Succinyl-CoA and NADH) and activated by $Ca^{2+}$. **Analysis of Incorrect Options:** * **Aconitase:** Catalyzes the reversible isomerization of Citrate to Isocitrate via cis-aconitate. * **Succinate thiokinase (Succinyl-CoA Synthetase):** Catalyzes the reversible conversion of Succinyl-CoA to Succinate. This step is unique as it involves **substrate-level phosphorylation** (generating GTP/ATP). * **Isocitrate dehydrogenase:** While this is the **rate-limiting step** and is physiologically irreversible, in the context of standard NEET-PG questions, α-ketoglutarate dehydrogenase and Citrate Synthase are the classic examples of committed, highly exergonic irreversible steps. (Note: If both are present, Isocitrate dehydrogenase is the rate-limiting enzyme, but α-ketoglutarate dehydrogenase is the most "irreversible" in terms of equilibrium). **High-Yield Clinical Pearls for NEET-PG:** 1. **Three Irreversible Steps:** Citrate Synthase, Isocitrate Dehydrogenase, and α-ketoglutarate Dehydrogenase. 2. **Cofactor Dependency:** α-ketoglutarate dehydrogenase requires the same five cofactors as the Pyruvate Dehydrogenase (PDH) complex. **Thiamine deficiency** (B1) inhibits these enzymes, leading to impaired ATP production and Wernicke-Korsakoff syndrome. 3. **Arsenic Poisoning:** Arsenite inhibits α-ketoglutarate dehydrogenase by binding to the -SH groups of **Lipoic acid**, leading to a buildup of upstream metabolites.

Question 136: Which of the following enzymes is a constituent of the HMP shunt?

A. Glucose-6-phosphatase
B. Hexokinase
C. G-6-P dehydrogenase (Correct Answer)
D. Phosphorylase

Explanation: ### Explanation **Correct Answer: C. G-6-P dehydrogenase (G6PD)** The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), occurs in the cytosol. **Glucose-6-phosphate dehydrogenase (G6PD)** is the **rate-limiting and committed enzyme** of the oxidative phase of this pathway. It catalyzes the conversion of Glucose-6-phosphate to 6-phosphogluconolactone, simultaneously reducing NADP+ to **NADPH**. This pathway is crucial for generating NADPH (used in reductive biosynthesis and maintaining reduced glutathione) and Ribose-5-phosphate (for nucleotide synthesis). **Why the other options are incorrect:** * **A. Glucose-6-phosphatase:** This enzyme is involved in **Gluconeogenesis** and **Glycogenolysis**. It converts Glucose-6-phosphate back to free Glucose, primarily in the liver and kidneys. * **B. Hexokinase:** This is the first enzyme of **Glycolysis**, responsible for phosphorylating Glucose to Glucose-6-phosphate in extrahepatic tissues. * **D. Phosphorylase (Glycogen Phosphorylase):** This is the rate-limiting enzyme of **Glycogenolysis**, which breaks down glycogen into Glucose-1-phosphate. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to **hemolytic anemia** under oxidative stress (e.g., Fava beans, Primaquine, Infection) because RBCs cannot generate enough NADPH to maintain reduced glutathione, leading to **Heinz bodies** and **Bite cells**. * **Tissue Distribution:** The HMP shunt is highly active in tissues requiring NADPH for fatty acid or steroid synthesis (Adrenal cortex, Liver, Lactating mammary glands) and in RBCs to combat oxidative stress. * **Non-oxidative Phase:** Uses **Transketolase**, which requires **Thiamine (Vitamin B1)** as a cofactor—a common point of integration with nutrition questions.

Question 137: Which of the following glycosaminoglycans plays a significant role in wound healing?

