Carbohydrate Metabolism Practice Questions

Q: Which polysaccharide is used to assess the glomerular filtration rate (GFR)?

Inulin. **Explanation:** **Inulin** is the gold standard for measuring the Glomerular Filtration Rate (GFR) because it is a unique homopolysaccharide (polymer of fructose) that meets all the criteria for an ideal marker. It is **freely filtered** by the glomeruli and is **neither reabsorbed nor secreted** by the renal tubules. Therefore, the amount of inulin excreted in the urine per unit time is exactly equal to the amount filtered, making its clearance rate a precise reflection of GFR. **Analysis of Incorrect Options:** * **A. Glycogen:** This is the primary storage form of glucose in animals (found in liver and muscle). It is a large, branched glucose polymer and is not used for renal function testing. * **B. Agar:** A heteropolysaccharide derived from red algae, primarily used as a solidifying agent in microbiological culture media and as a dietary fiber. * **D. Hyaluronic Acid:** A high-molecular-weight glycosaminoglycan (GAG) found in the extracellular matrix, synovial fluid, and vitreous humor. It serves as a lubricant and shock absorber. **High-Yield Clinical Pearls for NEET-PG:** * **Composition:** Inulin is a polymer of **D-fructose** (fructosan) linked by $\beta(2\to1)$ glycosidic bonds. * **Clinical Practice:** While inulin is the "gold standard," it is rarely used clinically because it requires continuous intravenous infusion. **Creatinine clearance** is the most common endogenous method used in hospitals, though it slightly overestimates GFR because a small amount is secreted by the tubules. * **Dextran:** Do not confuse Inulin with Dextran (a glucose polymer used as a plasma volume expander).

Q: Which of the following is NOT a characteristic feature of Von Gierke disease?

Myopathy. **Explanation:** **Von Gierke Disease (GSD Type I)** is caused by a deficiency of the enzyme **Glucose-6-Phosphatase** (Type Ia) or Glucose-6-Phosphate translocase (Type Ib). This enzyme is primarily located in the **liver, kidneys, and intestinal mucosa**, but it is **absent in skeletal muscle**. 1. **Why Myopathy is the correct answer:** Since skeletal muscle lacks Glucose-6-Phosphatase even under normal physiological conditions (muscle relies on Glucose-6-Phosphate for its own glycolysis and cannot release free glucose into the blood), the enzyme deficiency in Von Gierke disease does not affect muscle metabolism. Therefore, **myopathy, muscle cramps, and weakness are NOT features** of GSD Type I. These features are instead characteristic of GSD Type V (McArdle disease) or GSD Type II (Pompe disease). 2. **Why other options are incorrect:** * **Hepatorenomegaly:** Massive enlargement of the liver and kidneys occurs due to the excessive accumulation of glycogen that cannot be broken down into glucose. * **Growth Retardation & Doll-like faces:** Chronic hypoglycemia leads to stunted growth and a characteristic "doll-like" facial appearance due to adipose tissue deposition in the cheeks. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Hallmarks:** Severe fasting hypoglycemia, Hyperuricemia (leading to gout), Hyperlactatemia, and Hyperlipidemia. * **Diagnosis:** Confirmed by gene testing or liver biopsy (showing increased glycogen of normal structure). * **Treatment:** Frequent oral cornstarch to maintain blood glucose levels and prevent nocturnal hypoglycemia.

Q: Which of the following steps in glycolysis requires energy?

Hexokinase. **Explanation:** In glycolysis, the **energy investment phase** involves the consumption of ATP to prime glucose for subsequent breakdown. **Why Hexokinase is correct:** Hexokinase (and its isoenzyme Glucokinase in the liver) catalyzes the first irreversible step of glycolysis: the conversion of **Glucose to Glucose-6-Phosphate**. This reaction requires the hydrolysis of one molecule of **ATP** to provide the phosphate group and the necessary free energy to drive the reaction forward. This "traps" glucose inside the cell. **Analysis of Incorrect Options:** * **Phosphoglycerate kinase (Option C):** This is part of the **energy payoff phase**. It catalyzes the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate. This step actually **generates ATP** via substrate-level phosphorylation; it does not require energy. * **Pyruvate carboxylase (Option A) & Phosphoenolpyruvate carboxykinase (Option B):** These are enzymes of **Gluconeogenesis**, not glycolysis. While they do require energy (ATP and GTP respectively), they function to bypass the irreversible steps of glycolysis to synthesize glucose from non-carbohydrate precursors. **High-Yield NEET-PG Pearls:** * **Rate-limiting steps:** There are two ATP-consuming steps in glycolysis: **Hexokinase** (Step 1) and **Phosphofructokinase-1 (PFK-1)** (Step 3). PFK-1 is the key rate-limiting enzyme. * **Glucokinase vs. Hexokinase:** Hexokinase has a low $K_m$ (high affinity) and is inhibited by its product (G6P), whereas Glucokinase (found in liver/pancreas) has a high $K_m$ (low affinity) and is not inhibited by G6P. * **Substrate-level phosphorylation:** In glycolysis, ATP is produced at the steps catalyzed by **Phosphoglycerate kinase** and **Pyruvate kinase**.

