Carbohydrate Metabolism Practice Questions

Q: Von Gierke's disease occurs due to deficiency of which enzyme?

Glucose-6-phosphatase. **Explanation:** **Von Gierke’s Disease (GSD Type I)** is the most common glycogen storage disease. It is caused by a deficiency of **Glucose-6-phosphatase**, the enzyme responsible for converting Glucose-6-phosphate into free glucose. This enzyme is primarily located in the liver and kidneys. Since this is the final step in both glycogenolysis and gluconeogenesis, its deficiency leads to severe fasting hypoglycemia and massive accumulation of glycogen in the liver. **Analysis of Options:** * **Option A (Correct):** Glucose-6-phosphatase deficiency prevents the liver from releasing glucose into the blood, leading to the classic triad of hepatomegaly, hypoglycemia, and lactic acidosis. * **Option B (Incorrect):** Deficiency of **Liver phosphorylase** causes **Hers disease (GSD Type VI)**, which presents with a milder clinical course than Von Gierke’s. * **Option C (Incorrect):** Deficiency of **Muscle phosphorylase** causes **McArdle disease (GSD Type V)**, characterized by exercise-induced cramps and myoglobinuria, but no hypoglycemia. * **Option D (Incorrect):** Deficiency of the **Debranching enzyme** (α-1,6-glucosidase) causes **Cori disease (GSD Type III)**, where glycogen has abnormally short outer branches (limit dextrins). **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Hallmarks:** Hyperuricemia (due to increased PPP shunt), Hyperlipidemia (increased VLDL), and Lactic Acidosis. * **Clinical Presentation:** "Doll-like" facies (fat deposition), protuberant abdomen (massive hepatomegaly), and stunted growth. * **Treatment:** Frequent oral glucose/cornstarch and avoidance of fructose/galactose (which cannot be converted to glucose).

Q: What is the primary source of readily available energy for most cells in the body?

Glycogen. **Explanation:** The correct answer is **Glycogen**. **Why Glycogen is Correct:** Glycogen is a highly branched polymer of glucose that serves as the primary storage form of carbohydrates in animals. It is considered the "readily available" energy source because it can be rapidly mobilized through **glycogenolysis**. Unlike fats, glycogen can be broken down into glucose-6-phosphate, which enters glycolysis to produce ATP even in **anaerobic conditions**. This rapid mobilization is essential for maintaining blood glucose levels (via the liver) and providing immediate fuel for muscle contraction. **Why Other Options are Incorrect:** * **A. Fat (Triacylglycerols):** While fats provide the highest energy yield (9 kcal/g) and represent the body's largest energy reserve, they are mobilized slowly. They require oxygen for oxidation (aerobic only) and cannot be used as a rapid source of energy during high-intensity bursts. * **C. Lactate:** Lactate is a metabolic byproduct of anaerobic glycolysis. While it can be converted back to glucose in the liver via the **Cori Cycle**, it is a substrate for gluconeogenesis rather than a primary storage form of energy. * **D. Ketones:** Ketone bodies (e.g., acetoacetate, β-hydroxybutyrate) are alternative fuels produced during starvation or prolonged fasting. They are not the "primary" or "readily available" source under normal physiological conditions. **High-Yield NEET-PG Pearls:** * **Storage Sites:** The liver has the highest *concentration* of glycogen, but skeletal muscle contains the largest *total amount* due to its greater mass. * **Key Enzyme:** **Glycogen phosphorylase** is the rate-limiting enzyme of glycogenolysis, activated by glucagon and epinephrine. * **Muscle vs. Liver:** Muscle glycogen lacks **Glucose-6-phosphatase**; therefore, it cannot contribute to blood glucose and is used exclusively for local muscle contraction.

Q: The development of cataracts in patients with Diabetes Mellitus is primarily due to the accumulation of which of the following substances in the lens?

