Carbohydrate Metabolism Practice Questions

Q: Which of the following is a non-reducing sugar?

Sucrose. ### Explanation **Core Concept: Reducing vs. Non-Reducing Sugars** A sugar is classified as "reducing" if it has a free **anomeric carbon** (aldehyde or ketone group) capable of acting as a reducing agent. In chemical tests like Benedict’s or Fehling’s, these sugars reduce cupric ions ($Cu^{2+}$) to cuprous ions ($Cu^+$), resulting in a color change. **Why Sucrose is the Correct Answer:** Sucrose is a disaccharide composed of **Glucose and Fructose**. The glycosidic linkage occurs between the $C_1$ of glucose and the $C_2$ of fructose. Since both anomeric carbons are involved in the bond, there is **no free aldehyde or ketone group** available. Consequently, sucrose cannot reduce alkaline copper solutions, making it a **non-reducing sugar**. **Analysis of Incorrect Options:** * **Glucose (Option A):** A monosaccharide with a free aldehyde group at $C_1$; it is a classic reducing sugar. * **Maltose (Option B):** A disaccharide (Glucose + Glucose) with an $\alpha(1\to4)$ bond. The second glucose molecule retains a free anomeric carbon at $C_1$. * **Lactose (Option C):** A disaccharide (Galactose + Glucose) with a $\beta(1\to4)$ bond. The glucose unit has a free anomeric carbon at $C_1$. **High-Yield Clinical Pearls for NEET-PG:** * **Inversion:** Sucrose is dextrorotatory, but upon hydrolysis, it becomes levorotatory (due to fructose). Hence, hydrolyzed sucrose is called **"Invert Sugar."** * **Benedict’s Test:** Used clinically to detect reducing sugars in urine (e.g., glucose in Diabetes Mellitus or galactose in Galactosemia). * **Trehalose:** Another high-yield non-reducing disaccharide (found in mushrooms) where two glucose units are linked via their anomeric carbons ($1\to1$). * **All monosaccharides** are reducing sugars.

Q: Which enzyme is involved in fructose metabolism?

Glyceraldehyde-3-P Dehydrogenase. **Explanation:** Fructose metabolism (fructolysis) occurs primarily in the liver. The process bypasses the major rate-limiting step of glycolysis (PFK-1), which explains why fructose is metabolized faster than glucose. **Why Glyceraldehyde-3-P Dehydrogenase is correct:** In the liver, Fructose is converted to Fructose-1-Phosphate by **Fructokinase**. This is then cleaved by **Aldolase B** into Dihydroxyacetone phosphate (DHAP) and **Glyceraldehyde**. Glyceraldehyde is phosphorylated to **Glyceraldehyde-3-Phosphate (G3P)** by Triokinase. From this point forward, G3P enters the standard glycolytic pathway, where it is acted upon by **Glyceraldehyde-3-P Dehydrogenase** to form 1,3-bisphosphoglycerate. Thus, this enzyme is a shared and essential component of the downstream metabolism of fructose. **Analysis of Incorrect Options:** * **A. Glucokinase:** This enzyme is specific for glucose phosphorylation in the liver. While Hexokinase can phosphorylate fructose in muscles, Glucokinase cannot. * **C. Aldolase A:** This isoform is found in muscle and RBCs and prefers Fructose-1,6-BP. Fructose metabolism in the liver specifically requires **Aldolase B**, which can utilize Fructose-1-Phosphate as a substrate. * **D. PFK-1:** This is the rate-limiting enzyme of glycolysis. Fructose metabolism enters the pathway *below* this step, allowing it to bypass regulation by insulin and ATP. **High-Yield Clinical Pearls for NEET-PG:** 1. **Essential Fructosuria:** Deficiency of **Fructokinase**. It is a benign condition where fructose appears in the urine (reducing sugar positive). 2. **Hereditary Fructose Intolerance (HFI):** Deficiency of **Aldolase B**. It leads to the accumulation of Fructose-1-Phosphate, causing intracellular phosphate depletion, hypoglycemia, and jaundice. 3. **Speed of Metabolism:** Fructose is metabolized faster than glucose because it bypasses the PFK-1 regulatory step.

Q: A neonate presents with hypoglycemia and jaundice and is diagnosed to have a metabolic disorder. What is the most likely diagnosis?