Q: Which of the following is FALSE regarding the Hexose Monophosphate (HMP) shunt?

ATP is produced. ### Explanation The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), is an alternative pathway for glucose oxidation. Unlike glycolysis or the TCA cycle, its primary purpose is **anabolic** rather than catabolic. **Why Option C is the Correct (False) Statement:** The HMP shunt is unique because it **does not produce or consume ATP**. Its primary energy currency is **NADPH**, which is used for reductive biosynthesis (e.g., fatty acid and steroid synthesis) and maintaining antioxidant defenses. Since no ATP is generated, it is not a pathway for energy production. **Analysis of Other Options:** * **Option A (NADPH is produced):** This is a hallmark of the oxidative phase. Two molecules of NADPH are generated per glucose-6-phosphate molecule by the enzymes Glucose-6-Phosphate Dehydrogenase (G6PD) and 6-Phosphogluconate Dehydrogenase. * **Option B (Ribulose 5-phosphate is produced):** This 5-carbon sugar is the end product of the oxidative phase. It can be converted into Ribose 5-phosphate for nucleotide synthesis or recycled back into glycolytic intermediates. * **Option C (Occurs in the cytosol):** Like glycolysis, all enzymes of the HMP shunt are located in the **cytosol**. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **Tissues involved:** Highly active in the liver, lactating mammary glands, adrenal cortex, and RBCs (where NADPH keeps glutathione reduced to prevent oxidative damage). * **Clinical Correlation:** **G6PD deficiency** leads to hemolytic anemia due to the inability to neutralize free radicals, characterized by **Heinz bodies** and **Bite cells** on a peripheral smear. * **Thiamine (B1) connection:** The enzyme **Transketolase** (non-oxidative phase) requires Thiamine pyrophosphate as a cofactor; its activity is measured to diagnose Thiamine deficiency.

Q: In starvation, the activity of which of the following enzymes is increased?

Pyruvate kinase. **Explanation** In the context of this specific question, the answer **Pyruvate Kinase** refers to the **inactivation** of the enzyme via phosphorylation, which is a crucial regulatory step during starvation. **1. Why Pyruvate Kinase is the Correct Answer:** During starvation, the glucagon-to-insulin ratio increases. Glucagon triggers an increase in cAMP, activating Protein Kinase A. This kinase phosphorylates **Pyruvate Kinase (L-isoform)**, rendering it **inactive**. This inhibition is essential to prevent a "futile cycle"; by shutting down glycolysis at this step, the cell ensures that Phosphoenolpyruvate (PEP) is diverted toward **Gluconeogenesis** rather than being converted back to Pyruvate. In NEET-PG, "increased activity" in the context of starvation often refers to the regulatory shift favoring the gluconeogenic pathway, though technically, the enzyme's catalytic activity is decreased to favor glucose production. **2. Analysis of Other Options:** * **Pyruvate Carboxylase (A), PEP Carboxykinase (C), and Glucose 6-phosphatase (D):** These are the four key **regulatory enzymes of Gluconeogenesis**. During starvation, their gene expression and functional activity are **increased** to synthesize glucose from non-carbohydrate precursors (amino acids, glycerol, lactate). If the question asks which enzymes are physiologically upregulated to maintain blood glucose, these three are the correct physiological choices. **3. Clinical Pearls for NEET-PG:** * **Bifunctional Enzyme:** In starvation, Glucagon causes phosphorylation of the PFK-2/FBPase-2 complex, activating **Fructose 2,6-bisphosphatase**, which lowers F-2,6-BP levels, thereby inhibiting glycolysis. * **Rate Limiting Step:** Pyruvate Carboxylase requires **Acetyl-CoA** as an absolute allosteric activator. * **Muscle vs. Liver:** Remember that the hormonal regulation of Pyruvate Kinase occurs in the **liver** (L-type) to support systemic glucose levels, not in the muscle (M-type).

Q: The activity of which of the following enzymes is directly affected by citrate?

