Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 621: Which of the following metabolic processes does NOT take place in the mitochondria?

A. Fatty acid oxidation
B. Embden-Meyerhof-Parnas (EMP) pathway (Correct Answer)
C. Electron transport chain
D. Citric acid cycle

Explanation: ### Explanation The **Embden-Meyerhof-Parnas (EMP) pathway**, commonly known as **Glycolysis**, is the correct answer because it occurs entirely within the **cytosol** of the cell. This pathway involves the breakdown of glucose into pyruvate (in aerobic conditions) or lactate (in anaerobic conditions) and does not require mitochondrial machinery or oxygen to proceed. **Analysis of Options:** * **Fatty acid oxidation (Beta-oxidation):** This process occurs within the **mitochondrial matrix**. Long-chain fatty acids are transported across the mitochondrial membrane via the carnitine shuttle to be broken down into Acetyl-CoA. * **Electron transport chain (ETC):** This is located on the **inner mitochondrial membrane**. It is the final stage of aerobic respiration where oxidative phosphorylation occurs to generate ATP. * **Citric acid cycle (TCA Cycle):** All enzymes for this cycle (except succinate dehydrogenase, which is on the inner membrane) are located in the **mitochondrial matrix**. It serves as the hub for oxidizing Acetyl-CoA derived from carbohydrates, fats, and proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Purely Cytosolic Pathways:** Glycolysis, HMP Shunt, Fatty acid synthesis, and Cholesterol synthesis. * **Purely Mitochondrial Pathways:** TCA cycle, ETC, Beta-oxidation of fatty acids, and Ketogenesis. * **Dual Compartment Pathways (Both Cytosol & Mitochondria):** Remember the mnemonic **"HUG"** — **H**eme synthesis, **U**rea cycle, and **G**luconeogenesis. * **RBC Metabolism:** Since mature Red Blood Cells lack mitochondria, they depend exclusively on the EMP pathway (Glycolysis) for energy.

Question 622: What are the two important byproducts of the Hexose Monophosphate (HMP) shunt?

A. NADH and pentose sugars
B. NADPH and pentose sugars (Correct Answer)
C. Pentose sugars and 4-membered sugars
D. Pentose sugars and sedoheptulose

Explanation: The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), is an alternative pathway for glucose oxidation that occurs in the cytosol. Unlike glycolysis, its primary purpose is not the generation of ATP, but the production of two vital biosynthetic precursors: 1. **NADPH:** Generated by the oxidative phase (via Glucose-6-Phosphate Dehydrogenase), it serves as a reducing agent for fatty acid and steroid synthesis and maintains reduced glutathione to prevent oxidative damage. 2. **Pentose Sugars (Ribose-5-Phosphate):** Produced for the synthesis of nucleotides (DNA/RNA) and coenzymes. **Analysis of Options:** * **Option A (Incorrect):** NADH is primarily produced in glycolysis and the TCA cycle for ATP generation via the electron transport chain. The HMP shunt specifically produces **NADPH**. * **Option C & D (Incorrect):** While 4-membered sugars (Erythrose-4-P) and 7-membered sugars (Sedoheptulose-7-P) are intermediates in the non-oxidative phase, they are not the primary "functional byproducts" the pathway is designed to provide. They serve as links to return carbon skeletons to glycolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **G6PD Deficiency:** The most common enzymopathy. It leads to neonatal jaundice and drug-induced hemolytic anemia (e.g., after Primaquine or Fava beans) because RBCs cannot generate NADPH to combat oxidative stress. * **Tissue Distribution:** The pathway is highly active in the liver, lactating mammary glands, adrenal cortex, and RBCs. * **Transketolase:** An enzyme in the non-oxidative phase that requires **Thiamine (Vitamin B1)** as a cofactor; its activity is measured to diagnose Thiamine deficiency.

Question 623: Patients with diabetes frequently report changing visual acuities when their glucose levels are chronically high. Which of the following could explain the fluctuating acuity with high blood glucose levels?

