Carbohydrate Metabolism Practice Questions

Q: Reducing sugar in urine is seen in which of the following conditions?

Galactosemia. **Explanation:** **Why Galactosemia is correct:** Galactosemia is a disorder of galactose metabolism, most commonly due to a deficiency of **Galactose-1-phosphate uridyltransferase (GALT)**. This leads to an accumulation of galactose in the blood (galactosemia) and its subsequent excretion in the urine (**galactosuria**). Since galactose is a **reducing sugar** (containing a free aldehyde group), it reacts positively with Benedict’s test or Fehling’s solution, resulting in a positive test for reducing substances in the urine. **Analysis of Incorrect Options:** * **Fanconi Anemia:** This is a DNA repair defect characterized by bone marrow failure and physical anomalies. It should not be confused with *Fanconi Syndrome* (a renal proximal tubule defect), which can cause glucosuria. * **Lactose Intolerance:** This is a deficiency of the enzyme lactase in the brush border of the small intestine. It leads to the malabsorption of lactose in the gut, causing osmotic diarrhea. Lactose does not enter the bloodstream in significant amounts and is therefore not typically found in the urine. * **Phenylketonuria (PKU):** This is a disorder of amino acid metabolism (deficiency of Phenylalanine hydroxylase). It results in the excretion of phenylketones (like phenylpyruvate), which are not reducing sugars. **High-Yield Clinical Pearls for NEET-PG:** * **Benedict’s Test vs. Dipstick:** Benedict’s test detects all reducing sugars (glucose, galactose, fructose, lactose). The standard urine dipstick uses the **glucose oxidase** method, which is specific for glucose only. A "positive Benedict’s and negative Dipstick" is a classic clue for Galactosemia or Essential Fructosuria. * **Common Reducing Sugars in Urine:** Glucose (Diabetes), Galactose (Galactosemia), Fructose (Essential Fructosuria), and Pentose (Essential Pentosuria). * **Galactosemia Triad:** Cataracts (due to galactitol accumulation), Hepatomegaly, and Intellectual disability.

Q: Which glucose transporter in myocytes is stimulated by insulin?

GLUT 4. ### Explanation **Correct Option: D (GLUT 4)** GLUT 4 is the only **insulin-dependent** glucose transporter. It is primarily expressed in **skeletal muscle (myocytes)** and **adipose tissue**. In the resting state, GLUT 4 is sequestered in intracellular vesicles. When insulin binds to its receptor, it triggers a signaling cascade (via PI3-kinase) that causes these vesicles to fuse with the plasma membrane, increasing glucose uptake. In myocytes, GLUT 4 translocation is also stimulated by **exercise/muscle contraction** (AMPK pathway), which is why exercise helps manage blood glucose in diabetics. **Incorrect Options:** * **GLUT 1:** This is a basal glucose transporter found in almost all tissues, especially **RBCs** and the **Blood-Brain Barrier**. It is insulin-independent and ensures a constant baseline glucose supply. * **GLUT 2:** A high-capacity, low-affinity transporter found in the **Liver, Pancreatic beta-cells, and Kidney**. It acts as a "glucose sensor" in the pancreas and allows bidirectional transport in the liver. It is insulin-independent. * **GLUT 3:** A high-affinity transporter found primarily in **Neurons**. It ensures glucose uptake even during hypoglycemia, independent of insulin. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT 5:** Unique because it is a **Fructose** transporter (found in the small intestine and spermatozoa). * **SGLT 1/2:** These are active transporters (Sodium-Glucose Co-transporters). SGLT-2 inhibitors (e.g., Dapagliflozin) are modern drugs used in Diabetes to promote glucose excretion in urine. * **Mnemonics:** Remember "**4** is for the **Door**" (Insulin opens the door for glucose in muscle/fat).

Q: What is the source of ATP in red blood cells?

