Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 341: Galactosemia is due to the deficiency of which enzyme?

A. Galactose-1-Phosphatase
B. Glucose-1-Phosphatase
C. Galactose-1-Phosphate uridyl transferase (Correct Answer)
D. Glucose-6-Phosphatase

Explanation: **Explanation:** **Galactosemia** refers to a group of inherited disorders of galactose metabolism. The most common and severe form, known as **Classic Galactosemia (Type 1)**, is caused by a deficiency of the enzyme **Galactose-1-Phosphate Uridyl Transferase (GALT)**. In the normal Leloir pathway, GALT is responsible for converting Galactose-1-phosphate and UDP-glucose into UDP-galactose and Glucose-1-phosphate. When GALT is deficient, **Galactose-1-phosphate** and **galactitol** (produced via the polyol pathway) accumulate in tissues like the liver, brain, and lens of the eye, leading to organ damage. **Analysis of Incorrect Options:** * **A & B (Galactose/Glucose-1-Phosphatase):** These enzymes are not part of the standard Leloir pathway for galactose metabolism. * **D (Glucose-6-Phosphatase):** Deficiency of this enzyme causes **Von Gierke Disease (Glycogen Storage Disease Type I)**, characterized by severe hypoglycemia and hepatomegaly, not galactosemia. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Cataracts, Hepatomegaly (jaundice/cirrhosis), and Intellectual disability. * **Early Sign:** Infantile cataracts due to the accumulation of **galactitol** in the lens. * **Sepsis Risk:** Infants with classic galactosemia are at a significantly increased risk of **E. coli neonatal sepsis**. * **Diagnosis:** Suggested by the presence of **reducing sugars in urine** (Benedict’s test positive) but a negative glucose oxidase test (dipstick). * **Management:** Immediate and lifelong elimination of lactose and galactose from the diet (e.g., soy-based formula instead of breast milk).

Question 342: Diabetic cataract occurs due to the accumulation of which substance?

A. Glucose
B. Fructose
C. Galactitol
D. Sorbitol (Correct Answer)

Explanation: **Explanation:** The development of diabetic cataracts is primarily attributed to the **Polyol Pathway** (Sorbitol pathway). In states of chronic hyperglycemia, the hexokinase enzyme becomes saturated, and excess glucose is shunted into the polyol pathway. 1. **Mechanism (Why Sorbitol is correct):** The enzyme **Aldose Reductase** reduces glucose into **Sorbitol** using NADPH as a cofactor. In tissues like the lens, retina, and peripheral nerves, the subsequent enzyme (Sorbitol Dehydrogenase) is either absent or has very low activity. Consequently, sorbitol accumulates intracellularly. Because sorbitol is a sugar alcohol and is **osmotically active**, it draws water into the lens fibers, leading to swelling, lens opacification, and eventually, a cataract. **Analysis of Incorrect Options:** * **A. Glucose:** While high glucose triggers the process, it is the metabolic byproduct (sorbitol) that causes the osmotic damage, not the glucose molecule itself. * **B. Fructose:** Sorbitol is normally converted to fructose by sorbitol dehydrogenase. Fructose does not accumulate significantly in the lens and is not the primary osmotic agent in this context. * **C. Galactitol:** This is the osmotic byproduct of **galactose** (via aldose reductase). It causes cataracts in patients with **Galactosemia**, not Diabetes Mellitus. **High-Yield NEET-PG Pearls:** * **"Sorbitol is Slow":** Tissues with low Sorbitol Dehydrogenase (Lens, Retina, Kidney, Schwann cells) are most prone to diabetic complications (**L**ens, **R**etina, **K**idney = **LRK**). * **Enzyme involved:** Aldose Reductase (requires NADPH). * **Cataract Type:** Diabetic cataracts are typically described as **"Snowflake cataracts."** * **Galactosemia Connection:** Galactitol accumulation causes cataracts much faster than sorbitol.

