Carbohydrate Metabolism Practice Questions

Q: Which glycogen storage disease causes hypoglycemia, hepatomegaly, growth retardation, muscle weakness, and accumulation of limit dextrins?

Cori's disease. **Explanation:** The clinical presentation of hypoglycemia, hepatomegaly, and growth retardation is characteristic of several Glycogen Storage Diseases (GSDs). However, the presence of **limit dextrins** (abnormally short outer branches of glycogen) and **muscle weakness** specifically points to **Cori’s disease (GSD Type III)**. 1. **Why Cori’s Disease is Correct:** It is caused by a deficiency of the **Debranching enzyme** (α-1,6-glucosidase). Without this enzyme, glycogen can be broken down by phosphorylase only until it reaches a branch point, leaving behind "limit dextrins." Unlike Von Gierke’s, Cori’s involves both the liver and muscles, explaining the myopathy/muscle weakness. Gluconeogenesis remains intact, so hypoglycemia is generally milder than in Type I. 2. **Why Other Options are Incorrect:** * **Von Gierke’s disease (Type I):** Caused by Glucose-6-Phosphatase deficiency. It presents with severe hypoglycemia, lactic acidosis, and hyperuricemia, but **no muscle involvement** and no limit dextrins. * **Andersen disease (Type IV):** Caused by **Branching enzyme** deficiency. It results in long, unbranched glucose chains (amylopectin-like). It typically presents with infantile cirrhosis and liver failure, not hypoglycemia. * **Pompe’s disease (Type II):** Caused by Lysosomal acid maltase deficiency. It primarily affects the heart (cardiomegaly) and muscles. Blood glucose levels are typically **normal**. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "ABCD" – **A**ndersen is **B**ranching; **C**ori is **D**ebranching. * **Limit Dextrins:** Pathognomonic for Cori’s disease. * **Muscle Involvement:** If a GSD looks like Von Gierke’s but includes muscle weakness/wasting, think Cori’s (Type III). * **Type I vs. Type III:** Type I has elevated lactate; Type III has normal lactate levels.

Q: Which enzyme is specific for gluconeogenesis?

Glucose-6-phosphatase. **Explanation:** Gluconeogenesis is the metabolic pathway that results in the generation of glucose from non-carbohydrate precursors. While many steps of gluconeogenesis are simply the reverse of glycolysis, there are **three irreversible steps** in glycolysis that must be bypassed by four specific gluconeogenic enzymes. **1. Why Glucose-6-phosphatase is correct:** Glucose-6-phosphatase is one of the four key "bypass enzymes" of gluconeogenesis. It catalyzes the conversion of Glucose-6-phosphate to free Glucose, bypassing the irreversible hexokinase/glucokinase reaction of glycolysis. This enzyme is primarily located in the **lumen of the endoplasmic reticulum** of the liver and kidney, allowing these organs to release free glucose into the blood. **2. Why the other options are incorrect:** * **Aldolase (B):** This enzyme is involved in both glycolysis (cleaving Fructose-1,6-bisphosphate) and gluconeogenesis (condensing DHAP and Glyceraldehyde-3-phosphate). Because it functions in both directions, it is not "specific" to gluconeogenesis. * **Phosphoglycerate kinase (C) & Phosphoglycerate mutase (D):** These are reversible enzymes shared by both the glycolytic and gluconeogenic pathways. **High-Yield Clinical Pearls for NEET-PG:** * **The Four Unique Gluconeogenic Enzymes:** 1. Pyruvate carboxylase, 2. PEP carboxykinase (PEPCK), 3. Fructose-1,6-bisphosphatase (Rate-limiting step), and 4. Glucose-6-phosphatase. * **Von Gierke Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. It presents with severe fasting hypoglycemia, hepatomegaly, and hyperuricemia. * **Location:** Muscle lacks Glucose-6-phosphatase; therefore, muscle glycogen cannot contribute directly to blood glucose levels.

Q: All of the following tests can help in the diagnosis of Galactosemia EXCEPT?