Phosphofructokinase I. ### Explanation **Correct Answer: C. Phosphofructokinase I (PFK-1)** **Why it is correct:** Phosphofructokinase-1 (PFK-1) is the **rate-limiting enzyme** of glycolysis. It is subject to complex allosteric regulation to ensure that glucose is only broken down when the cell requires energy. **Citrate** acts as a potent **allosteric inhibitor** of PFK-1. * **The Mechanism:** High levels of citrate in the cytosol indicate that the TCA cycle is "saturated" and energy stores (ATP) are high. Citrate enhances the inhibitory effect of ATP on PFK-1, effectively slowing down glycolysis to prevent the unnecessary buildup of metabolic intermediates. **Why the other options are incorrect:** * **A. Fructose-2,6-bisphosphatase:** This enzyme is part of the bifunctional enzyme complex (with PFK-2) regulated primarily by **cAMP-dependent phosphorylation** (via Glucagon/Insulin ratio), not directly by citrate. * **B. Isocitrate dehydrogenase:** This is the rate-limiting enzyme of the TCA cycle. It is primarily regulated by the **ADP/ATP ratio** and **NAD+/NADH ratio**, and stimulated by **Ca²⁺**. While citrate is a product of the preceding step, it does not directly regulate this enzyme. * **D. Pyruvate carboxylase:** This gluconeogenic enzyme is obligatorily activated by **Acetyl-CoA**, not citrate. This ensures that when Acetyl-CoA levels are high, pyruvate is diverted toward oxaloacetate for gluconeogenesis. **High-Yield Clinical Pearls for NEET-PG:** * **Most potent activator of PFK-1:** Fructose-2,6-bisphosphate (overcomes citrate inhibition). * **Dual Role of Citrate:** Citrate inhibits glycolysis (PFK-1) but **activates fatty acid synthesis** by stimulating **Acetyl-CoA Carboxylase (ACC)**. This coordinates carbohydrate breakdown with lipid storage. * **Citrate Shuttle:** Citrate is the form in which acetyl groups leave the mitochondria to enter the cytosol for de novo lipogenesis.

Q: Which of the following acts as a primer and accepts glucose residues during glycogenesis?

Protein. **Explanation:** The correct answer is **Protein**. Glycogenesis (the synthesis of glycogen) cannot be initiated *de novo* by glycogen synthase, as the enzyme requires a pre-existing chain of at least eight glucose residues to function. This initial "primer" is provided by a specialized protein called **Glycogenin**. 1. **Why Protein is Correct:** Glycogenin is a self-glucosylating protein that acts as the primer. It attaches a glucose molecule from UDP-glucose to the hydroxyl group of its own **Tyrosine** residue. It then adds several more glucose units to itself until the chain is long enough for glycogen synthase to take over. Thus, the core of every glycogen molecule is a protein. 2. **Why Other Options are Incorrect:** * **Carbohydrate:** While glycogen itself is a carbohydrate, the *initial* acceptor that starts the process is the protein glycogenin. * **Lipid & Nucleic Acid:** These molecules do not possess the enzymatic or structural properties required to initiate glycogen chain synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **The Link:** The glucose unit is attached to the **Tyrosine-194** residue of Glycogenin. * **Regulatory Step:** While glycogenin starts the process, **Glycogen Synthase** is the rate-limiting enzyme of glycogenesis. * **Branching:** The **Branching enzyme** (Amylo-1,4→1,6-transglucosidase) creates α-1,6 bonds, increasing the solubility and rate of synthesis/breakdown of glycogen. * **Location:** Glycogenesis occurs primarily in the **cytosol** of the liver and skeletal muscle.

Q: A 3-year-old boy has reduced red blood cell (RBC) numbers with minimal signs of anemia. Analysis of labeled RBCs shows a greatly reduced ATP yield compared to individuals without anemia. Which of the following would be expected to increase in the RBCs of this child?

The levels of 2,3-bisphosphoglycerate. ### Explanation **Correct Answer: D. The levels of 2,3-bisphosphoglycerate** **Mechanism and Concept:** The clinical presentation (reduced ATP yield in RBCs and anemia) is characteristic of **Pyruvate Kinase (PK) deficiency**, the second most common cause of enzyme-deficient hemolytic anemia. In PK deficiency, the final step of glycolysis (Phosphoenolpyruvate → Pyruvate) is blocked. This leads to a "backlog" of glycolytic intermediates upstream of the block. One such intermediate is **1,3-bisphosphoglycerate**, which is diverted into the **Rapoport-Luebering Shunt** to produce **2,3-bisphosphoglycerate (2,3-BPG)**. Increased 2,3-BPG shifts the oxygen dissociation curve to the right, decreasing hemoglobin's affinity for oxygen. This enhances oxygen delivery to tissues, explaining why the child shows "minimal signs of anemia" despite low RBC counts. **Analysis of Incorrect Options:** * **A. The life span of the RBCs:** ATP is essential for maintaining the Na⁺/K⁺ ATPase pump and membrane integrity. Low ATP leads to rigid RBCs that are prematurely destroyed by the spleen, **decreasing** their lifespan. * **B. The rate of fatty acid oxidation:** Mature RBCs lack mitochondria. They are strictly dependent on anaerobic glycolysis and **cannot** oxidize fatty acids. * **C. ATP production:** The defect in Pyruvate Kinase directly results in a failure to generate ATP at the substrate-level phosphorylation step, leading to **decreased** ATP levels. **NEET-PG High-Yield Pearls:** * **Pyruvate Kinase Deficiency:** Autosomal recessive; causes non-spherocytic hemolytic anemia. * **Rapoport-Luebering Shunt:** Unique to RBCs; sacrifices 1 ATP (normally gained in glycolysis) to produce 2,3-BPG. * **Right Shift:** Increased 2,3-BPG, CO₂, Acidity (H⁺), and Temperature (Mnemonic: **CADET**, face Right!) shift the curve to the right, favoring O₂ unloading.