Sorbitol. ***Correct Option: Sorbitol*** - In the setting of chronic **hyperglycemia**, excess glucose is converted into **sorbitol** by the enzyme **aldose reductase** via the polyol pathway. - Sorbitol is poorly transported out of the lens cells and its accumulation creates an internal **osmotic gradient**, drawing water into the lens, which leads to cell swelling, lens fiber disruption, and eventual **cataract** formation. - This is the **primary mechanism** of diabetic cataract development. *Incorrect Option: Galactitol* - **Galactitol** (also known as dulcitol) is the specific sugar alcohol that accumulates when there is deficiency of **galactokinase** or **galactose-1-phosphate uridyltransferase**. - Its accumulation is characteristic of **galactosemia**, where high galactose levels lead to its formation via aldose reductase, causing congenital cataracts in that condition, not diabetic cataracts. *Incorrect Option: Fructose* - While **fructose** is produced from sorbitol by sorbitol dehydrogenase in the polyol pathway, it is readily metabolized and does not accumulate in lens tissue. - Fructose itself does not cause osmotic damage or cataract formation in diabetes. *Incorrect Option: Mannitol* - **Mannitol** is a hexahydric sugar alcohol primarily used pharmacologically as an osmotic diuretic for conditions like **cerebral edema**. - It is not an endogenous product of glucose metabolism in the lens and is not associated with diabetic cataract formation.

Q: A patient with a history of strenuous exercise skipped a meal and later consumed alcohol at a party. Alcohol inhibits which of the following biochemical processes, potentially leading to hypoglycemia?

Gluconeogenesis. ***Gluconeogenesis*** - Alcohol metabolism by **alcohol dehydrogenase** and **aldehyde dehydrogenase** generates a massive influx of **NADH**, drastically increasing the hepatic NADH/NAD+ ratio. - This high NADH/NAD+ ratio shifts metabolic intermediates, inhibiting key enzymes (e.g., **lactate dehydrogenase** and **malate dehydrogenase**) required to convert precursors like lactate and amino acids into glucose, leading to failure of new glucose synthesis and subsequent hypoglycemia. *Glycogenolysis* - This process, the breakdown of stored **glycogen** into glucose, is not directly inhibited by the increased NADH ratio produced during alcohol metabolism. - In fact, the initial phase of alcohol-induced hypoglycemia relies on the depletion of pre-existing glycogen stores, which is accelerated by factors like strenuous exercise or fasting. *Hexose Monophosphate (HMP) Shunt* - The primary function of the HMP shunt is to produce **NADPH** (for reductive biosynthesis and antioxidant defense) and five-carbon sugars (**ribose-5-phosphate**) for nucleotide synthesis. - Inhibition of the HMP shunt alters cell redox status and nucleotide production but does not directly impair blood glucose maintenance or cause acute hypoglycemia. *Ketone Body Formation* - Alcohol metabolism actually tends to **inhibit** ketone body formation because the high NADH ratio inhibits **beta-oxidation** of fatty acids in the liver, which is required to produce the Acetyl-CoA necessary for ketogenesis. - While ketone formation is inhibited, this is a consequence of high NADH, and the hypoglycemia itself results from the inability to synthesize glucose via gluconeogenesis, not the absence of ketones.

Q: A patient with a long-standing history of diabetes presents with cataracts. Which of the following metabolic products is primarily responsible for cataract formation in this condition?

Sorbitol. ***Sorbitol*** - When blood glucose is elevated in diabetes, the enzyme **aldose reductase** converts excess glucose into **sorbitol** (a polyol) in tissues like the lens, which do not require insulin for glucose uptake. - Sorbitol is poorly permeable across cell membranes. Its accumulation creates a significant **osmotic gradient** within the lens fibers, causing water influx and subsequent lens swelling and opacification (cataract formation). *Mannitol* - **Mannitol** is a polyol often used as an osmotic diuretic in clinical settings to reduce cerebral edema or intraocular pressure. - Although chemically similar to sorbitol, mannitol accumulation is not the primary mechanism of **cataract formation** specifically linked to chronic hyperglycemia. *Galactitol* - **Galactitol** is NOT responsible for cataract formation in diabetes, but is specifically responsible for cataracts in individuals with **galactosemia** (inability to metabolize galactose). - It is formed from **galactose** via aldose reductase, making it the incorrect metabolite in a patient with diabetes-related cataracts. *Fructose* - **Fructose** is formed when sorbitol is further metabolized by the enzyme **sorbitol dehydrogenase**. - While present in the polyol pathway, **sorbitol** itself is the product whose high intracellular concentration and osmotic activity is the direct cause of the diabetic cataract.

