Carbohydrate Metabolism — MCQs

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?

Q248Easy

Which of the following steps in glycolysis requires energy?

Q249Easy

Which enzyme is strongly activated by fructose 2,6 bisphosphate?

Q250Easy

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

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 241: A 2-month-old, breast-fed baby presents with gastrointestinal problems and cirrhosis of the liver. Molecular analysis indicates a normal amount of galactose-1-phosphate uridyl transferase (GALT) mRNA, but no observable enzyme activity. What is the most likely explanation for this observation?

A. Gene deletion
B. Nonsense mutation (Correct Answer)
C. Premature transcription termination sequence in the DNA
D. Promoter mutation

Explanation: ### Explanation The clinical presentation of gastrointestinal distress and cirrhosis in a 2-month-old infant suggests **Classic Galactosemia**, typically caused by a deficiency of **Galactose-1-phosphate uridyl transferase (GALT)**. **Why Nonsense Mutation is Correct:** The key to this question lies in the molecular findings: **normal mRNA levels but zero enzyme activity.** * A **nonsense mutation** involves a single nucleotide substitution that creates a premature stop codon (UAG, UAA, or UGA) within the coding region. * The transcription process remains unaffected, resulting in a **normal amount of mRNA**. * However, during translation, the ribosome dissociates early, leading to a truncated, non-functional protein that is often rapidly degraded. This explains the complete absence of enzyme activity despite the presence of mRNA. **Why the Other Options are Incorrect:** * **A. Gene Deletion:** If the gene were deleted, there would be no template for transcription, resulting in an **absence of mRNA**. * **C. Premature Transcription Termination:** This occurs at the DNA level (e.g., a mutation creating a poly-A signal too early). This would result in **truncated/abnormal mRNA**, not a "normal amount" of full-length mRNA. * **D. Promoter Mutation:** The promoter is responsible for the initiation of transcription. A mutation here would typically lead to **decreased or absent mRNA synthesis**. **Clinical Pearls for NEET-PG:** * **Classic Galactosemia (Type I):** Deficiency of GALT. Symptoms appear once breastfeeding starts (lactose = glucose + galactose). * **Key Findings:** Oil-drop cataracts, hepatosplenomegaly, jaundice, and *E. coli* sepsis. * **Diagnosis:** Reducing sugars in urine (Benedict's test positive) but negative glucose oxidase test. * **Management:** Immediate withdrawal of milk; switch to soy-based or lactose-free formula.

Question 242: Which of the following is the rate-limiting enzyme in glycogenesis?

A. Glucokinase
B. Glycogen phosphorylase
C. Glycogen synthase (Correct Answer)
D. Branching enzyme

Explanation: **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.

Question 243: Which polysaccharide is used to assess the glomerular filtration rate (GFR)?

A. Glycogen
B. Agar
C. Inulin (Correct Answer)
D. Hyaluronic acid

Explanation: **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).

Question 244: Where does the Kreb's cycle occur?

A. Cytoplasm
B. Mitochondria (Correct Answer)
C. Smooth Endoplasmic reticulum
D. Nucleus

Explanation: **Explanation:** The **Krebs cycle** (also known as the Citric Acid Cycle or TCA cycle) occurs in the **mitochondrial matrix**. This is the correct answer because all the necessary enzymes for the cycle—such as Citrate synthase and Isocitrate dehydrogenase—are localized within the matrix. The only exception is Succinate dehydrogenase, which is located on the inner mitochondrial membrane. This localization is functional, as it allows the NADH and $FADH_2$ produced during the cycle to directly enter the Electron Transport Chain (ETC) located on the inner mitochondrial membrane. **Analysis of Incorrect Options:** * **Cytoplasm:** This is the site for **Glycolysis**, HMP Shunt, and Fatty Acid synthesis. While the precursor (Pyruvate) is formed here, it must be transported into the mitochondria via the pyruvate symporter to enter the Krebs cycle. * **Smooth Endoplasmic Reticulum (SER):** This organelle is primarily involved in lipid synthesis, steroid hormone production, and detoxification (via Cytochrome P450 enzymes), not oxidative metabolism. * **Nucleus:** The nucleus houses genetic material and is the site for replication and transcription; it does not host major metabolic pathways like the TCA cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Amphibolic Nature:** The Krebs cycle is both catabolic (breaks down Acetyl-CoA) and anabolic (provides intermediates for gluconeogenesis and heme synthesis). * **Rate-limiting enzyme:** Isocitrate Dehydrogenase. * **Energy Yield:** One turn of the cycle produces **10 ATP** (3 NADH = 7.5, 1 $FADH_2$ = 1.5, 1 GTP = 1). * **Inhibitor:** Fluoroacetate inhibits Aconitase, while Arsenite inhibits the $\alpha$-Ketoglutarate dehydrogenase complex.

