Carbohydrate Metabolism Practice Questions

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].

Q: Regarding proteoglycans, which statement is false?

They hold a small amount of water.. **Explanation** Proteoglycans are complex macromolecules consisting of a core protein with one or more covalently attached **glycosaminoglycan (GAG)** chains. **Why Option B is the correct (false) statement:** Proteoglycans are highly hydrophilic. Due to the presence of sulfate and carboxyl groups, they carry a high density of negative charges. these charges repel each other and attract a large cloud of cations (like $Na^+$), which creates high osmotic pressure. Consequently, proteoglycans **hold a large amount of water**, forming a hydrated "gel" that provides structural support and shock absorption in tissues like cartilage and the vitreous humor. **Analysis of other options:** * **Option A:** Chondroitin sulfate is the most abundant GAG in the body and is a primary component of proteoglycans found in cartilage and bone. * **Option C:** They are composed of sugars (specifically repeating disaccharide units of GAGs) which make up about 95% of their weight. * **Option D:** They carry a strong **negative electrical charge** (polyanionic) due to the sulfate and uronic acid groups. **High-Yield NEET-PG Pearls:** * **Hyaluronic Acid:** The only GAG that is **not sulfated** and not covalently attached to a protein core (it exists as a free carbohydrate chain). * **Aggrecan:** The major proteoglycan in cartilage; it associates with hyaluronic acid to form massive aggregates. * **Clinical Correlation:** Deficiencies in enzymes that degrade GAGs lead to **Mucopolysaccharidoses** (e.g., Hurler and Hunter Syndromes), characterized by the accumulation of these molecules in lysosomes.

Q: What is the net generation of ATP up to pyruvate in aerobic glycolysis?

7. **Explanation:** In aerobic glycolysis, the net ATP yield is calculated by accounting for both direct substrate-level phosphorylation and the oxidative phosphorylation of reduced coenzymes. **The Calculation:** 1. **ATP Consumption:** 2 ATP are consumed in the preparatory phase (Hexokinase and Phosphofructokinase-1 reactions). 2. **ATP Production (Substrate-level):** 4 ATP are produced (2 from Phosphoglycerate kinase and 2 from Pyruvate kinase). 3. **NADH Production:** 2 NADH are generated at the Glyceraldehyde-3-phosphate dehydrogenase step. 4. **The Yield:** In aerobic conditions, these 2 NADH enter the electron transport chain via the **Malate-Aspartate shuttle**, yielding **2.5 ATP per NADH** (Total = 5 ATP). 5. **Net Total:** (4 Substrate ATP + 5 Oxidative ATP) – 2 Consumed ATP = **7 ATP**. *Note: If the Glycerol-3-phosphate shuttle is used (common in muscle/brain), the yield is 1.5 ATP per NADH, totaling 5 net ATP. However, 7 is the standard "high-yield" answer for aerobic glycolysis in most medical exams.* **Analysis of Incorrect Options:** * **Option A (2):** This is the net ATP yield for **anaerobic glycolysis**, where NADH is consumed to reduce pyruvate to lactate, leaving only substrate-level ATP. * **Option C (15):** This represents the ATP yield for one molecule of Acetyl-CoA entering the TCA cycle plus its associated oxidative phosphorylation. * **Option D (35):** This is closer to the total ATP yield for the complete oxidation of one glucose molecule to $CO_2$ and $H_2O$ (approx. 30–32 ATP). **Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Phosphofructokinase-1 (PFK-1). * **Rapoport-Luebering Cycle:** In RBCs, glycolysis can bypass the phosphoglycerate kinase step to produce 2,3-BPG, resulting in **zero net ATP** production for that molecule of glucose. * **Arsenic Poisoning:** Inhibits ATP production in glycolysis by competing with inorganic phosphate at the GAPDH step.

Q: A child with increased conjugated bilirubin develops seizures and cataract. What is the probable diagnosis?

Galactosemia. **Explanation:** The clinical triad of **conjugated hyperbilirubinemia (jaundice), cataracts, and seizures** in an infant is a classic presentation of **Classic Galactosemia**, caused by a deficiency of the enzyme **Galactose-1-phosphate uridyltransferase (GALT)**. 1. **Why Galactosemia is correct:** * **Cataracts:** Excess galactose is diverted to the polyol pathway, where **aldose reductase** converts it into **galactitol**. Galactitol is osmotically active, causing water to enter the lens, leading to cataracts (often "oil-drop" appearance). * **Jaundice:** Accumulation of Galactose-1-phosphate is toxic to hepatocytes, leading to liver dysfunction and conjugated hyperbilirubinemia. * **Seizures:** These occur due to secondary **hypoglycemia**, as accumulated galactose-1-phosphate inhibits phosphoglucomutase, impairing glycogenolysis. 2. **Why other options are incorrect:** * **Tyrosinemia:** Presents with liver failure, renal tubular dysfunction (Fanconi syndrome), and a "cabbage-like" odor, but cataracts are not a feature. * **Fructosemia (Hereditary Fructose Intolerance):** Presents with jaundice and hypoglycemia after introducing fruit juice/sucrose, but **does not cause cataracts** because fructose is not converted to a polyol in the lens. * **Glycogen Storage Disorders (GSD):** Primarily present with hepatomegaly and severe hypoglycemia (e.g., Von Gierke’s), but do not typically present with cataracts. **NEET-PG High-Yield Pearls:** * **Screening Test:** Benedict’s test is positive for reducing sugars in urine (but glucose oxidase dipstick is negative). * **Increased Risk:** Patients are highly susceptible to **E. coli neonatal sepsis**. * **Management:** Immediate withdrawal of lactose/galactose from the diet (switch to soy milk). * **Galactokinase deficiency:** Causes cataracts only, without the systemic "sick baby" features (no jaundice or seizures).

Question 1

Which gene is NOT associated with Diabetes Mellitus?

Accepted Answer

PDGF-R

Answer

PPARy

Answer

KCNJ11

Answer

CTLA4

Question 2

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 3

What is the major contributor to gluconeogenesis?

Accepted Answer

Lactate

Answer

Glutamate

Answer

Ketones

Answer

Alanine

Question 4

Muscle glycolysis is increased by which of the following substances?

Accepted Answer

Epinephrine

Answer

Acetylcholine

Answer

Histamine

Answer

Serotonin

Question 5

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 6

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

Accepted Answer

Cellulose

Answer

Amylose

Answer

Amylopectin

Answer

Glycogen

Question 7

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

Question 8

Regarding proteoglycans, which statement is false?

Accepted Answer

They hold a small amount of water.

Answer

Chondroitin sulfate is a component of proteoglycans.

Answer

They are composed of sugars.

Answer

They carry an electrical charge.

Question 9

What is the net generation of ATP up to pyruvate in aerobic glycolysis?

Accepted Answer

7

Answer

2

Answer

15

Answer

35

Question 10

A child with increased conjugated bilirubin develops seizures and cataract. What is the probable diagnosis?

Accepted Answer

Galactosemia

Answer

Tyrosinemia

Answer

Fructosemia

Answer

Glycogen storage disorder

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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