Carbohydrate Metabolism Practice Questions

Q: Chronic nonspherocytic hemolytic anemia is a manifestation of:

G6PD deficiency. **Explanation:** **Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency** is the most common enzyme deficiency causing **Chronic Nonspherocytic Hemolytic Anemia (CNSHA)**. G6PD is the rate-limiting enzyme of the Hexose Monophosphate (HMP) shunt, responsible for producing **NADPH**. NADPH is essential for maintaining a pool of reduced glutathione, which protects red blood cells (RBCs) from oxidative stress. In G6PD deficiency, oxidative damage leads to the denaturation of hemoglobin (**Heinz bodies**) and subsequent hemolysis. While most variants cause episodic hemolysis triggered by fava beans or infections, certain "Class I" mutations lead to chronic, baseline hemolysis without spherocytes on the peripheral smear. **Analysis of Incorrect Options:** * **Hereditary Elliptocytosis:** This is a structural membrane defect (usually involving spectrin) characterized by elliptical-shaped RBCs, not a primary enzyme deficiency. * **Sickle Cell Anemia:** This is a qualitative hemoglobinopathy (Glu → Val at position 6 of the β-globin chain). While it causes chronic hemolysis, it is characterized by "sickle cells" and vaso-occlusive crises rather than being classified as nonspherocytic enzyme-related anemia. * **Hexokinase Deficiency:** Although it can cause CNSHA, it is extremely rare compared to G6PD deficiency. In the context of standard medical examinations, G6PD is the classic association for this presentation. **High-Yield Pearls for NEET-PG:** * **Inheritance:** G6PD deficiency is **X-linked recessive**. * **Morphology:** Look for **Heinz bodies** (supravital stain) and **Bite cells** (degmacytes) on peripheral smear. * **Trigger factors:** AAA (Antimalarials like Primaquine, Antibiotics like Sulfa drugs, and Aspirin/Acidosis). * **Protection:** G6PD deficiency offers a selective advantage against *Plasmodium falciparum* malaria.

Q: What is invert sugar?

An equimolar mixture of glucose and fructose. **Explanation:** **Invert sugar** is an equimolar mixture of **D-glucose and D-fructose**. It is produced by the hydrolysis of sucrose (table sugar) by the enzyme **sucrase (invertase)** or by dilute acids. **Why the correct answer is right:** The term "invert" refers to the change in the **optical rotation** of the solution. * **Sucrose** is dextrorotatory ($+66.5^\circ$). * Upon hydrolysis, it yields equal parts glucose ($+52.7^\circ$) and fructose ($-92.4^\circ$). * Because the levorotatory (left-turning) power of fructose is much stronger than the dextrorotatory (right-turning) power of glucose, the overall net rotation of the mixture becomes **levorotatory**. This "inversion" from $(+)$ to $(-)$ gives the mixture its name. **Why the incorrect options are wrong:** * **Option B:** Sucrose is the parent disaccharide; it is not a component of the hydrolyzed mixture. * **Option C & D:** Lactose is a disaccharide of glucose and galactose. Hydrolysis of lactose yields these two, but this mixture is not called invert sugar and does not exhibit the specific optical inversion characteristic of sucrose hydrolysis. **High-Yield Facts for NEET-PG:** 1. **Sweetness:** Invert sugar is significantly sweeter than sucrose because free fructose is the sweetest naturally occurring sugar. 2. **Honey:** Honey is a natural source of invert sugar, containing high concentrations of free glucose and fructose. 3. **Specific Rotation:** Remember the values for the exam: Sucrose ($+66.5^\circ$), Glucose ($+52.7^\circ$), and Fructose ($-92.4^\circ$). 4. **Seliwanoff’s Test:** Invert sugar will give a positive result (cherry red color) due to the presence of fructose (a ketose).

Q: Which of the following is NOT a heteropolysaccharide?

