Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 31: Which of the following is affected by gluconeogenesis?

A. Lactate
B. Glycerol
C. Alanine
D. Growth hormone (Correct Answer)

Explanation: ### Explanation The core of this question lies in distinguishing between the **substrates** (raw materials) of gluconeogenesis and the **hormonal regulators** that influence the process. **Why Growth Hormone (GH) is the correct answer:** Gluconeogenesis is a metabolic pathway that generates glucose from non-carbohydrate precursors during fasting or stress. This process is tightly regulated by hormones. **Growth Hormone (GH)** is a counter-regulatory (diabetogenic) hormone. It **stimulates gluconeogenesis** in the liver and decreases peripheral glucose uptake to maintain blood glucose levels. Therefore, the activity and rate of gluconeogenesis are directly "affected" (stimulated) by the presence of Growth Hormone. **Why the other options are incorrect:** * **A, B, and C (Lactate, Glycerol, Alanine):** These are the primary **substrates** for gluconeogenesis. While they are *consumed* during the process, they are the building blocks rather than the factors that regulate or are "affected by" the pathway's induction in a physiological sense. * **Lactate** enters via the Cori Cycle. * **Glycerol** enters via phosphorylation to DHAP. * **Alanine** is the primary glucogenic amino acid (Cahill Cycle). **NEET-PG High-Yield Pearls:** 1. **Hormonal Control:** Gluconeogenesis is stimulated by **Glucagon, Epinephrine, Cortisol, and Growth Hormone**. It is inhibited by **Insulin**. 2. **Key Regulatory Enzyme:** The most important rate-limiting step is **Fructose-1,6-bisphosphatase**. 3. **Energy Requirement:** Gluconeogenesis is an endergonic process; it requires **6 ATP/GTP** molecules to produce one molecule of glucose from two molecules of pyruvate. 4. **Location:** It occurs primarily in the **Liver** (90%) and **Kidney** (10%), specifically within the mitochondria and cytosol.

Question 32: Which phase of the pentose phosphate pathway is responsible for oxidation?

A. Oxidative phase (Correct Answer)
B. Non-oxidative phase
C. Both phases
D. Neither phase

Explanation: **Explanation:** The Pentose Phosphate Pathway (PPP), also known as the Hexose Monophosphate (HMP) Shunt, occurs in the cytosol and is divided into two distinct functional phases: **1. Why Option A is Correct:** The **Oxidative Phase** is the irreversible stage of the pathway where glucose-6-phosphate undergoes oxidation. The key enzyme, **Glucose-6-Phosphate Dehydrogenase (G6PD)**, catalyzes the rate-limiting step, transferring electrons to NADP⁺ to form **NADPH**. This phase is responsible for producing the reducing equivalents (NADPH) required for fatty acid synthesis and maintaining glutathione in its reduced state. It also produces Ribulose-5-phosphate. **2. Why Other Options are Incorrect:** * **Option B (Non-oxidative phase):** This phase is reversible and does not involve redox reactions. Its primary function is the **interconversion of sugars** (using enzymes like Transketolase and Transaldolase) to produce Ribose-5-phosphate for nucleotide synthesis or to recycle pentoses back into glycolytic intermediates (Fructose-6-P and Glyceraldehyde-3-P). * **Option C & D:** Since oxidation is strictly confined to the first three steps of the pathway, these options are physiologically inaccurate. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** G6PD (induced by Insulin). * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to hemolytic anemia because RBCs cannot generate NADPH to combat oxidative stress (e.g., from Fava beans or Primaquine), leading to **Heinz bodies** and **Bite cells**. * **Transketolase:** Requires **Thiamine (Vitamin B1)** as a cofactor. Measuring erythrocyte transketolase activity is a diagnostic test for Thiamine deficiency (Wernicke-Korsakoff syndrome). * **Tissues involved:** Highly active in the adrenal cortex, liver, mammary glands, and RBCs (tissues requiring NADPH for steroid/lipid synthesis or antioxidant defense).

Question 33: Which one of the following enzymes provides a link between glycolysis and the citric acid cycle?

