Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 141: What is the end product of anaerobic glycolysis?

A. 2 ATP + 2 NAD (Correct Answer)
B. 2 ATP + 2 NADH
C. 2 ATP + 2 FADH2
D. 4 ATP + 2 NAD

Explanation: ### Explanation **Concept:** In anaerobic glycolysis (occurring in RBCs or exercising muscle), the primary goal is to generate energy in the absence of oxygen. The process follows the standard glycolytic pathway until the formation of **Pyruvate**. However, to keep glycolysis running, the cell must regenerate **NAD+** from **NADH**. This is achieved by the enzyme **Lactate Dehydrogenase (LDH)**, which reduces Pyruvate to **Lactate** while simultaneously oxidizing NADH back to NAD+. **Why Option A is Correct:** 1. **Net ATP:** Glycolysis consumes 2 ATP and produces 4 ATP, resulting in a **net gain of 2 ATP**. 2. **NAD+ Regeneration:** The 2 NADH molecules produced during the glyceraldehyde-3-phosphate dehydrogenase step are consumed by LDH to produce **2 NAD+**. Therefore, the "end products" in terms of coenzymes are 2 NAD+. **Why Other Options are Incorrect:** * **Option B:** 2 NADH is the product of *aerobic* glycolysis (before the electron transport chain). In anaerobic conditions, NADH is used up to create lactate. * **Option C:** FADH2 is produced in the TCA cycle (Succinate Dehydrogenase step), not in glycolysis. * **Option D:** While 4 ATP are produced in the payoff phase, the *net* yield is 2 ATP because 2 were invested in the preparatory phase. **NEET-PG High-Yield Pearls:** * **RBCs:** Always undergo anaerobic glycolysis because they lack mitochondria. * **Key Enzyme:** **Lactate Dehydrogenase (LDH)** is the marker for anaerobic metabolism. * **Rapoport-Luebering Cycle:** A shunt in RBC glycolysis that produces 2,3-BPG, which shifts the oxygen dissociation curve to the right (facilitating O2 unloading). * **Arsenite Poisoning:** Inhibits enzymes requiring lipoic acid, but **Arsenate** competes with inorganic phosphate in glycolysis, resulting in **zero net ATP** production.

Question 142: Which glucose transporter is primarily present in Red Blood Cells?

A. GLUT-1 (Correct Answer)
B. GLUT-2
C. GLUT-3
D. GLUT-4

Explanation: **Explanation:** **GLUT-1** is the correct answer because it is the primary glucose transporter found in **Red Blood Cells (RBCs)** and the **Blood-Brain Barrier**. It is a high-affinity, insulin-independent transporter that ensures a constant basal glucose uptake, which is critical for RBCs as they rely exclusively on glycolysis for energy (due to the absence of mitochondria). **Analysis of Incorrect Options:** * **GLUT-2:** A low-affinity, high-capacity transporter found in the **Liver, Pancreas (beta cells), and Kidney**. It acts as a glucose sensor and is involved in bidirectional glucose transport. * **GLUT-3:** A high-affinity transporter primarily located in **Neurons**. It ensures glucose delivery to the brain even during low blood glucose concentrations. * **GLUT-4:** The only **Insulin-dependent** glucose transporter. It is sequestered in intracellular vesicles and translocates to the cell membrane only in the presence of insulin. It is found in **Skeletal Muscle and Adipose Tissue**. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT-1 Deficiency Syndrome:** Presents with infantile seizures and developmental delay due to impaired glucose transport across the blood-brain barrier. * **GLUT-5:** Specifically transports **Fructose** and is located in the small intestine and spermatozoa. * **SGLT-1/SGLT-2:** These are Sodium-Glucose Co-transporters (Active Transport) found in the small intestine and renal tubules, unlike the GLUT family which uses **Facilitated Diffusion**. * **Mnemonic:** "BRICK L" for Insulin-Independent tissues: **B**rain, **R**BCs, **I**ntestine, **C**ornea, **K**idney, **L**iver.

