Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 541: A baby presents with early morning hypoglycemia. Glucagon administration raises blood glucose levels if given after meals but not during fasting. Liver biopsy shows increased glycogen deposits. What enzyme defect is most likely responsible?

A. Muscle phosphorylase
B. Glucose-6-phosphatase
C. Branching enzyme
D. Debranching enzyme (Correct Answer)

Explanation: ### Explanation The clinical presentation describes **Cori disease (Type III Glycogen Storage Disease)**, caused by a deficiency of the **Debranching enzyme** (Amylo-1,6-glucosidase). **1. Why Debranching Enzyme is correct:** In Cori disease, glycogenolysis is impaired because the debranching enzyme cannot remove the "limit dextrins" (short branches). * **Post-prandial state:** Glucagon raises blood glucose because the phosphorylase enzyme can still act on the outer linear chains of newly synthesized glycogen. * **Fasting state:** Once the outer chains are exhausted, phosphorylase stops at the branch points. Since the debranching enzyme is missing, no further glucose can be released, leading to **fasting hypoglycemia** and the accumulation of **limit dextrin-like glycogen** in the liver. **2. Analysis of Incorrect Options:** * **A. Muscle phosphorylase (McArdle Disease/Type V):** Affects muscles only. It presents with exercise intolerance and cramps, not fasting hypoglycemia or hepatomegaly. * **B. Glucose-6-phosphatase (Von Gierke Disease/Type I):** This is the most severe GSD. Glucagon administration **never** raises blood glucose (even after meals) because the final step of glucose release is blocked. It is also associated with severe lactic acidosis and hyperuricemia. * **C. Branching enzyme (Andersen Disease/Type IV):** Presents with "amylopectin-like" glycogen (long, unbranched chains). It typically leads to early liver cirrhosis and failure, rather than isolated hypoglycemia. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cori vs. Von Gierke:** Both have hepatomegaly and hypoglycemia, but Cori is **milder**, has **normal lactate** levels, and responds to glucagon in the fed state. * **Limit Dextrin:** The hallmark biochemical finding in Cori disease. * **Treatment:** Frequent high-protein meals (protein can be converted to glucose via gluconeogenesis, which remains intact in Cori disease).

Question 542: What is the net ATP yield from glycolysis?

A. 5
B. 8 (Correct Answer)
C. 10
D. 15

Explanation: **Explanation:** In the context of NEET-PG and standard medical biochemistry (based on Harper’s Illustrated Biochemistry), the **net ATP yield of aerobic glycolysis is 8 ATP**. This calculation is derived from the following steps: 1. **ATP Consumption Phase:** 2 ATP are consumed (Hexokinase and Phosphofructokinase-1 reactions). 2. **ATP Generation Phase (Substrate-level):** 4 ATP are produced (Phosphoglycerate kinase and Pyruvate kinase reactions). 3. **Oxidative Phosphorylation:** 2 molecules of NADH are produced (Glyceraldehyde-3-phosphate dehydrogenase). In the Malate-Aspartate shuttle (predominant in heart, liver, and kidney), each NADH yields 2.5 (rounded to 3 in older texts) ATP. Thus, 2 NADH × 3 = 6 ATP. **Net Yield:** (4 Substrate ATP + 6 Oxidative ATP) – 2 Consumed ATP = **8 ATP**. **Analysis of Incorrect Options:** * **Option A (5):** This is the net yield if the **Glycerol-3-phosphate shuttle** is used (common in skeletal muscle/brain), where 2 NADH yield only 3 ATP (1.5 each). (4+3-2 = 5). * **Option C (10):** This represents the total gross production (4 substrate + 6 oxidative) without subtracting the 2 ATP consumed in the initial steps. * **Option D (15):** This value does not correspond to glycolysis; it is closer to the yield of one turn of the TCA cycle plus the preceding pyruvate dehydrogenase reaction. **High-Yield Clinical Pearls for NEET-PG:** * **Anaerobic Glycolysis:** The net yield is only **2 ATP** because NADH is consumed to reduce pyruvate to lactate. * **Rapoport-Luebering Cycle:** In RBCs, bypassing the phosphoglycerate kinase step to form 2,3-BPG results in a **net yield of 0 ATP** for that specific shunt. * **Key Regulatory Step:** Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme of glycolysis.