Guthrie's test. **Explanation:** The correct answer is **Guthrie’s test** because it is a specific screening tool for **Phenylketonuria (PKU)**, not Galactosemia. It is a bacterial inhibition assay that detects elevated levels of phenylalanine in the blood using *Bacillus subtilis*. **Analysis of Options:** * **GALT Assay (Option A):** This is the **gold standard** for diagnosing Classical Galactosemia. It measures the activity of the enzyme *Galactose-1-phosphate uridyltransferase* in erythrocytes. Low or absent activity confirms the diagnosis. * **Mutation Analysis (Option B):** Molecular genetic testing (e.g., identifying mutations in the *GALT* gene like Q188R) is used for definitive diagnosis, carrier screening, and prenatal testing. * **Benedict’s Test (Option C):** This is a non-specific screening test for **reducing substances** in the urine. Since galactose is a reducing sugar, a child with galactosemia will show a positive Benedict’s test but a **negative Glucose Oxidase (Dipstick) test**, providing a strong clinical clue. **High-Yield Clinical Pearls for NEET-PG:** * **Classical Galactosemia** is an autosomal recessive deficiency of **GALT**. * **Clinical Triad:** Oil-drop cataracts, hepatosplenomegaly (jaundice), and intellectual disability. * **Early Sign:** Increased susceptibility to **E. coli sepsis** in the neonatal period. * **Management:** Immediate withdrawal of lactose/galactose from the diet (switch to soy-based formula). * **Duarte Variant:** A milder form of the disease with partial enzyme activity.

Q: A 17-year-old with type 1 diabetes presents with diabetic ketoacidosis after a weekend camping trip. He is noted to be jaundiced, and the plasma has a red color. Which of the following is a product of the enzyme for which this patient has a deficiency?

NADPH. ### Explanation **1. Why NADPH is the Correct Answer:** The clinical presentation describes a patient with **G6PD (Glucose-6-Phosphate Dehydrogenase) deficiency** triggered by **Diabetic Ketoacidosis (DKA)**. In G6PD deficiency, oxidative stress (caused here by acidosis and infection/stress from the trip) leads to the oxidation of hemoglobin into **Heinz bodies**, resulting in acute hemolysis (jaundice and red plasma/hemoglobinemia). G6PD is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. It catalyzes the conversion of Glucose-6-Phosphate to 6-Phosphogluconolactone, and in the process, it reduces NADP+ to **NADPH**. NADPH is essential for maintaining the pool of **reduced glutathione**, which protects erythrocytes from oxidative damage by neutralizing free radicals and hydrogen peroxide. **2. Why Other Options are Incorrect:** * **A. FADH2:** Produced primarily in the TCA cycle (Succinate dehydrogenase) and Beta-oxidation. It is not a product of the G6PD enzyme. * **B. NAD+:** This is an oxidized coenzyme. The HMP shunt is focused on producing *reduced* coenzymes (NADPH). * **C. NADH:** Produced during glycolysis and the TCA cycle. While it is a reducing equivalent, it is not the product of the HMP shunt and cannot be used by Glutathione Reductase to protect against oxidative stress in RBCs. **3. Clinical Pearls for NEET-PG:** * **Inheritance:** X-linked recessive (more common in males). * **Triggers:** Fava beans, Infections, DKA, and Drugs (AAA: **A**ntimalarials like Primaquine, **A**ntibiotics like Sulfa drugs, **A**spirin/NSAIDs). * **Morphology:** **Heinz Bodies** (denatured Hb) and **Bite Cells** (formed by splenic macrophages removing Heinz bodies). * **Diagnosis:** Enzyme assay (Note: Levels may be falsely normal during an acute hemolytic episode as young reticulocytes have higher enzyme levels).

Q: Enantiomers are isomers that differ in structure at which carbon?

