Carbohydrate Metabolism — MCQs
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Which of the following is a fructosan?
Which among the following enzymes is an allosteric inhibitor of the TCA cycle?
A 3-year-old girl presents with a history of fussy eating since weaning, particularly when fruit is part of her diet. She becomes cranky, sweats, and displays dizziness and lethargy after consuming meals containing fruit. Her mother has observed that these symptoms resolve when fruit is excluded from her diet. What is the most likely underlying cause of the problems the girl exhibits when eating fruit?
What is the first step in the liver's metabolism of fructose?
The hexose monophosphate shunt occurs in all of the following except?
Hyaluronic acid is found in which of the following locations?
Which metabolic pathway can utilize propionic acid?
Two sisters are diagnosed with hemolytic anemia. Their older brother was previously diagnosed with the same disorder. Two other brothers are asymptomatic. The mother and father are second cousins. Deficiency of which of the following enzymes would be most likely to cause this disorder?
What is the rate-limiting step of glycogenolysis?
A newborn presents with severe acidosis, vomiting, hypotonia, and neurologic deficits. Serum analysis reveals elevated levels of lactate and alanine. These observations suggest a deficiency in which of the following enzymes?
Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs
Question 201: Which of the following is a fructosan?
- A. Pectin
- B. Inulin (Correct Answer)
- C. Chitin
- D. Glycogen
Explanation: **Explanation:** **Inulin** is the correct answer because it is a **fructosan** (or fructan), a homopolysaccharide composed of repeating units of D-fructose linked by **β(2→1) glycosidic bonds**. It is found in the tubers and roots of plants like chicory, dahlias, and dandelions. **Analysis of Options:** * **Pectin (Option A):** A complex heteropolysaccharide primarily composed of galacturonic acid units. It is found in the cell walls of fruits (e.g., apples) and is used as a gelling agent. * **Chitin (Option C):** A structural homopolysaccharide found in the exoskeleton of arthropods and fungal cell walls. It consists of **N-acetyl-D-glucosamine** units linked by β(1→4) bonds. * **Glycogen (Option D):** The primary storage homopolysaccharide in animals. It is a polymer of **D-glucose** with α(1→4) linkages in the chains and α(1→6) linkages at branch points. **High-Yield Clinical Pearls for NEET-PG:** 1. **Renal Physiology:** Inulin is the "gold standard" for measuring **Glomerular Filtration Rate (GFR)** because it is freely filtered by the glomeruli but is neither secreted nor reabsorbed by the renal tubules. 2. **Solubility:** Unlike starch, inulin is readily soluble in warm water. 3. **Diagnostic Use:** While inulin is the most accurate, **Creatinine clearance** is more commonly used in clinical practice to estimate GFR as it is endogenous and does not require an intravenous infusion. 4. **Dietary Fiber:** Inulin is not digested by human enzymes and acts as a prebiotic, promoting the growth of beneficial gut bacteria.
Question 202: Which among the following enzymes is an allosteric inhibitor of the TCA cycle?
- A. Pyruvate dehydrogenase
- B. Alpha-ketoglutarate dehydrogenase
- C. Isocitrate dehydrogenase (Correct Answer)
- D. Malate dehydrogenase
Explanation: **Explanation:** The TCA (Tricarboxylic Acid) cycle is the central metabolic pathway for energy production. Its regulation is primarily governed by the energy status of the cell, signaled by the ratios of ATP/ADP and NADH/NAD+. **Why Isocitrate Dehydrogenase (ICDH) is the correct answer:** ICDH catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate. It is considered the **rate-limiting step** of the TCA cycle. It is strictly regulated by **allosteric effectors**: * **Allosteric Inhibitors:** **ATP** and **NADH** (signaling high energy stores). * **Allosteric Activators:** **ADP** and **Ca²⁺** (signaling energy demand). When ATP levels are high, ICDH is inhibited, leading to an accumulation of Citrate, which then leaves the mitochondria to inhibit glycolysis (via PFK-1) and stimulate fatty acid synthesis. **Analysis of Incorrect Options:** * **A. Pyruvate Dehydrogenase (PDH):** While PDH is inhibited by ATP and NADH, it is **not** part of the TCA cycle itself. It is a multienzyme complex that links glycolysis to the TCA cycle (the "Link Reaction"). * **B. Alpha-ketoglutarate Dehydrogenase:** This enzyme is inhibited by its products (**Succinyl-CoA** and **NADH**) and ATP. However, it is not the primary rate-limiting allosteric valve compared to ICDH. * **D. Malate Dehydrogenase:** This reaction is primarily regulated by the concentration of substrates and the NADH/NAD+ ratio (mass action kinetics) rather than complex allosteric modulation. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of TCA:** Isocitrate Dehydrogenase. * **CO₂ Release:** Occurs at the steps catalyzed by ICDH and Alpha-ketoglutarate dehydrogenase. * **Fluoroacetate:** A potent inhibitor of the TCA cycle that inhibits the enzyme **Aconitase** (suicide inhibition). * **Arsenite poisoning:** Inhibits Alpha-ketoglutarate dehydrogenase (and PDH) by binding to lipoic acid.