Anaerobic glycolysis. ### Explanation **Correct Answer: C. Anaerobic glycolysis** **1. Why Anaerobic Glycolysis is Correct:** Mature red blood cells (RBCs) lack **mitochondria**, which are the "powerhouses" required for aerobic respiration (TCA cycle and Electron Transport Chain). Consequently, RBCs cannot utilize oxygen for energy production, despite being the primary carriers of oxygen. They rely exclusively on **anaerobic glycolysis** (the Embden-Meyerhof pathway) in the cytosol to convert glucose into lactate, generating a net gain of **2 ATP** molecules per glucose molecule. This ATP is vital for maintaining ion gradients (Na⁺/K⁺ ATPase pump) and erythrocyte membrane integrity. **2. Why Other Options are Incorrect:** * **A. Beta-oxidation of fatty acids:** This process occurs within the mitochondrial matrix. Since RBCs lack mitochondria, they cannot oxidize fatty acids for energy. * **B. TCA cycle:** The enzymes for the Tricarboxylic Acid (TCA) cycle are located in the mitochondria. Without these organelles, RBCs cannot perform aerobic metabolism. * **D. Gluconeogenesis:** This is the synthesis of glucose from non-carbohydrate precursors (primarily occurring in the liver and kidneys). RBCs are consumers of glucose, not producers. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Rapoport-Luebering Shunt:** A side pathway of glycolysis in RBCs that produces **2,3-BPG**. This molecule is crucial as it decreases the affinity of hemoglobin for oxygen, facilitating oxygen delivery to tissues. * **Lactate Production:** The end product of glycolysis in RBCs is always lactate, which is released into the blood and taken up by the liver for gluconeogenesis (**Cori Cycle**). * **G6PD Deficiency:** The HMP shunt (Pentose Phosphate Pathway) is also vital in RBCs to produce **NADPH**, which maintains reduced glutathione to protect the cell against oxidative damage. Deficiency leads to hemolytic anemia.

Q: Which of the following represents the normal glucose tolerance curve?

Curve B. ***Curve B*** - Demonstrates the normal **glucose tolerance pattern** with fasting glucose around **80-90 mg/dL**, peak at **120-140 mg/dL** at 30-60 minutes, and return to near-fasting levels by **2 hours**. - Represents normal **insulin sensitivity** and **pancreatic beta-cell function** with appropriate glucose clearance from blood. *Curve A* - Shows a **diabetic pattern** with elevated fasting glucose and peak values exceeding **200 mg/dL** during oral glucose tolerance test. - Blood glucose remains **persistently elevated** at 2 hours (>200 mg/dL), indicating **impaired glucose tolerance** or diabetes mellitus. *Curve C* - Demonstrates an **abnormal flat curve** pattern, possibly indicating **renal glycosuria** or **alimentary hypoglycemia**. - May show **minimal glucose rise** despite oral glucose load, suggesting **malabsorption** or rapid glucose clearance without normal physiological response. *None of the above* - This option is incorrect as **Curve B** clearly represents the normal glucose tolerance curve pattern. - The **WHO/ADA criteria** for normal OGTT are well-established, making one of the given curves definitively correct.

Q: Which of the following is NOT a cause of fasting hypoglycemia?

Excess glucagon. **Explanation:** The core physiological mechanism of fasting hypoglycemia involves an inability to maintain blood glucose levels during periods of starvation, usually due to defects in glycogenolysis, gluconeogenesis, or hormonal regulation [3]. **Why Option A is correct:** **Glucagon** is a counter-regulatory hormone produced by the alpha cells of the pancreas. Its primary function is to **increase** blood glucose levels by stimulating glycogenolysis and gluconeogenesis in the liver [1]. Therefore, an **excess of glucagon** (as seen in a Glucagonoma) would lead to **hyperglycemia**, not hypoglycemia. **Why the other options are incorrect:** * **B. Glucose-6-phosphatase deficiency (Von Gierke Disease):** This is Type I Glycogen Storage Disease (GSD). This enzyme is essential for the final step of both glycogenolysis and gluconeogenesis [2]. Its absence leads to severe fasting hypoglycemia because the liver cannot release free glucose into the blood. * **C. Uremia:** Chronic kidney disease/uremia causes hypoglycemia through multiple mechanisms, including reduced renal gluconeogenesis (the kidney contributes ~20% of glucose production), impaired insulin clearance, and malnutrition. * **D. Glycogen synthase deficiency:** Known as Type 0 GSD, this condition prevents the liver from storing glycogen [3]. Without glycogen stores to draw upon during a fast, patients develop ketotic hypoglycemia. **High-Yield NEET-PG Pearls:** * **Whipple’s Triad for Hypoglycemia:** 1. Symptoms of hypoglycemia, 2. Low plasma glucose (<55 mg/dL), 3. Relief of symptoms after glucose administration. * **Common causes of fasting hypoglycemia:** Insulinoma, exogenous insulin (factitious), adrenal insufficiency (Addison’s), and severe liver disease. * **Glucagonoma Clinical Triad:** Hyperglycemia (Diabetes), Necrolytic Migratory Erythema (NME), and weight loss.

