Carbohydrate Metabolism Practice Questions

Q: Which of the following is NOT a factor responsible for ketosis in a patient of von Gierke's disease?

Low fat mobilization. **Explanation:** **Von Gierke’s Disease (GSD Type I)** is caused by a deficiency of **Glucose-6-Phosphatase**. This enzyme is the final common step for both glycogenolysis and gluconeogenesis, meaning the liver cannot release free glucose into the blood. **Why "Low fat mobilization" is the correct answer:** In Von Gierke’s disease, patients actually experience **increased fat mobilization**, not low. Due to the inability to maintain blood glucose, there is a chronic state of hypoglycemia. This leads to a low Insulin:Glucagon ratio, which stimulates **Hormone-Sensitive Lipase (HSL)** in adipose tissue. This results in massive mobilization of free fatty acids (FFAs) to the liver. These FFAs undergo β-oxidation, providing the acetyl-CoA necessary for **ketogenesis**. Therefore, "Low fat mobilization" is factually incorrect regarding the disease's pathophysiology. **Analysis of Incorrect Options:** * **Hypoglycemia:** This is the primary trigger. Low blood glucose stimulates glucagon and epinephrine, which drive the lipolysis required for ketosis. * **Impaired Gluconeogenesis & Glycogenolysis:** Both are hallmark features of GSD Type I. Because the liver cannot convert Glucose-6-Phosphate into free glucose, the body is forced to switch to alternative fuel sources (fats and ketones) to meet energy demands. **Clinical Pearls for NEET-PG:** * **The "Doll-like" Face:** Caused by fat deposition in cheeks due to hyperlipidemia. * **Biochemical Tetrad:** Hyperuricemia (gout), Hyperlactatemia (lactic acidosis), Hyperlipidemia, and Hypoglycemia. * **Key Distinction:** Unlike GSD Type III (Cori’s), Von Gierke’s presents with **severe lactic acidosis** because the lactate cannot be recycled into glucose via gluconeogenesis.

Q: Which of the following statements about NADPH is false?

NADPH produces ATP in Red Blood Cells.. **Explanation:** **1. Why Option A is the correct (False) statement:** NADPH (Nicotinamide Adenine Dinucleotide Phosphate) is primarily used for **reductive biosynthesis** and maintaining **antioxidant defenses**, not for energy production. Unlike NADH, which enters the Electron Transport Chain (ETC) to generate ATP via oxidative phosphorylation, NADPH does not serve as a substrate for ATP synthesis. In Red Blood Cells (RBCs), ATP is generated exclusively through **anaerobic glycolysis** (the Embden-Meyerhof pathway). **2. Analysis of other options:** * **Option B:** True. The **Hexose Monophosphate (HMP) Shunt** is the sole source of NADPH in RBCs. Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting enzyme of this pathway; its deficiency directly leads to decreased NADPH levels. * **Option C:** True. NADPH provides the reducing equivalents necessary for the synthesis of fatty acids, cholesterol, and steroid hormones. * **Option D:** True. NADPH is essential for regenerating **reduced glutathione** from its oxidized form. Reduced glutathione neutralizes reactive oxygen species (like H₂O₂) that would otherwise cause lipid peroxidation and oxidative damage to the RBC membrane, leading to hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzyme deficiency worldwide. It presents as episodic hemolytic anemia triggered by oxidative stress (e.g., Fava beans, Primaquine, or infections). * **Heinz Bodies:** Denatured hemoglobin precipitates seen in G6PD deficiency due to oxidative stress. * **Bite Cells:** Formed when splenic macrophages pluck out Heinz bodies from RBCs. * **Key NADPH Sources:** HMP Shunt (major) and Malic Enzyme (minor, in tissues with mitochondria).