Question 543: A patient's blood glucose levels are normal by the GOD-POD method, but their urine shows a positive Benedict's test. What is the most likely reason for this discrepancy in results?

A. False positive result
B. Fructosemia
C. Galactosemia (Correct Answer)
D. Glucose intolerance

Explanation: **Explanation:** The discrepancy between a normal blood glucose level and a positive Benedict’s test in the urine is a classic biochemical presentation of **reducing sugars other than glucose** being excreted. 1. **Why Galactosemia is correct:** The **GOD-POD method** (Glucose Oxidase-Peroxidase) is highly specific for **D-glucose**. It will not detect other sugars. However, **Benedict’s test** is a non-specific semi-quantitative test that detects any **reducing sugar** (glucose, fructose, galactose, lactose, etc.) by reducing cupric ions to cuprous oxide. In Galactosemia, galactose levels are elevated in the blood and spill into the urine (galactosuria). Since galactose is a reducing sugar, it gives a positive Benedict’s test, while the glucose-specific GOD-POD blood test remains normal. 2. **Analysis of Incorrect Options:** * **False positive result:** While certain drugs (like Vitamin C or Salicylates) can cause false positives in Benedict's test, in a clinical exam context, a metabolic disorder is the preferred answer. * **Fructosemia:** While fructose is a reducing sugar, Galactosemia is more frequently tested in this context and often presents earlier/more severely in pediatric cases. However, if Galactosemia is an option, it is the classic "textbook" answer for this discrepancy. * **Glucose intolerance:** This would result in high blood glucose levels, which would be detected by the GOD-POD method, contradicting the question stem. **High-Yield Clinical Pearls for NEET-PG:** * **Specific vs. Non-specific:** GOD-POD = Specific for Glucose; Benedict’s = Non-specific for all reducing sugars. * **Non-reducing sugar:** **Sucrose** is the most important non-reducing sugar (gives a negative Benedict's test). * **Galactosemia Triad:** Cataract (due to galactitol), Hepatomegaly, and Mental retardation. * **Inborn errors of metabolism:** Always suspect a reducing sugar in urine if a neonate presents with jaundice or cataracts despite "normal" glucose strips.

Question 544: A patient has a rare disease characterized by impaired secretory function of the alpha-cells of the pancreas. Direct stimulation of which of the following pathways in the liver will be impaired?

A. Citric acid cycle
B. Glycogenesis
C. Gluconeogenesis (Correct Answer)
D. Glycolysis

Explanation: **Explanation** The core of this question lies in understanding the hormonal regulation of hepatic metabolism. **Why Gluconeogenesis is Correct:** The alpha-cells of the pancreas are responsible for secreting **Glucagon**. Glucagon is a "counter-regulatory" hormone released during fasting states. Its primary role in the liver is to maintain blood glucose levels by stimulating **Gluconeogenesis** (the synthesis of glucose from non-carbohydrate precursors) and Glycogenolysis. It achieves this by increasing cAMP levels, activating Protein Kinase A, and inducing key enzymes like PEPCK and Fructose-1,6-bisphosphatase. If alpha-cell function is impaired, glucagon levels drop, leading to a failure in the direct stimulation of the gluconeogenic pathway. **Why the other options are incorrect:** * **Glycogenesis & Glycolysis:** These pathways are stimulated by **Insulin** (secreted by beta-cells), not glucagon. Glucagon actually *inhibits* glycolysis (by decreasing Fructose-2,6-bisphosphate) and glycogenesis to prevent a futile cycle during fasting. * **Citric Acid Cycle (TCA):** While glucagon influences substrate availability for the TCA cycle, it does not "directly stimulate" the cycle as a primary metabolic pathway in the same regulatory manner as gluconeogenesis. **NEET-PG High-Yield Pearls:** * **Glucagon’s Second Messenger:** It acts via the **Gαs - Adenylate Cyclase - cAMP** pathway. * **Key Regulatory Enzyme:** Glucagon inhibits **Pyruvate Kinase** via phosphorylation, diverting phosphoenolpyruvate (PEP) toward gluconeogenesis instead of glycolysis. * **Clinical Correlation:** Patients with glucagon deficiency (or alpha-cell failure) are highly prone to **fasting hypoglycemia**. * **Insulin/Glucagon Ratio:** It is the ratio of these two hormones, rather than absolute levels, that dictates the direction of hepatic metabolic flux.