A. Increased sorbitol in the lens (Correct Answer)
B. Decreased fructose in the lens
C. Increased oxidative phosphorylation in the lens
D. Macular degeneration

Explanation: **Explanation:** The correct answer is **A. Increased sorbitol in the lens.** **Mechanism:** In states of chronic hyperglycemia (Diabetes Mellitus), the insulin-independent tissues—such as the lens, retina, and peripheral nerves—take up glucose freely. When intracellular glucose levels are high, the enzyme **Aldose Reductase** reduces glucose into **Sorbitol** (a sugar alcohol) using NADPH as a cofactor. Under normal conditions, sorbitol is further converted to fructose by *sorbitol dehydrogenase*. However, in the lens, the activity of sorbitol dehydrogenase is very low. Consequently, sorbitol accumulates. Because sorbitol is osmotically active and cannot easily cross cell membranes, it draws water into the lens. This causes **osmotic swelling**, altering the curvature and refractive index of the lens, which leads to fluctuating visual acuity and eventually "sugar cataracts." **Why other options are incorrect:** * **B. Decreased fructose:** Fructose levels actually *increase* as a byproduct of the polyol pathway, though not as rapidly as sorbitol. * **C. Increased oxidative phosphorylation:** The lens is largely avascular and relies primarily on anaerobic glycolysis. High glucose leads to increased polyol pathway activity and oxidative stress, not efficient oxidative phosphorylation. * **D. Macular degeneration:** While diabetes causes retinopathy, fluctuating acuity related specifically to glucose spikes is typically a refractive issue caused by lens swelling, not a degenerative change in the macula. **High-Yield NEET-PG Pearls:** * **Polyol Pathway:** Glucose → Sorbitol (Aldose Reductase) → Fructose (Sorbitol Dehydrogenase). * **Tissues lacking Sorbitol Dehydrogenase:** "LoSeR" (Lens, Schwann cells, Retina). These are the primary sites of diabetic complications (cataracts, neuropathy, retinopathy). * **NADPH Depletion:** The use of NADPH by aldose reductase depletes the pool available for *Glutathione Reductase*, increasing oxidative stress in the cell.

Question 624: Forbes' disease is due to deficiency of which enzyme?

A. Branching enzyme
B. Debranching enzyme (Correct Answer)
C. Myophosphorylase
D. Hepatic phosphorylase

Explanation: **Explanation:** **Forbes' disease**, also known as **Cori’s disease (GSD Type III)**, is caused by a deficiency of the **Debranching enzyme** (Amylo-1,6-glucosidase). In this condition, glycogen can be partially broken down by phosphorylase, but the process stops at the branch points, leading to the accumulation of abnormal glycogen with short outer chains known as **limit dextrin**. * **Why Option B is correct:** The debranching enzyme has two activities: transferase and glucosidase. Its deficiency prevents the complete mobilization of glucose from glycogen stores, leading to hepatomegaly, growth retardation, and fasting hypoglycemia (though milder than von Gierke’s). **Analysis of Incorrect Options:** * **Option A (Branching enzyme):** Deficiency causes **Andersen’s disease (GSD Type IV)**. This leads to the accumulation of long, unbranched glucose chains (amylopectin-like) which trigger an immune response, causing progressive liver cirrhosis. * **Option C (Myophosphorylase):** Deficiency causes **McArdle’s disease (GSD Type V)**. This affects only skeletal muscle, presenting with exercise intolerance, muscle cramps, and myoglobinuria. * **Option D (Hepatic phosphorylase):** Deficiency causes **Hers’ disease (GSD Type VI)**. It presents with hepatomegaly and mild hypoglycemia, but unlike Forbes' disease, the glycogen structure remains normal. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "ABCD" — **A**ndersen = **B**ranching; **C**ori = **D**ebranching. * Forbes' disease involves both **liver and muscle** (Type IIIa) or just the liver (Type IIIb). * Unlike Type I (von Gierke), blood lactate and uric acid levels are usually **normal** in Forbes' disease. * Limit dextrinosis is the hallmark histological finding.

Question 625: Which of the following methods can NOT be used for glucose detection?