Inulin. **Explanation:** The distinction between polysaccharides is based on their monomeric composition. **Inulin** is a **homopolysaccharide**, specifically a polymer of **fructose** (fructosan) linked by β(2→1) glycosidic bonds. Because it consists of only one type of monosaccharide unit, it is not a heteropolysaccharide. **Why the other options are incorrect:** Options A, B, and C are all examples of **Glycosaminoglycans (GAGs)**, which are classic **heteropolysaccharides**. They are composed of repeating disaccharide units consisting of an amino sugar (e.g., glucosamine) and a uronic acid (e.g., glucuronic acid). * **Hyaluronic acid:** Composed of D-glucuronic acid and N-acetyl-D-glucosamine. It is unique among GAGs as it is non-sulfated and not protein-bound. * **Chondroitin sulfate:** The most abundant GAG in the body, found in cartilage and bone. * **Heparan sulfate:** Found on cell surfaces and in the extracellular matrix; it contains higher levels of acetylated glucosamine compared to heparin. **High-Yield Clinical Pearls for NEET-PG:** * **Inulin Clearance:** Inulin is the "Gold Standard" for measuring **Glomerular Filtration Rate (GFR)** because it is freely filtered by the glomeruli but neither reabsorbed nor secreted by the renal tubules. * **Storage forms:** Starch (plants) and Glycogen (animals) are homopolysaccharides of glucose. * **Dextran vs. Dextrin:** Dextran is a branched glucose polymer used as a plasma volume expander; Dextrin is an intermediate product of starch hydrolysis. * **Agar:** A heteropolysaccharide derived from seaweed, used in laboratories as a culture medium.

Q: Keratan sulphate is found in abundance in which of the following locations?

Cornea. **Explanation:** **Keratan Sulphate (KS)** is a unique Glycosaminoglycan (GAG) because it is the only one that does not contain uronic acid (it contains galactose instead). It exists in two forms: KS I and KS II. **Why Cornea is the Correct Answer:** **Keratan Sulphate I** is found predominantly in the **cornea**. Its primary physiological role is to maintain corneal transparency. The specific arrangement and hydration levels of KS within the corneal stroma minimize light scattering, ensuring a clear path for vision. Deficiency or abnormalities in KS metabolism (as seen in Macular Corneal Dystrophy) lead to corneal opacification. **Analysis of Incorrect Options:** * **Heart Muscle:** While the heart valves contain GAGs like Dermatan sulphate and Hyaluronic acid to maintain structural integrity, Keratan sulphate is not a major constituent of the myocardium (heart muscle). * **Liver:** The liver is the site of GAG degradation, but it does not store Keratan sulphate. Heparan sulphate is the more relevant GAG associated with cell surfaces and basement membranes in systemic organs. * **Adrenal Cortex:** This is an endocrine tissue primarily involved in steroidogenesis; it does not contain significant amounts of specialized structural GAGs like Keratan sulphate. **High-Yield NEET-PG Pearls:** * **KS I vs. KS II:** KS I is found in the **cornea**, while KS II is found in **cartilage and bone**. * **Unique Structure:** Remember, KS is the "odd one out" among GAGs—it contains **Galactose** instead of Uronic acid. * **Clinical Correlation:** **Morquio Syndrome (MPS IV)** is characterized by the inability to degrade Keratan sulphate, leading to skeletal deformities and corneal clouding. * **Most Abundant GAG:** Chondroitin sulphate (found in cartilage/bone). * **Only Non-Sulphated GAG:** Hyaluronic acid.

Q: During a hunger strike with a liquid diet and minimal calories, which of the following hormonal and metabolic changes would occur after 4 to 5 hours?

Increased cyclic AMP and increased liver glycogenolysis. **Explanation:** The core concept here is the body's response to the **post-absorptive state** (early fasting). Approximately 4–5 hours after a meal, blood glucose levels begin to decline. This triggers the pancreas to secrete **glucagon** and the adrenal medulla to release **epinephrine**. **1. Why Option B is Correct:** Glucagon and epinephrine bind to G-protein coupled receptors (GPCRs) on hepatocytes. This activates **adenylyl cyclase**, which converts ATP into **cyclic AMP (cAMP)**. Elevated cAMP activates Protein Kinase A (PKA), which phosphorylates and activates **Glycogen Phosphorylase**. This leads to **liver glycogenolysis**, the primary mechanism for maintaining blood glucose levels during short-term fasting. **2. Why Other Options are Incorrect:** * **Option A:** Decreased cAMP and increased glycogen synthesis occur in the **fed state** under the influence of insulin. * **Option C:** Epinephrine levels **increase** (not decrease) during fasting or stress to stimulate glucose mobilization. * **Option D:** While Ca²⁺ does increase in muscle during contraction to stimulate glycogenolysis, the question focuses on the systemic hormonal response to a "hunger strike" (fasting), where liver glycogenolysis is the priority for blood glucose maintenance. Furthermore, increased Ca²⁺ would **increase**, not decrease, glycogenolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glycogen Phosphorylase is the rate-limiting enzyme for glycogenolysis; it is activated by phosphorylation. * **Liver vs. Muscle:** Liver glycogen maintains **blood glucose**; muscle glycogen is used only for **local energy** (muscle lacks Glucose-6-Phosphatase). * **Timeline:** Liver glycogen stores are typically exhausted after **12–24 hours** of fasting, after which gluconeogenesis becomes the sole source of blood glucose.

