Carbohydrate Metabolism — MCQs
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Within the RBC, hypoxia stimulates glycolysis by which of the following regulating pathways?
Hypoglycemia is defined as a blood glucose value of less than?
Biosynthesis of glucuronic acid requires the:
An eight-month-old female infant presented with recurrent episodes of hypoglycemia, especially if the time interval between feedings is increased. Dicarboxylic acid is present in the urine, and urine ketone bodies are negative. The child responded well to IV glucose, a diet low in fat and high in carbohydrate, and frequent feedings. The child was diagnosed with MCAD deficiency. What is the primary reason for hypoglycemia in this condition?
A 6-month-old male infant presents with vomiting, lethargy, and severe jaundice upon initiation of weaning with fruit juice. Which of the following enzymes is defective?
A histological section of the left ventricle of a deceased 28-year-old male shows classic contraction band necrosis of the myocardium. Biological specimens confirm the presence of cocaine and metabolites. Activity of which of the following enzymes was most likely increased in the patient's myocardial cells shortly prior to his death?
Complex polysaccharides are converted to glucose and absorbed by the help of which enzyme?
When there is no other source of glucose, how long would liver and muscle glycogen stores typically be exhausted?
Which of the following is not a hexose sugar?
In the fed state, what is the major fate of Glucose-6-phosphate in tissues?
Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs
Question 631: Within the RBC, hypoxia stimulates glycolysis by which of the following regulating pathways?
- A. Hypoxia stimulates pyruvate dehydrogenase by increased 2,3 DPG.
- B. Hypoxia inhibits hexokinase.
- C. Hypoxia stimulates release of all glycolytic enzymes from band 3 on the RBC membrane. (Correct Answer)
- D. Activation of the regulatory enzymes by high pH.
Explanation: **Explanation:** The regulation of glycolysis in Red Blood Cells (RBCs) under hypoxic conditions is unique due to the structural role of the erythrocyte membrane. **Why Option C is Correct:** In the RBC membrane, **Band 3 (Anion Exchanger 1)** serves as a docking site for several key glycolytic enzymes (including PFK, Aldolase, and GAPDH). When these enzymes are bound to the cytoplasmic tail of Band 3, they are **inhibited**. Under hypoxic conditions, deoxyhemoglobin (deoxy-Hb) has a high affinity for Band 3. Deoxy-Hb displaces the glycolytic enzymes from Band 3 to bind there itself. This release into the cytosol **activates** the enzymes, thereby stimulating glycolysis to meet the cell's energy needs despite low oxygen. **Why Other Options are Incorrect:** * **Option A:** RBCs lack mitochondria; therefore, they do not possess the **Pyruvate Dehydrogenase (PDH)** complex. They rely solely on anaerobic glycolysis, converting pyruvate to lactate. * **Option B:** Hypoxia stimulates, rather than inhibits, hexokinase and the overall glycolytic flux to compensate for reduced metabolic efficiency. * **Option D:** Hypoxia typically leads to lactic acid accumulation, which **lowers** pH (acidosis). High pH (alkalosis) actually stimulates PFK-1, but this is not the mechanism triggered by hypoxia. **High-Yield Clinical Pearls for NEET-PG:** * **Rapoport-Luebering Cycle:** A bypass of glycolysis in RBCs that produces **2,3-BPG**, which shifts the oxygen-dissociation curve to the **right** (promoting O2 release to tissues). * **Mature RBC Metabolism:** They lack a nucleus, ribosomes, and mitochondria. They derive 100% of their ATP from **anaerobic glycolysis** (Embden-Meyerhof pathway). * **Band 3 Protein:** It is the most abundant integral membrane protein in RBCs, responsible for the "chloride shift."
Question 632: Hypoglycemia is defined as a blood glucose value of less than?