Question 545: Lactic acidosis in thiamine deficiency is due to which enzyme dysfunction?

A. Phosphoenol pyruvate carboxykinase
B. Pyruvate dehydrogenase (Correct Answer)
C. Pyruvate carboxylase
D. Aldolase

Explanation: **Explanation:** **1. Why Pyruvate Dehydrogenase (PDH) is the correct answer:** The **Pyruvate Dehydrogenase Complex (PDH)** is a multi-enzyme complex that converts Pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. This enzyme requires **Thiamine Pyrophosphate (TPP)**, the active form of Vitamin B1, as a mandatory cofactor. In thiamine deficiency, PDH activity is severely impaired. Consequently, pyruvate cannot enter the TCA cycle and instead accumulates in the cytosol. To regenerate NAD+ and maintain glycolysis, the body shunts this excess pyruvate into the **Lactic Acid pathway** via the enzyme Lactate Dehydrogenase. This leads to an accumulation of lactic acid, resulting in **Lactic Acidosis**. **2. Why other options are incorrect:** * **Phosphoenolpyruvate carboxykinase (PEPCK):** This is a key enzyme in gluconeogenesis (converting OAA to PEP). It requires GTP, not thiamine. * **Pyruvate carboxylase:** This enzyme converts pyruvate to oxaloacetate. It is a biotin-dependent (Vitamin B7) enzyme, not thiamine-dependent. * **Aldolase:** This enzyme functions in glycolysis (cleaving Fructose-1,6-bisphosphate). It does not require thiamine. **3. NEET-PG High-Yield Pearls:** * **TPP-dependent enzymes:** Remember the mnemonic **"ATP"**: **A**lpha-ketoglutarate dehydrogenase, **T**ransketolase, and **P**yruvate dehydrogenase (also Branched-chain ketoacid dehydrogenase). * **Clinical Correlation:** Lactic acidosis is a hallmark of **Beri-beri** and **Wernicke-Korsakoff syndrome**. * **Diagnostic Tip:** In thiamine deficiency, **Erythrocyte Transketolase activity** is decreased; this is the gold standard biochemical test. * **Management Warning:** Always administer thiamine *before* glucose in malnourished patients to prevent precipitating Wernicke’s encephalopathy (as glucose loading increases the demand for TPP).

Question 546: Which of the following enzymes is involved in gluconeogenesis, except?

A. Phosphoglycerate kinase
B. Fructose 1, 6-biphosphatase
C. Phosphoglucomutase (Correct Answer)
D. Pyruvate carboxylase

Explanation: ### Explanation **Gluconeogenesis** is the metabolic pathway that results in the generation of glucose from non-carbohydrate precursors. It essentially reverses glycolysis but must bypass three irreversible steps using four unique enzymes. #### Why Phosphoglucomutase is the Correct Answer **Phosphoglucomutase** is an enzyme involved in **glycogenesis** and **glycogenolysis**. It catalyzes the reversible conversion of Glucose-1-Phosphate to Glucose-6-Phosphate. While it handles glucose derivatives, it is not a component of the gluconeogenic pathway, which focuses on converting pyruvate/lactate/amino acids into glucose. #### Analysis of Other Options * **Pyruvate Carboxylase (Option D):** This is the first regulatory enzyme of gluconeogenesis. It converts pyruvate to oxaloacetate in the mitochondria (requires Biotin and ATP). * **Fructose 1,6-bisphosphatase (Option B):** This is the **rate-limiting enzyme** of gluconeogenesis. It bypasses the irreversible PFK-1 step of glycolysis by converting Fructose 1,6-bisphosphate to Fructose 6-phosphate. * **Phosphoglycerate Kinase (Option A):** This enzyme catalyzes a **reversible** step in glycolysis. Because it is reversible, the same enzyme is utilized in gluconeogenesis to convert 1,3-bisphosphoglycerate to 3-phosphoglycerate (and vice versa). #### NEET-PG High-Yield Pearls * **The Four Key Gluconeogenic Enzymes:** 1. Pyruvate Carboxylase 2. PEP Carboxykinase (PEPCK) 3. Fructose 1,6-bisphosphatase (Rate-limiting) 4. Glucose 6-phosphatase (Absent in muscle, hence muscle cannot contribute to blood glucose). * **Location:** Gluconeogenesis occurs primarily in the **Liver** (90%) and Kidney (10%). * **Subcellular site:** It is a "mixed" pathway; Pyruvate carboxylase is **mitochondrial**, while the rest are **cytosolic** (except Glucose 6-phosphatase, which is in the ER).

