Carbohydrate Metabolism Practice Questions

Q: A 3-month-old infant presented with irritability, lethargy between feedings, and a potbelly. Physical examination revealed an enlarged liver. Blood work between feedings showed elevated lactate and uric acid levels, along with hypoglycemia. This child most likely has a mutation in which one of the following enzymes?

Glucose-6-phosphatase. This clinical presentation is classic for **Von Gierke Disease (Glycogen Storage Disease Type I)**. ### **Explanation of the Correct Answer** **Glucose-6-phosphatase (G6Pase)** is the final enzyme in both glycogenolysis and gluconeogenesis, responsible for converting glucose-6-phosphate into free glucose in the liver. * **Hypoglycemia:** A deficiency prevents the liver from releasing glucose into the blood during fasting. * **Hepatomegaly:** Excess glucose-6-phosphate is diverted into glycogen synthesis and lipid production, leading to a "fatty" enlarged liver (potbelly). * **Lactic Acidosis:** Accumulated G6P enters the glycolytic pathway, increasing pyruvate and lactate. * **Hyperuricemia:** Increased G6P shunts into the Pentose Phosphate Pathway, increasing ribose-5-phosphate, which drives purine synthesis and subsequent degradation to uric acid. ### **Why Other Options are Incorrect** * **A. Liver glycogen phosphorylase (Hers Disease):** While it causes hepatomegaly and mild hypoglycemia, it does **not** typically present with significant lactic acidosis or hyperuricemia because gluconeogenesis remains intact. * **B. Glycogen synthase:** Deficiency leads to fasting hypoglycemia but results in a **small liver** (no glycogen storage) and does not cause lactic acidosis. * **D. Muscle glycogen phosphorylase (McArdle Disease):** This enzyme is muscle-specific. Deficiency causes exercise intolerance and cramps, but **no hypoglycemia or hepatomegaly**, as the liver is unaffected. ### **NEET-PG High-Yield Pearls** * **GSD Type I subtypes:** Type Ia (G6Pase deficiency); Type Ib (G6P translocase deficiency—presents with **neutropenia** and recurrent infections). * **Biochemical Markers:** "Doll-like facies," hyperlipidemia (xanthomas), hyperuricemia, and lactic acidosis. * **Treatment:** Frequent oral cornstarch (slow-release glucose) and avoidance of fructose/galactose (which worsen G6P accumulation).

Q: Type II glycogen storage disorder is due to deficiency of which enzyme?

alpha-Glucosidase. **Explanation:** **Type II Glycogen Storage Disease (GSD)**, also known as **Pompe disease**, is caused by a deficiency of the lysosomal enzyme **$\alpha$-1,4-glucosidase** (also called **Acid Maltase**). Unlike other GSDs, Pompe disease is unique because it is a **Lysosomal Storage Disorder**. While most glycogen breakdown occurs in the cytosol via glycogen phosphorylase, about 1-3% of glycogen is degraded within lysosomes by $\alpha$-glucosidase. A deficiency leads to the massive accumulation of glycogen within vacuoles in nearly all tissues, most significantly affecting the heart and skeletal muscles. **Analysis of Options:** * **Option A (Correct):** $\alpha$-Glucosidase is the enzyme responsible for breaking down glycogen into glucose within lysosomes. Its deficiency leads to Pompe disease. * **Option B (Incorrect):** $\alpha$-galactosidase A deficiency causes **Fabry disease**, a sphingolipidosis characterized by angiokeratomas and renal failure. * **Option C (Incorrect):** Muscle phosphorylase deficiency causes **Type V GSD (McArdle disease)**, which presents with exercise-induced cramps and myoglobinuria. * **Option D (Incorrect):** Acid Lipase deficiency leads to **Wolman disease** or Cholesteryl Ester Storage Disease. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Pompe affects the **Pump** (Heart)." * **Clinical Triad:** Cardiomegaly (HOCM), hypotonia ("Floppy baby"), and macroglossia. * **Key Distinction:** It is the only GSD that is also a Lysosomal Storage Disorder. * **Blood Glucose:** Unlike Type I (von Gierke), blood glucose levels are typically **normal** in Pompe disease because cytosolic glycogenolysis remains intact.

Q: Which of the following is a fructosan?