Q: Which of the following is excreted in the urine of patients with pentosuria?

L-Xylulose. ***L-Xylulose***- Pentosuria is a benign, autosomal recessive disorder caused by a deficiency in the enzyme **L-xylulose reductase** (or xylitol dehydrogenase), which is part of the **glucuronic acid pathway (GAA)**.- This deficiency prevents the conversion of **L-xylulose** (a pentose intermediate) to xylitol, leading to the accumulation and subsequent excretion of L-xylulose in the urine.*D-Ribose*- D-Ribose is an **aldopentose** sugar crucial for forming the backbone of **RNA**, **ATP**, and various coenzymes (e.g., NAD, FAD).- Its metabolism is primarily handled by the **pentose phosphate pathway**; its excretion is not the hallmark of the enzyme deficiency in pentosuria.*D-Ribulose*- D-Ribulose is a **ketopentose** that plays a key role as an intermediate in the **pentose phosphate pathway** (PPP), specifically forming xylulose 5-phosphate.- Although it is a pentose, its accumulation and excretion are not characteristic findings of the defective **L-xylulose reductase** enzyme in pentosuria.*D-Lactose*- D-Lactose is a **disaccharide** composed of glucose and galactose, derived from milk, and is not a pentose sugar.- Lactosuria (lactose excretion) is typically related to high dietary intake or severe disorders affecting **lactose metabolism** or hepatic function, which is unrelated to the biochemical defect causing pentosuria.

Q: A 3-month-old baby presents with severe hepatomegaly, cataracts in both eyes, lethargy, and hypotonia. Based on these symptoms, which enzyme deficiency is most likely involved?

Galactose-1-Phosphate Uridyl Transferase. ***Galactose-1-Phosphate Uridyl Transferase*** - **GALT** deficiency causes **Classic Galactosemia**, which is characterized by the accumulation of **galactose-1-phosphate**, leading to widespread organ damage. - The classic presentation in neonates includes **hepatomegaly**, **jaundice**, failure to thrive, hypoglycemia, **E. coli sepsis**, developmental delay, and pathognomonic **cataracts**. - This is the **most severe form** of galactosemia and matches all the clinical features in this case. *Galactokinase* - Deficiency of **Galactokinase (GALK)** leads to a milder form of galactosemia where **galactitol** accumulates, primarily causing **cataracts** in infancy. - Systemic symptoms like **hepatomegaly**, liver failure, and severe lethargy, as seen in this 3-month-old, are typically **absent** in GALK deficiency. *UDP-Galactose-4-Epimerase* - **GALE** deficiency is the rarest form of galactosemia with variable clinical severity. - While severe forms can present with symptoms similar to classic galactosemia, they are extremely rare and account for less than 5% of galactosemia cases. - The combination of severe hepatomegaly and cataracts at 3 months is most characteristic of **GALT deficiency**, not GALE deficiency. *Hepatic Glucose-6-Phosphatase* - Deficiency of this enzyme causes **Von Gierke disease** (GSD Type I), characterized by significant **hepatomegaly** and severe **fasting hypoglycemia**. - While it causes hepatomegaly and lethargy (from hypoglycemia), it is **not** associated with the development of **cataracts**, which differentiates it from galactosemia.

Q: A 68-year-old female patient has had a diabetic cataract for 3 months. Accumulation of which of the following substances is responsible for this?