Question 343: Anaerobic glycolysis occurs in all of the following tissues except:

A. RBCs
B. Lens
C. Brain (Correct Answer)
D. Testis

Explanation: ### Explanation The correct answer is **Brain (Option C)**. **1. Why Brain is the correct answer:** While the brain is a major consumer of glucose, it relies almost exclusively on **aerobic metabolism**. Brain cells (neurons) have a very high density of mitochondria and require a constant, high-yield supply of ATP to maintain membrane potentials. Anaerobic glycolysis (converting glucose to lactate) yields only 2 ATP per glucose molecule, which is insufficient to meet the brain's metabolic demands. Therefore, the brain performs complete oxidation of glucose via the TCA cycle and Oxidative Phosphorylation to generate 30-32 ATP. **2. Why the other options are incorrect:** * **RBCs (Option A):** Mature erythrocytes lack **mitochondria**. Therefore, they are obligate anaerobic glycolyzers, as they cannot perform the TCA cycle or electron transport chain. * **Lens (Option B):** The lens of the eye is largely **avascular** to maintain transparency. Due to low oxygen tension, it relies primarily on anaerobic glycolysis for energy. * **Testis (Option D):** Tissues like the testis, renal medulla, and leucocytes have relatively low oxygen supply or specific metabolic adaptations that make anaerobic glycolysis a significant pathway for their energy needs. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **End product:** The end product of anaerobic glycolysis is **Lactate** (via Lactate Dehydrogenase), whereas aerobic glycolysis ends in **Pyruvate**. * **RBC Metabolism:** Since RBCs only perform anaerobic glycolysis, they are the body's primary producers of lactate. They also utilize the **Rapoport-Luebering cycle** to produce 2,3-BPG. * **Brain Fuel:** In prolonged starvation, the brain can adapt to use **ketone bodies** (specifically acetoacetate and β-hydroxybutyrate), but it never switches to purely anaerobic metabolism. * **Warburg Effect:** Remember that cancer cells often prefer anaerobic glycolysis even in the presence of oxygen (aerobic glycolysis).

Question 344: Seliwanoff's test is positive in which of the following?

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

Explanation: **Explanation:** Seliwanoff’s test is a biochemical color reaction used to distinguish between **aldoses** (sugars with an aldehyde group) and **ketoses** (sugars with a ketone group). **Why Fructose is Correct:** Fructose is a **ketohexose**. The test involves heating the sugar with Seliwanoff’s reagent, which contains resorcinol and concentrated hydrochloric acid (HCl). The acid dehydrates ketoses much faster than aldoses to form **5-hydroxymethylfurfural**. This compound then reacts with resorcinol to produce a characteristic **cherry-red (fiery red) precipitate**. Because fructose is a ketose, it gives a rapid positive result. **Why Other Options are Incorrect:** * **Glucose, Galactose, and Mannose:** These are all **aldoses**. While aldoses can eventually react to produce a faint pink color if heated for a prolonged period (due to slow conversion to ketoses), they do not produce the rapid cherry-red color characteristic of a positive Seliwanoff’s test. **High-Yield NEET-PG Clinical Pearls:** * **Specific for Ketoses:** Seliwanoff’s test is the gold standard for identifying fructose and sucrose (since sucrose hydrolyzes into glucose and fructose). * **Bial’s Test:** Often confused with Seliwanoff’s, Bial’s test is used to detect **Pentoses** (like ribose), yielding a blue-green color. * **Clinical Correlation:** Fructose metabolism is clinically significant in **Hereditary Fructose Intolerance** (deficiency of Aldolase B) and **Essential Fructosuria** (deficiency of Fructokinase). * **Semen Analysis:** Seliwanoff’s test is used to detect fructose in semen; its absence indicates an obstruction or absence of the seminal vesicles.

Question 345: Pyruvate can be a substrate for which of the following processes?