Inulin. **Explanation:** **Inulin** is the correct answer because it is a **fructosan** (or fructan), a homopolysaccharide composed of repeating units of D-fructose linked by **β(2→1) glycosidic bonds**. It is found in the tubers and roots of plants like chicory, dahlias, and dandelions. **Analysis of Options:** * **Pectin (Option A):** A complex heteropolysaccharide primarily composed of galacturonic acid units. It is found in the cell walls of fruits (e.g., apples) and is used as a gelling agent. * **Chitin (Option C):** A structural homopolysaccharide found in the exoskeleton of arthropods and fungal cell walls. It consists of **N-acetyl-D-glucosamine** units linked by β(1→4) bonds. * **Glycogen (Option D):** The primary storage homopolysaccharide in animals. It is a polymer of **D-glucose** with α(1→4) linkages in the chains and α(1→6) linkages at branch points. **High-Yield Clinical Pearls for NEET-PG:** 1. **Renal Physiology:** Inulin is the "gold standard" for measuring **Glomerular Filtration Rate (GFR)** because it is freely filtered by the glomeruli but is neither secreted nor reabsorbed by the renal tubules. 2. **Solubility:** Unlike starch, inulin is readily soluble in warm water. 3. **Diagnostic Use:** While inulin is the most accurate, **Creatinine clearance** is more commonly used in clinical practice to estimate GFR as it is endogenous and does not require an intravenous infusion. 4. **Dietary Fiber:** Inulin is not digested by human enzymes and acts as a prebiotic, promoting the growth of beneficial gut bacteria.

Q: Which among the following enzymes is an allosteric inhibitor of the TCA cycle?

Isocitrate dehydrogenase. **Explanation:** The TCA (Tricarboxylic Acid) cycle is the central metabolic pathway for energy production. Its regulation is primarily governed by the energy status of the cell, signaled by the ratios of ATP/ADP and NADH/NAD+. **Why Isocitrate Dehydrogenase (ICDH) is the correct answer:** ICDH catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate. It is considered the **rate-limiting step** of the TCA cycle. It is strictly regulated by **allosteric effectors**: * **Allosteric Inhibitors:** **ATP** and **NADH** (signaling high energy stores). * **Allosteric Activators:** **ADP** and **Ca²⁺** (signaling energy demand). When ATP levels are high, ICDH is inhibited, leading to an accumulation of Citrate, which then leaves the mitochondria to inhibit glycolysis (via PFK-1) and stimulate fatty acid synthesis. **Analysis of Incorrect Options:** * **A. Pyruvate Dehydrogenase (PDH):** While PDH is inhibited by ATP and NADH, it is **not** part of the TCA cycle itself. It is a multienzyme complex that links glycolysis to the TCA cycle (the "Link Reaction"). * **B. Alpha-ketoglutarate Dehydrogenase:** This enzyme is inhibited by its products (**Succinyl-CoA** and **NADH**) and ATP. However, it is not the primary rate-limiting allosteric valve compared to ICDH. * **D. Malate Dehydrogenase:** This reaction is primarily regulated by the concentration of substrates and the NADH/NAD+ ratio (mass action kinetics) rather than complex allosteric modulation. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of TCA:** Isocitrate Dehydrogenase. * **CO₂ Release:** Occurs at the steps catalyzed by ICDH and Alpha-ketoglutarate dehydrogenase. * **Fluoroacetate:** A potent inhibitor of the TCA cycle that inhibits the enzyme **Aconitase** (suicide inhibition). * **Arsenite poisoning:** Inhibits Alpha-ketoglutarate dehydrogenase (and PDH) by binding to lipoic acid.

Q: What is the first step in the liver's metabolism of fructose?

Phosphorylation to fructose 1 phosphate by ATP. **Explanation:** The metabolism of fructose in the liver follows a specialized pathway distinct from glycolysis. The first and rate-limiting step is the **phosphorylation of fructose at the C1 position** to form **Fructose 1-phosphate**. This reaction is catalyzed by the enzyme **Fructokinase** (also known as Ketohexokinase), utilizing one molecule of ATP as the phosphate donor. **Analysis of Options:** * **Option D (Correct):** Fructokinase has a high affinity (low Km) for fructose and specifically produces Fructose 1-phosphate. This bypasses the major regulatory step of glycolysis (Phosphofructokinase-1), leading to more rapid metabolism of fructose compared to glucose. * **Option A:** Isomerization of glucose refers to the conversion of Glucose 6-phosphate to Fructose 6-phosphate in glycolysis; it is not the initial step of fructose metabolism. * **Option B:** Fructose 1,6-bisphosphate is formed later in the pathway after Fructose 1-phosphate is cleaved into glyceraldehyde and DHAP. * **Option C:** While **Hexokinase** can phosphorylate fructose to Fructose 6-phosphate, it has a very low affinity for fructose. This pathway only becomes significant in extrahepatic tissues (like muscle) or when fructose levels are extremely high. **NEET-PG High-Yield Pearls:** * **Essential Fructosuria:** Caused by a deficiency of **Fructokinase**. It is a benign, asymptomatic condition where fructose is excreted in the urine. * **Hereditary Fructose Intolerance (HFI):** Caused by a deficiency of **Aldolase B**. This leads to the toxic accumulation of Fructose 1-phosphate, causing intracellular phosphate depletion, hypoglycemia, and liver failure. * **Metabolic Speed:** Fructose metabolism is faster than glucose metabolism because it bypasses the PFK-1 rate-limiting step.