Question 245: Which of the following is NOT a characteristic feature of Von Gierke disease?

A. Hepatorenomegaly
B. Myopathy (Correct Answer)
C. Growth Retardation
D. Doll-like faces

Explanation: **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.

Question 246: Blood glucose levels cannot be augmented by mobilization of muscle glycogen due to lack of which enzyme?

A. G-6-P dehydrogenase
B. G-6-phosphatase (Correct Answer)
C. Aldolase
D. Glucokinase

Explanation: **Explanation** The correct answer is **G-6-phosphatase**. **1. Why G-6-phosphatase is correct:** Muscle glycogen serves as a source of energy for the muscle itself during contraction, but it cannot contribute to blood glucose levels. This is because muscles lack the enzyme **Glucose-6-phosphatase**. In glycogenolysis, glycogen is broken down into Glucose-1-phosphate and then converted to **Glucose-6-phosphate (G-6-P)**. To enter the bloodstream, G-6-P must be dephosphorylated into free glucose. This reaction is catalyzed by G-6-phosphatase, which is present in the **liver and kidneys** but absent in muscle tissue. Consequently, G-6-P in the muscle is forced into the glycolytic pathway to produce ATP. **2. Why the other options are incorrect:** * **A. G-6-P dehydrogenase:** This is the rate-limiting enzyme of the Hexose Monophosphate (HMP) Shunt. Its deficiency leads to G6PD deficiency (hemolytic anemia) but does not affect glucose release from glycogen. * **C. Aldolase:** This enzyme is involved in glycolysis (cleaving Fructose-1,6-bisphosphate). It is present in muscles and is not involved in the release of free glucose into the blood. * **D. Glucokinase:** This enzyme phosphorylates glucose to G-6-P in the liver and pancreas. It helps in glucose utilization, not mobilization. **High-Yield Clinical Pearls for NEET-PG:** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of G-6-phosphatase. It presents with severe fasting hypoglycemia, hepatomegaly, and hyperlactatemia. * **Muscle Glycogen:** Lacks G-6-phosphatase; therefore, it provides "selfish" energy for local use only. * **Lactate Shuttle:** During exercise, muscle glycogen can indirectly contribute to blood glucose via the **Cori Cycle**, where muscle-derived lactate is converted to glucose in the liver.

Question 247: 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?

A. Glycogen phosphorylase (Correct Answer)
B. Carnitine palmitoyl transferase II
C. Glucose-6-phosphatase
D. Glycogen synthase