A. Glucose oxidase
B. Ferric Chloride test (Correct Answer)
C. Dextrostix
D. Follin and Wu method

Explanation: The detection of glucose is a fundamental topic in clinical biochemistry. Here is the breakdown of the methods mentioned: ### **Why Ferric Chloride (FeCl₃) Test is the Correct Answer** The **Ferric Chloride test** is not used for glucose detection. It is a classic screening test used to detect **phenols** or specific metabolites in urine. In a clinical context, it is most famously used to screen for **Phenylketonuria (PKU)**, where it reacts with phenylpyruvic acid to produce a characteristic blue-green color. It also reacts with ketones (acetoacetate), salicylates, and melanin. ### **Analysis of Incorrect Options** * **Glucose Oxidase (Option A):** This is the most **specific** method for glucose detection. The enzyme glucose oxidase converts glucose to gluconic acid and hydrogen peroxide ($H_2O_2$). The $H_2O_2$ then reacts with a chromogen to produce a color change. * **Dextrostix (Option C):** These are plastic strips used for rapid bedside monitoring of blood glucose. They utilize the **glucose oxidase-peroxidase** enzymatic reaction. * **Follin-Wu Method (Option D):** This is an older, **non-specific reduction method**. It relies on the ability of glucose (a reducing sugar) to reduce cupric ions to cuprous ions in an alkaline medium, which then reacts with phosphomolybdic acid to form a blue complex (molybdenum blue). ### **High-Yield Clinical Pearls for NEET-PG** * **Specific vs. Non-specific:** Glucose oxidase is specific for $\beta$-D-glucose. Benedict’s test and Follin-Wu are non-specific as they react with any reducing substance (e.g., fructose, galactose, or even Vitamin C). * **Renal Threshold:** Glucose appears in urine (glycosuria) when blood glucose exceeds approximately **180 mg/dL**. * **FeCl₃ Test Summary:** Remember it for **PKU** (Green), **Alkaptonuria** (Transient Blue), and **Salicylate poisoning** (Stable Purple).

Question 626: What is the end product of glycolysis under anaerobic conditions?

A. Lactic acid (Correct Answer)
B. Pyruvic acid
C. Acetoacetic acid
D. Oxaloacetic acid

Explanation: **Explanation:** In glycolysis, glucose is converted into two molecules of pyruvate. The fate of this pyruvate depends on the availability of oxygen and the presence of mitochondria. **1. Why Lactic Acid is Correct:** Under **anaerobic conditions** (lack of oxygen), the electron transport chain cannot function. To allow glycolysis to continue and generate ATP, the cell must regenerate **NAD+** from NADH. The enzyme **Lactate Dehydrogenase (LDH)** reduces pyruvate to **Lactic acid**, simultaneously oxidizing NADH back to NAD+. This is the primary pathway in mature erythrocytes (which lack mitochondria) and exercising skeletal muscle. **2. Why the other options are incorrect:** * **Pyruvic acid:** This is the end product of **aerobic** glycolysis. In the presence of oxygen, pyruvate enters the mitochondria to be converted into Acetyl-CoA. * **Acetoacetic acid:** This is a **ketone body** produced during ketogenesis (primarily in the liver during starvation or uncontrolled diabetes), not a product of glycolysis. * **Oxaloacetic acid:** This is an intermediate of the TCA cycle and gluconeogenesis. While pyruvate can be converted to oxaloacetate via *pyruvate carboxylase*, it is not the end product of the glycolytic pathway. **Clinical Pearls & High-Yield Facts:** * **Net ATP Yield:** Anaerobic glycolysis yields only **2 ATP** per glucose molecule, whereas aerobic respiration yields 30-32 ATP. * **Cori Cycle:** The lactic acid produced in muscles travels to the liver, where it is converted back to glucose via gluconeogenesis. * **Lactic Acidosis:** This occurs when there is excessive anaerobic metabolism (e.g., septic shock, severe hypoxia), leading to a drop in blood pH. * **Key Enzyme:** Lactate Dehydrogenase (LDH) is a tetramer; LDH-1 is prominent in the heart, while LDH-5 is prominent in the liver and skeletal muscle.