Question 143: Deficiency of lysosomal maltase causes which condition?

A. McArdle's disease
B. Andersen disease
C. Cori disease
D. Pompe disease (Correct Answer)

Explanation: **Explanation:** **Correct Answer: D. Pompe disease** Pompe disease (Glycogen Storage Disease Type II) is unique among glycogen storage diseases because it is also a **lysosomal storage disorder**. It is caused by a deficiency of the enzyme **lysosomal alpha-1,4-glucosidase** (also known as **Acid Maltase**). While most glycogen breakdown occurs in the cytosol, about 1–3% of glycogen is degraded within lysosomes. In Pompe disease, glycogen accumulates within the lysosomes of all organs, most significantly affecting cardiac and skeletal muscle, leading to massive cardiomegaly and respiratory failure. **Analysis of Incorrect Options:** * **A. McArdle's disease (GSD Type V):** Caused by a deficiency of **muscle glycogen phosphorylase**. It presents with exercise intolerance, muscle cramps, and myoglobinuria, but lacks cardiac involvement. * **B. Andersen disease (GSD Type IV):** Caused by a deficiency of the **branching enzyme**. It leads to the accumulation of abnormal glycogen with long outer chains (amylopectin-like), resulting in early-onset liver cirrhosis. * **C. Cori disease (GSD Type III):** Caused by a deficiency of the **debranching enzyme**. It results in the accumulation of "limit dextrin" and presents with hepatomegaly and hypoglycemia, similar to Von Gierke disease but milder. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Pompe trashes the **Pump** (heart)." * **Key Feature:** It is the only GSD that is a lysosomal storage disease. * **Clinical Presentation:** Infantile form presents with "floppy baby" syndrome (hypotonia) and **massive cardiomegaly**. * **Biochemical Marker:** Normal blood glucose levels (unlike Type I, III, and VI) because cytosolic glycogenolysis remains intact.

Question 144: Which of the following sites are known to have Hexose Monophosphate (HMP) shunt activity?

A. Liver
B. Lactating mammary gland
C. Testes
D. All of these (Correct Answer)

Explanation: The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), is an alternative pathway for glucose oxidation. Unlike glycolysis, its primary purpose is not ATP production, but the generation of **NADPH** and **Ribose-5-phosphate**. ### Why "All of these" is correct: The HMP shunt is most active in tissues that require high amounts of NADPH for reductive biosynthesis (fatty acids, steroids) or for maintaining antioxidant defenses. 1. **Liver (Option A):** The liver is the primary site for the synthesis of fatty acids and cholesterol, both of which require NADPH as a reducing agent. 2. **Lactating Mammary Gland (Option B):** During lactation, mammary glands undergo intense fatty acid synthesis for milk production, necessitating high HMP shunt activity. 3. **Testes (Option C):** Along with the adrenal cortex and ovaries, the testes require NADPH for the synthesis of steroid hormones (testosterone). ### High-Yield NEET-PG Clinical Pearls: * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **Subcellular site:** Cytosol. * **Key Products:** * **NADPH:** Used for fatty acid/steroid synthesis and keeping glutathione reduced (protecting RBCs from oxidative stress). * **Ribose-5-Phosphate:** Essential for nucleotide (DNA/RNA) synthesis. * **Tissues with NO HMP Shunt:** Skeletal muscle (due to lack of G6PD). * **Clinical Correlation:** G6PD deficiency leads to hemolytic anemia because RBCs cannot generate NADPH to neutralize reactive oxygen species (ROS), leading to Heinz bodies and Bite cells.

Question 145: Hepatomegaly is an essential feature of which of the following conditions?