A. Lactate dehydrogenase
B. Pyruvate Kinase
C. Citrate synthase
D. Pyruvate dehydrogenase (Correct Answer)

Explanation: ### Explanation The **Pyruvate Dehydrogenase Complex (PDH)** is the critical "bridge" or "link" reaction that connects anaerobic glycolysis (occurring in the cytosol) to the aerobic Citric Acid Cycle (occurring in the mitochondrial matrix). **Why Pyruvate Dehydrogenase is correct:** Glycolysis ends with the production of **Pyruvate** in the cytosol. For energy production to continue via the TCA cycle, pyruvate must enter the mitochondria and be converted into **Acetyl-CoA**. PDH catalyzes the irreversible oxidative decarboxylation of pyruvate into Acetyl-CoA. Since Acetyl-CoA is the primary substrate that enters the TCA cycle, PDH serves as the essential gateway between these two major metabolic pathways. **Analysis of Incorrect Options:** * **Lactate Dehydrogenase (LDH):** Converts pyruvate to lactate under anaerobic conditions. This is a "dead-end" pathway for pyruvate to regenerate NAD+, rather than a link to the TCA cycle. * **Pyruvate Kinase:** The final enzyme of glycolysis that converts Phosphoenolpyruvate (PEP) to pyruvate. It stays within the glycolytic pathway. * **Citrate Synthase:** The first enzyme of the TCA cycle itself (condensing Acetyl-CoA with Oxaloacetate). It acts *after* the link has already been established. **High-Yield Clinical Pearls for NEET-PG:** * **Co-factors:** PDH requires five co-enzymes (**T**ender **L**oving **C**are **F**or **N**ancy): **T**hiamine (B1), **L**ipoic acid, **C**oA (B5), **F**AD (B2), and **N**AD+ (B3). * **Deficiency:** Thiamine deficiency (Beri-beri/Wernicke-Korsakoff) impairs PDH, leading to ATP depletion and lactic acidosis, as pyruvate is shunted to lactate. * **Regulation:** PDH is inhibited by its products (Acetyl-CoA, NADH) and activated by ADP and $Ca^{2+}$.

Question 34: Which of the following is a polymer of fructose?

A. Dextrose
B. Cellulose
C. Inulin (Correct Answer)
D. Glycogen

Explanation: ### Explanation **Correct Answer: C. Inulin** **Why Inulin is correct:** Inulin is a naturally occurring polysaccharide consisting of a chain of fructose units (fructans), typically ending in a terminal glucose molecule. The fructose units are linked by **$\beta(2 \to 1)$ glycosidic bonds**. Because the human body lacks the enzyme (inulinase) to break these specific bonds, inulin is not digested in the small intestine and reaches the colon intact, where it acts as a prebiotic. **Analysis of Incorrect Options:** * **A. Dextrose:** This is simply another name for **D-glucose**, a monosaccharide. It is the primary fuel source for the body and not a polymer. * **B. Cellulose:** This is a structural homopolysaccharide of **glucose** units linked by $\beta(1 \to 4)$ glycosidic bonds. It is found in plant cell walls. * **D. Glycogen:** This is the primary storage form of **glucose** in animals (found in liver and muscle). It consists of glucose units linked by $\alpha(1 \to 4)$ bonds with $\alpha(1 \to 6)$ branching. **High-Yield Clinical Pearls for NEET-PG:** 1. **GFR Gold Standard:** Inulin is the "Gold Standard" for measuring **Glomerular Filtration Rate (GFR)** because it is freely filtered by the glomerulus and is neither reabsorbed nor secreted by the renal tubules. 2. **Diagnostic Use:** While inulin is the gold standard, **Creatinine clearance** is more commonly used in clinical practice because inulin requires continuous intravenous infusion. 3. **Dahlia Tubers:** Inulin is commercially extracted from chicory roots or Dahlia tubers. 4. **Dextran vs. Dextrose:** Do not confuse Dextrose (glucose) with **Dextran** (a complex branched glucan used as a plasma volume expander).

Question 35: All of the following are examples of liver glycogenosis except?