A. Fatty acid synthesis
B. TCA cycle
C. Cholesterol synthesis
D. All of the above (Correct Answer)

Explanation: **Explanation:** Pyruvate serves as a critical metabolic hub, acting as the bridge between glycolysis and several other metabolic pathways. The correct answer is **All of the above** because pyruvate is the primary precursor for **Acetyl-CoA**, which is the fundamental building block for the TCA cycle, fatty acid synthesis, and cholesterol synthesis. 1. **TCA Cycle:** Under aerobic conditions, pyruvate enters the mitochondria and is oxidatively decarboxylated by the **Pyruvate Dehydrogenase (PDH) complex** to form Acetyl-CoA. This Acetyl-CoA then condenses with oxaloacetate to enter the TCA cycle for ATP production. 2. **Fatty Acid Synthesis:** When energy levels are high, Acetyl-CoA (derived from pyruvate) is transported out of the mitochondria into the cytosol (via the Citrate-Malate shuttle). In the cytosol, it serves as the substrate for *de novo* lipogenesis. 3. **Cholesterol Synthesis:** Acetyl-CoA is also the starting material for the mevalonate pathway, which leads to the synthesis of cholesterol. **Why individual options are correct (and thus "All of the above" is the choice):** * **A & C:** Both fatty acids and cholesterol are synthesized from Acetyl-CoA units derived from pyruvate. * **B:** The TCA cycle is the immediate oxidative destination for pyruvate-derived Acetyl-CoA. **High-Yield Clinical Pearls for NEET-PG:** * **PDH Complex:** This is a multi-enzyme complex requiring five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (Mnemonic: **T**ender **R**oving **N**ights **P**lease **L**uck). * **Anaplerotic Reaction:** Pyruvate can also be converted directly to oxaloacetate by **Pyruvate Carboxylase** (requires Biotin), which is the first step of gluconeogenesis. * **Lactate Link:** In anaerobic conditions, pyruvate is reduced to lactate by Lactate Dehydrogenase (LDH), regenerating NAD+ for glycolysis.

Question 346: What is the substrate for the only physiologically irreversible reaction in the Kreb's Citric acid cycle?

A. Citrate
B. Alpha ketoglutarate (Correct Answer)
C. Succinate
D. Malate

Explanation: ### Explanation In the Citric Acid Cycle (TCA), there are three reactions characterized by a large negative Gibbs free energy change ($\Delta G$), making them functionally irreversible under physiological conditions: 1. **Citrate Synthase** (Oxaloacetate + Acetyl-CoA → Citrate) 2. **Isocitrate Dehydrogenase** (Isocitrate → $\alpha$-Ketoglutarate) 3. **$\alpha$-Ketoglutarate Dehydrogenase Complex** ($\alpha$-KG → Succinyl-CoA) However, the conversion of **$\alpha$-Ketoglutarate** to Succinyl-CoA is considered the **only physiologically irreversible** step in the sense that it is the "point of no return." This reaction involves oxidative decarboxylation and is heavily regulated. Unlike the other two steps, which can be bypassed or have alternative metabolic fates, this step is the primary rate-limiting commitment to the final oxidation phase of the cycle. #### Analysis of Options: * **A. Citrate:** Citrate is the product of the first irreversible reaction, but the reaction itself is technically reversible under specific non-physiological concentrations (though not in the TCA cycle). * **B. $\alpha$-Ketoglutarate (Correct):** The $\alpha$-Ketoglutarate Dehydrogenase reaction is highly exergonic and irreversible. It requires five cofactors (Thiamine, Lipoic acid, CoA, FAD, NAD+) and is the key regulatory site. * **C. Succinate:** The conversion of Succinate to Fumarate (via Succinate Dehydrogenase) is a reversible reaction. * **D. Malate:** The conversion of Malate to Oxaloacetate (via Malate Dehydrogenase) is reversible and actually has a positive $\Delta G$ under standard conditions; it proceeds forward only because oxaloacetate is rapidly consumed. #### NEET-PG High-Yield Pearls: * **Rate-limiting enzyme:** Isocitrate Dehydrogenase is the overall rate-limiting enzyme of the TCA cycle. * **Cofactor Requirement:** $\alpha$-KG Dehydrogenase requires the same five cofactors as Pyruvate Dehydrogenase (**T**ender **L**oving **C**are **F**or **N**o-one: **T**PP, **L**ipoate, **C**oA, **F**AD, **N**AD). * **Inhibition:** This step is inhibited by high levels of Arsenite, which binds to the -SH groups of lipoic acid.

Question 347: In glycogen metabolism, a metabolically active enzyme found in the liver is converted from its inactive dephosphorylated state to its active phosphorylated state. Which of the following is true about this enzyme?

A. Phosphorylation sometimes activates the enzyme.
B. Catecholamines directly stimulate it.
C. More commonly seen in the fasting state than in the fed state. (Correct Answer)
D. Always activated by cAMP-dependent protein kinase.

