Carbohydrate Metabolism Practice Questions

Q: Glucose-6-phosphate dehydrogenase (G6PD) is primarily found in which cellular compartment?

Cytosol. **Explanation:** **1. Why Cytosol is Correct:** Glucose-6-phosphate dehydrogenase (G6PD) is the **rate-limiting enzyme** of the **Hexose Monophosphate (HMP) Shunt** (also known as the Pentose Phosphate Pathway). The HMP shunt is a unique pathway because it does not involve the mitochondria and occurs entirely within the **cytosol** of the cell. This location is strategic, as the pathway generates **NADPH**, which is required for cytosolic processes such as fatty acid synthesis, steroid synthesis, and the reduction of glutathione to protect cells against oxidative stress. **2. Why Other Options are Incorrect:** * **Mitochondria:** This compartment houses the TCA cycle, Beta-oxidation of fatty acids, and the Electron Transport Chain (ETC). G6PD is not involved in these oxidative phosphorylation processes. * **Golgi Apparatus:** This organelle is primarily involved in the post-translational modification, sorting, and packaging of proteins; it does not host the enzymes of primary carbohydrate metabolism. * **Endoplasmic Reticulum (ER):** While the ER is involved in protein synthesis (Rough ER) and lipid/steroid synthesis (Smooth ER), the initial enzymatic steps of the HMP shunt occur in the surrounding cytosol. (Note: Glucose-6-phosphatase is located in the ER, but G6PD is cytosolic). **3. Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to **hemolytic anemia** triggered by oxidative stress (e.g., Fava beans, Primaquine, Infection) because RBCs lack mitochondria and rely solely on the HMP shunt for NADPH to maintain reduced glutathione. * **Tissue Distribution:** G6PD activity is highest in tissues requiring NADPH for reductive biosynthesis (Liver, Adipose tissue, Lactating mammary glands, Adrenal cortex) and in RBCs for antioxidant defense. * **Key Product:** The HMP shunt produces **Ribose-5-phosphate** (for nucleotide synthesis) and **NADPH** (not NADH).

Q: The oxidative phase of the HMP shunt pathway is least active in which of the following tissues?

Skeletal muscle. **Explanation:** The oxidative phase of the **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway) is primarily responsible for the production of **NADPH**. Therefore, this pathway is most active in tissues that require high amounts of NADPH for reductive biosynthesis or protection against oxidative stress. **Why Skeletal Muscle is the Correct Answer:** Skeletal muscle lacks the enzymes for significant fatty acid or steroid synthesis. Its primary metabolic requirement is ATP production via glycolysis and the TCA cycle. Since it does not perform large-scale reductive biosynthesis, the activity of Glucose-6-Phosphate Dehydrogenase (G6PD)—the rate-limiting enzyme of the oxidative phase—is extremely low in resting skeletal muscle. **Analysis of Incorrect Options:** * **Adrenal Cortex:** Highly active in the HMP shunt because NADPH is essential for the hydroxylation reactions involved in **steroid hormone synthesis**. * **Lactating Mammary Gland:** Requires massive amounts of NADPH for the **de novo synthesis of fatty acids** found in breast milk. * **Red Blood Cells (RBCs):** Utilize NADPH to maintain a pool of **reduced glutathione**, which is critical for neutralizing reactive oxygen species (ROS) and preventing hemolysis. **High-Yield Facts for NEET-PG:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD), regulated by NADP+ levels. * **Key Products:** NADPH (for biosynthesis/antioxidant defense) and Ribose-5-phosphate (for nucleotide synthesis). * **Tissues with High HMP Activity:** Liver, Adrenal cortex, Testes/Ovaries, Lactating mammary gland, RBCs, and Phagocytic cells (for respiratory burst). * **Clinical Correlation:** G6PD deficiency leads to hemolytic anemia under oxidative stress (e.g., fava beans, primaquine) because RBCs cannot generate enough NADPH to maintain glutathione in its reduced state.

Q: A patient develops dark colored urine one day after administration of primaquine. What is the most likely diagnosis?