Explanation: ### Explanation The clinical presentation of exercise-induced muscle cramps, weakness, and a **failure of serum lactate to rise during an ischemic exercise test** is the hallmark of **McArdle Disease (Glycogen Storage Disease Type V)**. **1. Why Glycogen Phosphorylase is Correct:** In muscle, glycogen phosphorylase (myophosphorylase) is responsible for breaking down glycogen into glucose-1-phosphate to provide energy during anaerobic exercise. In McArdle disease, this enzyme is deficient. During the ischemic exercise test (where blood flow is restricted), the muscle is forced to rely on anaerobic glycolysis. Since glycogen cannot be mobilized, no pyruvate is produced, and consequently, **no lactate is formed**. This lack of fuel leads to ATP depletion, resulting in painful cramps and potential myoglobinuria. **2. Why the Other Options are Incorrect:** * **Carnitine Palmitoyl Transferase II (CPT II) Deficiency:** While this also causes exercise-induced cramps and myoglobinuria, the symptoms usually occur after *prolonged* exercise (fasting state) rather than short bursts of strenuous activity. Crucially, the ischemic exercise test shows a **normal** rise in lactate. * **Glucose-6-Phosphatase (Von Gierke Disease):** This enzyme is absent in the **liver and kidney**, not muscle. It presents with severe fasting hypoglycemia, hepatomegaly, and hyperlactatemia, not exercise-induced muscle cramps. * **Glycogen Synthase Deficiency (GSD Type 0):** This leads to decreased glycogen stores, resulting in early morning hypoglycemia and ketosis, but not the specific "block" in glycogenolysis seen here. **Clinical Pearls for NEET-PG:** * **"Second Wind" Phenomenon:** A classic feature of McArdle disease where symptoms improve after a few minutes of exercise once the body switches to using blood-borne glucose and free fatty acids. * **Ischemic Exercise Test:** A flat lactate curve with a **disproportionate rise in ammonia** is diagnostic for McArdle disease. * **Biochemical Marker:** Elevated serum creatine kinase (CK) is common due to muscle damage.

Question 248: Which of the following steps in glycolysis requires energy?

A. Pyruvate carboxylase
B. Phosphoenolpyruvate carboxykinase
C. Phosphoglycerate kinase
D. Hexokinase (Correct Answer)

Explanation: **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**.

Question 249: Which enzyme is strongly activated by fructose 2,6 bisphosphate?

A. Cyclic AMP
B. Adenosine triphosphate
C. Citrate
D. Phosphofructokinase 1 (Correct Answer)

Explanation: **Explanation:** **Phosphofructokinase-1 (PFK-1)** is the key rate-limiting enzyme of glycolysis. It is regulated by the energy status of the cell and hormonal signals. **Fructose 2,6-bisphosphate (F2,6-BP)** is the most potent allosteric activator of PFK-1. It increases the enzyme's affinity for its substrate (Fructose-6-phosphate) and helps overcome the inhibitory effects of ATP, thereby accelerating glycolysis. **Analysis of Options:** * **Phosphofructokinase-1 (Correct):** F2,6-BP acts as a "molecular switch" that signals high glucose availability (via insulin), strongly activating PFK-1 to drive glycolysis forward. * **Cyclic AMP (Incorrect):** cAMP is a second messenger, not an enzyme. In the liver, high cAMP levels (triggered by glucagon) actually lead to a *decrease* in F2,6-BP levels, thereby inhibiting PFK-1. * **Adenosine Triphosphate (Incorrect):** ATP acts as an **allosteric inhibitor** of PFK-1. High ATP levels signal that the cell has sufficient energy, slowing down glycolysis. * **Citrate (Incorrect):** Citrate is an intermediate of the TCA cycle and acts as an **allosteric inhibitor** of PFK-1. High citrate levels signal that biosynthetic precursors are abundant, reducing glycolytic flux. **High-Yield Clinical Pearls for NEET-PG:** * **Bifunctional Enzyme:** F2,6-BP levels are regulated by a single protein with two activities: **PFK-2** (synthesis) and **FBPase-2** (breakdown). * **Insulin vs. Glucagon:** Insulin dephosphorylates this bifunctional enzyme, activating PFK-2 and increasing F2,6-BP (promoting glycolysis). Glucagon phosphorylates it, activating FBPase-2 and decreasing F2,6-BP (promoting gluconeogenesis). * **Reciprocal Regulation:** F2,6-BP simultaneously activates PFK-1 (glycolysis) and inhibits Fructose 1,6-bisphosphatase (gluconeogenesis), preventing a futile cycle.

Question 250: Which of the following is FALSE regarding the Hexose Monophosphate (HMP) shunt?

A. NADPH is produced
B. Ribulose 5-phosphate is produced
C. ATP is produced (Correct Answer)
D. Occurs in the cytosol

Explanation: ### 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.

Practice by Chapter

Carbohydrate Chemistry and Classification

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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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