Explanation: ### Explanation The enzyme described is **Glycogen Phosphorylase**, the rate-limiting enzyme of glycogenolysis. In the liver, this enzyme is activated via phosphorylation (converting the inactive 'b' form to the active 'a' form) to maintain blood glucose levels. **1. Why Option C is Correct:** In the **fasting state**, the insulin-to-glucagon ratio decreases. Glucagon triggers a signaling cascade that leads to the phosphorylation of Glycogen Phosphorylase. This ensures the breakdown of liver glycogen into glucose to prevent hypoglycemia. Therefore, the active phosphorylated state is a hallmark of the fasting (post-absorptive) state. **2. Analysis of Incorrect Options:** * **Option A:** In glycogen metabolism, phosphorylation **always** activates Glycogen Phosphorylase and **always** inactivates Glycogen Synthase. The statement "sometimes" is too vague for this specific regulatory mechanism. * **Option B:** Catecholamines (Epinephrine) do not *directly* stimulate the enzyme. They act via G-protein coupled receptors (GPCRs) to increase cAMP, which activates Protein Kinase A (PKA), which then activates Phosphorylase Kinase, which finally phosphorylates Glycogen Phosphorylase. * **Option D:** Glycogen Phosphorylase is directly phosphorylated by **Phosphorylase Kinase**, not cAMP-dependent protein kinase (PKA). PKA acts one step upstream in the cascade. Additionally, in muscle, it can be activated by Calcium and AMP without phosphorylation. **High-Yield Clinical Pearls for NEET-PG:** * **Reciprocal Regulation:** Phosphorylation acts as a molecular switch—it turns **ON** catabolic pathways (Glycogenolysis) and turns **OFF** anabolic pathways (Glycogenesis). * **Von Gierke’s Disease (Type I GSD):** Deficiency of Glucose-6-Phosphatase; liver can break down glycogen to G6P, but cannot release free glucose into the blood. * **McArdle Disease (Type V GSD):** Deficiency of **Muscle** Glycogen Phosphorylase; presents with exercise-induced cramps and myoglobinuria, but normal blood glucose.

Question 348: Which of the following is required to bring about gluconeogenesis from pyruvate?

A. Pyruvate dehydrogenase
B. Biotin (Correct Answer)
C. Alpha ketoglutarate dehydrogenase
D. Fructose-6 phosphate

Explanation: **Explanation:** The conversion of pyruvate to glucose (gluconeogenesis) begins with the bypass of the irreversible glycolytic step catalyzed by pyruvate kinase. This requires the enzyme **Pyruvate Carboxylase**, which converts pyruvate into oxaloacetate (OAA) in the mitochondria. **Why Biotin is the correct answer:** Pyruvate Carboxylase is a ligase that requires **Biotin (Vitamin B7)** as a co-enzyme. Biotin acts as a carrier of activated carbon dioxide ($CO_2$). The reaction occurs in two stages: first, the carboxylation of biotin (ATP-dependent), and second, the transfer of the carboxyl group to pyruvate to form oxaloacetate. Without biotin, this first committed step of gluconeogenesis cannot occur. **Analysis of Incorrect Options:** * **Pyruvate Dehydrogenase (PDH):** This enzyme complex converts pyruvate to Acetyl-CoA. This is a "committed step" toward the TCA cycle and energy production, effectively moving away from gluconeogenesis. * **Alpha-ketoglutarate Dehydrogenase:** This is an enzyme of the TCA cycle. While it also requires several cofactors (like Thiamine and Lipoic acid), it is not involved in the gluconeogenic pathway. * **Fructose-6 Phosphate:** This is an intermediate of glycolysis/gluconeogenesis, but it is a product/substrate of the reaction, not a requirement for the initiation of the pathway from pyruvate. **High-Yield Clinical Pearls for NEET-PG:** * **All Carboxylases** (Pyruvate carboxylase, Acetyl-CoA carboxylase, Propionyl-CoA carboxylase) require **Biotin** (The "ABC" rule: ATP, Biotin, $CO_2$). * **Acetyl-CoA** is an obligatory allosteric **activator** of Pyruvate Carboxylase. * **Avidin**, a protein in raw egg whites, binds biotin tightly and can induce deficiency, leading to impaired gluconeogenesis.

