Carbohydrate Metabolism Practice Questions

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.

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.

Question 1

Oxidative decarboxylation of pyruvate requires which of the following?

Accepted Answer

COASH

Answer

NADP+

Answer

Cytochromes

Answer

Pyridoxal phosphate

Question 2

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 3

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 4

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 5

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 6

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

Accepted Answer

Fructose

Answer

Glucose

Answer

Mannose

Answer

Galactose

Question 7

Which enzyme is common to both gluconeogenesis and glycolysis pathways?

Accepted Answer

Phosphofructokinase

Answer

Fructose 2,6-bisphosphatase

Answer

Hexokinase

Answer

Glucose 6-phosphatase

Question 8

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

Accepted Answer

Lactulose

Answer

Lactose

Answer

Sucrose

Answer

Isomaltose

Question 9

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 10

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

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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