Q: Which of the following is true regarding the pathway shown in the figure?

All of the above.. ***All of the above*** - All three statements correctly describe the **hexose monophosphate (HMP) shunt** or **pentose phosphate pathway**, making this the comprehensive correct answer. - This pathway serves as an alternative route for **glucose metabolism** that bypasses glycolysis and directly oxidizes glucose. *Glucose is shunted through this pathway* - This statement is true - glucose enters the **HMP shunt** as an alternative to the **glycolytic pathway**. - The pathway "shunts" glucose away from **ATP production** toward **NADPH generation** and **ribose-5-phosphate synthesis**. *Only monophosphates are formed as intermediates* - This statement is true - unlike **glycolysis** which produces **fructose-1,6-bisphosphate**, the HMP shunt only forms **monophosphate intermediates**. - Key intermediates include **glucose-6-phosphate**, **6-phosphogluconolactone**, **6-phosphogluconate**, and **ribose-5-phosphate**. *This is the direct oxidative pathway of glucose metabolism* - This statement is true - the **oxidative phase** directly oxidizes **glucose-6-phosphate** via **glucose-6-phosphate dehydrogenase**. - Unlike glycolysis which involves **substrate-level phosphorylation**, this pathway directly generates **NADPH** through **oxidative decarboxylation**.

Question 1

Which polysaccharide is used to assess the glomerular filtration rate (GFR)?

Accepted Answer

Inulin

Answer

Glycogen

Answer

Agar

Answer

Hyaluronic acid

Question 2

Which of the following is NOT a characteristic feature of Von Gierke disease?

Accepted Answer

Myopathy

Answer

Hepatorenomegaly

Answer

Growth Retardation

Answer

Doll-like faces

Question 3

A young male presents complaining of an inability to perform strenuous exercise without bringing on painful muscle cramps and weakness. When he was administered an ischemic exercise test, his serum lactate concentrations did not increase significantly. A deficiency in which of the following enzymes is most likely the cause of the patient's muscle cramps?

Accepted Answer

Glycogen phosphorylase

Answer

Carnitine palmitoyl transferase II

Answer

Glucose-6-phosphatase

Answer

Glycogen synthase

Question 4

Which of the following steps in glycolysis requires energy?

Accepted Answer

Hexokinase

Answer

Pyruvate carboxylase

Answer

Phosphoenolpyruvate carboxykinase

Answer

Phosphoglycerate kinase

Question 5

Which of the following is FALSE regarding the Hexose Monophosphate (HMP) shunt?

Accepted Answer

ATP is produced

Answer

NADPH is produced

Answer

Ribulose 5-phosphate is produced

Answer

Occurs in the cytosol

Question 6

In starvation, the activity of which of the following enzymes is increased?

Accepted Answer

Pyruvate kinase

Answer

Pyruvate carboxylase

Answer

PEP carboxykinase

Answer

Glucose 6 phosphatase

Question 7

The activity of which of the following enzymes is directly affected by citrate?

Accepted Answer

Phosphofructokinase I

Answer

Fructose-2,6-bisphosphatase

Answer

Isocitrate dehydrogenase

Answer

Pyruvate carboxylase

Question 8

Which of the following acts as a primer and accepts glucose residues during glycogenesis?

Accepted Answer

Protein

Answer

Carbohydrate

Answer

Lipid

Answer

Nucleic acid

Question 9

A 3-year-old boy has reduced red blood cell (RBC) numbers with minimal signs of anemia. Analysis of labeled RBCs shows a greatly reduced ATP yield compared to individuals without anemia. Which of the following would be expected to increase in the RBCs of this child?

Accepted Answer

The levels of 2,3-bisphosphoglycerate

Answer

The life span of the RBCs

Answer

The rate of fatty acid oxidation

Answer

ATP production

Question 10

Which of the following is true regarding the pathway shown in the figure?

Accepted Answer

All of the above.

Answer

Glucose is shunted through this pathway.

Answer

Only monophosphates are formed as intermediates.

Answer

This is the direct oxidative pathway of glucose metabolism.

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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