Carbohydrate Metabolism Practice Questions

Q: Which disorder of carbohydrate metabolism typically involves cardiac involvement?

Glycogen Storage Disease Type II (Pompe). ### Explanation **Correct Answer: A. Glycogen Storage Disease Type II (Pompe)** **Why it is correct:** Glycogen Storage Disease Type II (Pompe disease) is unique among GSDs because it is a **lysosomal storage disorder**. It is caused by a deficiency of **Acid α-1,4-glucosidase (Acid Maltase)**, the enzyme responsible for breaking down glycogen within lysosomes. Unlike other GSDs that primarily affect the liver or skeletal muscle, Pompe disease leads to the massive accumulation of glycogen in the **cardiac muscle**, skeletal muscle, and liver. The infantile-onset form typically presents with **hypertrophic cardiomyopathy**, leading to early heart failure and death. **Why the other options are incorrect:** * **B. Galactosemia:** Caused by GALT deficiency. It primarily presents with cataracts, hepatosplenomegaly, and intellectual disability, but does not involve the heart. * **C. GSD Type I (Von Gierke):** Caused by Glucose-6-Phosphatase deficiency. It affects the liver and kidneys, presenting with severe hypoglycemia, lactic acidosis, and "doll-like" facies. Cardiac involvement is absent. * **D. Hereditary Fructose Intolerance:** Caused by Aldolase B deficiency. It leads to hypoglycemia and jaundice after fructose ingestion but has no cardiac manifestations. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Pompe trashes the **Pump** (Heart)." * **ECG Finding:** Characteristically shows **short PR interval** and giant QRS complexes. * **Enzyme:** Acid Maltase (Lysosomal enzyme). * **Histology:** PAS-positive material (glycogen) in vacuoles. * **Key differentiator:** Pompe is the only GSD that is also a Lysosomal Storage Disease. Blood glucose levels are typically **normal** in Pompe disease, unlike Type I or III.

Q: In the Krebs cycle, from which step is CO2 released, catalyzed by which of the following enzymes?

Isocitrate dehydrogenase. In the Krebs cycle (TCA cycle), carbon dioxide ($CO_2$) is released during **oxidative decarboxylation** reactions. There are two such steps in the cycle: 1. **Isocitrate to $\alpha$-Ketoglutarate:** Catalyzed by **Isocitrate Dehydrogenase**. 2. **$\alpha$-Ketoglutarate to Succinyl-CoA:** Catalyzed by the $\alpha$-Ketoglutarate Dehydrogenase complex. **Isocitrate dehydrogenase** is the correct answer as it catalyzes the first decarboxylation step, converting a 6-carbon molecule (isocitrate) into a 5-carbon molecule ($\alpha$-ketoglutarate) while reducing $NAD^+$ to $NADH$. ### Analysis of Incorrect Options: * **B. Succinate dehydrogenase:** This enzyme converts succinate to fumarate. It is a redox reaction that produces $FADH_2$, not $CO_2$. It is unique because it is the only TCA enzyme located in the inner mitochondrial membrane (part of Complex II of the ETC). * **C. Aconitase:** This enzyme catalyzes the isomerization of citrate to isocitrate via the intermediate *cis*-aconitate. No $CO_2$ or energy equivalents are produced here. * **D. Succinate thiokinase (Succinyl-CoA synthetase):** This enzyme converts Succinyl-CoA to Succinate. This is a **substrate-level phosphorylation** step, producing GTP (or ATP), not $CO_2$. ### High-Yield Clinical Pearls for NEET-PG: * **Rate-Limiting Step:** Isocitrate dehydrogenase is the **rate-limiting enzyme** of the TCA cycle. It is allosterically activated by ADP and $Ca^{2+}$, and inhibited by ATP and NADH. * **Carbon Count:** The TCA cycle begins with a 6-carbon citrate and ends with a 4-carbon oxaloacetate; the 2 carbons lost are those released as $CO_2$ by Isocitrate DH and $\alpha$-Ketoglutarate DH. * **Fluoroacetate Poisoning:** Aconitase is inhibited by fluoroacetate (rat poison), which is converted to fluorocitrate, halting the cycle.

Q: Phospho-dephosphorylation of phosphofructokinase and fructose 1, 6-bisphosphatase by fructose 2, 6-bisphosphate regulation is seen in which of the following?