Q: α-D-glucose and β-D-glucose are related as:

Anomers. **Explanation:** **Why Anomers is the correct answer:** Anomers are a specific type of stereoisomer found in cyclic sugars. When glucose forms a ring structure (pyranose), the carbonyl carbon (C1) becomes a new chiral center, known as the **anomeric carbon**. The difference between α-D-glucose and β-D-glucose lies solely in the orientation of the hydroxyl (-OH) group at this C1 position. In the α-form, the -OH is below the plane of the ring (trans to the CH₂OH group), while in the β-form, it is above the plane (cis to the CH₂OH group). **Analysis of Incorrect Options:** * **Stereoisomers:** While anomers are technically a subset of stereoisomers, "Anomers" is the most specific and accurate term. In NEET-PG, always choose the most specific classification. * **Epimers:** These are isomers that differ at only one chiral center *other than* the anomeric carbon. For example, Glucose and Galactose are C4 epimers; Glucose and Mannose are C2 epimers. * **Keto-aldo pairs:** These are functional isomers. Glucose is an aldose (aldehyde group), while Fructose is a ketose (ketone group). **High-Yield Clinical Pearls for NEET-PG:** * **Mutarotation:** This is the change in specific optical rotation when α and β anomers are interconverted in an aqueous solution until an equilibrium is reached (approx. 36% α and 64% β). * **Reducing Sugars:** All monosaccharides (including both anomers of glucose) are reducing sugars because the cyclic form can open back into a free aldehyde chain to reduce Benedict’s or Fehling’s reagent. * **Starch vs. Cellulose:** Humans can digest α-1,4-glycosidic bonds (starch/glycogen) but lack the enzyme (cellulase) to break β-1,4-glycosidic bonds (cellulose).

Q: Which gene is NOT associated with Diabetes Mellitus?

PDGF-R. **Explanation:** The correct answer is **PDGF-R (Platelet-Derived Growth Factor Receptor)**. This receptor is primarily involved in cell proliferation, wound healing, and angiogenesis. While it plays a role in diabetic complications (like proliferative retinopathy or atherosclerosis), the gene itself is not a primary genetic locus associated with the etiology of Diabetes Mellitus. **Analysis of Options:** * **PPARγ (Peroxisome Proliferator-Activated Receptor gamma):** This is a nuclear receptor that regulates fatty acid storage and glucose metabolism. Mutations or polymorphisms in the *PPARG* gene are strongly linked to Type 2 Diabetes and insulin resistance. It is also the pharmacological target for Thiazolidinediones (e.g., Pioglitazone). * **KCNJ11:** This gene encodes the Kir6.2 subunit of the ATP-sensitive potassium channel in pancreatic beta cells. Mutations in this gene are a well-known cause of **Neonatal Diabetes Mellitus** and are associated with an increased risk of Type 2 Diabetes. * **CTLA4 (Cytotoxic T-Lymphocyte Associated Protein 4):** This is an immune checkpoint molecule. Polymorphisms in the *CTLA4* gene are associated with susceptibility to **Type 1 Diabetes Mellitus**, as it plays a critical role in T-cell mediated autoimmunity. **High-Yield Clinical Pearls for NEET-PG:** * **MODY (Maturity-Onset Diabetes of the Young):** Most common gene involved is **HNF1A** (MODY 3), followed by **Glucokinase** (MODY 2). * **HLA Association:** Type 1 DM is strongly linked to **HLA-DR3 and HLA-DR4**. * **Wolfram Syndrome:** A rare genetic cause of diabetes characterized by **DIDMOAD** (Diabetes Insipidus, Diabetes Mellitus, Optic Atrophy, and Deafness).

Q: Which of the following occurs during glycolysis?

Pyruvate, NADH, and ATP are produced.. **Explanation:** **Why Option D is Correct:** Glycolysis (the Embden-Meyerhof pathway) is the primary metabolic pathway that breaks down one molecule of glucose into two molecules of **pyruvate**. This process occurs in the cytosol and results in a net gain of **2 ATP** (via substrate-level phosphorylation) and **2 NADH** (via the oxidation of glyceraldehyde-3-phosphate). These products are essential for cellular energy production and downstream aerobic respiration. **Analysis of Incorrect Options:** * **Option A:** While glycolysis occurs in the cytosol, it does not always end in pyruvate. In cells lacking mitochondria (e.g., **mature RBCs**) or under anaerobic conditions (e.g., exercising muscle), pyruvate is reduced to **lactate** to regenerate NAD+. * **Option B:** The rate-limiting and committed step of glycolysis is the conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate, catalyzed by **Phosphofructokinase-1 (PFK-1)**, not the formation of glucose-6-phosphate. * **Option C:** While glucose is indeed phosphorylated by hexokinase (or glucokinase), this is a single step within the pathway, not a summary of what "occurs during glycolysis" as a whole. Option D provides a more comprehensive description of the pathway's output. **High-Yield Clinical Pearls for NEET-PG:** * **Key Regulatory Enzymes:** Hexokinase/Glucokinase, PFK-1 (Rate-limiting), and Pyruvate Kinase. * **Rapoport-Luebering Cycle:** In RBCs, 1,3-BPG can be converted to **2,3-BPG**, which shifts the oxygen dissociation curve to the right (facilitating O2 unloading). * **Arsenic Poisoning:** Inhibits the net ATP gain in glycolysis by competing with inorganic phosphate at the glyceraldehyde-3-phosphate dehydrogenase step. * **Fluoride:** Inhibits the enzyme **Enolase**; this is why fluoride is added to blood collection tubes (grey top) to prevent glycolysis during glucose estimation.