Sorbitol + Fructose. ***Sorbitol + Fructose*** This option correctly identifies the products of the **polyol pathway**, which is significantly activated in lens fibers during **hyperglycemia**. **Sorbitol** is the key substance that accumulates due to high **aldose reductase** activity and low sorbitol dehydrogenase activity, leading to an **osmotic gradient** and water accumulation, causing lens swelling and opacification (cataract). *Glucose* Although high blood **glucose** initiates the process by serving as the substrate for aldose reductase, glucose itself is not the primary substance responsible for the osmotic damage in the lens. In normal lens metabolism, glucose is primarily metabolized via **glycolysis**; only when levels are excessive is the polyol pathway significantly utilized. *Fructose* Fructose is the breakdown product of **sorbitol** (catalyzed by sorbitol dehydrogenase), but its concentration is typically much lower than sorbitol in the lens. **Sorbitol** accumulation is the primary driver of the powerful osmotic effect that leads to water influx into the lens fiber cells and subsequent cataract formation. *Lactose + Glucose* Accumulation of **lactose** is not associated with diabetic cataract; this combination is irrelevant to the pathogenesis of hyperglycemia-induced lens damage. **Galactosemic cataract**, a different type of osmotic cataract, is caused by the accumulation of **galactitol** (a polyol derived from galactose, not glucose).

Question 1

Von Gierke's disease occurs due to deficiency of which enzyme?

Accepted Answer

Glucose-6-phosphatase

Answer

Liver phosphorylase

Answer

Muscle phosphorylase

Answer

Debranching enzyme

Question 2

What is the primary source of readily available energy for most cells in the body?

Accepted Answer

Glycogen

Answer

Fat

Answer

Lactate

Answer

Ketone

Question 3

Arrange the following 4 enzymes of gluconeogenesis in sequence:

Accepted Answer

Pyruvate carboxylase - Phosphoenol pyruvate carboxy kinase - Fructose 1,6 Bisphosphatase - Glucose - 6 - phosphatase

Answer

Fructose 1,6 Bisphosphatase - Phosphoenol pyruvate carboxy kinase - Pyruvate carboxylase - Glucose - 6 - phosphatase

Answer

Glucose - 6 - phosphatase - Pyruvate carboxylase - Fructose 1,6 Bisphosphatase - Phosphoenol pyruvate carboxy kinase

Answer

Phosphoenol pyruvate carboxy kinase - Fructose 1,6 Bisphosphatase - Glucose - 6 - phosphatase - Pyruvate carboxylase

Question 4

A breastfed infant presents with lethargy, hepatomegaly, and cataracts. Which of the following enzyme deficiencies is most likely responsible for this presentation?

Accepted Answer

Galactose-1-phosphate uridyltransferase (GALPUT)

Answer

Galactokinase

Answer

Aldolase B

Answer

Fructokinase

Question 5

The development of cataracts in patients with Diabetes Mellitus is primarily due to the accumulation of which of the following substances in the lens?

Accepted Answer

Sorbitol

Answer

Galactitol

Answer

Fructose

Answer

Mannitol

Question 6

A patient with a history of strenuous exercise skipped a meal and later consumed alcohol at a party. Alcohol inhibits which of the following biochemical processes, potentially leading to hypoglycemia?

Accepted Answer

Gluconeogenesis

Answer

Glycogenolysis

Answer

Ketone Body Formation

Answer

Hexose Monophosphate (HMP) Shunt

Question 7

A patient with a long-standing history of diabetes presents with cataracts. Which of the following metabolic products is primarily responsible for cataract formation in this condition?

Accepted Answer

Sorbitol

Answer

Galactitol

Answer

Mannitol

Answer

Fructose

Question 8

Which of the following is excreted in the urine of patients with pentosuria?

Accepted Answer

L-Xylulose

Answer

D-Lactose

Answer

D-Ribulose

Answer

D-Ribose

Question 9

A 3-month-old baby presents with severe hepatomegaly, cataracts in both eyes, lethargy, and hypotonia. Based on these symptoms, which enzyme deficiency is most likely involved?

Accepted Answer

Galactose-1-Phosphate Uridyl Transferase

Answer

Hepatic Glucose-6-Phosphatase

Answer

UDP-Galactose-4-Epimerase

Answer

Galactokinase

Question 10

A 68-year-old female patient has had a diabetic cataract for 3 months. Accumulation of which of the following substances is responsible for this?

Accepted Answer

Sorbitol + Fructose

Answer

Fructose

Answer

Lactose + Glucose

Answer

Glucose

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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