Liver. ### Explanation The regulation of glycolysis and gluconeogenesis is primarily controlled by the bifunctional enzyme **PFK-2/FBPase-2**, which determines the levels of **Fructose 2,6-bisphosphate (F2,6-BP)**. **Why Liver is the Correct Answer:** The liver plays a central role in maintaining systemic blood glucose levels. In the liver, the bifunctional enzyme is regulated by **cAMP-dependent phosphorylation** (via Protein Kinase A). * **In the fasting state:** Glucagon increases cAMP, activating Protein Kinase A, which **phosphorylates** the enzyme. This activates the phosphatase domain (FBPase-2) and inactivates the kinase domain (PFK-2), leading to decreased F2,6-BP levels, thereby inhibiting glycolysis and promoting gluconeogenesis. * **In the fed state:** Insulin promotes **dephosphorylation**, activating PFK-2, increasing F2,6-BP, and stimulating glycolysis. **Why Other Options are Incorrect:** * **Brain:** The brain lacks the machinery for gluconeogenesis and does not regulate glucose metabolism via the hormonal phospho-dephosphorylation of F2,6-BP; it relies on a continuous supply of glucose. * **Adrenal Cortex:** While metabolically active, it does not serve as a primary glucose-regulating organ like the liver. * **RBC:** Red blood cells lack mitochondria and gluconeogenic enzymes. Their glycolysis is regulated primarily by the ATP/AMP ratio and 2,3-BPG levels, not by the hormonal regulation of F2,6-BP seen in the liver. **High-Yield Clinical Pearls for NEET-PG:** 1. **Fructose 2,6-bisphosphate** is the most potent allosteric activator of **PFK-1** (the rate-limiting enzyme of glycolysis). 2. **Muscle Isoenzyme:** Unlike the liver, the muscle isoform of PFK-2 is **not** inhibited by phosphorylation; instead, it is activated by epinephrine to ensure rapid energy production during exercise. 3. **Reciprocal Regulation:** F2,6-BP simultaneously activates PFK-1 and inhibits Fructose 1,6-bisphosphatase, preventing a "futile cycle."

Q: UDP Glucose is formed from which precursor molecule?

Glucose-1-phosphate. **Explanation:** **1. Why Glucose-1-phosphate is correct:** UDP-Glucose (Uridine diphosphate glucose) is the activated form of glucose required for glycogen synthesis, galactose metabolism, and the synthesis of glycoproteins. It is synthesized by the enzyme **UDP-glucose pyrophosphorylase**. This enzyme catalyzes the reaction between **Glucose-1-phosphate (G1P)** and **UTP** (Uridine triphosphate). During this process, the phosphate on G1P and the innermost phosphate of UTP form the UDP-linkage, releasing inorganic pyrophosphate (PPi). The subsequent hydrolysis of PPi by pyrophosphatase makes this reaction irreversible, driving glycogen synthesis forward. **2. Why the other options are incorrect:** * **Glycogen:** This is the storage polymer of glucose. UDP-glucose is a *precursor* used by glycogen synthase to add glucose units to an existing glycogen chain; glycogen is the end product, not the precursor of UDP-glucose. * **Lactose phosphate:** Lactose is a disaccharide of glucose and galactose. While UDP-galactose is involved in lactose synthesis in mammary glands, "lactose phosphate" is not a standard intermediate in the formation of UDP-glucose. * **Starch:** This is the storage form of glucose in plants. Human metabolism breaks down starch into free glucose in the gut, which must then be phosphorylated to Glucose-6-P and converted to Glucose-1-P before it can form UDP-glucose. **Clinical Pearls for NEET-PG:** * **Galactosemia:** Deficiency of *Galactose-1-phosphate uridyltransferase* (GALT) prevents the conversion of Galactose-1-P and UDP-Glucose into UDP-Galactose and Glucose-1-P, leading to Classic Galactosemia. * **Glycogenesis:** UDP-glucose is the immediate donor of glucose residues for the enzyme **Glycogen Synthase**. * **Bilirubin Conjugation:** UDP-glucuronic acid (derived from UDP-glucose) is essential for conjugating bilirubin in the liver via the enzyme *UDP-glucuronosyltransferase*.

Q: What is the Pasteur effect?