Q: What is the major contributor to gluconeogenesis?

Lactate. **Explanation:** Gluconeogenesis is the metabolic pathway that generates glucose from non-carbohydrate precursors during periods of fasting or intense exercise. **Why Lactate is the correct answer:** Lactate is considered the **major quantitative contributor** to gluconeogenesis. It is primarily produced by anaerobic glycolysis in skeletal muscles and red blood cells. Through the **Cori Cycle**, lactate is transported to the liver, where it is converted back into pyruvate by lactate dehydrogenase (LDH) and subsequently into glucose. This cycle is essential for maintaining blood glucose levels and recycling lactate to prevent lactic acidosis. **Analysis of Incorrect Options:** * **Glutamate:** While glutamate is a glucogenic amino acid (entering the TCA cycle as $\alpha$-ketoglutarate), it is not the primary precursor compared to the sheer volume of lactate produced daily. * **Ketones:** Ketone bodies (acetoacetate and $\beta$-hydroxybutyrate) **cannot** be converted into glucose in humans because the reaction catalyzed by pyruvate dehydrogenase is irreversible; acetyl-CoA cannot be converted back to pyruvate. * **Alanine:** Alanine is the most important **amino acid** precursor for gluconeogenesis (via the Glucose-Alanine cycle), but in terms of total daily flux, lactate contributes a higher percentage of the glucose pool. **NEET-PG High-Yield Pearls:** * **Major Precursors:** Lactate > Glycerol > Alanine. * **Rate-limiting enzyme:** Fructose-1,6-bisphosphatase. * **Location:** Primarily the liver (90%), followed by the kidney cortex (10%). * **Key distinction:** While Alanine is the primary precursor during **prolonged fasting**, Lactate remains the overall major contributor under normal physiological conditions.

Q: Muscle glycolysis is increased by which of the following substances?

Epinephrine. **Explanation:** **1. Why Epinephrine is Correct:** In skeletal muscle, epinephrine (adrenaline) acts as a potent stimulator of glycolysis during the "fight or flight" response. It binds to **$\beta_2$-adrenergic receptors**, leading to an increase in intracellular cAMP. This triggers a signaling cascade that activates **Glycogen Phosphorylase**, accelerating glycogenolysis to provide Glucose-1-Phosphate. Crucially, in muscle, the increased cAMP and calcium levels (from contraction) activate **Phosphofructokinase-1 (PFK-1)**, the rate-limiting enzyme of glycolysis. Unlike the liver, where epinephrine inhibits glycolysis to save glucose for the brain, in muscle, it promotes glycolysis to generate immediate ATP for contraction. **2. Why the Other Options are Incorrect:** * **B. Acetylcholine:** This is the primary neurotransmitter at the neuromuscular junction responsible for muscle contraction. While contraction indirectly increases glycolysis (via $Ca^{2+}$ ions), acetylcholine itself does not directly regulate the glycolytic enzymatic pathway. * **C. Histamine:** Primarily involved in inflammatory responses, vasodilation, and gastric acid secretion; it has no direct regulatory role in skeletal muscle carbohydrate metabolism. * **D. Serotonin:** Acts mainly as a neurotransmitter in the CNS and a local hormone in the GI tract; it does not modulate muscle glycolytic flux. **3. High-Yield NEET-PG Pearls:** * **Organ-Specific Regulation:** Remember the "Reciprocal Regulation" difference: Epinephrine **stimulates** glycolysis in **muscle** but **inhibits** it in the **liver** (via PFK-2 inhibition). * **PFK-1 Activators:** The most potent allosteric activator of PFK-1 is **Fructose-2,6-bisphosphate**. * **AMP vs. ATP:** High AMP levels (signaling low energy) activate glycolysis, while high ATP and Citrate levels inhibit it. * **Key Enzyme:** PFK-1 is the "Pacemaker" of glycolysis.