Q: Which enzyme catalyzes the irreversible step in the citric acid cycle?

α-ketoglutarate dehydrogenase. ### Explanation The Citric Acid Cycle (TCA cycle) is regulated primarily at its **irreversible steps**, which have a large negative Gibbs free energy ($\Delta G$). **Why α-ketoglutarate dehydrogenase is correct:** The conversion of α-ketoglutarate to Succinyl-CoA by the **α-ketoglutarate dehydrogenase complex** is a key regulatory, irreversible oxidative decarboxylation step. This multienzyme complex requires five cofactors (Thiamine, Lipoic acid, CoA, FAD, and NAD+). It is inhibited by its products (Succinyl-CoA and NADH) and activated by $Ca^{2+}$. **Analysis of Incorrect Options:** * **Aconitase:** Catalyzes the reversible isomerization of Citrate to Isocitrate via cis-aconitate. * **Succinate thiokinase (Succinyl-CoA Synthetase):** Catalyzes the reversible conversion of Succinyl-CoA to Succinate. This step is unique as it involves **substrate-level phosphorylation** (generating GTP/ATP). * **Isocitrate dehydrogenase:** While this is the **rate-limiting step** and is physiologically irreversible, in the context of standard NEET-PG questions, α-ketoglutarate dehydrogenase and Citrate Synthase are the classic examples of committed, highly exergonic irreversible steps. (Note: If both are present, Isocitrate dehydrogenase is the rate-limiting enzyme, but α-ketoglutarate dehydrogenase is the most "irreversible" in terms of equilibrium). **High-Yield Clinical Pearls for NEET-PG:** 1. **Three Irreversible Steps:** Citrate Synthase, Isocitrate Dehydrogenase, and α-ketoglutarate Dehydrogenase. 2. **Cofactor Dependency:** α-ketoglutarate dehydrogenase requires the same five cofactors as the Pyruvate Dehydrogenase (PDH) complex. **Thiamine deficiency** (B1) inhibits these enzymes, leading to impaired ATP production and Wernicke-Korsakoff syndrome. 3. **Arsenic Poisoning:** Arsenite inhibits α-ketoglutarate dehydrogenase by binding to the -SH groups of **Lipoic acid**, leading to a buildup of upstream metabolites.

Q: Which of the following enzymes is a constituent of the HMP shunt?

G-6-P dehydrogenase. ### Explanation **Correct Answer: C. G-6-P dehydrogenase (G6PD)** The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), occurs in the cytosol. **Glucose-6-phosphate dehydrogenase (G6PD)** is the **rate-limiting and committed enzyme** of the oxidative phase of this pathway. It catalyzes the conversion of Glucose-6-phosphate to 6-phosphogluconolactone, simultaneously reducing NADP+ to **NADPH**. This pathway is crucial for generating NADPH (used in reductive biosynthesis and maintaining reduced glutathione) and Ribose-5-phosphate (for nucleotide synthesis). **Why the other options are incorrect:** * **A. Glucose-6-phosphatase:** This enzyme is involved in **Gluconeogenesis** and **Glycogenolysis**. It converts Glucose-6-phosphate back to free Glucose, primarily in the liver and kidneys. * **B. Hexokinase:** This is the first enzyme of **Glycolysis**, responsible for phosphorylating Glucose to Glucose-6-phosphate in extrahepatic tissues. * **D. Phosphorylase (Glycogen Phosphorylase):** This is the rate-limiting enzyme of **Glycogenolysis**, which breaks down glycogen into Glucose-1-phosphate. **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, leading to **Heinz bodies** and **Bite cells**. * **Tissue Distribution:** The HMP shunt is highly active in tissues requiring NADPH for fatty acid or steroid synthesis (Adrenal cortex, Liver, Lactating mammary glands) and in RBCs to combat oxidative stress. * **Non-oxidative Phase:** Uses **Transketolase**, which requires **Thiamine (Vitamin B1)** as a cofactor—a common point of integration with nutrition questions.