Question 203: A 3-year-old girl presents with a history of fussy eating since weaning, particularly when fruit is part of her diet. She becomes cranky, sweats, and displays dizziness and lethargy after consuming meals containing fruit. Her mother has observed that these symptoms resolve when fruit is excluded from her diet. What is the most likely underlying cause of the problems the girl exhibits when eating fruit?
- A. Decreased levels of fructose in the blood
- B. Elevated levels of glyceraldehyde in liver cells
- C. High levels of sucrose in the stool
- D. Elevated levels of fructose-1-phosphate in liver cells (Correct Answer)
Explanation: ### Explanation The clinical presentation describes a classic case of **Hereditary Fructose Intolerance (HFI)**. This autosomal recessive disorder is caused by a deficiency of **Aldolase B**, the enzyme responsible for cleaving Fructose-1-Phosphate (F1P) into dihydroxyacetone phosphate (DHAP) and glyceraldehyde. **1. Why Option D is Correct:** In HFI, the deficiency of Aldolase B leads to the **intracellular accumulation of Fructose-1-Phosphate** in the liver, kidneys, and small intestine. This accumulation is toxic because it "traps" inorganic phosphate ($P_i$). The resulting depletion of $P_i$ inhibits **Glycogen Phosphorylase** (preventing glycogenolysis) and impairs **Gluconeogenesis**. This leads to severe postprandial hypoglycemia, sweating, dizziness, and lethargy following fructose or sucrose ingestion. **2. Why Other Options are Incorrect:** * **Option A:** In HFI, blood fructose levels actually *increase* (fructosemia) because the liver cannot process it, leading to its excretion in urine (fructosuria). * **Option B:** Glyceraldehyde is a product of F1P cleavage. Since Aldolase B is deficient, glyceraldehyde levels would be **decreased**, not elevated. * **Option C:** Sucrose is a disaccharide (glucose + fructose). While it triggers symptoms, it is broken down by sucrase in the intestine. High levels of sucrose in the stool are seen in **Sucrase-Isomaltase deficiency**, which causes osmotic diarrhea but not systemic hypoglycemia. **Clinical Pearls for NEET-PG:** * **The "Weaning" Clue:** Symptoms of HFI typically appear when a baby is weaned from breast milk (which contains lactose) to formulas or fruits containing **sucrose or fructose**. * **Essential Fructosuria:** Caused by **Fructokinase deficiency**. It is a benign, asymptomatic condition (fructose appears in urine, but no hypoglycemia occurs). * **Biochemical Trap:** The sequestration of $P_i$ also leads to a decrease in ATP production, causing increased uric acid production and potential liver failure. * **Management:** Strict avoidance of fructose, sucrose, and sorbitol.
Question 204: What is the first step in the liver's metabolism of fructose?
- A. Isomerization of glucose
- B. Phosphorylation to fructose 1, 6 bisphosphate by ATP
- C. Phosphorylation to fructose 6 phosphate by ATP
- D. Phosphorylation to fructose 1 phosphate by ATP (Correct Answer)
Explanation: **Explanation:** The metabolism of fructose in the liver follows a specialized pathway distinct from glycolysis. The first and rate-limiting step is the **phosphorylation of fructose at the C1 position** to form **Fructose 1-phosphate**. This reaction is catalyzed by the enzyme **Fructokinase** (also known as Ketohexokinase), utilizing one molecule of ATP as the phosphate donor. **Analysis of Options:** * **Option D (Correct):** Fructokinase has a high affinity (low Km) for fructose and specifically produces Fructose 1-phosphate. This bypasses the major regulatory step of glycolysis (Phosphofructokinase-1), leading to more rapid metabolism of fructose compared to glucose. * **Option A:** Isomerization of glucose refers to the conversion of Glucose 6-phosphate to Fructose 6-phosphate in glycolysis; it is not the initial step of fructose metabolism. * **Option B:** Fructose 1,6-bisphosphate is formed later in the pathway after Fructose 1-phosphate is cleaved into glyceraldehyde and DHAP. * **Option C:** While **Hexokinase** can phosphorylate fructose to Fructose 6-phosphate, it has a very low affinity for fructose. This pathway only becomes significant in extrahepatic tissues (like muscle) or when fructose levels are extremely high. **NEET-PG High-Yield Pearls:** * **Essential Fructosuria:** Caused by a deficiency of **Fructokinase**. It is a benign, asymptomatic condition where fructose is excreted in the urine. * **Hereditary Fructose Intolerance (HFI):** Caused by a deficiency of **Aldolase B**. This leads to the toxic accumulation of Fructose 1-phosphate, causing intracellular phosphate depletion, hypoglycemia, and liver failure. * **Metabolic Speed:** Fructose metabolism is faster than glucose metabolism because it bypasses the PFK-1 rate-limiting step.