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

Explanation: ### Explanation **Correct Answer: C. Hyaluronic acid** **Why Hyaluronic acid is correct:** Hyaluronic acid (HA) is a unique glycosaminoglycan (GAG) because it is non-sulfated and not bound to a protein core. It plays a pivotal role in **wound healing** by creating a highly hydrated, porous extracellular matrix. During the early inflammatory phase, HA levels rise, facilitating the migration and proliferation of fibroblasts and keratinocytes. Its "molecular sieve" property allows for the diffusion of nutrients and signaling molecules, which is essential for tissue repair and remodeling. **Why the other options are incorrect:** * **A. Keratan sulfate:** Found primarily in the **cornea** (maintaining transparency) and cartilage. It does not play a primary role in the dynamic process of wound repair. * **B. Dermatan sulfate:** Predominantly found in the **skin, blood vessels, and heart valves**. While it is present in the skin, its primary function relates to structural integrity and anticoagulation (via interaction with Heparin Cofactor II) rather than the active healing phase. * **C. Chondroitin sulfate:** The most abundant GAG in the body, found in **cartilage and bone**. It provides tensile strength and resistance to compression but is not the primary mediator of wound healing. **High-Yield NEET-PG Pearls:** * **Hyaluronic Acid:** Only GAG that is **not sulfated**, not covalently linked to protein (not a proteoglycan), and contains **Glucuronic acid + N-acetylglucosamine**. * **Dermatan Sulfate:** Associated with **Hurler Syndrome** (accumulation due to alpha-L-iduronidase deficiency). * **Heparin:** The only GAG that is **intracellular** (found in mast cells) and acts as a potent natural anticoagulant. * **Chondroitin Sulfate:** Most abundant GAG in the human body.

Question 138: Which of the following are products formed in the glycolytic pathway?

A. Fructose 2,6 biphosphate
B. Fructose 1,6 biphosphate
C. Glyceraldehyde-3-phosphate
D. All of the above (Correct Answer)

Explanation: **Explanation:** The glycolytic pathway (Embden-Meyerhof pathway) is the primary sequence of reactions for glucose oxidation. The correct answer is **All of the above** because each molecule mentioned plays a critical role in the pathway, either as a direct intermediate or a key regulator. 1. **Fructose 1,6-bisphosphate (F1,6-BP):** This is a direct intermediate formed in the third step of glycolysis. The enzyme **Phosphofructokinase-1 (PFK-1)** phosphorylates Fructose 6-phosphate to F1,6-BP. This is the "committed step" and the rate-limiting step of glycolysis. 2. **Glyceraldehyde-3-phosphate (G3P):** This is a 3-carbon intermediate formed when F1,6-BP is cleaved by the enzyme **Aldolase A**. It is the substrate for the first energy-yielding step of glycolysis (catalyzed by G3P Dehydrogenase). 3. **Fructose 2,6-bisphosphate (F2,6-BP):** While not a direct intermediate that breaks down into pyruvate, it is a **by-product** formed from Fructose 6-phosphate by the enzyme **PFK-2**. It is the most potent allosteric activator of PFK-1. In the context of "products formed in the pathway," it is synthesized within the metabolic milieu of glycolysis to regulate its flux. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** PFK-1 (inhibited by ATP and Citrate; activated by AMP and F2,6-BP). * **Rapoport-Luebering Cycle:** In RBCs, the intermediate 1,3-BPG can be converted to **2,3-BPG**, which decreases hemoglobin's affinity for oxygen (shifts dissociation curve to the right). * **Arsenic Poisoning:** Arsenate competes with inorganic phosphate in the G3P Dehydrogenase reaction, resulting in zero net ATP production during glycolysis. * **Essential Fructosuria:** Caused by Fructokinase deficiency; it is a benign condition compared to Hereditary Fructose Intolerance (Aldolase B deficiency).

Question 139: Which of the following statements regarding the TCA cycle is true?