Q: The level of glucose transporters is reduced after an overnight fast in which of the following cells?

Cardiac muscle cells. ### Explanation The correct answer is **D. Cardiac muscle cells.** **1. Why Cardiac Muscle Cells is Correct:** The regulation of glucose transporters (GLUT) depends on whether they are insulin-dependent or insulin-independent. **GLUT-4** is the primary insulin-responsive glucose transporter found in **cardiac muscle, skeletal muscle, and adipose tissue.** During an overnight fast, insulin levels drop while glucagon levels rise. In the absence of insulin, GLUT-4 transporters are sequestered from the cell membrane and moved into intracellular vesicles. This physiological mechanism ensures that glucose is conserved for glucose-dependent organs (like the brain) while muscles shift to fatty acid oxidation for energy. **2. Why the Other Options are Incorrect:** * **A. Liver cells:** These contain **GLUT-2**, which is insulin-independent. GLUT-2 is always present on the membrane to allow for bidirectional glucose flux (glycogenolysis and gluconeogenesis during fasting). * **B. Brain cells:** These primarily use **GLUT-1 and GLUT-3**, which are insulin-independent and have a high affinity for glucose. This ensures the brain receives a constant supply of glucose even during low blood sugar states. * **C. Red blood cells:** These contain **GLUT-1**, which is insulin-independent. Since RBCs lack mitochondria and rely solely on glycolysis, they require constant glucose uptake regardless of insulin levels. **3. High-Yield Clinical Pearls for NEET-PG:** * **GLUT-4** is the only GLUT regulated by insulin. * **Exercise** can also trigger the translocation of GLUT-4 to the cell membrane in skeletal muscle, independent of insulin (important for managing Diabetes Mellitus). * **GLUT-2** has a high $K_m$ (low affinity) and acts as a "glucose sensor" in Pancreatic $\beta$-cells. * **SGLT-1/SGLT-2** are sodium-dependent active transporters found in the small intestine and kidneys, unlike the GLUT family which facilitates passive diffusion.

Q: In which of the following polysaccharides are monomers linked by a beta-glycosidic bond?

Cellulose. ### Explanation The correct answer is **Cellulose**. **1. Why Cellulose is Correct:** Cellulose is a structural homopolysaccharide found in the cell walls of plants. It consists of long, unbranched chains of D-glucose units linked by **$\beta(1 \to 4)$ glycosidic bonds**. This $\beta$-linkage results in a linear, rigid structure that allows for the formation of hydrogen bonds between adjacent chains, providing high tensile strength. Humans lack the enzyme **cellulase**, which is specific for hydrolyzing $\beta$-glycosidic bonds; therefore, cellulose cannot be digested and serves as dietary fiber. **2. Why the Other Options are Incorrect:** * **Amylose (A):** A component of starch consisting of unbranched glucose chains linked by **$\alpha(1 \to 4)$ glycosidic bonds**. * **Amylopectin (B):** The branched component of starch. It contains **$\alpha(1 \to 4)$ bonds** in the linear chain and **$\alpha(1 \to 6)$ bonds** at the branching points. * **Glycogen (C):** The primary storage polysaccharide in animals (liver and muscle). Like amylopectin, it uses **$\alpha(1 \to 4)$ bonds** for the backbone and **$\alpha(1 \to 6)$ bonds** for branches, though it is more highly branched than amylopectin. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Digestibility:** Human salivary and pancreatic $\alpha$-amylase can only hydrolyze $\alpha(1 \to 4)$ bonds. * **Dietary Fiber:** Cellulose increases stool bulk and decreases intestinal transit time, protective against colon cancer and diverticulosis. * **Inulin:** Another high-yield polysaccharide (fructose polymer) with $\beta(2 \to 1)$ bonds, used to measure **GFR** because it is freely filtered but neither reabsorbed nor secreted. * **Lactose:** A disaccharide (Glucose + Galactose) that also contains a **$\beta(1 \to 4)$ bond**, requiring the enzyme lactase for digestion.

