Carbohydrate Metabolism Practice Questions

Q: What is the maximum carbohydrate concentration typically found in a strict vegetarian diet?

Glycogen. **Explanation:** The question focuses on the storage forms of carbohydrates and their dietary sources. **Glycogen** is the correct answer because it is the primary storage polysaccharide in animals (found in liver and muscle). In the context of a **strict vegetarian (vegan) diet**, which excludes all animal products, the dietary intake of pre-formed glycogen is effectively **zero** or negligible. Therefore, the "maximum concentration" of glycogen found in such a diet is the lowest among the options provided. **Analysis of Options:** * **Glycogen (Correct):** As an animal-derived storage homopolymer of glucose, it is absent in plant-based foods. A strict vegetarian diet lacks this component entirely. * **Amylase (Incorrect):** This is an enzyme (protein), not a carbohydrate. While present in plants (e.g., $\beta$-amylase in seeds), it does not represent a carbohydrate concentration. * **Maltose (Incorrect):** A disaccharide produced during the breakdown of starch. It is found in germinating cereals (malt) and is a common component of a vegetarian diet. * **Fructose (Incorrect):** A monosaccharide abundant in fruits and honey. It is a major carbohydrate component in any plant-based diet. **NEET-PG Clinical Pearls:** * **Structure:** Glycogen features $\alpha(1\to4)$ glycosidic bonds for linear chains and $\alpha(1\to6)$ bonds for branching (every 8-12 residues). * **Storage:** In humans, the liver maintains blood glucose (via glycogenolysis), while muscle glycogen provides local energy during contraction. * **Biochemical Marker:** Glycogen is a homopolymer of D-glucose. Its absence in plants is a key distinction between animal and plant energy storage (starch vs. glycogen).

Q: Glycosylated hemoglobin reflects the mean blood glucose level of the previous?

3 months. **Explanation:** **1. Why Option C is Correct:** Glycosylated hemoglobin (HbA1c) is formed by the non-enzymatic, irreversible attachment of glucose to the hemoglobin molecule within red blood cells (RBCs). Because this binding is permanent for the life of the cell, the HbA1c level is directly proportional to the average blood glucose concentration over the lifespan of the erythrocyte. Since the **average lifespan of an RBC is approximately 120 days (4 months)**, HbA1c provides a weighted average of blood glucose levels over the preceding **2 to 3 months**, with the most recent 30 days contributing the most to the final value. **2. Why Other Options are Incorrect:** * **Option A (15 days) & B (1 month):** These durations are too short to reflect the cumulative glucose exposure captured by hemoglobin. However, **Fructosamine** (glycated albumin) is used to monitor glucose control over a shorter period of **2–3 weeks**, as albumin has a shorter half-life than RBCs. * **Option D (6 months):** This exceeds the 120-day physiological lifespan of the RBC. By 6 months, the original population of glycated RBCs would have been cleared by the spleen and replaced. **3. High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Cut-off:** According to WHO/ADA, an **HbA1c ≥ 6.5%** is diagnostic for Diabetes Mellitus. * **Pre-diabetes:** HbA1c range of **5.7% to 6.4%**. * **False Lows:** Conditions with high RBC turnover (e.g., Hemolytic anemia, pregnancy, recent blood transfusion, or treatment for iron/B12 deficiency). * **False Highs:** Conditions that prolong RBC lifespan (e.g., Splenectomy) or iron deficiency anemia (due to altered glycation kinetics). * **Target:** For most diabetic patients, the goal is to keep HbA1c **< 7%** to prevent microvascular complications.

Q: Oxidative decarboxylation of pyruvate requires which of the following?