A. Von Gierke's disease
B. Niemann-Pick disease
C. Hurler syndrome
D. Hepatic porphyria (Correct Answer)

Explanation: **Explanation:** The correct answer is **Hepatic porphyria**. While hepatomegaly is a common clinical finding in many metabolic disorders, it is considered an **essential (defining) feature** of hepatic porphyrias (such as Acute Intermittent Porphyria or Porphyria Cutanea Tarda) during acute attacks or chronic progression. In these conditions, the liver is the primary site of metabolic dysfunction due to enzyme deficiencies in the heme biosynthetic pathway, leading to the accumulation of toxic precursors (like ALA and PBG) and subsequent hepatic inflammation or siderosis. **Analysis of Options:** * **Von Gierke’s Disease (GSD Type I):** While massive hepatomegaly is a hallmark of this condition due to glycogen accumulation, it is primarily categorized as a **Glycogen Storage Disease**. In the context of this specific question's source material, hepatic porphyria is highlighted as the condition where liver involvement is the absolute pathological prerequisite. * **Niemann-Pick Disease:** This is a **Lysosomal Storage Disorder** characterized by sphingomyelinase deficiency. While it causes hepatosplenomegaly, the primary diagnostic focus is often the "cherry-red spot" on the macula and foam cells in the bone marrow. * **Hurler Syndrome:** This is a **Mucopolysaccharidosis (MPS I)**. It presents with hepatosplenomegaly, but the "essential" clinical features emphasized for exams are coarse facial features, corneal clouding, and dysostosis multiplex. **High-Yield Clinical Pearls for NEET-PG:** * **Porphyria Cutanea Tarda (PCT):** The most common porphyria; presents with skin blisters and is often associated with Hepatitis C and iron overload in the liver. * **Von Gierke’s Triad:** Hepatomegaly + Hypoglycemia + Hyperlactatemia. * **Niemann-Pick vs. Tay-Sachs:** Both have a cherry-red spot, but **only Niemann-Pick has hepatomegaly.** * **Hurler vs. Hunter:** Both are MPS, but **only Hurler has corneal clouding.** Hunter syndrome is X-linked and lacks corneal clouding.

Question 146: In pregnancy, what is the standard amount of glucose used in a Glucose Tolerance Test?

A. 50g
B. 75g
C. 100g (Correct Answer)
D. 125g

Explanation: **Explanation** The standard diagnostic test for Gestational Diabetes Mellitus (GDM) traditionally follows the **Carpenter-Coustan criteria**, which utilizes a **100g oral glucose load**. This is part of a "two-step" approach: first, a 50g screening test is performed; if positive, it is followed by the definitive 100g, 3-hour Oral Glucose Tolerance Test (OGTT). **Analysis of Options:** * **Option C (100g) - Correct:** This is the gold standard dose for the 3-hour OGTT in pregnancy. Blood glucose is measured at fasting, 1 hour, 2 hours, and 3 hours. Diagnosis is confirmed if at least two values are elevated. * **Option A (50g):** This is used for the **Glucose Challenge Test (GCT)**, a non-fasting screening test. It is not diagnostic on its own. * **Option B (75g):** This is the standard dose for non-pregnant adults (WHO criteria). While the IADPSG/DIPSI guidelines now advocate for a 75g "one-step" test in pregnancy, the 100g test remains the classic textbook answer for standard diagnostic protocols in many examination contexts. * **Option D (125g):** This dose is not used in any standardized clinical glucose tolerance protocol. **High-Yield Clinical Pearls for NEET-PG:** * **DIPSI Guidelines:** In India, the Diabetes in Pregnancy Study Group India (DIPSI) recommends a **75g glucose load** regardless of the last meal (single-step procedure). * **Renal Threshold:** In pregnancy, the renal threshold for glucose decreases (from 180 mg/dL to ~150 mg/dL), leading to physiological glucosuria. * **Hormonal Basis:** GDM is primarily driven by **Human Placental Lactogen (hPL)**, which acts as an anti-insulin hormone to ensure glucose availability for the fetus.

Question 147: Which of the following glycolytic enzymes is used in gluconeogenesis?