Inhibition of glycolysis by oxygen. **Explanation:** The **Pasteur Effect** refers to the observation that the rate of glycolysis is significantly decreased in the presence of oxygen compared to anaerobic conditions. **Why Option A is Correct:** In the presence of oxygen (aerobic conditions), cells can utilize the **Electron Transport Chain (ETC)** and oxidative phosphorylation. This process is far more efficient, yielding **30–32 ATP** per glucose molecule, compared to only **2 ATP** produced during anaerobic glycolysis. Because the cell’s energy requirements are met more efficiently, high levels of ATP and citrate act as allosteric inhibitors of **Phosphofructokinase-1 (PFK-1)**, the rate-limiting enzyme of glycolysis. Thus, oxygen "inhibits" the rapid consumption of glucose. **Why Other Options are Incorrect:** * **Option B:** The absence of oxygen actually *stimulates* glycolysis (the opposite of the Pasteur effect) to compensate for low ATP yields. * **Option C:** Glycolysis contains three irreversible steps (Hexokinase, PFK-1, and Pyruvate Kinase); the Pasteur effect is a regulatory phenomenon, not a description of reversibility. * **Option D:** Oxygen decelerates glycolysis; the *acceleration* of glycolysis in the presence of oxygen is known as the **Warburg Effect**, typically seen in cancer cells. **NEET-PG High-Yield Pearls:** * **Key Enzyme:** The Pasteur effect primarily targets **PFK-1**. * **Warburg Effect:** This is the "clinical opposite" where cancer cells prefer glycolysis even when oxygen is plentiful (aerobic glycolysis), allowing them to use glycolytic intermediates for cell growth. * **Crabtree Effect:** The inhibition of oxygen consumption by high concentrations of glucose (seen in yeast and some tumor cells).

Q: Alpha-amylase acts on which type of glycosidic bond?

Alpha 1-4 glycosidic bond. **Explanation:** **Alpha-amylase** (both salivary and pancreatic) is an **endo-glycosidase** that specifically hydrolyzes the internal **alpha 1-4 glycosidic bonds** of polysaccharides like starch and glycogen. By breaking these linear bonds, it converts complex carbohydrates into smaller units like maltose, maltotriose, and alpha-limit dextrins. **Analysis of Options:** * **Option A (Correct):** Alpha-amylase acts exclusively on alpha 1-4 bonds. It requires a pH of 6.7–7.0 and chloride ions ($Cl^-$) for optimal activity. * **Option B (Incorrect):** Alpha 1-6 glycosidic bonds are the "branch points" in amylopectin and glycogen. Alpha-amylase **cannot** hydrolyze these bonds. These are broken by "debranching enzymes" (like isomaltase in the gut). * **Option C (Incorrect):** Beta 1-4 bonds are found in **cellulose**. Humans lack the enzyme (cellulase) to break these bonds, which is why cellulose serves as dietary fiber. * **Option D (Incorrect):** Beta 1-6 bonds are not typically found in major dietary starches or glycogen. **High-Yield Clinical Pearls for NEET-PG:** * **Chloride Activation:** Alpha-amylase is a metalloenzyme that requires **Calcium ($Ca^{2+}$)** for stability and **Chloride ($Cl^-$)** for activation. * **Limit Dextrins:** Because amylase cannot bypass or break alpha 1-6 branch points, the remaining branched fragments are called **alpha-limit dextrins**. * **Diagnostic Marker:** Serum amylase rises in **Acute Pancreatitis** (within 2–12 hours), though lipase is considered more specific. * **Isoenzymes:** Salivary amylase (Ptyalin) is inactivated by gastric HCl, while Pancreatic amylase continues digestion in the duodenum.

Q: Which of the following is NOT seen in lactose intolerance?

Alkaline urine. **Explanation:** Lactose intolerance occurs due to a deficiency of the **lactase enzyme** (Option D) in the brush border of the small intestine. When lactose is not hydrolyzed into glucose and galactose, it remains in the intestinal lumen, acting as an osmotic agent that draws water in, leading to osmotic diarrhea. **Why "Alkaline Urine" is the correct answer:** In lactose intolerance, the unabsorbed lactose reaches the colon, where bacteria ferment it into **Short-Chain Fatty Acids (SCFAs)** (like lactic, acetic, and propionic acid) and gases ($H_2, CO_2$). These acids are absorbed into the bloodstream, leading to a mild **metabolic acidosis**. Consequently, the kidneys compensate by excreting excess hydrogen ions, resulting in **acidic urine**, not alkaline urine. **Analysis of other options:** * **Acidic Stool (Option C):** The bacterial fermentation of lactose produces lactic acid and other SCFAs, which significantly lowers the stool pH (typically **< 5.5**). * **Reducing Substances in Stool (Option A):** Lactose is a reducing sugar. Since it remains unabsorbed, it can be detected in the stool using Benedict’s reagent or Clinitest tablets. * **Lactase Enzyme Deficiency (Option B):** This is the primary pathophysiology, whether congenital, primary (developmental), or secondary (post-gastroenteritis). **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Hydrogen Breath Test (increased $H_2$ due to bacterial fermentation). * **Stool pH:** Characteristically < 5.5. * **Clinical Presentation:** Explosive, watery diarrhea, abdominal distension, and perianal excoriation (due to acidic stools). * **Secondary Lactose Intolerance:** Most commonly occurs after a viral rotavirus infection due to sloughing of the villi tips where lactase is located.