COASH. **Explanation:** The oxidative decarboxylation of pyruvate to Acetyl-CoA is a critical link between glycolysis and the TCA cycle. This reaction is catalyzed by the **Pyruvate Dehydrogenase (PDH) Complex**, a multi-enzyme system located in the mitochondrial matrix. **Why COASH is correct:** The PDH complex requires five specific cofactors to function: **T**hiamine pyrophosphate (TPP/B1), **L**ipoic acid, **C**oenzyme A (CoA-SH/B5), **F**AD (B2), and **N**AD+ (B3). **CoA-SH** acts as the final acceptor of the acetyl group, forming **Acetyl-CoA**, which then enters the Krebs cycle. Without CoA-SH, the two-carbon unit cannot be activated for further metabolism. **Why other options are incorrect:** * **NADP+:** This is primarily used in reductive biosynthesis (e.g., fatty acid synthesis) and the HMP shunt. The PDH complex specifically uses **NAD+** as an electron acceptor. * **Cytochromes:** These are components of the Electron Transport Chain (ETC) involved in redox reactions for ATP production, not in the decarboxylation of keto-acids. * **Pyridoxal phosphate (PLP):** This is the active form of Vitamin B6, essential for **transamination** and decarboxylation of amino acids, but it plays no role in the PDH complex. **Clinical Pearls for NEET-PG:** * **Mnemonic:** "The Lovely Co-enzymes For Nerds" (TPP, Lipoate, CoA, FAD, NAD). * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the -SH groups of **Lipoic acid**, leading to lactic acidosis and neurological symptoms. * **Deficiency:** Thiamine (B1) deficiency (Wernicke-Korsakoff) impairs PDH activity, significantly affecting ATP-dependent organs like the brain.

Q: What is the most common glycogen storage disorder to present in adolescents?

McArdle disease. **Explanation:** The correct answer is **McArdle disease (GSD Type V)**. The primary reason it presents in adolescence or early adulthood is that it affects **muscle glycogen phosphorylase (myophosphorylase)**, an enzyme required only during high-intensity anaerobic exercise. Unlike hepatic glycogen storage diseases that manifest in infancy with systemic metabolic crises (hypoglycemia), McArdle disease is a purely myogenic disorder. Symptoms typically emerge when the patient begins engaging in more strenuous physical activity during teenage years. **Why the other options are incorrect:** * **Von Gierke disease (Type I):** This is the most common GSD overall but presents in **infancy** (3–6 months) with severe fasting hypoglycemia, hepatomegaly, and doll-like facies. * **Cori disease (Type III):** Presents in **early childhood** with hepatomegaly and growth retardation. While it involves muscle weakness, the metabolic symptoms appear much earlier than adolescence. * **Andersen disease (Type IV):** A very severe form presenting in **infancy** with failure to thrive and progressive liver cirrhosis, often leading to death by age 2. **NEET-PG High-Yield Pearls:** * **Clinical Triad:** Exercise intolerance, muscle cramps, and **myoglobinuria** (burgundy-colored urine) after strenuous exercise. * **Second-Wind Phenomenon:** A pathognomonic feature where symptoms improve after a short period of exercise due to the mobilization of fatty acids and increased blood flow. * **Biochemical Marker:** Failure of blood lactate to rise during an ischemic exercise test (flat lactate curve). * **Key Enzyme:** Myophosphorylase (Muscle glycogen phosphorylase).

Q: An infant presents to OPD with signs of heart failure. On examination, there is hypotonia, hepatomegaly, and cardiomegaly. ECG shows tall QRS complex and a short PR interval. Which enzyme is deficient in this condition?