Question 547: What is true about gluconeogenesis?

A. Occurs mainly in the muscle
B. It is the reverse of glycolysis
C. Alanine and lactate can both serve as substrates (Correct Answer)
D. Glycerol is not a substrate

Explanation: **Explanation:** Gluconeogenesis is the metabolic pathway that results in the generation of glucose from non-carbohydrate precursors. It is essential for maintaining blood glucose levels during fasting and intense exercise. **Why Option C is correct:** Gluconeogenesis utilizes several non-carbohydrate substrates. **Lactate** (produced by anaerobic glycolysis in muscles and RBCs) is converted to pyruvate via the Cori Cycle. **Alanine** (the primary glucogenic amino acid) is transported from muscles to the liver and converted to pyruvate via the Glucose-Alanine Cycle. Both enter the gluconeogenic pathway at the level of pyruvate. **Analysis of Incorrect Options:** * **Option A:** Gluconeogenesis occurs primarily in the **Liver** (90%) and to a lesser extent in the **Kidney cortex** (10%). Muscle lacks Glucose-6-Phosphatase, meaning it cannot release free glucose into the blood. * **Option B:** It is **not a simple reversal** of glycolysis. While they share many enzymes, gluconeogenesis must bypass the three irreversible steps of glycolysis (Hexokinase, PFK-1, and Pyruvate Kinase) using four unique enzymes: Pyruvate Carboxylase, PEP Carboxykinase, Fructose-1,6-Bisphosphatase, and Glucose-6-Phosphatase. * **Option D:** **Glycerol is a substrate.** It is derived from the breakdown of triacylglycerols in adipose tissue, phosphorylated to glycerol-3-phosphate, and converted to Dihydroxyacetone phosphate (DHAP), an intermediate in gluconeogenesis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Fructose-1,6-Bisphosphatase. * **Requirement:** It is an energy-expensive process requiring 6 ATP/GTP per molecule of glucose formed. * **Key Activator:** Acetyl-CoA obligatorily activates Pyruvate Carboxylase. * **Clinical Link:** Alcohol inhibits gluconeogenesis by increasing the NADH/NAD+ ratio, diverting pyruvate to lactate and leading to fasting hypoglycemia.

Question 548: Which metabolite is involved in glycolysis, the HMP shunt pathway, and gluconeogenesis?

A. Glucose-1-phosphate
B. Glucose-6-phosphate (Correct Answer)
C. Fructose-6-phosphate
D. Pyruvate

Explanation: **Explanation:** **Glucose-6-phosphate (G6P)** is the central metabolic hub of carbohydrate metabolism, acting as a common intermediate for several intersecting pathways. 1. **Glycolysis:** G6P is the first intermediate formed after glucose enters the cell (catalyzed by Hexokinase/Glucokinase). It is then isomerized to Fructose-6-phosphate. 2. **HMP Shunt (Pentose Phosphate Pathway):** G6P is the starting substrate for this pathway. It is oxidized by **G6P Dehydrogenase (G6PD)** to generate NADPH and ribose-5-phosphate. 3. **Gluconeogenesis:** It is the final intermediate produced before the release of free glucose. The enzyme **Glucose-6-phosphatase** (found in the liver and kidney) removes the phosphate group to allow glucose to enter the bloodstream. 4. **Glycogenesis/Glycogenolysis:** It also serves as the link to glycogen metabolism via conversion to Glucose-1-phosphate. **Analysis of Incorrect Options:** * **A. Glucose-1-phosphate:** Primarily involved in Glycogenesis and Glycogenolysis; it is not a direct intermediate of glycolysis or the HMP shunt. * **C. Fructose-6-phosphate:** While involved in glycolysis and gluconeogenesis, it is not the starting substrate for the HMP shunt (though it can be a recycling product). * **D. Pyruvate:** The end product of aerobic glycolysis and a substrate for gluconeogenesis, but it plays no role in the HMP shunt. **NEET-PG High-Yield Pearls:** * **G6PD Deficiency:** The most common enzyme deficiency worldwide, leading to hemolytic anemia due to the inability of RBCs to generate NADPH via the HMP shunt. * **Von Gierke Disease (GSD Type I):** Caused by a deficiency of **Glucose-6-phosphatase**, leading to severe fasting hypoglycemia and hepatomegaly because G6P cannot be converted back to glucose. * **Muscle Metabolism:** Muscle lacks Glucose-6-phosphatase; therefore, muscle glycogen cannot contribute to blood glucose levels.