Q: In which of the following tissues is glycogen incapable of contributing directly to blood glucose?

Muscle. **Explanation:** The correct answer is **Muscle** because it lacks the enzyme **Glucose-6-Phosphatase**. **1. Why Muscle is the Correct Answer:** While both the liver and skeletal muscle store glycogen, their physiological roles differ. In the liver, glycogenolysis produces Glucose-6-Phosphate (G6P), which is then converted into free glucose by the enzyme **Glucose-6-Phosphatase**. This free glucose can exit the cell via GLUT-2 transporters to maintain blood glucose levels. In contrast, skeletal muscle lacks Glucose-6-Phosphatase. Therefore, the G6P generated from muscle glycogenolysis is trapped within the muscle cell and must enter the **glycolytic pathway** to provide ATP for local muscle contraction. Consequently, muscle glycogen cannot contribute directly to blood glucose. **2. Why Other Options are Incorrect:** * **A. Liver:** The liver is the primary organ responsible for maintaining blood glucose during fasting. It contains high concentrations of Glucose-6-Phosphatase, allowing it to release glucose into the bloodstream. * **C & D:** These are incorrect based on the specific enzymatic deficiency in muscle tissue described above. **High-Yield Clinical Pearls for NEET-PG:** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of Glucose-6-Phosphatase. It results in severe fasting hypoglycemia because neither glycogenolysis nor gluconeogenesis can release glucose from the liver. * **Cori Cycle:** Muscle glycogen can *indirectly* contribute to blood glucose via the Cori Cycle, where muscle-derived lactate travels to the liver to be converted back into glucose. * **Key Enzyme:** Glycogen phosphorylase is the rate-limiting enzyme for glycogenolysis, but Glucose-6-Phosphatase is the "gatekeeper" for glucose release into the blood.

Q: Which enzyme is responsible for the complete oxidation of glucose to CO2 and water?

Mitochondria. **Explanation:** The complete oxidation of glucose involves three major stages: Glycolysis, the TCA (Krebs) cycle, and the Electron Transport Chain (ETC). While glycolysis occurs in the cytosol, it only partially oxidizes glucose to pyruvate. The **Mitochondria** is the definitive site for complete oxidation because it houses the Pyruvate Dehydrogenase (PDH) complex, the TCA cycle enzymes, and the machinery for Oxidative Phosphorylation. Within the mitochondrial matrix, acetyl-CoA is oxidized to **CO2**, and the resulting reducing equivalents (NADH/FADH2) are used by the ETC on the inner mitochondrial membrane to produce **water** and ATP. **Analysis of Incorrect Options:** * **A. Cytosol:** This is the site for glycolysis (partial oxidation) and the HMP shunt. It lacks the oxygen-dependent machinery required to break down pyruvate into CO2 and water. * **C. Lysosomes:** These are "suicide bags" involved in the degradation of macromolecules (proteolysis, lipolysis) via acid hydrolases, not energy metabolism. * **D. Endoplasmic Reticulum:** The ER is primarily involved in protein synthesis (RER), lipid synthesis, and detoxification (SER). It plays a role in gluconeogenesis (Glucose-6-phosphatase) but not glucose oxidation. **High-Yield NEET-PG Pearls:** * **Mitochondria** are often called the "Powerhouse of the cell" because they generate >90% of cellular ATP. * **RBCs** lack mitochondria; therefore, they can never oxidize glucose completely and rely solely on anaerobic glycolysis (lactate production). * **Key Mitochondrial Enzymes:** Pyruvate Dehydrogenase, Citrate Synthase, and Cytochrome Oxidase. * **Site Marker:** Succinate dehydrogenase is a marker enzyme for the inner mitochondrial membrane.