Acid maltase. ### Explanation The clinical presentation of **cardiomegaly, hepatomegaly, and hypotonia** in an infant, combined with pathognomonic ECG findings (**short PR interval and tall QRS complexes**), is a classic description of **Pompe Disease (Glycogen Storage Disease Type II)**. **1. Why Acid Maltase is Correct:** Pompe disease is caused by a deficiency of **Lysosomal $\alpha$-1,4-glucosidase (Acid Maltase)**. Unlike other glycogen storage diseases (GSDs), this is a **lysosomal storage disorder**. Acid maltase is responsible for breaking down glycogen within lysosomes. Its deficiency leads to massive accumulation of glycogen in the heart, skeletal muscle, and liver. The cardiac involvement is the hallmark, leading to hypertrophic cardiomyopathy and early heart failure. **2. Why Other Options are Incorrect:** * **Glucose 6-phosphatase (Option A):** Deficient in **Von Gierke Disease (GSD I)**. While it presents with hepatomegaly and hypoglycemia, it **does not** involve the heart or cause skeletal muscle hypotonia. * **Muscle phosphorylase (Option B):** Deficient in **McArdle Disease (GSD V)**. This presents in adolescence/adulthood with exercise intolerance and cramps; it does not cause infantile heart failure. * **Liver phosphorylase (Option C):** Deficient in **Hers Disease (GSD VI)**. This is a milder condition presenting with hepatomegaly and growth retardation, without cardiac involvement. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Pompe trashes the **Pump** (Heart), **Liver**, and **Muscle**." * **ECG Hallmark:** Short PR interval due to accelerated conduction through glycogen-laden myocardium and massive QRS voltage (biventricular hypertrophy). * **Histology:** PAS-positive material in lysosomes. * **Treatment:** Enzyme Replacement Therapy (ERT) with Alglucosidase alfa.

Q: In glycolysis, the conversion of 1 mol of fructose 1,6-bisphosphate to 2 mol of pyruvate results in the formation of which of the following molecules?

2 mol NADH and 4 mol of ATP. **Explanation:** In the glycolytic pathway, the conversion of **Fructose 1,6-bisphosphate (F1,6BP)** to Pyruvate represents the "Pay-off Phase." 1. **Cleavage:** F1,6BP (6 carbons) is cleaved by Aldolase A into two 3-carbon units: Glyceraldehyde 3-phosphate (G3P) and Dihydroxyacetone phosphate (DHAP). DHAP is isomerized to G3P, resulting in **2 moles of G3P**. 2. **Oxidation (NADH generation):** Each G3P is oxidized by *G3P Dehydrogenase*. This step reduces NAD+ to NADH. Since there are 2 moles of G3P, **2 mol of NADH** are produced. 3. **ATP Generation (Substrate-Level Phosphorylation):** * Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate (by Phosphoglycerate kinase) yields 1 ATP per G3P. * Conversion of Phosphoenolpyruvate (PEP) to Pyruvate (by Pyruvate kinase) yields 1 ATP per G3P. * Total yield: 2 ATP × 2 moles of G3P = **4 mol of ATP**. **Why incorrect options are wrong:** * **A & B:** These suggest the formation of NAD+. In glycolysis, NAD+ is *consumed* (reduced), not formed. NAD+ is only regenerated during anaerobic glycolysis (lactate formation). * **C:** This option accounts for only 2 ATP. While the *net* gain of glycolysis starting from Glucose is 2 ATP, the question starts from **Fructose 1,6-bisphosphate**, bypassing the preparatory phase where 2 ATP are normally consumed. **Clinical Pearls for NEET-PG:** * **Net ATP Yield:** From Glucose to Pyruvate is 2 ATP; from F1,6BP to Pyruvate is 4 ATP. * **Arsenate Poisoning:** Arsenate competes with inorganic phosphate in the G3P Dehydrogenase reaction, resulting in 0 net ATP production despite glycolysis continuing. * **Rapoport-Luebering Cycle:** In RBCs, 1,3-BPG can be converted to 2,3-BPG, bypassing the first ATP-generating step, which shifts the O2 dissociation curve to the right.

Q: GLUT-5 is a transporter for which of the following?