Question 205: The hexose monophosphate shunt occurs in all of the following except?
- A. Liver
- B. Adipose tissue
- C. Mammary gland
- D. Skin (Correct Answer)
Explanation: The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), is a metabolic pathway parallel to glycolysis. Its primary functions are the generation of **NADPH** (for reductive biosynthesis) and **Ribose-5-phosphate** (for nucleotide synthesis). ### Why Skin is the Correct Answer The HMP shunt is most active in tissues that require high amounts of NADPH for lipid synthesis or to maintain a reduced state of glutathione. **Skin** does not have a high demand for these specific reductive biosynthetic processes compared to the other organs listed. Therefore, while minimal activity may exist, it is not a primary site for the HMP shunt. ### Analysis of Incorrect Options * **Liver:** This is a major site for the HMP shunt because NADPH is required for the de novo synthesis of fatty acids and cholesterol, as well as for detoxification reactions involving Cytochrome P450. * **Adipose Tissue:** Highly active in the HMP shunt to provide the NADPH necessary for the synthesis of long-chain fatty acids (lipogenesis). * **Mammary Gland:** Specifically during lactation, the mammary glands show high HMP shunt activity to supply NADPH for the synthesis of milk lipids. ### High-Yield Clinical Pearls for NEET-PG * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **Key Tissues:** Liver, Adipose tissue, Adrenal cortex, Erythrocytes (to maintain reduced glutathione), and Lactating mammary glands. * **Non-oxidative phase:** Transketolase is a key enzyme in this phase and requires **Thiamine (Vitamin B1)** as a cofactor. Measuring transketolase activity in RBCs is used to diagnose Thiamine deficiency. * **No ATP:** Unlike glycolysis, the HMP shunt does not produce or consume ATP directly.
Question 206: Hyaluronic acid is found in which of the following locations?
- A. Vitreous humor (Correct Answer)
- B. Synovial fluid
- C. Aqueous humor
- D. Cornea/Lens
Explanation: **Explanation:** **Hyaluronic acid (Hyaluronan)** is a high-molecular-weight **nonsulfated glycosaminoglycan (GAG)**. Unlike other GAGs, it is not covalently linked to a protein core and is not synthesized in the Golgi. Its primary function is to serve as a lubricant and shock absorber due to its immense water-binding capacity. 1. **Why Option A is Correct:** Hyaluronic acid is a major structural component of the **vitreous humor** of the eye. It forms a complex network with collagen fibers, creating the gel-like consistency required to maintain the shape of the eyeball and provide optical clarity. 2. **Analysis of Other Options:** * **Synovial Fluid (Option B):** While hyaluronic acid *is* found in synovial fluid (providing lubrication), in the context of standard medical entrance exams, it is most classically associated with the **vitreous humor** and **umbilical cord (Wharton’s jelly)**. (Note: In some exams, this may be a "multiple correct" scenario, but Vitreous humor is the textbook primary location). * **Aqueous Humor (Option C):** This is a watery fluid with very low protein and GAG content; it does not contain significant hyaluronic acid. * **Cornea/Lens (Option D):** The cornea contains **Keratan sulfate I** and **Dermatan sulfate**, which are essential for transparency. The lens is composed primarily of crystallin proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Unique Feature:** Hyaluronic acid is the only GAG that is **not sulfated** and not bound to a protein (no proteoglycan monomer). * **Wharton’s Jelly:** It is the chief constituent of the umbilical cord matrix. * **Enzyme Link:** **Hyaluronidase** (the "spreading factor") is secreted by bacteria and found in sperm (to penetrate the ovum) to break down this acid. * **Tumor Marker:** Elevated levels are sometimes seen in **Mesothelioma**.
Question 207: Which metabolic pathway can utilize propionic acid?