Question 627: Insulin causes all of the following effects except:

A. Glycogenesis
B. Glycolysis
C. Lipogenesis
D. Ketogenesis (Correct Answer)

Explanation: ### Explanation **Concept:** Insulin is the body’s primary **anabolic hormone**, secreted by the pancreatic beta cells in the "fed state." Its primary goal is to lower blood glucose levels by promoting energy storage and inhibiting the breakdown of stored fuels. **1. Why Ketogenesis is the correct answer:** Insulin **inhibits** ketogenesis. Ketogenesis (the formation of ketone bodies) occurs during starvation or uncontrolled Diabetes Mellitus when insulin levels are low. Insulin suppresses ketogenesis by: * Inhibiting **Hormone-Sensitive Lipase (HSL)**, which reduces the supply of free fatty acids (the substrate for ketones). * Decreasing the activity of **HMG-CoA synthase**, the rate-limiting enzyme of ketogenesis. Therefore, insulin is an anti-ketogenic hormone. **2. Why the other options are incorrect:** * **Glycogenesis (A):** Insulin promotes glucose storage. It activates **Glycogen Synthase**, leading to increased glycogen synthesis in the liver and muscles. * **Glycolysis (B):** Insulin stimulates the utilization of glucose for energy. It induces key glycolytic enzymes like **Glucokinase, PFK-1, and Pyruvate Kinase**. * **Lipogenesis (C):** Insulin promotes the synthesis of fatty acids and triglycerides. It activates **Acetyl-CoA Carboxylase (ACC)** and increases glucose uptake in adipose tissue via **GLUT-4** transporters. **High-Yield Clinical Pearls for NEET-PG:** * **The "Insulin:Glucagon Ratio":** Metabolism is governed by this ratio. A high ratio (Insulin > Glucagon) favors synthesis; a low ratio favors breakdown (Glycogenolysis, Gluconeogenesis, Ketogenesis). * **Rate-Limiting Enzyme:** HMG-CoA Synthase (Mitochondrial) is the rate-limiting step for Ketogenesis. * **Clinical Correlation:** Diabetic Ketoacidosis (DKA) occurs due to absolute insulin deficiency, leading to unchecked ketogenesis.

Question 628: Which enzyme aids in the production of more glucose?

A. Pyruvate kinase
B. Pyruvate carboxylase (Correct Answer)
C. Pyruvate dehydrogenase (PDH)
D. Pyruvate decarboxylase

Explanation: ### Explanation The production of glucose from non-carbohydrate precursors is known as **Gluconeogenesis**. This process is not a simple reversal of glycolysis because three steps in glycolysis are irreversible. To bypass these steps, specific gluconeogenic enzymes are required. **1. Why Pyruvate Carboxylase is Correct:** The first irreversible step in glycolysis is the conversion of Phosphoenolpyruvate (PEP) to Pyruvate by Pyruvate kinase. To bypass this in gluconeogenesis, **Pyruvate carboxylase** converts Pyruvate into **Oxaloacetate (OAA)** inside the mitochondria. This OAA is then converted to PEP by PEP carboxykinase (PEPCK). Thus, Pyruvate carboxylase is a key regulatory enzyme that initiates the production of more glucose. **2. Why the Other Options are Incorrect:** * **Pyruvate kinase (A):** This is a glycolytic enzyme that breaks down PEP into pyruvate to produce ATP. It acts in the opposite direction of glucose production. * **Pyruvate dehydrogenase (C):** This enzyme complex converts Pyruvate into Acetyl-CoA to enter the TCA cycle. Once converted to Acetyl-CoA, the carbons cannot be used for net glucose synthesis in humans. * **Pyruvate decarboxylase (D):** This enzyme is involved in ethanol fermentation (converting pyruvate to acetaldehyde), primarily in yeast and some bacteria, not in human gluconeogenesis. **Clinical Pearls for NEET-PG:** * **Cofactor Requirement:** Pyruvate carboxylase requires **Biotin (B7)** and ATP. It is also allosterically **activated by Acetyl-CoA**. * **Location:** It is a mitochondrial enzyme, whereas the rest of gluconeogenesis occurs primarily in the cytosol. * **Deficiency:** Pyruvate carboxylase deficiency leads to lactic acidosis and fasting hypoglycemia because pyruvate cannot be diverted toward glucose production.