- A. 60 mg/dl
- B. 50 mg/dl
- C. 40 mg/dl (Correct Answer)
- D. 30 mg/dl
Explanation: **Explanation:** In the context of clinical biochemistry and standard medical examinations like NEET-PG, hypoglycemia is biochemically defined as a blood glucose level **less than 40 mg/dl**. While physiological symptoms may begin at higher thresholds, this specific value is the recognized cutoff for diagnosing significant hypoglycemia in adults. **Breakdown of Options:** * **40 mg/dl (Correct):** This is the classical biochemical definition. At this level, the brain's glucose supply becomes critically compromised, leading to neuroglycopenic symptoms (confusion, seizures, or coma). * **60 mg/dl (Incorrect):** This is often considered the "lower limit of normal" fasting glucose. While levels between 40–60 mg/dl may trigger counter-regulatory hormones (like glucagon and epinephrine), they do not strictly define clinical hypoglycemia. * **50 mg/dl (Incorrect):** This is a common threshold used in the **Whipple’s Triad** for symptomatic patients, but 40 mg/dl remains the definitive biochemical standard for the exam. * **30 mg/dl (Incorrect):** This represents severe, life-threatening hypoglycemia, often seen in profound insulin overdose or advanced insulinoma, but it is too low to be the baseline definition. **High-Yield Clinical Pearls for NEET-PG:** 1. **Whipple’s Triad:** Essential for diagnosing hypoglycemia. It includes: (1) Symptoms consistent with hypoglycemia, (2) Low plasma glucose (typically <50 mg/dl), and (3) Relief of symptoms when glucose is raised. 2. **Hormonal Response:** The first response to falling glucose is the suppression of insulin, followed by the release of **Glucagon** and **Epinephrine** (the most important acute counter-regulatory hormones). 3. **Neonatal Hypoglycemia:** Note that the definition differs in newborns; it is generally defined as <45 mg/dl after the first 24 hours of life.
Question 633: Biosynthesis of glucuronic acid requires the:
- A. Oxidation of UDP glucose (Correct Answer)
- B. Oxidation of glucose 6-phosphate
- C. Oxidation of 6-phosphogluconate
- D. Oxidation of glucose
Explanation: ### Explanation The biosynthesis of **Glucuronic acid** occurs via the **Uronic Acid Pathway**. This pathway is essential for the production of UDP-glucuronate, which serves as a precursor for proteoglycans and plays a critical role in detoxification reactions. **Why Option A is Correct:** The key regulatory step in this pathway is the conversion of **UDP-glucose to UDP-glucuronate**. This reaction is catalyzed by the enzyme **UDP-glucose dehydrogenase**. During this process, the primary alcohol group at carbon-6 of the glucose moiety is oxidized to a carboxyl group. This reaction requires **NAD+** as a co-factor (reducing it to NADH). **Why Other Options are Incorrect:** * **Option B (Glucose 6-phosphate):** Oxidation of G6P occurs in the Pentose Phosphate Pathway (PPP) by G6PD to form 6-phosphogluconolactone, not glucuronic acid. * **Option C (6-phosphogluconate):** This is an intermediate of the PPP. Its oxidation leads to the formation of Ribulose 5-phosphate and CO₂. * **Option D (Glucose):** Direct oxidation of glucose at C-1 yields gluconic acid, while oxidation at C-6 yields glucuronic acid; however, in the human body, this does not occur via "free" glucose but specifically through the **UDP-bound form**. **High-Yield Clinical Pearls for NEET-PG:** 1. **Conjugation:** UDP-glucuronate is essential for the conjugation of **bilirubin** (forming bilirubin diglucuronide) and various drugs (e.g., morphine, steroids) to make them water-soluble for excretion. 2. **Vitamin C Connection:** In most animals, the uronic acid pathway leads to **Ascorbic acid** synthesis. However, **humans lack the enzyme L-gulonolactone oxidase**, making Vitamin C an essential dietary requirement. 3. **Essential Pentosuria:** A deficiency of **Xylitol dehydrogenase** in this pathway leads to the excretion of L-xylulose in urine (a benign condition).
Question 634: An eight-month-old female infant presented with recurrent episodes of hypoglycemia, especially if the time interval between feedings is increased. Dicarboxylic acid is present in the urine, and urine ketone bodies are negative. The child responded well to IV glucose, a diet low in fat and high in carbohydrate, and frequent feedings. The child was diagnosed with MCAD deficiency. What is the primary reason for hypoglycemia in this condition?
- A. Increased dicarboxylic acid inhibits glycogenolysis.
- B. Lack of sufficient acetyl-CoA to support gluconeogenesis. (Correct Answer)
- C. Impaired beta-oxidation leading to decreased acetyl-CoA availability for gluconeogenesis.
- D. Inadequate glycogen stores due to a defect in glycogen synthesis.