- A. Glycolysis
- B. Gluconeogenesis (Correct Answer)
- C. Glycogenolysis
- D. Glycogenesis
Explanation: **Explanation:** Propionic acid (propionate) is a three-carbon fatty acid produced primarily from the oxidation of **odd-chain fatty acids** and the catabolism of certain amino acids (Valine, Isoleucine, Threonine, and Methionine). **Why Gluconeogenesis is correct:** Propionate enters the gluconeogenic pathway via a specific conversion sequence: 1. Propionate is converted to **Propionyl-CoA**. 2. Propionyl-CoA is carboxylated to **Methylmalonyl-CoA** (requires Biotin/B7). 3. Methylmalonyl-CoA is isomerized to **Succinyl-CoA** (requires Vitamin B12). Succinyl-CoA enters the TCA cycle and is converted to Malate, which exits the mitochondria to enter **Gluconeogenesis**. This makes propionate the only part of a fatty acid (in odd-chain species) that is truly glucogenic. **Why other options are incorrect:** * **Glycolysis:** This is the breakdown of glucose to pyruvate; it does not utilize propionate as a substrate. * **Glycogenolysis:** This is the breakdown of stored glycogen into glucose-1-phosphate; it involves carbohydrate polymers, not fatty acid derivatives. * **Glycogenesis:** This is the synthesis of glycogen from glucose; while propionate can eventually become glucose, it is not a direct substrate for glycogen synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin Requirements:** The conversion of propionate to Succinyl-CoA requires **Biotin (B7)** and **Cobalamin (B12)**. * **Methylmalonic Aciduria:** A deficiency in Methylmalonyl-CoA mutase or Vitamin B12 leads to the accumulation of methylmalonic acid, causing metabolic acidosis and developmental delays. * **Glucogenic vs. Ketogenic:** While even-chain fatty acids are purely ketogenic (converted to Acetyl-CoA), odd-chain fatty acids are **glucogenic** because of their conversion to propionate.
Question 208: Two sisters are diagnosed with hemolytic anemia. Their older brother was previously diagnosed with the same disorder. Two other brothers are asymptomatic. The mother and father are second cousins. Deficiency of which of the following enzymes would be most likely to cause this disorder?
- A. Debranching enzyme
- B. Glucose-6-phosphatase
- C. Glucose-6-phosphate dehydrogenase
- D. Pyruvate kinase (Correct Answer)
Explanation: **Explanation:** The clinical presentation describes a family with multiple siblings (both male and female) affected by hemolytic anemia, born to consanguineous parents (second cousins). This pedigree strongly suggests an **Autosomal Recessive (AR)** inheritance pattern. **1. Why Pyruvate Kinase (PK) is correct:** Pyruvate Kinase deficiency is the most common enzyme deficiency in the **Embden-Meyerhof (glycolytic) pathway** causing non-spherocytic hemolytic anemia. It is inherited in an **Autosomal Recessive** manner, explaining why both sisters and the brother are affected. Mature RBCs lack mitochondria and depend entirely on glycolysis for ATP. A deficiency in PK leads to decreased ATP production, causing failure of the Na+/K+ ATPase pumps, leading to cell dehydration, rigid "echinocytes" (spiculated cells), and premature destruction in the spleen. **2. Why other options are incorrect:** * **Glucose-6-Phosphate Dehydrogenase (G6PD):** While it is the most common cause of enzyme-induced hemolysis, it is **X-linked Recessive**. In this scenario, it is unlikely for two sisters to be affected unless the father was affected and the mother was a carrier, which is less probable than the AR pattern of PK deficiency. * **Debranching enzyme (Type III GSD) & Glucose-6-phosphatase (Type I GSD):** These are Glycogen Storage Diseases. While they are AR, they primarily present with hepatomegaly, hypoglycemia, and growth retardation, not primary hemolytic anemia. **Clinical Pearls for NEET-PG:** * **PK Deficiency:** Look for "Echinocytes" or "Burr cells" on peripheral smear and an increase in **2,3-BPG** levels (due to proximal metabolite buildup), which shifts the oxygen dissociation curve to the **right**, helping patients tolerate anemia better. * **G6PD Deficiency:** Look for "Heinz bodies" and "Bite cells" triggered by oxidative stress (fava beans, drugs like Primaquine). * **Inheritance Rule:** Most enzyme deficiencies are Autosomal Recessive (except G6PD and Hunter Syndrome).
Question 209: What is the rate-limiting step of glycogenolysis?
