Carbohydrate Metabolism Practice Questions

Q: Which disorder is associated with convulsions, cataract, and mental retardation?

Galactosaemia. **Explanation:** **Classic Galactosemia** is an autosomal recessive disorder caused by a deficiency of the enzyme **Galactose-1-phosphate uridyltransferase (GALT)**. The accumulation of galactose and its metabolites (galactose-1-phosphate and galactitol) leads to multi-organ damage: * **Cataract:** Galactitol accumulates in the lens, causing osmotic swelling and "oil-drop" cataracts. * **Convulsions & Mental Retardation:** Accumulation of toxic metabolites in the brain leads to cerebral edema, seizures, and progressive intellectual disability if untreated. * **Other features:** Jaundice, hepatosplenomegaly, and a high risk of *E. coli* sepsis. **Why other options are incorrect:** * **Phenylketonuria (PKU):** Characterized by intellectual disability, seizures, and a "mousy" odor, but **cataracts are not a feature**. Patients typically have fair skin and blue eyes due to melanin deficiency. * **Tyrosinemia (Type 1):** Primarily presents with severe liver failure, renal tubular dysfunction (Fanconi syndrome), and a "cabbage-like" odor. It does not typically cause cataracts. * **Maple Syrup Urine Disease (MSUD):** Presents early with poor feeding, vomiting, and a "maple syrup" odor in urine. While it causes severe neurological distress and seizures, it is not associated with cataracts. **High-Yield Clinical Pearls for NEET-PG:** * **Screening:** Reducing substances in urine (Benedict's test positive) but a negative dipstick for glucose is a classic diagnostic clue. * **Cataract Type:** Described as **"Oil-drop" cataract**. * **Management:** Immediate exclusion of lactose and galactose from the diet (switching to soy-based formula). * **Complication:** Even with treatment, many patients develop speech deficits and females often face **premature ovarian failure**.

Q: Which of the following is not a glycogen storage disorder?

Lesch Nyhan syndrome. **Explanation:** The correct answer is **Lesch-Nyhan syndrome** because it is a disorder of **purine metabolism**, not carbohydrate metabolism. It is caused by a deficiency of the enzyme **Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT)**, leading to the overproduction of uric acid. Clinically, it is characterized by hyperuricemia, intellectual disability, and a hallmark sign of **self-mutilation**. **Analysis of Incorrect Options (Glycogen Storage Diseases - GSDs):** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of **Glucose-6-Phosphatase**. It results in severe fasting hypoglycemia, hepatomegaly, and elevated levels of lactate and uric acid. * **Pompe’s Disease (GSD Type II):** Caused by a deficiency of **Lysosomal acid alpha-1,4-glucosidase (Acid Maltase)**. It is unique because it is a lysosomal storage disorder affecting the heart (cardiomegaly) and muscles. * **McArdle’s Disease (GSD Type V):** Caused by a deficiency of **Skeletal Muscle Glycogen Phosphorylase**. It presents with exercise-induced muscle cramps and myoglobinuria (Burgundy-colored urine). **NEET-PG High-Yield Pearls:** * **Mnemonic for GSDs:** "**V**ery **P**oor **C**arbohydrate **A**nd **M**etabolism **H**urt" (Type I: **V**on Gierke, II: **P**ompe, III: **C**ori, IV: **A**ndersen, V: **M**cArdle, VI: **H**ers). * **Lesch-Nyhan** is X-linked recessive; remember the "3 Ts": **T**ophaceous gout, **T**witching (dystonia), and **T**errible self-injury. * **Von Gierke** is the most common GSD; **Pompe** is the only GSD that is also a Lysosomal Storage Disease.

Q: What is the primary function of Glucose-6-phosphate dehydrogenase (G6PD)?

Reduces NADP+. **Explanation:** Glucose-6-phosphate dehydrogenase (G6PD) is the **rate-limiting enzyme** of the Pentose Phosphate Pathway (Hexose Monophosphate Shunt). Its primary function is to catalyze the oxidation of Glucose-6-phosphate to 6-phosphogluconolactone. During this reaction, electrons are transferred to the coenzyme **NADP+**, thereby **reducing it to NADPH**. **Why Option B is correct:** The reaction catalyzed by G6PD is: *Glucose-6-P + NADP+ → 6-phosphogluconolactone + NADPH + H+* By reducing NADP+, G6PD ensures a steady supply of NADPH, which is essential for reductive biosynthesis (e.g., fatty acids) and for maintaining the pool of **reduced glutathione** to protect cells against oxidative stress. **Why other options are incorrect:** * **Option A:** G6PD does not oxidize NADPH; it produces it. NADPH is oxidized back to NADP+ by enzymes like Glutathione Reductase or in biosynthetic pathways. * **Options C & D:** G6PD is highly specific for the NADP+/NADPH cofactor system. It does not utilize the NAD+/NADH system, which is primarily involved in ATP-generating catabolic pathways (like Glycolysis or the TCA cycle). **Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to **Non-immune Hemolytic Anemia** because RBCs lack mitochondria and rely solely on G6PD for NADPH to neutralize reactive oxygen species (ROS). * **Heinz Bodies:** Denatured hemoglobin precipitates seen in G6PD deficiency. * **Bite Cells:** Result from splenic macrophages removing Heinz bodies. * **Triggers:** Hemolysis is typically triggered by infections, Fava beans, or drugs (e.g., Primaquine, Sulphonamides, Dapsone).

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.

Question 1

Which disorder is associated with convulsions, cataract, and mental retardation?

Accepted Answer

Galactosaemia

Answer

Phenylketonuria

Answer

Tyrosinemia

Answer

Maple syrup disease

Question 2

Which of the following is not a glycogen storage disorder?

Accepted Answer

Lesch Nyhan syndrome

Answer

McArdle's disease

Answer

Pompe's disease

Answer

Von-Gierke's disease

Question 3

What is the primary function of Glucose-6-phosphate dehydrogenase (G6PD)?

Accepted Answer

Reduces NADP+

Answer

Oxidizes NADPH

Answer

Oxidizes NAD

Answer

Reduces NAD

Question 4

Which of the following is a non-reducing sugar?

Accepted Answer

Sucrose

Answer

Glucose

Answer

Maltose

Answer

Lactose

Question 5

Which enzyme is involved in fructose metabolism?

Accepted Answer

Glyceraldehyde-3-P Dehydrogenase

Answer

Glucokinase

Answer

Aldolase A

Answer

PFK-1

Question 6

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 7

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 8

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 9

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 10

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

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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