Glucose-6-phosphate dehydrogenase deficiency. **Explanation:** The clinical presentation of dark-colored urine (hemoglobinuria) following the administration of **Primaquine** is a classic indicator of acute hemolysis due to **Glucose-6-phosphate dehydrogenase (G6PD) deficiency**. **Why Option A is Correct:** G6PD is a critical enzyme in the Pentose Phosphate Pathway, responsible for generating **NADPH**. In red blood cells, NADPH is essential for maintaining a pool of **reduced glutathione**, which neutralizes reactive oxygen species (ROS). Primaquine is an oxidizing drug that increases oxidative stress. In G6PD-deficient individuals, the inability to regenerate reduced glutathione leads to the oxidation of hemoglobin, forming **Heinz bodies**. These damaged RBCs are destroyed in the spleen (hemolysis), resulting in hemoglobinuria (dark urine) and anemia. **Why Other Options are Incorrect:** * **Option B (Glucose-6-phosphate deficiency):** This is not a recognized clinical entity in this context. Glucose-6-phosphatase deficiency (Von Gierke disease) is a glycogen storage disease, not related to drug-induced hemolysis. * **Options C & D (Galactose-related enzymes):** These enzymes are involved in galactose metabolism. Deficiencies (e.g., Classic Galactosemia) present with cataracts, liver failure, and intellectual disability in infancy, not drug-induced hemolysis. **NEET-PG High-Yield Pearls:** * **Inheritance:** G6PD deficiency is an **X-linked recessive** disorder. * **Peripheral Smear:** Look for **Heinz bodies** (denatured hemoglobin) and **Bite cells** (degluticytes) formed by splenic macrophages. * **Other Trigger Drugs:** Sulfonamides, Nitrofurantoin, Dapsone, and Chloroquine (rarely). Fava beans (Favism) are a common dietary trigger. * **Primaquine Rule:** Always screen for G6PD deficiency before prescribing Primaquine for the radical cure of *P. vivax* or *P. ovale*.

Q: A 30-year-old patient presents with intractable vomiting and inability to eat or drink for the past 3 days. Their blood glucose level remains normal. Which of the following is most important for the maintenance of blood glucose in this patient?

Liver. **Explanation:** The maintenance of blood glucose during fasting or starvation is primarily the responsibility of the **Liver**. In this patient, who has been unable to eat for 3 days, the body has transitioned from the post-absorptive state to the early starvation phase. **Why the Liver is Correct:** The liver is the central organ for glucose homeostasis. It maintains blood glucose through two key pathways: 1. **Glycogenolysis:** The breakdown of stored glycogen (exhausted within 12–24 hours of fasting). 2. **Gluconeogenesis:** The synthesis of glucose from non-carbohydrate precursors (lactate, glycerol, and glucogenic amino acids). By day 3 of starvation, gluconeogenesis in the liver is the **primary source** of blood glucose to support glucose-dependent tissues like the brain and RBCs. The liver uniquely possesses the enzyme **Glucose-6-Phosphatase**, allowing it to release free glucose into the bloodstream. **Why Other Options are Incorrect:** * **Heart:** The heart is a consumer of energy, not a producer. It primarily utilizes fatty acids and ketone bodies during starvation to spare glucose for the brain. * **Skeletal Muscle:** While muscle stores significant glycogen, it lacks **Glucose-6-Phosphatase**. Therefore, muscle glycogen can only be used locally for energy and cannot be released as glucose into the blood. * **Lysosome:** These are organelles involved in cellular degradation (autophagy) and do not play a direct role in systemic blood glucose regulation. **NEET-PG High-Yield Pearls:** * **Key Enzyme:** Glucose-6-Phosphatase is the "marker enzyme" for gluconeogenesis, found in the liver and kidney cortex, but absent in muscle. * **Timeline:** Liver glycogen is depleted in ~18 hours; thereafter, gluconeogenesis becomes the sole source of blood glucose. * **Kidney Role:** In prolonged starvation (>5–6 days), the kidney cortex also contributes significantly (up to 40%) to gluconeogenesis.

Q: Glucuronic acid and Iduronic acid are:

Epimers. ### Explanation **Correct Answer: D. Epimers** **Underlying Concept:** Epimers are stereoisomers that differ in configuration around only **one** specific carbon atom (other than the anomeric carbon). Glucuronic acid and Iduronic acid are both uronic acids derived from glucose. They are **C-5 epimers**. * In **D-Glucuronic acid**, the carboxyl group (-COOH) at C-5 is positioned above the ring (in the D-configuration). * In **L-Iduronic acid**, the configuration at C-5 is inverted. **Why other options are incorrect:** * **Anomers (A):** These differ only at the hemiacetal/hemiketal carbon (C-1 for glucose). Glucuronic and Iduronic acids differ at C-5. * **Enantiomers (B):** These are non-superimposable mirror images where every chiral center is inverted (e.g., D-Glucose and L-Glucose). These two acids are not mirror images. * **Functional Isomers (C):** These have the same molecular formula but different functional groups (e.g., Glucose and Fructose). Both molecules here are uronic acids. **Clinical Pearls for NEET-PG:** 1. **GAG Composition:** Iduronic acid is a major component of Glycosaminoglycans (GAGs) like **Heparin** and **Dermatan sulfate**. 2. **Epimerization:** In the body, D-glucuronic acid residues in heparin chains are converted to L-iduronic acid by the enzyme **uronyl C-5 epimerase**. 3. **Other High-Yield Epimers:** * **Glucose and Galactose:** C-4 epimers. * **Glucose and Mannose:** C-2 epimers. * **Ribulose and Xylulose:** C-3 epimers. 4. **Glucuronic Acid Function:** It is essential for the **conjugation of bilirubin** and the detoxification of xenobiotics in the liver.

