Carbohydrate Metabolism Practice Questions

Q: Keratin sulfate in urine is found in which of the following conditions?

Morquio's syndrome. **Explanation:** The question pertains to **Mucopolysaccharidoses (MPS)**, a group of metabolic disorders caused by the deficiency of lysosomal enzymes needed to break down Glycosaminoglycans (GAGs). **1. Why Morquio’s Syndrome is Correct:** Morquio’s syndrome (MPS IV) is uniquely characterized by the accumulation and urinary excretion of **Keratan Sulfate**. It occurs in two forms: Type A (Galactose-6-sulfatase deficiency) and Type B (β-galactosidase deficiency). Unlike other MPS types, Morquio’s presents with severe skeletal dysplasia (spondyloepiphyseal dysplasia) and ligamentous laxity, but notably **spares intelligence**. **2. Analysis of Incorrect Options:** * **Hurler’s Syndrome (MPS IH):** Characterized by the deficiency of α-L-iduronidase. The GAGs found in urine are **Dermatan sulfate** and **Heparan sulfate**. It is the most severe form, presenting with corneal clouding and mental retardation. * **Hunter’s Syndrome (MPS II):** Caused by Iduronate sulfatase deficiency. Like Hurler’s, it involves **Dermatan and Heparan sulfate**, but it is **X-linked recessive** and lacks corneal clouding. * **Sanfilippo’s Syndrome (MPS III):** Primarily involves the accumulation of **Heparan sulfate**. It is clinically distinct due to profound central nervous system involvement (aggressive behavior, dementia) with relatively mild physical symptoms. **High-Yield Clinical Pearls for NEET-PG:** * **Keratan Sulfate** is also found in the cornea and cartilage; hence, its accumulation leads to the skeletal features of Morquio’s. * **Mnemonic for GAGs:** "Mor-K" (Morquio = Keratan); "San-H" (Sanfilippo = Heparan). * All MPS are Autosomal Recessive **except Hunter’s**, which is X-linked Recessive (Mnemonic: *The Hunter needs X-ray vision to see the target*). * **Corneal clouding** is absent in Hunter’s and Sanfilippo’s.

Q: During caloric deprivation, how many hours are typically required for complete depletion of hepatic glycogen stores?

18 hours. **Explanation:** **1. Why 18 hours is correct:** Hepatic glycogen serves as the primary source of blood glucose during the early stages of fasting. In a healthy adult, the liver stores approximately 75–100 grams of glycogen. Following a meal, blood glucose levels are maintained by dietary intake for about 4 hours. As the body enters the post-absorptive state, **glycogenolysis** (breakdown of glycogen) becomes the dominant process. Under standard caloric deprivation, these stores are progressively depleted and typically run out within **12 to 18 hours**. By this point, the body must transition almost entirely to **gluconeogenesis** (synthesis of glucose from non-carbohydrate sources like amino acids and glycerol) to maintain glycemia. **2. Analysis of Incorrect Options:** * **9 hours (Option A):** At this stage, hepatic glycogenolysis is still at its peak. The stores are significantly reduced but not yet exhausted. * **24 hours (Option C):** While some textbooks suggest a range of 12–24 hours, 18 hours is the physiological "tipping point" emphasized in standard medical biochemistry (like Harper’s). By 24 hours, gluconeogenesis is already the sole source of blood glucose. * **48 hours (Option D):** This represents prolonged fasting. By 48 hours, the body has shifted toward ketosis to spare muscle protein. **3. NEET-PG High-Yield Pearls:** * **Muscle Glycogen:** Unlike the liver, muscle glycogen **cannot** contribute to blood glucose because muscles lack the enzyme **Glucose-6-Phosphatase**. * **Rate-Limiting Enzyme:** The rate-limiting enzyme for glycogenolysis is **Glycogen Phosphorylase**. * **Gluconeogenesis Trigger:** As glycogen levels fall, the **Insulin:Glucagon ratio** decreases, activating PEPCK and Fructose-1,6-bisphosphatase. * **Clinical Correlation:** In Von Gierke’s Disease (G6Pase deficiency), severe hypoglycemia occurs much earlier because neither glycogenolysis nor gluconeogenesis can release glucose into the blood.

Q: Which of the following is the rate-limiting enzyme in glycogenesis?

Glycogen synthase. **Explanation:** **Glycogenesis** is the process of glycogen synthesis from glucose. The correct answer is **Glycogen synthase** because it catalyzes the formation of $\alpha$-1,4-glycosidic bonds, adding glucose units from UDP-glucose to a pre-existing glycogen primer. It is the primary site of regulation through both allosteric activation (by Glucose-6-Phosphate) and covalent modification (inhibition by phosphorylation via Protein Kinase A). **Analysis of Incorrect Options:** * **Glucokinase (Option A):** While it initiates glucose metabolism by phosphorylating glucose to Glucose-6-Phosphate in the liver, it is not specific to glycogenesis; it also feeds into glycolysis and the HMP shunt. * **Glycogen phosphorylase (Option B):** This is the rate-limiting enzyme for **Glycogenolysis** (glycogen breakdown), not synthesis. It cleaves $\alpha$-1,4-glycosidic bonds to release Glucose-1-Phosphate. * **Branching enzyme (Option D):** Also known as Amylo-$(1,4 \to 1,6)$-transglucosidase, it creates $\alpha$-1,6-glycosidic bonds. While essential for increasing glycogen solubility, it is not the rate-limiting step. **High-Yield Clinical Pearls for NEET-PG:** * **Hormonal Regulation:** Insulin stimulates glycogen synthase (via dephosphorylation), while Glucagon and Epinephrine inhibit it. * **Glycogenin:** This protein acts as the initial primer required for glycogen synthase to start adding glucose units. * **Clinical Correlation:** A deficiency in Glycogen Synthase leads to **Glycogen Storage Disease (GSD) Type 0**, characterized by fasting hypoglycemia and postprandial hyperglycemia. * **Branching Enzyme Deficiency:** Leads to **Andersen’s Disease (GSD Type IV)**, resulting in the accumulation of abnormal glycogen with long outer chains (resembling amylopectin), causing liver cirrhosis.

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.

Question 1

Keratin sulfate in urine is found in which of the following conditions?

Accepted Answer

Morquio's syndrome

Answer

Hurler's syndrome

Answer

Hunter's syndrome

Answer

Sanfillipo's syndrome

Question 2

During caloric deprivation, how many hours are typically required for complete depletion of hepatic glycogen stores?

Accepted Answer

18 hours

Answer

9 hours

Answer

24 hours

Answer

48 hours

Question 3

Which of the following is the rate-limiting enzyme in glycogenesis?

Accepted Answer

Glycogen synthase

Answer

Glucokinase

Answer

Glycogen phosphorylase

Answer

Branching enzyme

Question 4

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

Accepted Answer

Inulin

Answer

Glycogen

Answer

Agar

Answer

Hyaluronic acid

Question 5

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 6

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 7

Which of the following steps in glycolysis requires energy?

Accepted Answer

Hexokinase

Answer

Pyruvate carboxylase

Answer

Phosphoenolpyruvate carboxykinase

Answer

Phosphoglycerate kinase

Question 8

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 9

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 10

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

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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