Q: Skeletal muscle is deficient in which of the following enzymes?

Glucose-6-phosphatase. ### Explanation **1. Why Glucose-6-phosphatase is the correct answer:** Glucose-6-phosphatase is the enzyme responsible for converting Glucose-6-phosphate into free glucose. This enzyme is primarily located in the **liver** and **kidneys**. Skeletal muscle lacks this enzyme; therefore, it cannot release free glucose into the bloodstream. Instead, the glucose-6-phosphate derived from muscle glycogenolysis enters the glycolytic pathway to provide ATP locally for muscle contraction. This ensures that muscle glycogen is a "selfish" fuel source, reserved exclusively for the muscle's own energy needs. **2. Analysis of Incorrect Options:** * **Hexokinase (Option B):** This enzyme is present in skeletal muscle. It catalyzes the first step of glycolysis (Glucose → Glucose-6-phosphate). It has a low Km (high affinity) for glucose, allowing muscles to utilize glucose even at low blood concentrations. * **Isomerase (Option C):** Specifically Phosphohexose Isomerase, this enzyme is essential for the second step of glycolysis (Glucose-6-P ↔ Fructose-6-P) and is present in all tissues, including muscle. * **Phosphofructokinase (Option D):** PFK-1 is the rate-limiting enzyme of glycolysis and is highly active in skeletal muscle to support rapid energy production during exercise. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. It presents with severe fasting hypoglycemia because neither glycogenolysis nor gluconeogenesis can export glucose from the liver. * **Cori Cycle:** Since muscles cannot release glucose, they release **lactate** during anaerobic exercise. This lactate travels to the liver, where it is converted back to glucose via gluconeogenesis. * **Glucokinase vs. Hexokinase:** Remember that the liver has Glucokinase (high Km), while muscles have Hexokinase (low Km).

Question 1

Which disorder of carbohydrate metabolism typically involves cardiac involvement?

Accepted Answer

Glycogen Storage Disease Type II (Pompe)

Answer

Galactosemia

Answer

Glycogen Storage Disease Type I (Von-Gierke)

Answer

Hereditary fructose intolerance

Question 2

In the Krebs cycle, from which step is CO2 released, catalyzed by which of the following enzymes?

Accepted Answer

Isocitrate dehydrogenase

Answer

Succinate dehydrogenase

Answer

Aconitase

Answer

Succinate thiokinase

Question 3

Phospho-dephosphorylation of phosphofructokinase and fructose 1, 6-bisphosphatase by fructose 2, 6-bisphosphate regulation is seen in which of the following?

Accepted Answer

Liver

Answer

Brain

Answer

Adrenal cortex

Answer

RBC

Question 4

UDP Glucose is formed from which precursor molecule?

Accepted Answer

Glucose-1-phosphate

Answer

Glycogen

Answer

Lactose phosphate

Answer

Starch

Question 5

What is the Pasteur effect?

Accepted Answer

Inhibition of glycolysis by oxygen

Answer

Inhibition of glycolysis due to absence of oxygen

Answer

Reversible reaction of glycolysis

Answer

Acceleration of glycolysis by oxygen

Question 6

Alpha-amylase acts on which type of glycosidic bond?

Accepted Answer

Alpha 1-4 glycosidic bond

Answer

Alpha 1-6 glycosidic bond

Answer

Beta 1-4 glycosidic bond

Answer

Beta 1-6 glycosidic bond

Question 7

Which of the following is NOT seen in lactose intolerance?

Accepted Answer

Alkaline urine

Answer

Reducing substances present in stools

Answer

Acidic stool

Answer

Lactase enzyme deficiency

Question 8

In which of the following tissues is glycogen incapable of contributing directly to blood glucose?

Accepted Answer

Muscle

Answer

Liver

Answer

Both

Answer

None

Question 9

Which enzyme is responsible for the complete oxidation of glucose to CO2 and water?

Accepted Answer

Mitochondria

Answer

Cytosol

Answer

Lysosomes

Answer

Endoplasmic Reticulum

Question 10

Skeletal muscle is deficient in which of the following enzymes?

Accepted Answer

Glucose-6-phosphatase

Answer

Hexokinase

Answer

Isomerase

Answer

Phosphofructokinase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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