Explanation: ### Explanation **1. Why Option B is Correct:** Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is the most common defect of fatty acid oxidation. In this condition, the body cannot break down medium-chain fatty acids into **Acetyl-CoA**. Acetyl-CoA is a critical **obligatory activator of Pyruvate Carboxylase**, the first rate-limiting enzyme of gluconeogenesis. During fasting, when glycogen stores are depleted, the liver relies on fatty acid oxidation to provide the energy (ATP) and the Acetyl-CoA necessary to drive gluconeogenesis. Without sufficient Acetyl-CoA, gluconeogenesis is impaired, leading to **hypoketotic hypoglycemia** (low blood sugar without the presence of ketones, as ketone body synthesis also requires Acetyl-CoA). **2. Why Other Options are Incorrect:** * **Option A:** While dicarboxylic acids are produced via omega-oxidation (a compensatory pathway), they are a *marker* of the disease, not the primary inhibitor of glycogenolysis. * **Option C:** While this statement is technically true regarding the mechanism, Option B is the more specific biochemical reason for the *failure of gluconeogenesis* (the lack of Pyruvate Carboxylase activation). In NEET-PG, the most direct biochemical link to the enzyme defect is preferred. * **Option D:** Glycogen synthesis is unaffected in MCAD deficiency. Hypoglycemia occurs only after glycogen stores are exhausted (prolonged fasting). **3. High-Yield Clinical Pearls for NEET-PG:** * **Classic Presentation:** An infant with "Reye-like" symptoms (vomiting, lethargy, seizures) triggered by fasting or illness. * **Key Lab Findings:** Hypoketotic hypoglycemia, C6-C10 dicarboxylic acids in urine, and elevated medium-chain acylcarnitines (Octanoylcarnitine) in blood. * **Management:** Avoid fasting; provide a high-carbohydrate, low-fat diet. * **Mnemonic:** MCAD = **M**edium chain, **C**an't **A**ctivate **D**-gluconeogenesis (due to low Acetyl-CoA).
Question 635: A 6-month-old male infant presents with vomiting, lethargy, and severe jaundice upon initiation of weaning with fruit juice. Which of the following enzymes is defective?
- A. Fructokinase
- B. Aldolase A
- C. Aldolase B (Correct Answer)
- D. Sucrase
Explanation: **Explanation:** The clinical presentation of vomiting, lethargy, and jaundice triggered by the introduction of fruit juice (which contains fructose) in a 6-month-old infant is a classic description of **Hereditary Fructose Intolerance (HFI)**. **Why Aldolase B is correct:** HFI is caused by a deficiency of **Aldolase B**. In the liver, fructose is converted to Fructose-1-Phosphate (F1P) by fructokinase. Aldolase B is responsible for cleaving F1P into DHAP and glyceraldehyde. When Aldolase B is defective, **Fructose-1-Phosphate accumulates** intracellularly. This "traps" inorganic phosphate, leading to ATP depletion. The lack of ATP inhibits gluconeogenesis and glycogenolysis, resulting in severe postprandial hypoglycemia, liver damage (jaundice), and proximal renal tubular dysfunction. **Why other options are incorrect:** * **Fructokinase (Essential Fructosuria):** This is a benign, asymptomatic condition. Fructose is not trapped in cells but is excreted in the urine. There is no hypoglycemia or liver damage. * **Aldolase A:** This enzyme is primarily found in muscle and erythrocytes; its deficiency leads to hemolytic anemia and myopathy, not fructose intolerance. * **Sucrase:** Deficiency leads to osmotic diarrhea and flatulence due to malabsorption of sucrose, but it does not cause systemic toxicity or jaundice. **NEET-PG High-Yield Pearls:** * **The Trigger:** Symptoms appear only after weaning (introduction of fruits/sucrose-containing formula). * **The Mechanism:** "Phosphate Trapping" is the hallmark of HFI. * **Diagnosis:** Reducing sugars in urine (Clinitest positive) but Glucose Oxidase test (Dipstick) negative. * **Management:** Strict avoidance of Fructose, Sucrose, and Sorbitol.
Question 636: A histological section of the left ventricle of a deceased 28-year-old male shows classic contraction band necrosis of the myocardium. Biological specimens confirm the presence of cocaine and metabolites. Activity of which of the following enzymes was most likely increased in the patient's myocardial cells shortly prior to his death?