Q: McArdle's disease is due to deficiency of which enzyme?

Myophosphorylase. **Explanation:** McArdle’s disease, also known as **GSD Type V**, is an autosomal recessive disorder characterized by the deficiency of **Myophosphorylase** (muscle glycogen phosphorylase). This enzyme is responsible for the rate-limiting step of glycogenolysis in skeletal muscle, breaking down glycogen into glucose-1-phosphate. Without it, muscles cannot mobilize glucose during strenuous exercise, leading to exercise intolerance, muscle cramps, and myoglobinuria. **Analysis of Options:** * **A. Myophosphorylase (Correct):** This is the muscle-specific isoform of glycogen phosphorylase. Its absence prevents the breakdown of muscle glycogen. * **B. Liver phosphorylase:** Deficiency of this enzyme leads to **Hers disease (GSD Type VI)**, which primarily presents with hepatomegaly and mild hypoglycemia, rather than muscle symptoms. * **C. Glucose-6-phosphatase:** Deficiency leads to **Von Gierke’s disease (GSD Type I)**, the most common GSD, characterized by severe fasting hypoglycemia, lactic acidosis, and "doll-like" facies. * **D. Acid maltase (α-1,4-glucosidase):** Deficiency leads to **Pompe disease (GSD Type II)**, which affects the heart and muscles due to lysosomal glycogen accumulation. **High-Yield Clinical Pearls for NEET-PG:** 1. **Ischemic Forearm Exercise Test:** Patients with McArdle’s show a **failure of blood lactate to rise** after exercise (since glycogen cannot be converted to lactate), while ammonia levels rise significantly. 2. **Second Wind Phenomenon:** Patients often experience a sudden improvement in exercise tolerance after 10–15 minutes as the body switches to using free fatty acids and blood glucose for energy. 3. **Burgundy-colored urine:** Post-exercise myoglobinuria can lead to acute renal failure.

Question 1

A 28-year-old man complains of abdominal bloating and diarrhea following a meal containing paneer, a dairy product. These symptoms are absent with other foods. Which of the following enzymes is likely deficient in this patient?

Accepted Answer

Sucrase

Answer

a-amylase

Answer

b-galactosidase

Answer

a-glucosidase

Question 2

Glucose-6-phosphate dehydrogenase (G6PD) is primarily found in which cellular compartment?

Accepted Answer

Cytosol

Answer

Mitochondria

Answer

Golgi apparatus

Answer

Endoplasmic reticulum

Question 3

The oxidative phase of the HMP shunt pathway is least active in which of the following tissues?

Accepted Answer

Skeletal muscle

Answer

Adrenal cortex

Answer

Lactating mammary gland

Answer

Red blood cells

Question 4

A 30-year-old male has excessive urinary glucose levels, but his blood glucose levels are normal. Investigations reveal high levels of L-xylulose in urine. A genetic defect in which of the following pathways is most likely responsible?

Accepted Answer

Uronic acid pathway

Answer

Tricarboxylic acid cycle

Answer

Glycolysis

Answer

Hexose monophosphate shunt

Question 5

What is the primary action of epinephrine on the liver?

Accepted Answer

Glycogenolysis

Answer

Glycolysis

Answer

Lipolysis

Answer

Gluconeogenesis

Question 6

A patient develops dark colored urine one day after administration of primaquine. What is the most likely diagnosis?

Accepted Answer

Glucose-6-phosphate dehydrogenase deficiency

Answer

Glucose-6-phosphate deficiency

Answer

Galactose-6-phosphate dehydrogenase deficiency

Answer

Galactose-6-phosphatase deficiency

Question 7

An infant presents with hepatosplenomegaly, hypoglycemia, hyperuricemia, and lactic acidosis. Liver biopsy shows normal structured glycogen deposition. Which enzyme is deficient in this infant?

Accepted Answer

Glucose-6-phosphatase

Answer

Debranching enzyme

Answer

Branching enzyme

Answer

Muscle phosphorylase

Question 8

A 30-year-old patient presents with intractable vomiting and inability to eat or drink for the past 3 days. Their blood glucose level remains normal. Which of the following is most important for the maintenance of blood glucose in this patient?

Accepted Answer

Liver

Answer

Heart

Answer

Skeletal muscle

Answer

Lysosome

Question 9

Glucuronic acid and Iduronic acid are:

Accepted Answer

Epimers

Answer

Anomers

Answer

Enantiomers

Answer

Functional isomers

Question 10

McArdle's disease is due to deficiency of which enzyme?

Accepted Answer

Myophosphorylase

Answer

Liver phosphorylase

Answer

Glucose-6-phosphatase

Answer

Acid maltase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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