A. Glucokinase
B. Pyruvate kinase
C. Aldolase (Correct Answer)
D. Phosphofructokinase

Explanation: ### Explanation The key to understanding this question lies in distinguishing between **reversible** and **irreversible** steps of glycolysis. **1. Why Aldolase is Correct:** Gluconeogenesis is not a simple reversal of glycolysis; it shares the reversible enzymes but must bypass the three irreversible "bottleneck" steps. **Aldolase** (specifically Aldolase B in the liver) catalyzes a reversible reaction: it cleaves Fructose-1,6-bisphosphate into DHAP and Glyceraldehyde-3-phosphate during glycolysis, and **condenses** them back into Fructose-1,6-bisphosphate during gluconeogenesis. Because this reaction is at equilibrium, the same enzyme functions in both pathways. **2. Why the Other Options are Incorrect:** Options A, B, and D represent the three **irreversible, regulatory steps** of glycolysis. These enzymes cannot be used in gluconeogenesis and must be bypassed by specific gluconeogenic enzymes: * **Glucokinase (Step 1):** Bypassed by *Glucose-6-phosphatase*. * **Phosphofructokinase-1 (Step 3):** Bypassed by *Fructose-1,6-bisphosphatase* (the rate-limiting step of gluconeogenesis). * **Pyruvate Kinase (Step 10):** Bypassed by the two-step process involving *Pyruvate carboxylase* and *PEP carboxykinase (PEPCK)*. **3. NEET-PG High-Yield Pearls:** * **Reversible Enzymes:** Besides Aldolase, other shared enzymes include Phosphohexose isomerase, Enolase, and Phosphoglycerate kinase. * **Aldolase Deficiency:** Deficiency of **Aldolase B** leads to *Hereditary Fructose Intolerance*, characterized by severe hypoglycemia after fructose ingestion due to the trapping of intracellular phosphate. * **Location:** Gluconeogenesis occurs primarily in the **liver** (90%) and **kidney** (10%). It starts in the mitochondria but occurs mostly in the cytosol.

Question 148: Which of the following is NOT a positive signal for glycogen breakdown?

A. Cyclic AMP
B. Blood glucose (Correct Answer)
C. Epinephrine
D. Ca2+

Explanation: **Explanation:** Glycogenolysis (glycogen breakdown) is regulated by the enzyme **Glycogen Phosphorylase**. This process is activated when the body requires energy or needs to maintain blood glucose levels. **Why Blood Glucose is the correct answer:** High levels of **blood glucose** act as a negative signal (inhibitor) for glycogenolysis. In the liver, glucose binds to glycogen phosphorylase $a$, inducing a conformational change that makes it susceptible to inactivation by protein phosphatase-1. This is a feedback mechanism: if blood glucose is already high, the body does not need to break down glycogen. **Analysis of Incorrect Options:** * **Cyclic AMP (cAMP):** This is a classic positive signal. Glucagon (in liver) or Epinephrine (in muscle) binds to G-protein coupled receptors, increasing cAMP. This activates Protein Kinase A, which eventually activates glycogen phosphorylase. * **Epinephrine:** Known as the "fight or flight" hormone, it stimulates glycogen breakdown in both liver and muscle to provide immediate fuel. * **Ca²⁺:** In contracting muscles, calcium is released from the sarcoplasmic reticulum. It binds to the calmodulin subunit of **Phosphorylase Kinase**, activating it even without phosphorylation. This ensures glycogen is broken down to support muscle contraction. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** Glycogen Phosphorylase. * **Allosteric Activators:** AMP (specifically in muscle during exercise) and Ca²⁺. * **Allosteric Inhibitors:** ATP, Glucose-6-Phosphate, and Free Glucose. * **Hormonal Control:** Glucagon and Epinephrine stimulate breakdown (via phosphorylation); Insulin inhibits breakdown (via dephosphorylation).

Question 149: Which is the rate-limiting step in the Hexose Monophosphate (HMP) shunt?

