Carbohydrate Metabolism Practice Questions

Q: Which disaccharide is not broken down in the gastrointestinal tract by enzymatic action?

Lactulose. ### Explanation **1. Why Lactulose is the Correct Answer:** Lactulose is a synthetic disaccharide composed of **galactose and fructose**. Unlike natural disaccharides, the human small intestine lacks the specific enzyme (disaccharidase) required to hydrolyze the synthetic bond in lactulose. Consequently, it remains unabsorbed and passes into the colon. There, resident bacteria ferment it into short-chain fatty acids (like lactic and acetic acid), which exerts an osmotic effect. This property makes it a potent **osmotic laxative**. **2. Analysis of Incorrect Options:** * **Lactose:** A natural milk sugar (galactose + glucose) broken down by the enzyme **Lactase** located in the brush border of the small intestine. * **Sucrose:** Common table sugar (glucose + fructose) hydrolyzed by the enzyme **Sucrase**. * **Isomaltose:** A product of starch digestion (two glucose units with an α-1,6 bond) broken down by the enzyme **Isomaltase** (part of the sucrase-isomaltase complex). **3. Clinical Pearls for NEET-PG:** * **Hepatic Encephalopathy:** Lactulose is the first-line treatment. It acidifies the colonic contents ($NH_3 \rightarrow NH_4^+$), "trapping" ammonia in the gut and preventing its absorption into the systemic circulation. * **Hydrogen Breath Test:** Since lactulose is only broken down by bacteria, it is used in breath tests to measure **Oro-cecal transit time** and to diagnose **SIBO** (Small Intestinal Bacterial Overgrowth). * **Brush Border Enzymes:** Remember that all physiological disaccharide digestion occurs at the **brush border** of the enterocytes, not in the intestinal lumen.

Q: Which of the following statements about glycosaminoglycans is FALSE?

Glycosaminoglycans are highly positively charged.. **Explanation:** **1. Why Option C is the Correct (False) Statement:** Glycosaminoglycans (GAGs) are **highly negatively charged** molecules, not positively charged. This intense negative charge is due to the presence of **sulfate groups** and **uronic acid (carboxyl) groups**. This property is physiologically vital: the negative charges repel each other, causing the GAG chains to remain extended in solution. Furthermore, they attract water molecules (hydration), creating a "cushioning" effect or lubrication, which is essential for joint health and structural integrity. **2. Analysis of Other Options:** * **Option A (True):** Most GAGs (except hyaluronic acid) are covalently linked to a **core protein**, forming a complex known as a **proteoglycan**. * **Option B (True):** GAGs are major components of the **extracellular matrix (ECM)** and connective tissues (e.g., cartilage, bone, skin, and vitreous humor), providing structural support. * **Option D (True):** As explained above, GAGs are polyanionic due to sulfate and carboxyl groups; thus, they are indeed negatively charged. **3. High-Yield Clinical Pearls for NEET-PG:** * **Hyaluronic Acid:** The only GAG that is **non-sulfated**, not covalently attached to a protein, and not limited to animal tissues (also found in bacteria). * **Heparin:** The GAG with the **highest negative charge density**; it acts as a natural anticoagulant by activating Antithrombin III. * **Chondroitin Sulfate:** The most abundant GAG in the body (found in cartilage and bone). * **Mucopolysaccharidoses (MPS):** Genetic disorders (e.g., Hurler and Hunter syndromes) caused by the deficiency of lysosomal enzymes required to degrade GAGs, leading to their accumulation in tissues.

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*.

Question 1

Which disaccharide is not broken down in the gastrointestinal tract by enzymatic action?

Accepted Answer

Lactulose

Answer

Lactose

Answer

Sucrose

Answer

Isomaltose

Question 2

Which of the following statements about glycosaminoglycans is FALSE?

Accepted Answer

Glycosaminoglycans are highly positively charged.

Answer

Glycosaminoglycans are associated with core proteins.

Answer

Glycosaminoglycans may be associated with connective tissues.

Answer

Glycosaminoglycans are negatively charged.

Question 3

A newborn infant refuses breast milk from the second day of birth, vomits on force-feeding but accepts glucose-water, develops diarrhea on the third day. By the fifth day, the infant is jaundiced with liver enlargement, and the eyes show signs of cataract. Urinary reducing sugar was positive, but blood glucose estimated by the glucose oxidation method was found to be low. What is the most likely cause of this condition?

Accepted Answer

Galactose-1-phosphate uridyl transferase deficiency

Answer

Beta galactosidase deficiency

Answer

Glucose-6-phosphatase deficiency

Answer

Galactokinase deficiency

Question 4

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 5

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

Accepted Answer

Cytosol

Answer

Mitochondria

Answer

Golgi apparatus

Answer

Endoplasmic reticulum

Question 6

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 7

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 8

What is the primary action of epinephrine on the liver?

Accepted Answer

Glycogenolysis

Answer

Glycolysis

Answer

Lipolysis

Answer

Gluconeogenesis

Question 9

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 10

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

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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