A. It is an anaerobic process.
B. It occurs in the cytosol.
C. It contains no intermediates for gluconeogenesis.
D. It is amphibolic in nature. (Correct Answer)

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The TCA cycle (Krebs cycle) is described as **amphibolic** because it plays a dual role in metabolism: * **Catabolic role:** It involves the oxidation of Acetyl-CoA into $CO_2$ and $H_2O$ to generate energy in the form of ATP, NADH, and $FADH_2$. * **Anabolic role:** Its intermediates serve as precursors for various biosynthetic pathways. For example, **Succinyl-CoA** is used for heme synthesis, and **$\alpha$-ketoglutarate** and **Oxaloacetate** are used for the synthesis of non-essential amino acids. **2. Why the Other Options are Incorrect:** * **Option A:** The TCA cycle is strictly **aerobic**. While oxygen isn't used directly in the cycle, it is required to regenerate $NAD^+$ and $FAD$ via the Electron Transport Chain (ETC). Without oxygen, the cycle halts. * **Option B:** The TCA cycle occurs in the **mitochondrial matrix**. The only exception is the enzyme *Succinate Dehydrogenase*, which is located on the inner mitochondrial membrane. Glycolysis occurs in the cytosol. * **Option C:** The TCA cycle provides several intermediates for gluconeogenesis. Most notably, **Oxaloacetate** is a direct precursor for phosphoenolpyruvate (PEP) synthesis. **3. NEET-PG High-Yield Clinical Pearls:** * **Rate-limiting enzyme:** Isocitrate Dehydrogenase. * **Substrate-level phosphorylation:** Occurs at the step where Succinyl-CoA is converted to Succinate (catalyzed by Succinate thiokinase), producing **GTP**. * **Inhibitors to remember:** **Fluoroacetate** (inhibits Aconitase), **Arsenite** (inhibits $\alpha$-ketoglutarate dehydrogenase), and **Malonate** (competitive inhibitor of Succinate dehydrogenase). * **Vitamins required:** The $\alpha$-ketoglutarate dehydrogenase complex requires five cofactors: Thiamine ($B_1$), Riboflavin ($B_2$), Niacin ($B_3$), Pantothenic acid ($B_5$), and Lipoic acid.

Question 140: The malate shuttle is important in which of the following processes?

A. Glycogenolysis
B. Glycolysis (Correct Answer)
C. Gluconeogenesis
D. HMP shunt

Explanation: ### Explanation **Correct Option: B. Glycolysis** The Malate-Aspartate shuttle is essential for aerobic **glycolysis**. During glycolysis in the cytosol, the enzyme Glyceraldehyde-3-phosphate dehydrogenase reduces $NAD^+$ to **NADH**. Since the inner mitochondrial membrane is impermeable to NADH, the malate shuttle "transports" these reducing equivalents into the mitochondria. Once inside, NADH enters the Electron Transport Chain (Complex I), yielding approximately **2.5 ATP** per molecule. This process ensures a continuous supply of cytosolic $NAD^+$ to keep glycolysis functioning. **Analysis of Incorrect Options:** * **A. Glycogenolysis:** This is the breakdown of glycogen into glucose-1-phosphate; it does not primarily rely on mitochondrial shuttles for redox balance. * **C. Gluconeogenesis:** While the malate shuttle is involved in transporting oxaloacetate out of the mitochondria (as malate), the question asks for the process where the shuttle is *functionally critical for ATP yield and redox recycling* of glycolytic NADH. (Note: In some contexts, it supports gluconeogenesis, but its primary textbook association for "shuttling reducing equivalents" is glycolysis). * **D. HMP Shunt:** This pathway occurs entirely in the cytosol and produces **NADPH**, not NADH. It does not require mitochondrial shuttles. **High-Yield Clinical Pearls for NEET-PG:** * **Two Shuttles:** The **Malate-Aspartate shuttle** (Heart, Liver, Kidney) yields **32 ATP** per glucose, while the **Glycerol-3-Phosphate shuttle** (Muscle, Brain) yields **30 ATP** (via $FADH_2$). * **Irreversibility:** The Malate shuttle is reversible, whereas the Glycerol-3-Phosphate shuttle is irreversible. * **Key Enzyme:** Malate dehydrogenase is the primary enzyme involved in both the cytosol and mitochondria.

Practice by Chapter

Carbohydrate Chemistry and Classification

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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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