Question 629: What is the approximate carbohydrate reserve of the human body?

A. 350 gm (Correct Answer)
B. 600 gm
C. 950 gm
D. 1500 gm

Explanation: **Explanation:** The total carbohydrate reserve in a healthy adult (approx. 70 kg) is roughly **350–500 gm**, primarily stored as glycogen. This reserve is distributed as follows: 1. **Muscle Glycogen (~250–300 gm):** The largest reservoir. It is used locally for muscle contraction and cannot contribute to blood glucose levels because muscle lacks the enzyme *Glucose-6-Phosphatase*. 2. **Liver Glycogen (~75–100 gm):** Maintains blood glucose levels during fasting. 3. **Blood Glucose (~15–20 gm):** A very small, transient fraction. **Analysis of Options:** * **Option A (350 gm):** This is the most accurate estimate for the baseline carbohydrate reserve in a standard adult. * **Option B (600 gm):** This value is slightly higher than the average physiological range, though it may be reached in highly trained athletes with "carb-loading." * **Options C & D (950 gm & 1500 gm):** These values far exceed the storage capacity of the human body. Excess carbohydrate intake beyond the glycogen storage limit is converted into triacylglycerols (fat) via *de novo lipogenesis*. **High-Yield NEET-PG Pearls:** * **Energy Yield:** Glycogen provides ~4 kcal/gm. The total reserve (350 gm) provides ~1400 kcal, which is exhausted within 12–18 hours of fasting. * **Hydration:** Glycogen is osmotic; it is stored with water (1 gm glycogen binds ~3 gm water). This is why rapid weight loss occurs in the first days of a keto diet (water loss). * **Rate-limiting enzyme:** Glycogen synthase (Glycogenesis) and Glycogen phosphorylase (Glycogenolysis).

Question 630: What is the most important amino acid substrate for gluconeogenesis?

A. Leucine
B. Lysine
C. Histidine
D. Alanine (Correct Answer)

Explanation: **Explanation:** **Alanine** is the most important glucogenic amino acid because it serves as the primary vehicle for transporting nitrogen and carbon skeletons from skeletal muscle to the liver. This occurs via the **Cahill Cycle (Glucose-Alanine Cycle)**. During fasting or intense exercise, muscle protein is broken down; the resulting amino groups are transferred to pyruvate to form alanine. In the liver, alanine undergoes transamination back into pyruvate, which is then directly utilized as a substrate for gluconeogenesis. **Analysis of Options:** * **Alanine (Correct):** It is the principal gluconeogenic precursor among amino acids. It has the highest rate of extraction by the liver compared to all other amino acids. * **Leucine & Lysine (Incorrect):** These are the only two **purely ketogenic** amino acids. They are metabolized into acetyl-CoA or acetoacetate and cannot be used for glucose synthesis. * **Histidine (Incorrect):** While histidine is a glucogenic amino acid (it enters the TCA cycle via $\alpha$-ketoglutarate), its quantitative contribution to total glucose production is significantly less than that of alanine. **High-Yield Clinical Pearls for NEET-PG:** * **Glucogenic vs. Ketogenic:** Most amino acids are both; Leucine and Lysine are *only* ketogenic. * **Key Precursors:** The three major non-carbohydrate precursors for gluconeogenesis are **Lactate** (Cori Cycle), **Glycerol** (from lipolysis), and **Alanine** (Cahill Cycle). * **Rate-Limiting Step:** The conversion of Fructose-1,6-bisphosphate to Fructose-6-phosphate by **Fructose-1,6-bisphosphatase** is the key regulatory step of gluconeogenesis.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

Practice Questions

Blood Glucose Regulation

Practice Questions

Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

Practice Questions

Lactose Intolerance and Galactosemia

Practice Questions

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