A. Von Girke disease
B. Hers disease
C. Type III glycogenosis
D. Pompe disease (Correct Answer)

Explanation: **Explanation:** Glycogen Storage Diseases (GSDs) are classified based on the primary organ system affected: **Hepatic** (presenting with hypoglycemia and hepatomegaly) or **Myopathic** (presenting with muscle cramps and exercise intolerance). **Why Pompe Disease is the correct answer:** **Pompe disease (Type II GSD)** is unique because it is a **lysosomal storage disorder** caused by a deficiency of **α-1,4-glucosidase (Acid Maltase)**. Unlike other GSDs, it does not primarily affect blood glucose homeostasis. Instead, glycogen accumulates in the lysosomes of all tissues, most significantly affecting the **heart** and **skeletal muscles**. It presents with massive cardiomegaly, hypotonia, and respiratory failure, making it a generalized/myopathic GSD rather than a hepatic one. **Analysis of incorrect options:** * **Von Gierke Disease (Type I):** Deficiency of Glucose-6-Phosphatase. It is the most common hepatic GSD, characterized by severe fasting hypoglycemia, hepatomegaly, and lactic acidosis. * **Hers Disease (Type VI):** Deficiency of Liver Phosphorylase. It is a relatively mild hepatic GSD presenting with hepatomegaly and growth retardation. * **Type III Glycogenosis (Cori Disease):** Deficiency of Debranching enzyme. It affects both liver and muscle, but the hepatic involvement (hypoglycemia and hepatomegaly) is a hallmark feature. **High-Yield Clinical Pearls for NEET-PG:** * **Pompe's Mnemonic:** "Pompe trashes the **Pump** (Heart)." * **Von Gierke's:** Look for "Doll-like facies" and hyperuricemia. * **McArdle Disease (Type V):** The classic myopathic GSD (Muscle phosphorylase deficiency) presenting with "second wind phenomenon." * **Anderson Disease (Type IV):** The only GSD that leads to liver cirrhosis (due to abnormal glycogen structure).

Question 36: Which step in glycolysis involves substrate-level phosphorylation?

A. Glyceraldehyde 3-phosphate dehydrogenase
B. Pyruvate kinase (Correct Answer)
C. Phosphofructokinase
D. Enolase

Explanation: **Explanation:** **Substrate-level phosphorylation** 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, independent of the electron transport chain. In glycolysis, there are **two steps** where this occurs: 1. **Phosphoglycerate Kinase:** Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate. 2. **Pyruvate Kinase (Option B):** This is the final irreversible step of glycolysis where Phosphoenolpyruvate (PEP) is converted to Pyruvate. PEP contains a high-energy phosphate bond; its cleavage provides sufficient energy to phosphorylate ADP to ATP. **Analysis of Incorrect Options:** * **A. Glyceraldehyde 3-phosphate dehydrogenase:** This step involves the oxidation of the substrate and the addition of inorganic phosphate to form 1,3-BPG. It produces **NADH**, not ATP. * **C. Phosphofructokinase (PFK-1):** This is the rate-limiting step of glycolysis. It **consumes** one molecule of ATP to phosphorylate Fructose-6-phosphate; it does not generate it. * **D. Enolase:** This enzyme catalyzes a dehydration reaction (2-phosphoglycerate to PEP). While it creates a high-energy bond, it does not involve phosphate transfer to ADP. **NEET-PG High-Yield Pearls:** * **Net ATP Yield:** Aerobic glycolysis yields 5 or 7 ATP (depending on the shuttle), while anaerobic glycolysis yields a net of **2 ATP**. * **Clinical Correlation:** **Pyruvate Kinase deficiency** is the second most common cause of enzyme-deficient hemolytic anemia (after G6PD deficiency). Since RBCs lack mitochondria, they rely entirely on glycolysis for ATP; a deficiency leads to ATP depletion, causing rigid cells and premature destruction (hemolysis). * **Arsenate Poisoning:** Arsenate competes with inorganic phosphate in the GAPDH step, bypassing the first substrate-level phosphorylation and resulting in **zero net ATP** production.

Question 37: Which Glycogen Storage Disease does not affect muscles?