Fructose. **Explanation:** The correct answer is **Fructose**. Glucose transporters (GLUTs) are a family of transmembrane proteins that facilitate the passive diffusion of monosaccharides across cell membranes. **GLUT-5** is unique among the GLUT family because it has a high affinity specifically for **fructose** and lacks the ability to transport glucose or galactose under physiological conditions. It is primarily expressed in the apical membrane of enterocytes in the small intestine, where it facilitates the absorption of dietary fructose. **Analysis of Options:** * **A. Glucose:** Transported primarily by GLUT-1 to GLUT-4. GLUT-1 and GLUT-3 are for basal uptake, GLUT-2 is high-capacity (liver/pancreas), and GLUT-4 is insulin-dependent (muscle/adipose). * **C. Mannose:** While mannose can be transported by some hexose transporters, it is not the primary substrate for GLUT-5. * **D. Galactose:** Along with glucose, galactose is transported into the enterocyte via **SGLT-1** (active transport) and exits into the blood via **GLUT-2**. GLUT-5 does not recognize galactose. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT-1 vs. GLUT-5:** Remember that SGLT-1 is a secondary active transporter (Sodium-dependent) for Glucose and Galactose, whereas GLUT-5 is a facilitated diffuser (Sodium-independent) for Fructose. * **GLUT-2:** This is the "bidirectional" transporter found in the liver, pancreas, and the basolateral membrane of the intestine; it transports glucose, galactose, and fructose. * **Spermatozoa:** GLUT-5 is also found in mature spermatozoa, as fructose is their primary energy source. * **Insulin Independence:** GLUT-5, like most GLUTs (except GLUT-4), is **not** regulated by insulin.

Question 1

What is the maximum carbohydrate concentration typically found in a strict vegetarian diet?

Accepted Answer

Glycogen

Answer

Amylase

Answer

Maltose

Answer

Fructose

Question 2

Congenital lactic acidosis may occur due to a defect in which of the following?

Accepted Answer

Pyruvate dehydrogenase complex

Answer

Pyruvate decarboxylase

Answer

Transketolase

Answer

Alpha-ketoglutarate dehydrogenase

Question 3

Glycosylated hemoglobin reflects the mean blood glucose level of the previous?

Accepted Answer

3 months

Answer

15 days

Answer

1 month

Answer

6 months

Question 4

Oxidative decarboxylation of pyruvate requires which of the following?

Accepted Answer

COASH

Answer

NADP+

Answer

Cytochromes

Answer

Pyridoxal phosphate

Question 5

What is the most common glycogen storage disorder to present in adolescents?

Accepted Answer

McArdle disease

Answer

Cori disease

Answer

Andersen disease

Answer

Von Gierke disease

Question 6

An infant presents to OPD with signs of heart failure. On examination, there is hypotonia, hepatomegaly, and cardiomegaly. ECG shows tall QRS complex and a short PR interval. Which enzyme is deficient in this condition?

Accepted Answer

Acid maltase

Answer

Glucose 6-phosphatase

Answer

Muscle phosphorylase

Answer

Liver phosphorylase

Question 7

In glycolysis, the conversion of 1 mol of fructose 1,6-bisphosphate to 2 mol of pyruvate results in the formation of which of the following molecules?

Accepted Answer

2 mol NADH and 4 mol of ATP

Answer

1 mol NAD+ and 2 mol of ATP

Answer

2 mol NAD+ and 4 mol of ATP

Answer

2 mol NADH and 2 mol of ATP

Question 8

The uptake of glucose by the liver increases following a carbohydrate meal because:

Accepted Answer

There is an increase in the phosphorylation of glucose by glucokinase.

Answer

GLUT-2 is stimulated by insulin.

Answer

Glucokinase has a low Km for glucose.

Answer

Hexokinase in the liver has a high affinity for glucose.

Question 9

GLUT-5 is a transporter for which of the following?

Accepted Answer

Fructose

Answer

Glucose

Answer

Mannose

Answer

Galactose

Question 10

Which enzyme is common to both gluconeogenesis and glycolysis pathways?

Accepted Answer

Phosphofructokinase

Answer

Fructose 2,6-bisphosphatase

Answer

Hexokinase

Answer

Glucose 6-phosphatase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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