Penultimate Carbon. ### Explanation **Concept Overview:** Enantiomers are a specific type of stereoisomer that are **non-superimposable mirror images** of each other. In carbohydrate chemistry, the classification of a sugar as a **D-isomer** or **L-isomer** is determined by the orientation of the hydroxyl (-OH) group on the **penultimate carbon** (the chiral carbon furthest from the carbonyl group). **Why Option C is Correct:** The penultimate carbon (the "next-to-last" carbon) is the reference point for stereoisomerism in sugars. For example, in a 6-carbon glucose molecule, this is **C-5**. If the -OH group on this carbon is on the right, it is a D-isomer; if on the left, it is an L-isomer. Since D and L forms are mirror images of each other, they are enantiomers. **Analysis of Incorrect Options:** * **A. Last Carbon:** The last carbon (e.g., C-6 in glucose) is usually a primary alcohol group (CH₂OH) and is **achiral** (not bonded to four different groups), so it cannot determine isomerism. * **B. First Carbon:** In aldoses, the first carbon is the aldehyde group. While it is functional, it is not the reference for D/L nomenclature. * **D. Carbonyl Carbon:** This is the functional group (C-1 in aldoses, C-2 in ketoses). When sugars cyclize, this becomes the **anomeric carbon**, giving rise to **anomers** (alpha and beta forms), not enantiomers. **NEET-PG High-Yield Pearls:** * **Most abundant form:** Most naturally occurring sugars in the human body are **D-isomers** (enzymes are stereospecific for D-sugars). * **Amino Acids:** Unlike sugars, naturally occurring amino acids are primarily **L-isomers**. * **Epimers:** Isomers differing at a single asymmetric carbon *other* than the penultimate carbon (e.g., Glucose and Galactose are C-4 epimers). * **Racemic Mixture:** An equimolar mixture of D and L enantiomers that shows no optical activity.

Q: The enzyme phosphofructo kinase-1 is strongly activated by which of the following?

Fructose 2,6 bisphosphate. **Explanation:** Phosphofructokinase-1 (PFK-1) is the **rate-limiting and key committed step** of Glycolysis, converting Fructose 6-phosphate to Fructose 1,6-bisphosphate. Its regulation is a high-yield topic for NEET-PG. **Why Fructose 2,6-bisphosphate (F2,6-BP) is correct:** F2,6-BP is the **most potent allosteric activator** of PFK-1. It is produced by the bifunctional enzyme PFK-2. F2,6-BP functions by increasing the affinity of PFK-1 for its substrate (Fructose 6-phosphate) and decreasing the inhibitory effect of ATP. This ensures that glycolysis proceeds rapidly even when energy levels are relatively high. **Analysis of Incorrect Options:** * **B. Adenosine Triphosphate (ATP):** ATP acts as an **allosteric inhibitor**. High ATP levels signal that the cell has sufficient energy, thus "turning off" glycolysis to conserve glucose. * **C. Citrate:** Citrate is an intermediate of the TCA cycle. High levels indicate that energy production is meeting demand, acting as an **allosteric inhibitor** of PFK-1. * **A. Cyclic AMP (cAMP):** In the liver, high cAMP (triggered by Glucagon) leads to the phosphorylation and inactivation of PFK-2, which **decreases** F2,6-BP levels, thereby inhibiting PFK-1 and glycolysis. **High-Yield Clinical Pearls for NEET-PG:** * **PFK-1 Inhibitors:** ATP, Citrate, and H+ ions (low pH/acidosis). * **PFK-1 Activators:** Fructose 2,6-BP and AMP. * **Insulin vs. Glucagon:** Insulin increases F2,6-BP (stimulating glycolysis), while Glucagon decreases it (stimulating gluconeogenesis). * **Tauri Disease (GSD Type VII):** Caused by a deficiency of the PFK-1 enzyme in muscles, leading to exercise intolerance and hemolysis.

Q: What are the steps involved in the production of fructose in seminal fluid?