Question 549: What are isomers that differ in the position of -H and -OH at the 2nd, 3rd, and 4th carbon atoms of glucose called?

A. Optical isomers
B. Epimers (Correct Answer)
C. Anomers
D. D,L isomers

Explanation: ### Explanation **1. Why Epimers is the Correct Answer:** Epimers are a subtype of diastereomers that differ in configuration around only **one specific chiral carbon atom** (other than the anomeric carbon). Glucose, mannose, and galactose are the most clinically relevant examples: * **C-2 Epimer:** Glucose and **Mannose** differ only at the 2nd carbon. * **C-4 Epimer:** Glucose and **Galactose** differ only at the 4th carbon. * **C-3 Epimer:** Glucose and **Allose** (though less clinically common) differ at the 3rd carbon. Since the question refers to isomers differing at these specific positions (2nd, 3rd, or 4th), they are classified as epimers. **2. Why Other Options are Incorrect:** * **Optical Isomers:** This is a broad term for compounds that rotate plane-polarized light (dextrorotatory or levorotatory). While epimers are optical isomers, the term is too general and does not specifically describe the positional difference at a single carbon. * **Anomers:** These are isomers that differ specifically at the **anomeric carbon** (C-1 for glucose, C-2 for fructose) during ring closure, resulting in $\alpha$ and $\beta$ forms. * **D,L Isomers (Enantiomers):** These are non-superimposable mirror images. They differ at **all** chiral centers, determined by the orientation of the -OH group on the most distant chiral carbon from the carbonyl group (C-5 in glucose). **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Enzyme Fact:** The interconversion of epimers (e.g., Galactose to Glucose) is catalyzed by **Epimerases** (e.g., UDP-galactose 4-epimerase). * **Mnemonic:** **"Ga-4-G"** (Galactose is the C-4 epimer of Glucose) and **"Ma-2-G"** (Mannose is the C-2 epimer of Glucose). * **Clinical Link:** Classic Galactosemia involves a deficiency in GALT, but the body can still synthesize galactose for glycolipids via the 4-epimerase enzyme.

Question 550: NADPH is produced by which metabolic pathway?

A. Glycolysis
B. Citric acid cycle
C. Hexose monophosphate shunt (Correct Answer)
D. Glycogenesis

Explanation: **Explanation:** The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), is the primary metabolic source of **NADPH** in the body. This pathway occurs in the cytosol and consists of an irreversible oxidative phase where the enzyme **Glucose-6-Phosphate Dehydrogenase (G6PD)** reduces NADP+ to NADPH. **Why the other options are incorrect:** * **Glycolysis:** This pathway produces **NADH** (specifically during the conversion of Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate) and ATP, but not NADPH. * **Citric Acid Cycle (TCA):** This mitochondrial cycle produces **NADH** and **FADH2**, which are used in the electron transport chain for ATP production. * **Glycogenesis:** This is the process of glycogen synthesis from glucose; it consumes energy (UTP/ATP) but does not involve the production of reducing equivalents like NADPH. **Clinical Pearls & High-Yield Facts for NEET-PG:** 1. **Functions of NADPH:** It is essential for **reductive biosynthesis** (fatty acids, steroids) and maintaining **reduced glutathione** to protect cells against reactive oxygen species (ROS). 2. **Tissue Distribution:** The HMP shunt is highly active in tissues requiring NADPH, such as the **adrenal cortex** (steroidogenesis), **lactating mammary glands** (fatty acid synthesis), and **erythrocytes** (antioxidant defense). 3. **Clinical Correlation:** **G6PD deficiency** is the most common enzyme deficiency worldwide. Without sufficient NADPH, RBCs cannot regenerate reduced glutathione, leading to hemolysis under oxidative stress (e.g., fava beans, primaquine). 4. **Rate-limiting enzyme:** G6PD is the key regulatory enzyme of this pathway.

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

Practice Questions

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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