- A. Phosphoenolpyruvate carboxykinase
- B. Phosphofructokinase-1 (Correct Answer)
- C. Pyruvate dehydrogenase
- D. Succinate dehydrogenase
Explanation: **Explanation:** The clinical presentation describes a cocaine-induced myocardial infarction. Cocaine is a potent sympathomimetic that causes severe coronary vasospasm and increased myocardial oxygen demand, leading to **ischemia**. In ischemic conditions, oxygen delivery to the myocardium is severely restricted, halting oxidative phosphorylation. To maintain ATP levels, the cell shifts from aerobic metabolism to **anaerobic glycolysis**. The rate-limiting enzyme of glycolysis is **Phosphofructokinase-1 (PFK-1)**. During ischemia, the drop in ATP and the rise in AMP (adenosine monophosphate) potently activate PFK-1 to accelerate glucose breakdown for energy. Therefore, PFK-1 activity would be significantly increased shortly before death. **Analysis of Incorrect Options:** * **A. Phosphoenolpyruvate carboxykinase (PEPCK):** This is a key enzyme in gluconeogenesis (primarily in the liver/kidney). It is not induced by myocardial ischemia. * **C. Pyruvate dehydrogenase (PDH):** This enzyme converts pyruvate to Acetyl-CoA for the TCA cycle. In anaerobic conditions, PDH is inhibited by high NADH/NAD+ ratios and lactic acidosis; pyruvate is instead diverted to lactate. * **D. Succinate dehydrogenase:** A component of the TCA cycle and Complex II of the electron transport chain. Its activity requires oxygen and would be decreased, not increased, during ischemia. **High-Yield Clinical Pearls for NEET-PG:** * **PFK-1 Regulation:** Stimulated by AMP and Fructose-2,6-bisphosphate; inhibited by ATP and Citrate. * **Contraction Band Necrosis:** A hallmark of reperfusion injury or catecholamine-induced (e.g., cocaine, pheochromocytoma) myocardial damage. It results from hypercontraction of myofibrils due to calcium influx. * **Ischemic Shift:** In ischemia, the heart shifts from its preferred fuel (Fatty Acids) to Glucose (Glycolysis) because glycolysis requires less oxygen per mole of ATP produced.
Question 637: Complex polysaccharides are converted to glucose and absorbed by the help of which enzyme?
- A. Na+ K+ ATPase
- B. Sucrase (Correct Answer)
- C. Enterokinase
- D. Carboxypeptidase
Explanation: ### Explanation **Correct Option: B. Sucrase** The digestion of complex carbohydrates involves a sequential breakdown process. While salivary and pancreatic **α-amylase** break down starches into smaller oligosaccharides (like maltose and maltotriose), the final step occurs at the **brush border of the small intestine**. **Sucrase-Isomaltase** is a multi-enzyme complex located on the intestinal microvilli. It is responsible for hydrolyzing sucrose into glucose and fructose, and it also possesses significant maltase activity. These "brush border enzymes" are essential for converting disaccharides and oligosaccharides into **monosaccharides** (glucose, galactose, fructose), which are the only forms that can be absorbed into the portal circulation. **Analysis of Incorrect Options:** * **A. Na+ K+ ATPase:** This is a membrane pump that maintains ionic gradients. While it provides the driving force for glucose absorption via the SGLT-1 transporter (secondary active transport), it is not an enzyme that converts polysaccharides into glucose. * **C. Enterokinase (Enteropeptidase):** This enzyme is responsible for activating trypsinogen into **trypsin**, thereby initiating the cascade of protein digestion. It has no role in carbohydrate metabolism. * **D. Carboxypeptidase:** This is an exopeptidase secreted by the pancreas that cleaves peptide bonds at the C-terminal end of proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The absorption of carbohydrates is limited by the rate of hydrolysis by brush border enzymes, not by the transport mechanisms. * **SGLT-1 vs. GLUT-5:** Glucose and Galactose are absorbed via **SGLT-1** (Sodium-dependent), whereas Fructose is absorbed via **GLUT-5** (Facilitated diffusion). * **Deficiency:** Sucrase-isomaltase deficiency leads to osmotic diarrhea and abdominal distension upon ingestion of sucrose, similar to lactose intolerance.
Question 638: When there is no other source of glucose, how long would liver and muscle glycogen stores typically be exhausted?
- A. 12 hours
- B. 18 hours (Correct Answer)
- C. 24 hours
- D. 36 hours
Explanation: **Explanation:** The correct answer is **18 hours**. This question tests the physiological timeline of fuel utilization during the transition from the post-absorptive state to early starvation. **1. Why 18 hours is correct:** Glycogen is the primary storage form of glucose. Liver glycogen (approx. 100g) maintains blood glucose levels, while muscle glycogen (approx. 400g) is used locally for energy. During fasting, liver glycogenolysis begins immediately to maintain glycemia. However, these stores are limited. By **12–18 hours** of fasting, liver glycogen is significantly depleted. At the 18-hour mark, the body reaches a "crossover point" where **gluconeogenesis** (synthesis of glucose from non-carbohydrate sources like amino acids and glycerol) becomes the dominant source of blood glucose to compensate for the exhausted glycogen stores. **2. Why other options are incorrect:** * **12 hours:** While glycogenolysis is high, stores are not yet fully exhausted; they are still providing a significant portion of blood glucose. * **24–36 hours:** By this time, the body is firmly in the "starvation state." Liver glycogen is entirely depleted long before this point, and the brain has begun adapting to using ketone bodies alongside glucose produced via gluconeogenesis. **NEET-PG High-Yield Pearls:** * **Key Enzyme:** Glycogen phosphorylase is the rate-limiting enzyme for glycogenolysis (activated by Glucagon/Epinephrine). * **Muscle vs. Liver:** Muscle glycogen cannot contribute to blood glucose because muscles lack the enzyme **Glucose-6-Phosphatase**. * **Gluconeogenesis:** Becomes the sole source of glucose after approximately 24 hours of fasting. * **Clinical Correlation:** In Von Gierke’s Disease (G6Pase deficiency), severe hypoglycemia occurs much faster (within 2-4 hours) because neither glycogenolysis nor gluconeogenesis can release glucose into the blood.