Q: Which of the following glycosaminoglycans does not contain a sulfur-containing bond?

Hyaluronic acid. **Explanation:** **Hyaluronic acid** (Hyaluronan) is the correct answer because it is the only Glycosaminoglycan (GAG) that is **non-sulfated**. Unlike other GAGs, it is not synthesized in the Golgi apparatus by sulfotransferases; instead, it is synthesized by integral membrane proteins called hyaluronan synthases directly into the extracellular space. Additionally, it is the only GAG that is not covalently attached to a protein core (i.e., it does not form proteoglycans). **Analysis of Incorrect Options:** * **Chondroitin (Chondroitin Sulfate):** The most abundant GAG in the body, found in cartilage and bone. It is heavily sulfated at the carbon-4 or carbon-6 positions. * **Keratan (Keratan Sulfate):** Found in the cornea and loose connective tissue. It contains sulfate groups and is unique because its repeating disaccharide unit contains galactose instead of uronic acid. * **Dermatan (Dermatan Sulfate):** Found in skin, blood vessels, and heart valves. It is a sulfated GAG formed by the epimerization of glucuronic acid to iduronic acid. **High-Yield Clinical Pearls for NEET-PG:** * **Hyaluronic Acid:** Acts as a lubricant and shock absorber in synovial fluid and the vitreous humor. It plays a vital role in cell migration during embryogenesis and wound healing. * **Heparin:** The GAG with the **highest negative charge** and highest degree of sulfation; it acts as an intracellular anticoagulant. * **Mucopolysaccharidoses (MPS):** Genetic deficiencies in lysosomal enzymes that degrade sulfated GAGs (e.g., Hurler and Hunter syndromes). Note that Hyaluronic acid is typically not involved in these storage disorders. * **Unique Feature:** Hyaluronic acid is the largest GAG by molecular weight, often consisting of thousands of sugar residues.

Question 1

Glycogen storage diseases include all of the following except?

Accepted Answer

None of the above

Answer

Krabbe's disease

Answer

Fabry's disease

Answer

Glycogen storage disease

Question 2

Reducing sugar in urine is seen in which of the following conditions?

Accepted Answer

Galactosemia

Answer

Fanconi anemia

Answer

Lactose intolerance

Answer

Phenylketonuria

Question 3

Which glucose transporter in myocytes is stimulated by insulin?

Accepted Answer

GLUT 4

Answer

GLUT 1

Answer

GLUT 2

Answer

GLUT 3

Question 4

What is the source of ATP in red blood cells?

Accepted Answer

Anaerobic glycolysis

Answer

Beta oxidation of fatty acids

Answer

TCA cycle

Answer

Gluconeogenesis

Question 5

Which of the following represents the normal glucose tolerance curve?

Accepted Answer

Curve B

Answer

Curve A

Answer

Curve C

Answer

None of the above

Question 6

In the TCA cycle, which two carbon atoms leave in the form of CO2 are derived from:

Accepted Answer

Oxaloacetate

Answer

Acetyl CoA

Answer

CO2

Answer

Citrate

Question 7

Which of the following is NOT a cause of fasting hypoglycemia?

Accepted Answer

Excess glucagon

Answer

Glucose-6-phosphatase deficiency

Answer

Uremia

Answer

Glycogen synthase deficiency

Question 8

Which of the following glycosaminoglycans does not contain a sulfur-containing bond?

Accepted Answer

Hyaluronic acid

Answer

Chondroitin

Answer

Keratan

Answer

Dermatan

Question 9

Glycogenolysis is best described by which of the following statements?

Accepted Answer

It requires a bifunctional enzyme (debranching and transferase).

Answer

It involves enzymes cleaving beta(1-4) glycosidic linkages.

Answer

It requires activation of glycogen synthase.

Answer

It requires inactivation of phosphorylase kinase.

Question 10

Familial fructokinase deficiency causes no symptoms because:

Accepted Answer

Hexokinase can phosphorylate fructose

Answer

Liver aldolase can metabolize it

Answer

Excess fructose does not escape into the urine

Answer

Excess fructose is excreted through feces

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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