Galactosemia. **Explanation:** **Galactosemia** (specifically Classic Galactosemia due to GALT deficiency) is the most likely diagnosis because it classically presents in the **neonatal period** shortly after the introduction of milk (lactose). 1. **Why Galactosemia is correct:** The accumulation of Galactose-1-phosphate in the liver causes hepatotoxicity, leading to **jaundice** and hepatomegaly. **Hypoglycemia** occurs because high levels of Galactose-1-phosphate inhibit *phosphoglucomutase* and *glucose-6-phosphatase*, impairing both glycogenolysis and gluconeogenesis. 2. **Why other options are incorrect:** * **Hypothyroidism:** While it can cause prolonged physiological jaundice in neonates, it typically presents with lethargy, constipation, and a large fontanelle, rather than acute hypoglycemia. * **Glycogen Storage Disease (GSD):** While GSD Type I (von Gierke) causes severe hypoglycemia and hepatomegaly, it usually presents at **3–6 months** of age when feeding intervals increase. It does not typically present with conjugated jaundice in the immediate neonatal period. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficiency:** Most common is **Galactose-1-phosphate uridyltransferase (GALT)**. * **Key Clinical Triad:** Cataracts (due to *galactitol* accumulation in the lens), Jaundice, and Liver failure. * **Diagnostic Clue:** Presence of **reducing sugars** in urine (Clinitest positive) but a negative glucose oxidase test (Dipstick negative). * **Infection Risk:** These neonates are at a significantly increased risk of **E. coli sepsis**. * **Management:** Immediate withdrawal of breast milk/lactose-containing formula and switching to soy-based formula.

Q: A 15-year-old female patient presents with pain in the calf muscles on exercise. Routine investigation reveals burgundy-colored urine after exercise. Histopathology of muscle shows deposits of glycogen in excess. What is the diagnosis?

McArdle's disease. ### Explanation **Correct Option: C. McArdle’s Disease (GSD Type V)** The clinical presentation of **exercise-induced muscle pain** (cramps) and **burgundy-colored urine** (myoglobinuria) is classic for McArdle’s disease. * **Mechanism:** It is caused by a deficiency of **muscle phosphorylase** (myophosphorylase). This enzyme is essential for breaking down muscle glycogen into glucose-1-phosphate. * **Pathology:** Since glycogen cannot be degraded during exercise, it accumulates in the muscle (subsarcolemmal deposits). The lack of ATP leads to muscle cell damage (rhabdomyolysis), releasing myoglobin into the urine, causing the characteristic burgundy color. **Incorrect Options:** * **A. Von Gierke’s Disease (GSD Type I):** Caused by Glucose-6-Phosphatase deficiency. It primarily affects the **liver**, presenting with severe fasting hypoglycemia, hepatomegaly, and hyperuricemia, but no muscle symptoms. * **B. Pompe’s Disease (GSD Type II):** Caused by Lysosomal acid alpha-glucosidase deficiency. It affects the **heart** and muscles globally. It typically presents in infancy with cardiomegaly and hypotonia ("floppy baby"), not exercise-induced cramps in teenagers. * **D. Cori’s Disease (GSD Type III):** Caused by Debranching enzyme deficiency. It presents similarly to Von Gierke’s but is milder and involves both liver and muscle; however, it does not typically cause myoglobinuria. **High-Yield Clinical Pearls for NEET-PG:** * **"Second Wind" Phenomenon:** Patients with McArdle’s often feel better after a few minutes of exercise as the body switches to using free fatty acids and blood glucose. * **Ischemic Forearm Exercise Test:** Shows a **failure of blood lactate to rise** (hallmark diagnostic feature), as glycogen cannot be converted to lactate. * **Biochemical marker:** Elevated serum Creatine Kinase (CK) levels.

Q: Ribose phosphate is a product in which of the following pathways?

Pentose phosphate pathway. **Explanation:** The **Pentose Phosphate Pathway (PPP)**, also known as the Hexose Monophosphate (HMP) Shunt, is a unique pathway of glucose oxidation that occurs in the cytosol. Unlike glycolysis, its primary purpose is not the generation of ATP, but the production of two vital intermediates: **NADPH** and **Pentoses (Ribose-5-phosphate)**. 1. **Why Option C is Correct:** In the **oxidative phase** of the PPP, Glucose-6-phosphate is converted into Ribulose-5-phosphate. In the subsequent **non-oxidative phase**, this is isomerized into **Ribose-5-phosphate**. This 5-carbon sugar is the essential precursor for the synthesis of nucleotides (DNA and RNA), ATP, NADH, and Coenzyme A. 2. **Why Other Options are Incorrect:** * **Lactic acid cycle (Cori Cycle):** This involves the conversion of lactate (produced by anaerobic glycolysis in muscles) back to glucose in the liver. It does not involve 5-carbon sugars. * **Citric acid cycle (TCA Cycle):** This is the final common pathway for the oxidation of carbohydrates, fats, and proteins. Its primary products are NADH, $FADH_2$, and $CO_2$; it does not produce ribose phosphate. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **G6PD Deficiency:** Leads to hemolytic anemia due to the inability to produce NADPH, which is required to keep glutathione reduced to neutralize reactive oxygen species (ROS) in RBCs. * **Tissue Distribution:** The PPP is highly active in tissues requiring NADPH for fatty acid or steroid synthesis (e.g., Adrenal cortex, Liver, Mammary glands) and in RBCs to maintain reduced glutathione. * **Transketolase:** An enzyme in the non-oxidative phase that requires **Thiamine (Vitamin B1)** as a cofactor; its activity is used to clinically diagnose Thiamine deficiency.