Question 639: Which of the following is not a hexose sugar?
- A. Ribose (Correct Answer)
- B. Glucose
- C. Fructose
- D. Galactose
Explanation: ### Explanation **Concept:** Monosaccharides are classified based on the number of carbon atoms in their chain. **Hexoses** contain six carbon atoms ($C_6H_{12}O_6$), while **Pentoses** contain five carbon atoms ($C_5H_{10}O_5$). **Why Ribose is the Correct Answer:** **Ribose** is a **pentose sugar** (5-carbon). It is a vital component of RNA and various coenzymes like ATP, NAD, and FAD. Its derivative, deoxyribose, forms the backbone of DNA. Since it has only five carbons, it is not a hexose. **Analysis of Incorrect Options:** * **Glucose (B):** An aldohexose (6-carbon sugar with an aldehyde group). It is the primary metabolic fuel for the body. * **Fructose (C):** A ketohexose (6-carbon sugar with a ketone group). It is the sweetest natural sugar and is metabolized primarily in the liver. * **Galactose (D):** An aldohexose. It is a constituent of lactose (milk sugar) and is an epimer of glucose at the $C_4$ position. **High-Yield Clinical Pearls for NEET-PG:** * **Epimers:** Glucose and Galactose are **$C_4$ epimers**, while Glucose and Mannose are **$C_2$ epimers**. * **Functional Isomers:** Glucose (aldose) and Fructose (ketose) are functional isomers. * **Pentose Phosphate Pathway (PPP):** This is the primary metabolic pathway that generates Ribose-5-phosphate for nucleotide synthesis and NADPH for reductive biosynthesis. * **Essential Pentosuria:** A rare, benign genetic deficiency of the enzyme **L-xylulose reductase**, leading to the excretion of the pentose sugar L-xylulose in urine.
Question 640: In the fed state, what is the major fate of Glucose-6-phosphate in tissues?
- A. Storage as fructose
- B. Storage as glyceraldehyde 3-phosphate
- C. Enters HMP shunt to produce ribulose 5-phosphate
- D. Storage as glycogen (Correct Answer)
Explanation: **Explanation:** In the **fed state**, the body is under the influence of **insulin**, which promotes anabolic processes and energy storage. Glucose-6-phosphate (G6P) sits at a metabolic crossroads. When glucose levels are high, G6P is primarily diverted toward **Glycogenesis** (glycogen synthesis) in the liver and skeletal muscle for future energy needs. This is the major storage fate of glucose in the body. **Analysis of Options:** * **Option D (Correct):** Insulin activates **Glycogen Synthase** and induces **Glucokinase**, facilitating the conversion of G6P to Glucose-1-phosphate and subsequently into glycogen. * **Option A:** Fructose is a monosaccharide, not a storage form of glucose. While glucose can be converted to fructose via the polyol pathway (sorbitol pathway), this is not a major "storage" fate and is clinically significant mainly in diabetic complications. * **Option B:** Glyceraldehyde 3-phosphate (G3P) is a transient intermediate in glycolysis. It is used to generate ATP or provide the glycerol backbone for TAG synthesis, but it is not a storage molecule itself. * **Option C:** While G6P does enter the HMP shunt (Pentose Phosphate Pathway) to produce Ribose-5-phosphate and NADPH, this is a **functional pathway** for biosynthesis and antioxidant defense, not a "storage" fate. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme of Glycogenesis:** Glycogen Synthase (activated by Insulin). * **Glucokinase vs. Hexokinase:** Glucokinase (Liver/Pancreas) has a high $K_m$ and high $V_{max}$, allowing it to handle large glucose loads in the fed state. * **The "Metabolic Hub":** G6P can enter Glycolysis, Glycogenesis, HMP Shunt, or the Uronic Acid Pathway depending on the cell's energy status and hormonal signals.
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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