A. Type 1 (Correct Answer)
B. Type 2
C. Type 3
D. Type 4

Explanation: **Explanation:** The correct answer is **Type 1 (von Gierke disease)**. This is because the deficient enzyme, **Glucose-6-Phosphatase**, is primarily expressed in the liver and kidneys. Muscle tissue lacks this enzyme naturally; instead, muscles utilize Glucose-6-Phosphate directly for glycolysis to generate ATP rather than releasing free glucose into the bloodstream. Consequently, Type 1 GSD presents with severe fasting hypoglycemia and hepatomegaly, but **no muscle symptoms** (no weakness or cramping). **Analysis of Incorrect Options:** * **Type 2 (Pompe disease):** Caused by a deficiency in **Lysosomal α-1,4-glucosidase (Acid Maltase)**. Since lysosomes are present in all cells, this disease affects the heart and skeletal muscles severely, leading to hypertrophic cardiomyopathy and hypotonia. * **Type 3 (Cori disease):** Caused by a deficiency in the **Debranching enzyme**. Unlike Type 1, this affects both the liver and muscles, often presenting with hepatomegaly along with progressive distal muscle weakness. * **Type 4 (Andersen disease):** Caused by a deficiency in the **Branching enzyme**, leading to the accumulation of abnormal glycogen (polyglucosan). This affects the liver, heart, and skeletal muscles, often resulting in cirrhosis and muscular hypotonia. **High-Yield Clinical Pearls for NEET-PG:** * **Type 1 (von Gierke):** Look for the "Doll-like face," hyperuricemia (gout), lactic acidosis, and hyperlipidemia. * **Type 5 (McArdle):** Affects **only** muscles (Myophosphorylase deficiency). Look for exercise-induced cramps and myoglobinuria. * **Mnemonic:** Remember that Type 1 and Type 6 (Hers) are primarily hepatic, while Type 5 is purely muscular. Type 2, 3, and 4 have overlapping systemic/muscular involvement.

Question 38: A 6-month-old infant, exclusively fed infant formula, develops jaundice, hepatomegaly, vomiting, lethargy, irritability, and seizures upon introduction of fruit juices. Urine-reducing substances are positive. Which of the following is the likely diagnosis explaining this child's condition?

A. Tyrosinemia
B. Galactosemia
C. Hereditary fructose intolerance (Correct Answer)
D. a1-Antitrypsin deficiency

Explanation: **Explanation:** The clinical presentation points toward **Hereditary Fructose Intolerance (HFI)**. The key diagnostic clue is the onset of symptoms (vomiting, jaundice, hepatomegaly, and seizures) specifically following the **introduction of fruit juices** (which contain fructose and sucrose) in an infant previously asymptomatic on formula or breast milk. **1. Why Hereditary Fructose Intolerance is correct:** HFI is caused by a deficiency of **Aldolase B**. When fructose is ingested, it is phosphorylated to **Fructose-1-Phosphate (F1P)** by fructokinase. Due to the lack of Aldolase B, F1P accumulates in hepatocytes, trapping intracellular inorganic phosphate. This depletion of ATP and Pi inhibits both **glycogenolysis and gluconeogenesis**, leading to profound postprandial hypoglycemia (causing seizures/lethargy) and liver damage (jaundice/hepatomegaly). The presence of **urine-reducing substances** (fructose) with a negative glucose dipstick is a classic finding. **2. Why other options are incorrect:** * **Galactosemia:** Symptoms appear much earlier (within days of birth) as soon as the infant starts breastfeeding or taking lactose-containing formula. * **Tyrosinemia:** While it causes hepatomegaly and jaundice, it is not specifically triggered by the introduction of fruit juices. * **α1-Antitrypsin deficiency:** Presents with neonatal cholestasis or chronic liver disease, but does not cause acute hypoglycemia or positive urine-reducing substances upon sugar ingestion. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficient:** Aldolase B (Chromosome 9q). * **The "Trapping" Mechanism:** Accumulation of F1P is the toxic event. * **Dietary Management:** Total avoidance of Fructose, Sucrose (Glucose + Fructose), and Sorbitol. * **Distinction:** Essential Fructosuria (Fructokinase deficiency) is a benign, asymptomatic condition, unlike HFI.