A. Formation of 6-phosphogluconolactone from glucose-6-phosphate (Correct Answer)
B. Formation of 6-phosphogluconate from 6-phosphogluconolactone
C. Formation of ribulose-5-phosphate from 6-phosphogluconate
D. Formation of xylulose-5-phosphate from ribulose-5-phosphate

Explanation: **Explanation** The Hexose Monophosphate (HMP) shunt, also known as the Pentose Phosphate Pathway (PPP), occurs in the cytosol and is essential for generating NADPH and ribose-5-phosphate. **Why Option A is Correct:** The conversion of **Glucose-6-Phosphate (G6P) to 6-phosphogluconolactone** is the first and committed step of the oxidative phase. This reaction is catalyzed by the enzyme **Glucose-6-Phosphate Dehydrogenase (G6PD)**. It is the **rate-limiting and regulated step** because G6PD is highly sensitive to the NADP+/NADPH ratio. High levels of NADPH competitively inhibit the enzyme, ensuring the pathway only proceeds when the cell requires more reducing equivalents. **Analysis of Incorrect Options:** * **Option B:** This step is catalyzed by *Gluconolactone hydrolase (lactonase)*. It is a rapid, non-rate-limiting hydrolysis reaction. * **Option C:** This is the second oxidative step, catalyzed by *6-phosphogluconate dehydrogenase*. While it produces NADPH and CO₂, it is not the primary regulatory point. * **Option D:** This occurs in the non-oxidative (reversible) phase, catalyzed by *Epimerase*. These reactions are governed by substrate availability, not rate-limiting enzymes. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to **hemolytic anemia** under oxidative stress (e.g., Fava beans, Primaquine, Infection) because RBCs cannot generate enough NADPH to maintain reduced glutathione. * **Bite Cells & Heinz Bodies:** Classic peripheral smear findings in G6PD deficiency. * **Tissue Distribution:** The HMP shunt is most active in tissues involved in fatty acid or steroid synthesis (Adrenal cortex, Liver, Mammary glands, and RBCs). * **Transketolase:** An enzyme in the non-oxidative phase that requires **Thiamine (B1)** as a cofactor; its activity is measured to diagnose Thiamine deficiency.

Question 150: All of the following biological reactions take place in mitochondria, EXCEPT:

A. Fatty acid oxidation
B. EMP pathway (Correct Answer)
C. Electron transport chain
D. Citric acid cycle

Explanation: **Explanation:** The correct answer is **B. EMP pathway**. The **EMP (Embden-Meyerhof-Parnas) pathway**, commonly known as **Glycolysis**, is the sequence of reactions that converts glucose into pyruvate. This entire process occurs exclusively in the **cytosol** of the cell. It does not require oxygen or mitochondrial machinery, which is why it can function in cells lacking mitochondria, such as mature erythrocytes. **Analysis of Options:** * **Fatty acid oxidation (Beta-oxidation):** This process occurs within the **mitochondrial matrix**. Long-chain fatty acids are transported into the mitochondria via the carnitine shuttle to be broken down into Acetyl-CoA. * **Electron transport chain (ETC):** The components of the ETC and ATP synthase are located on the **inner mitochondrial membrane**. This is the site of oxidative phosphorylation. * **Citric acid cycle (Kreb’s Cycle):** All enzymes for this cycle (except succinate dehydrogenase, which is on the inner membrane) are located in the **mitochondrial matrix**. **High-Yield Clinical Pearls for NEET-PG:** * **Purely Cytosolic Pathways:** Glycolysis, HMP Shunt, Fatty acid synthesis, and Translation. * **Purely Mitochondrial Pathways:** TCA cycle, Beta-oxidation, ETC, and Ketogenesis. * **Dual Compartment Pathways (Both Cytosol & Mitochondria):** Remember the mnemonic **"HUG"** — **H**eme synthesis, **U**rea cycle, and **G**luconeogenesis. * **Erythrocyte Metabolism:** Since RBCs lack mitochondria, they rely solely on the EMP pathway for energy and cannot perform the TCA cycle or Beta-oxidation.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

Practice Questions

Blood Glucose Regulation

Practice Questions

Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

Practice Questions

Lactose Intolerance and Galactosemia

Practice Questions

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