Glucose-6-phosphate glucose sorbitol fructose. **Explanation:** The production of fructose in the seminal vesicles occurs via the **Polyol Pathway** (also known as the Sorbitol Pathway). This pathway is essential because fructose is the primary energy source for sperm motility. **Why Option C is Correct:** The synthesis follows a specific sequence starting from blood glucose: 1. **Glucose-6-phosphate** is converted back to **Glucose** by Glucose-6-phosphatase. 2. **Glucose** is reduced to **Sorbitol** (a polyol) by the enzyme **Aldose Reductase**, using NADPH as a cofactor. 3. **Sorbitol** is then oxidized to **Fructose** by the enzyme **Sorbitol Dehydrogenase**, using NAD+ as a cofactor. This bypasses the glycolytic hexokinase step, allowing for efficient fructose production in the seminal vesicles. **Why Other Options are Incorrect:** * **Option A:** Describes a simple isomerization (like in glycolysis), but the seminal vesicles utilize the polyol pathway to ensure high concentrations of fructose specifically. * **Option B:** "Sorbitol phosphate" is not a physiological intermediate in this pathway; the pathway involves free sugars, not phosphorylated intermediates. * **Option D:** Fructose-1-phosphate is an intermediate of fructose metabolism in the liver (fructolysis), not its synthesis in seminal fluid. **Clinical Pearls for NEET-PG:** * **Enzymes:** Aldose Reductase (Glucose → Sorbitol) and Sorbitol Dehydrogenase (Sorbitol → Fructose). * **Tissue Distribution:** The seminal vesicles and lens have both enzymes. However, tissues like the **retina, kidneys, and nerves** lack Sorbitol Dehydrogenase. * **Pathology:** In **Diabetes Mellitus**, hyperglycemia leads to excessive sorbitol accumulation in tissues lacking Sorbitol Dehydrogenase. Since sorbitol is osmotically active, it causes water influx, leading to **cataracts, retinopathy, and neuropathy**.

Question 1

A 12-year-old intellectually disabled boy presents with short stature, protuberant abdomen with an umbilical hernia, and a prominent forehead. His vision is normal, and his parents are unaffected. What is the metabolic defect in this disorder?

Accepted Answer

Iduronate Sulfatase

Answer

L-Iduronidase

Answer

Aryl Sulfatase B

Answer

Beta-Glucuronidase

Question 2

Which glycogen storage disease causes hypoglycemia, hepatomegaly, growth retardation, muscle weakness, and accumulation of limit dextrins?

Accepted Answer

Cori's disease

Answer

Von Gierke's disease

Answer

Anderson disease

Answer

Pompe's disease

Question 3

Which enzyme is specific for gluconeogenesis?

Accepted Answer

Glucose-6-phosphatase

Answer

Aldolase

Answer

Phosphoglycerate kinase

Answer

Phosphoglycerate mutase

Question 4

All of the following tests can help in the diagnosis of Galactosemia EXCEPT?

Accepted Answer

Guthrie's test

Answer

GALT assay

Answer

Mutation analysis

Answer

Benedict's test

Question 5

A 17-year-old with type 1 diabetes presents with diabetic ketoacidosis after a weekend camping trip. He is noted to be jaundiced, and the plasma has a red color. Which of the following is a product of the enzyme for which this patient has a deficiency?

Accepted Answer

NADPH

Answer

FADH2

Answer

NAD+

Answer

NADH

Question 6

Enantiomers are isomers that differ in structure at which carbon?

Accepted Answer

Penultimate Carbon

Answer

Last Carbon

Answer

First Carbon

Answer

Carbonyl Carbon

Question 7

The enzyme phosphofructo kinase-1 is strongly activated by which of the following?

Accepted Answer

Fructose 2,6 bisphosphate

Answer

Cyclic AMP

Answer

Adenosine triphosphate

Answer

Citrate

Question 8

Which of the following statements is false regarding glycolysis?

Accepted Answer

The net production of ATP from anaerobic glycolysis is 2 ATP.

Answer

Glycolysis occurs in the cytosol of all cells.

Answer

The net production of ATP from aerobic glycolysis is approximately 7 ATP.

Answer

None of the above statements are false.

Question 9

What are the steps involved in the production of fructose in seminal fluid?

Accepted Answer

Glucose-6-phosphate 	 glucose 	 sorbitol 	 fructose

Answer

Glucose-6-phosphate 	 fructose-6-phosphate 	 fructose

Answer

Glucose-6-phosphate 	 sorbitol phosphate 	 fructose phosphate 	 fructose

Answer

Glucose-6-phosphate 	 fructose-1-phosphate 	 fructose

Question 10

What is the effect of oral administration of 50 gm of glucose on the body?

Accepted Answer

Decreased gluconeogenesis

Answer

Decreased ketone body production

Answer

Increased lactate production upon exercise

Answer

Increased gluconeogenesis

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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