Q: Which of the following glycosaminoglycans plays a significant role in wound healing?

Hyaluronic acid. ### Explanation **Correct Answer: C. Hyaluronic acid** **Why Hyaluronic acid is correct:** Hyaluronic acid (HA) is a unique glycosaminoglycan (GAG) because it is non-sulfated and not bound to a protein core. It plays a pivotal role in **wound healing** by creating a highly hydrated, porous extracellular matrix. During the early inflammatory phase, HA levels rise, facilitating the migration and proliferation of fibroblasts and keratinocytes. Its "molecular sieve" property allows for the diffusion of nutrients and signaling molecules, which is essential for tissue repair and remodeling. **Why the other options are incorrect:** * **A. Keratan sulfate:** Found primarily in the **cornea** (maintaining transparency) and cartilage. It does not play a primary role in the dynamic process of wound repair. * **B. Dermatan sulfate:** Predominantly found in the **skin, blood vessels, and heart valves**. While it is present in the skin, its primary function relates to structural integrity and anticoagulation (via interaction with Heparin Cofactor II) rather than the active healing phase. * **C. Chondroitin sulfate:** The most abundant GAG in the body, found in **cartilage and bone**. It provides tensile strength and resistance to compression but is not the primary mediator of wound healing. **High-Yield NEET-PG Pearls:** * **Hyaluronic Acid:** Only GAG that is **not sulfated**, not covalently linked to protein (not a proteoglycan), and contains **Glucuronic acid + N-acetylglucosamine**. * **Dermatan Sulfate:** Associated with **Hurler Syndrome** (accumulation due to alpha-L-iduronidase deficiency). * **Heparin:** The only GAG that is **intracellular** (found in mast cells) and acts as a potent natural anticoagulant. * **Chondroitin Sulfate:** Most abundant GAG in the human body.

Question 1

Chronic nonspherocytic hemolytic anemia is a manifestation of:

Accepted Answer

G6PD deficiency

Answer

Hereditary elliptocytosis

Answer

Sickle cell anemia

Answer

Hexokinase deficiency

Question 2

What is invert sugar?

Accepted Answer

An equimolar mixture of glucose and fructose

Answer

An equimolar mixture of sucrose and glucose

Answer

An equimolar mixture of glucose and lactose

Answer

An equimolar mixture of glucose and galactose

Question 3

All of the following are true regarding G6PD deficiency except?

Accepted Answer

Females are commonly affected

Answer

A recessive X-linked trait

Answer

Oxidative stress causes hemolysis

Answer

Protective against Plasmodium falciparum malaria

Question 4

Which of the following is NOT a heteropolysaccharide?

Accepted Answer

Inulin

Answer

Hyaluronic acid

Answer

Chondroitin sulfate

Answer

Heparan sulfate

Question 5

Alcohol causes hypoglycemia due to:

Accepted Answer

Decreased gluconeogenesis

Answer

Increased NADH

Answer

Decreased lipogenesis

Answer

Decreased glycogenesis

Question 6

Keratan sulphate is found in abundance in which of the following locations?

Accepted Answer

Cornea

Answer

Heart muscle

Answer

Liver

Answer

Adrenal cortex

Question 7

During a hunger strike with a liquid diet and minimal calories, which of the following hormonal and metabolic changes would occur after 4 to 5 hours?

Accepted Answer

Increased cyclic AMP and increased liver glycogenolysis

Answer

Decreased cyclic AMP and increased liver glycogen synthesis

Answer

Decreased epinephrine levels and increased liver glycogenolysis

Answer

Increased Ca2+ in muscle and decreased glycogenolysis

Question 8

Which enzyme catalyzes the irreversible step in the citric acid cycle?

Accepted Answer

α-ketoglutarate dehydrogenase

Answer

Aconitase

Answer

Succinate thiokinase

Answer

Isocitrate dehydrogenase

Question 9

Which of the following enzymes is a constituent of the HMP shunt?

Accepted Answer

G-6-P dehydrogenase

Answer

Glucose-6-phosphatase

Answer

Hexokinase

Answer

Phosphorylase

Question 10

Which of the following glycosaminoglycans plays a significant role in wound healing?

Accepted Answer

Hyaluronic acid

Answer

Keratan sulfate

Answer

Dermatan sulfate

Answer

Chondroitin sulfate

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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