Q: Fanconi-Bickel syndrome is due to a mutation in the gene encoding which of the following transporters?

GLUT2. **Explanation:** **Fanconi-Bickel Syndrome (FBS)** is a rare autosomal recessive disorder of carbohydrate metabolism caused by a mutation in the **SLC2A2 gene**, which encodes the **GLUT2** transporter. **1. Why GLUT2 is correct:** GLUT2 is a high-capacity, low-affinity bidirectional transporter found in the liver, pancreas (beta cells), small intestine, and proximal renal tubules. In FBS, the deficiency of GLUT2 leads to: * **Liver:** Impaired export of glucose, leading to glycogen accumulation (Glycogen Storage Disease Type XI) and hepatomegaly. * **Kidney:** Failure to reabsorb glucose, galactose, and other solutes in the proximal tubule, resulting in **Proximal Renal Tubular Acidosis (Fanconi Syndrome)** characterized by glucosuria, phosphaturia, and aminoaciduria. **2. Why the other options are incorrect:** * **GLUT1:** Primarily found in RBCs and the Blood-Brain Barrier. Deficiency leads to seizures and developmental delay (De Vivo disease). * **GLUT4:** The insulin-responsive transporter found in skeletal muscle and adipose tissue. It is not involved in FBS. * **Na+/Iodine Symporter (NIS):** Responsible for iodine uptake in the thyroid gland; mutations lead to congenital hypothyroidism, not carbohydrate metabolism disorders. **Clinical Pearls for NEET-PG:** * **Classic Triad:** Hepatomegaly (due to glycogen accumulation), rickets (due to hypophosphatemia), and growth retardation. * **Laboratory Hallmark:** Post-prandial hyperglycemia and fasting hypoglycemia (due to inability to mobilize liver glucose). * **Key Distinction:** Unlike other GSDs, FBS involves both glucose and galactose intolerance.

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).

Question 1

Which of the following is a non-reducing sugar?

Accepted Answer

Sucrose

Answer

Glucose

Answer

Maltose

Answer

Lactose

Question 2

Which enzyme is involved in fructose metabolism?

Accepted Answer

Glyceraldehyde-3-P Dehydrogenase

Answer

Glucokinase

Answer

Aldolase A

Answer

PFK-1

Question 3

A neonate presents with hypoglycemia and jaundice and is diagnosed to have a metabolic disorder. What is the most likely diagnosis?

Accepted Answer

Galactosemia

Answer

Hypothyroidism

Answer

Glycogen storage disease

Answer

None of the above

Question 4

The differences between the liver and muscle in glucose utilization from blood are due to the different regulatory properties of which enzyme?

Accepted Answer

Hexokinase

Answer

Pyruvate kinase

Answer

Phosphoglucomutase

Answer

Aldolase

Question 5

A 15-year-old female patient presents with pain in the calf muscles on exercise. Routine investigation reveals burgundy-colored urine after exercise. Histopathology of muscle shows deposits of glycogen in excess. What is the diagnosis?

Accepted Answer

McArdle's disease

Answer

Von Gierke's disease

Answer

Pompe's disease

Answer

Cori's disease

Question 6

Ribose phosphate is a product in which of the following pathways?

Accepted Answer

Pentose phosphate pathway

Answer

Lactic acid cycle

Answer

Citric acid cycle

Answer

None of the above

Question 7

Fanconi-Bickel syndrome is due to a mutation in the gene encoding which of the following transporters?

Accepted Answer

GLUT2

Answer

GLUT1

Answer

GLUT4

Answer

Na+/iodine channels

Question 8

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 9

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 10

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

Accepted Answer

Anomers

Answer

Stereoisomers

Answer

Epimers

Answer

Keto-aldo pairs

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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