Question 39: What is the maximum carbohydrate concentration typically found in a strict vegetarian diet?

A. Amylase
B. Maltose
C. Fructose
D. Glycogen (Correct Answer)

Explanation: **Explanation:** The question focuses on the storage forms of carbohydrates and their dietary sources. **Glycogen** is the correct answer because it is the primary storage polysaccharide in animals (found in liver and muscle). In the context of a **strict vegetarian (vegan) diet**, which excludes all animal products, the dietary intake of pre-formed glycogen is effectively **zero** or negligible. Therefore, the "maximum concentration" of glycogen found in such a diet is the lowest among the options provided. **Analysis of Options:** * **Glycogen (Correct):** As an animal-derived storage homopolymer of glucose, it is absent in plant-based foods. A strict vegetarian diet lacks this component entirely. * **Amylase (Incorrect):** This is an enzyme (protein), not a carbohydrate. While present in plants (e.g., $\beta$-amylase in seeds), it does not represent a carbohydrate concentration. * **Maltose (Incorrect):** A disaccharide produced during the breakdown of starch. It is found in germinating cereals (malt) and is a common component of a vegetarian diet. * **Fructose (Incorrect):** A monosaccharide abundant in fruits and honey. It is a major carbohydrate component in any plant-based diet. **NEET-PG Clinical Pearls:** * **Structure:** Glycogen features $\alpha(1\to4)$ glycosidic bonds for linear chains and $\alpha(1\to6)$ bonds for branching (every 8-12 residues). * **Storage:** In humans, the liver maintains blood glucose (via glycogenolysis), while muscle glycogen provides local energy during contraction. * **Biochemical Marker:** Glycogen is a homopolymer of D-glucose. Its absence in plants is a key distinction between animal and plant energy storage (starch vs. glycogen).

Question 40: Hemolytic anemia is most commonly seen due to a deficiency in which enzyme?

A. Pyruvate kinase (Correct Answer)
B. Phosphofructokinase I
C. Phosphofructokinase II
D. Pyruvate dehydrogenase

Explanation: **Explanation:** **1. Why Pyruvate Kinase (PK) is the Correct Answer:** Pyruvate kinase deficiency is the **most common glycolytic enzyme defect** causing hereditary non-spherocytic hemolytic anemia. Mature erythrocytes lack mitochondria and depend entirely on **anaerobic glycolysis** for ATP production. PK catalyzes the final step of glycolysis (Phosphoenolpyruvate → Pyruvate), generating ATP. * **Mechanism:** A deficiency leads to decreased ATP production. This causes failure of the **Na+/K+ ATPase pumps** on the RBC membrane, leading to loss of intracellular potassium and water. The cells become rigid (echinocytes), lose deformability, and are prematurely destroyed by the spleen, resulting in extravascular hemolysis. **2. Why Other Options are Incorrect:** * **Phosphofructokinase I (PFK-1):** Deficiency (Tarui disease/GSD Type VII) primarily affects muscles, causing exercise intolerance and cramps. While mild hemolysis can occur, it is significantly rarer than PK deficiency. * **Phosphofructokinase II (PFK-2):** This is a regulatory enzyme that controls levels of Fructose-2,6-bisphosphate. It is not a primary cause of hemolytic anemia. * **Pyruvate Dehydrogenase (PDH):** This enzyme links glycolysis to the TCA cycle. Deficiency leads to **Lactic Acidosis** and neurological impairment (Leigh syndrome), but not hemolysis, as RBCs do not utilize the TCA cycle. **3. NEET-PG High-Yield Pearls:** * **Inheritance:** Autosomal Recessive. * **Biochemical Marker:** Increased levels of **2,3-BPG** (due to the proximal backup in glycolysis). This shifts the Oxygen-Dissociation Curve to the **right**, facilitating oxygen unloading to tissues (patients may tolerate anemia better than expected). * **Blood Smear:** Presence of **Echinocytes** (Burr cells/spiculated cells). * **Note:** While Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency is the overall most common enzyme deficiency causing hemolysis, **Pyruvate Kinase** is the most common deficiency within the **glycolytic pathway** specifically.

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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