Question 349: Which enzyme catalyzes substrate-level phosphorylation in the Krebs cycle?

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

Explanation: **Explanation:** **1. Why Succinate Thiokinase is Correct:** Substrate-level phosphorylation (SLP) is the direct synthesis of ATP (or GTP) from ADP (or GDP) without the involvement of the electron transport chain. In the Krebs cycle, this occurs during the conversion of **Succinyl-CoA to Succinate**. This reaction is catalyzed by **Succinate thiokinase** (also known as Succinyl-CoA synthetase). The high-energy thioester bond of Succinyl-CoA is cleaved, and the energy released is used to phosphorylate GDP to GTP (which is later converted to ATP). This is the **only** step in the Citric Acid Cycle that generates high-energy phosphate directly. **2. Why the Other Options are Incorrect:** * **Alpha-ketoglutarate dehydrogenase:** This enzyme catalyzes the oxidative decarboxylation of $\alpha$-ketoglutarate to Succinyl-CoA, producing NADH, not ATP/GTP. * **Succinate dehydrogenase:** This enzyme converts Succinate to Fumarate. It is unique because it is the only Krebs cycle enzyme embedded in the inner mitochondrial membrane (Complex II of ETC) and produces $FADH_2$. * **Isocitrate dehydrogenase:** This is the rate-limiting enzyme of the Krebs cycle. It catalyzes the oxidative decarboxylation of Isocitrate to $\alpha$-ketoglutarate, producing $CO_2$ and NADH. **3. NEET-PG High-Yield Pearls:** * **Total SLP sites:** In aerobic glycolysis (1 glucose molecule), there are **3 sites** of SLP: two in Glycolysis (Phosphoglycerate kinase and Pyruvate kinase) and one in the Krebs cycle (Succinate thiokinase). * **Energy Yield:** One turn of the Krebs cycle produces **10 ATP** equivalents (3 NADH = 7.5, 1 $FADH_2$ = 1.5, 1 GTP = 1). * **Arsenite Inhibition:** Alpha-ketoglutarate dehydrogenase (like Pyruvate Dehydrogenase) is inhibited by Arsenite.

Question 350: Which of the following molecules can participate in gluconeogenesis?

A. Acetyl CoA
B. Propionyl CoA (Correct Answer)
C. Muscle glycogen
D. All of the above

Explanation: **Explanation:** Gluconeogenesis is the metabolic pathway that results in the generation of glucose from non-carbohydrate substrates. **Why Propionyl CoA is Correct:** Propionyl CoA is produced during the oxidation of **odd-chain fatty acids** and the catabolism of certain amino acids (Valine, Isoleucine, Methionine, Threonine). It is converted into **Succinyl CoA** (a TCA cycle intermediate) via a Vitamin B12-dependent pathway. Succinyl CoA eventually forms Oxaloacetate, which enters the gluconeogenic pathway. This makes Propionyl CoA the only part of a fatty acid that can contribute to a net gain of glucose. **Why Other Options are Incorrect:** * **Acetyl CoA:** In humans, the Pyruvate Dehydrogenase (PDH) reaction is irreversible. Acetyl CoA cannot be converted back to Pyruvate. Furthermore, for every two carbons of Acetyl CoA entering the TCA cycle, two are lost as $CO_2$; thus, there is no net synthesis of glucose from Acetyl CoA. * **Muscle Glycogen:** While muscle glycogen breaks down into Glucose-1-Phosphate, it cannot contribute to blood glucose levels because muscle lacks the enzyme **Glucose-6-Phosphatase**. It is used exclusively for local energy production via glycolysis. **High-Yield NEET-PG Pearls:** * **Key Substrates:** Lactate (Cori Cycle), Glycerol (from TAGs), Glucogenic amino acids (mainly Alanine), and Propionate. * **Rate-limiting enzyme:** Fructose-1,6-bisphosphatase. * **Clinical Link:** Deficiency of **Propionyl-CoA carboxylase** (requires Biotin) leads to Propionic Acidemia, presenting with vomiting, ketosis, and developmental delay. * **Odd vs. Even:** Even-chain fatty acids are **never** gluconeogenic; only odd-chain fatty acids are.

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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