Q: A false positive oral glucose tolerance test (OGTT) is seen in all of the following conditions EXCEPT?

Exercise. **Explanation:** The Oral Glucose Tolerance Test (OGTT) is a dynamic test used to diagnose Diabetes Mellitus and Impaired Glucose Tolerance. A **false positive** result occurs when a patient’s glucose levels appear abnormally high (diabetic range) due to factors other than true diabetes [2]. **Why Exercise is the Correct Answer:** Exercise increases insulin sensitivity and promotes glucose uptake by skeletal muscles via the translocation of GLUT-4 receptors [3]. This leads to **lower** blood glucose levels. Therefore, exercise would more likely cause a false negative or a lower-than-expected reading, rather than a false positive. For an accurate OGTT, patients are advised to remain seated and avoid strenuous activity during the test. **Analysis of Incorrect Options (Causes of False Positives):** * **Malnutrition:** Prolonged starvation or a low-carbohydrate diet leads to "starvation diabetes." The body shifts to fat metabolism, and the pancreas becomes "sluggish" in releasing insulin when suddenly challenged with a glucose load. * **Infection and Severe Emotional Stress:** These are states of high physiological stress. They trigger the release of **counter-regulatory hormones** (Cortisol, Adrenaline, Glucagon, and Growth Hormone). These hormones antagonize insulin and promote gluconeogenesis, leading to transient hyperglycemia [1]. **NEET-PG Clinical Pearls:** * **Preparation for OGTT:** The patient must consume at least **150g of carbohydrates per day** for 3 days prior to the test to ensure the pancreas is adequately "primed." * **WHO Criteria:** For a 75g OGTT, a 2-hour post-load plasma glucose of **≥200 mg/dL** is diagnostic of Diabetes, while **140–199 mg/dL** indicates Impaired Glucose Tolerance (IGT) [2]. * **Drugs causing False Positives:** Thiazide diuretics, Glucocorticoids, and Oral Contraceptive Pills [1].

Question 1

Which of the following is NOT a factor responsible for ketosis in a patient of von Gierke's disease?

Accepted Answer

Low fat mobilization

Answer

Hypoglycemia

Answer

Impaired gluconeogenesis

Answer

Impaired glycogenolysis

Question 2

Which of the following statements about NADPH is false?

Accepted Answer

NADPH produces ATP in Red Blood Cells.

Answer

Glucose-6-phosphate dehydrogenase deficiency causes decreased synthesis of NADPH.

Answer

NADPH is required for reductive biosynthesis.

Answer

NADPH stabilizes the membrane of Red Blood Cells.

Question 3

α-D-glucose and β-D-glucose are related as:

Accepted Answer

Anomers

Answer

Stereoisomers

Answer

Epimers

Answer

Keto-aldo pairs

Question 4

Which gene is NOT associated with Diabetes Mellitus?

Accepted Answer

PDGF-R

Answer

PPARy

Answer

KCNJ11

Answer

CTLA4

Question 5

Which of the following occurs during glycolysis?

Accepted Answer

Pyruvate, NADH, and ATP are produced.

Answer

Glucose is oxidized to pyruvate in the cytosol of all cells.

Answer

The rate-limiting step is the formation of glucose-6-phosphate.

Answer

Glucose is phosphorylated by hexokinase.

Question 6

What is the major contributor to gluconeogenesis?

Accepted Answer

Lactate

Answer

Glutamate

Answer

Ketones

Answer

Alanine

Question 7

Muscle glycolysis is increased by which of the following substances?

Accepted Answer

Epinephrine

Answer

Acetylcholine

Answer

Histamine

Answer

Serotonin

Question 8

The level of glucose transporters is reduced after an overnight fast in which of the following cells?

Accepted Answer

Cardiac muscle cells

Answer

Liver cells

Answer

Brain cells

Answer

Red blood cells

Question 9

In which of the following polysaccharides are monomers linked by a beta-glycosidic bond?

Accepted Answer

Cellulose

Answer

Amylose

Answer

Amylopectin

Answer

Glycogen

Question 10

A false positive oral glucose tolerance test (OGTT) is seen in all of the following conditions EXCEPT?

Accepted Answer

Exercise

Answer

Malnutrition

Answer

Infection

Answer

Severe emotional stress

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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