- A. Phosphorylase (Correct Answer)
- B. Glucan transferase
- C. Debranching enzyme
- D. Glucose-6-phosphatase
Explanation: ### Explanation **Correct Answer: A. Phosphorylase** **1. Why Phosphorylase is Correct:** Glycogenolysis is the breakdown of glycogen into glucose-1-phosphate. The **rate-limiting and committed step** is catalyzed by **Glycogen Phosphorylase**. This enzyme breaks the $\alpha(1\to4)$ glycosidic bonds by adding an inorganic phosphate (phosphorolysis) to release glucose-1-phosphate. It continues until four glucose residues remain before a branch point (the limit dextrin). Its activity is strictly regulated by covalent modification (phosphorylation by phosphorylase kinase) and allosteric effectors (AMP, ATP, and Glucose). **2. Why Other Options are Incorrect:** * **B. Glucan transferase:** This is a component of the debranching enzyme complex. It moves a trisaccharide unit from one branch to another to expose the $\alpha(1\to6)$ bond. It is a remodeling enzyme, not the rate-limiting one. * **C. Debranching enzyme:** This bifunctional enzyme (transferase and $\alpha$-1,6-glucosidase) is essential for complete degradation, but it does not control the overall flux of the pathway. * **D. Glucose-6-phosphatase:** This enzyme converts glucose-6-phosphate to free glucose in the liver and kidneys. While crucial for maintaining blood glucose, it is the final step of both glycogenolysis and gluconeogenesis, not the rate-limiting step of glycogenolysis itself. **3. Clinical Pearls & High-Yield Facts:** * **Cofactor:** Glycogen phosphorylase requires **Pyridoxal Phosphate (Vitamin B6)** as an essential cofactor. * **McArdle Disease (GSD Type V):** Caused by a deficiency of **muscle** glycogen phosphorylase, leading to exercise intolerance and cramps. * **Von Gierke Disease (GSD Type I):** Caused by a deficiency of **Glucose-6-phosphatase**, leading to severe fasting hypoglycemia and hepatomegaly. * **Hormonal Control:** Glucagon (liver) and Epinephrine (muscle/liver) activate phosphorylase via the cAMP pathway to increase blood glucose or energy availability.
Question 210: A newborn presents with severe acidosis, vomiting, hypotonia, and neurologic deficits. Serum analysis reveals elevated levels of lactate and alanine. These observations suggest a deficiency in which of the following enzymes?
- A. Alanine aminotransferase
- B. Glutamate dehydrogenase
- C. Lactate dehydrogenase
- D. Pyruvate dehydrogenase (Correct Answer)
Explanation: ### Explanation **Correct Option: D. Pyruvate dehydrogenase (PDH)** The clinical presentation of severe lactic acidosis, vomiting, and neurological deficits in a newborn is a classic manifestation of **Pyruvate Dehydrogenase Complex (PDHC) deficiency**. **Mechanism:** PDH is the bridge enzyme that converts Pyruvate to Acetyl-CoA, allowing entry into the TCA cycle. When PDH is deficient, pyruvate cannot be converted to Acetyl-CoA. Consequently, pyruvate accumulates and is shunted into alternative pathways: 1. **Lactate Pathway:** Pyruvate is reduced to **Lactate** via Lactate Dehydrogenase (LDH), leading to lactic acidosis. 2. **Alanine Pathway:** Pyruvate undergoes transamination to **Alanine** via Alanine Aminotransferase (ALT). This explains the simultaneous elevation of lactate and alanine in the serum. --- ### Why other options are incorrect: * **A. Alanine aminotransferase (ALT):** A deficiency would lead to *decreased* alanine levels, not an elevation. ALT is primarily a marker of hepatocellular injury. * **B. Glutamate dehydrogenase:** This enzyme is involved in nitrogen metabolism (converting glutamate to α-ketoglutarate). Deficiency does not typically cause lactic acidosis. * **C. Lactate dehydrogenase (LDH):** If LDH were deficient, the body would be unable to produce lactate from pyruvate; therefore, lactate levels would be low, not high. --- ### High-Yield Clinical Pearls for NEET-PG: * **Inheritance:** PDH deficiency is the most common cause of congenital lactic acidosis; the E1-alpha subunit deficiency is **X-linked dominant**. * **Management:** Treatment involves a **Ketogenic Diet** (high fat, low carbohydrate). This provides energy via ketone bodies and acetyl-CoA (from fatty acid oxidation), bypassing the PDH block. * **Ketogenic Amino Acids:** Supplementation with **Lysine and Leucine** is high-yield, as they are purely ketogenic and do not increase pyruvate levels. * **Cofactors:** PDH requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (**T**ender **R**oving **N**ights **P**lease **L**oosen).
Practice by Chapter
Carbohydrate Chemistry and Classification
Practice Questions
Glycolysis: Reactions and Regulation
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Gluconeogenesis: Reactions and Regulation
Practice Questions
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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