Q: The hexose monophosphate shunt occurs in all of the following except?

Skin. The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), is a metabolic pathway parallel to glycolysis. Its primary functions are the generation of **NADPH** (for reductive biosynthesis) and **Ribose-5-phosphate** (for nucleotide synthesis). ### Why Skin is the Correct Answer The HMP shunt is most active in tissues that require high amounts of NADPH for lipid synthesis or to maintain a reduced state of glutathione. **Skin** does not have a high demand for these specific reductive biosynthetic processes compared to the other organs listed. Therefore, while minimal activity may exist, it is not a primary site for the HMP shunt. ### Analysis of Incorrect Options * **Liver:** This is a major site for the HMP shunt because NADPH is required for the de novo synthesis of fatty acids and cholesterol, as well as for detoxification reactions involving Cytochrome P450. * **Adipose Tissue:** Highly active in the HMP shunt to provide the NADPH necessary for the synthesis of long-chain fatty acids (lipogenesis). * **Mammary Gland:** Specifically during lactation, the mammary glands show high HMP shunt activity to supply NADPH for the synthesis of milk lipids. ### High-Yield Clinical Pearls for NEET-PG * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **Key Tissues:** Liver, Adipose tissue, Adrenal cortex, Erythrocytes (to maintain reduced glutathione), and Lactating mammary glands. * **Non-oxidative phase:** Transketolase is a key enzyme in this phase and requires **Thiamine (Vitamin B1)** as a cofactor. Measuring transketolase activity in RBCs is used to diagnose Thiamine deficiency. * **No ATP:** Unlike glycolysis, the HMP shunt does not produce or consume ATP directly.

Q: Which metabolic pathway can utilize propionic acid?

Gluconeogenesis. **Explanation:** Propionic acid (propionate) is a three-carbon fatty acid produced primarily from the oxidation of **odd-chain fatty acids** and the catabolism of certain amino acids (Valine, Isoleucine, Threonine, and Methionine). **Why Gluconeogenesis is correct:** Propionate enters the gluconeogenic pathway via a specific conversion sequence: 1. Propionate is converted to **Propionyl-CoA**. 2. Propionyl-CoA is carboxylated to **Methylmalonyl-CoA** (requires Biotin/B7). 3. Methylmalonyl-CoA is isomerized to **Succinyl-CoA** (requires Vitamin B12). Succinyl-CoA enters the TCA cycle and is converted to Malate, which exits the mitochondria to enter **Gluconeogenesis**. This makes propionate the only part of a fatty acid (in odd-chain species) that is truly glucogenic. **Why other options are incorrect:** * **Glycolysis:** This is the breakdown of glucose to pyruvate; it does not utilize propionate as a substrate. * **Glycogenolysis:** This is the breakdown of stored glycogen into glucose-1-phosphate; it involves carbohydrate polymers, not fatty acid derivatives. * **Glycogenesis:** This is the synthesis of glycogen from glucose; while propionate can eventually become glucose, it is not a direct substrate for glycogen synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin Requirements:** The conversion of propionate to Succinyl-CoA requires **Biotin (B7)** and **Cobalamin (B12)**. * **Methylmalonic Aciduria:** A deficiency in Methylmalonyl-CoA mutase or Vitamin B12 leads to the accumulation of methylmalonic acid, causing metabolic acidosis and developmental delays. * **Glucogenic vs. Ketogenic:** While even-chain fatty acids are purely ketogenic (converted to Acetyl-CoA), odd-chain fatty acids are **glucogenic** because of their conversion to propionate.

Question 1

Activity of which of the following enzymes is increased in a diabetic state?

Accepted Answer

Pyruvate dehydrogenase

Answer

Pyruvate carboxylase

Answer

PEP Carboxykinase

Answer

Glucose 6 phosphatase

Question 2

A 3-month-old infant presented with irritability, lethargy between feedings, and a potbelly. Physical examination revealed an enlarged liver. Blood work between feedings showed elevated lactate and uric acid levels, along with hypoglycemia. This child most likely has a mutation in which one of the following enzymes?

Accepted Answer

Glucose-6-phosphatase

Answer

Liver glycogen phosphorylase

Answer

Glycogen synthase

Answer

Muscle glycogen phosphorylase

Question 3

Type II glycogen storage disorder is due to deficiency of which enzyme?

Accepted Answer

alpha-Glucosidase

Answer

alpha-galactosidase

Answer

Muscle phosphorylase

Answer

Acid Lipase

Question 4

Which of the following is a fructosan?

Accepted Answer

Inulin

Answer

Pectin

Answer

Chitin

Answer

Glycogen

Question 5

Which among the following enzymes is an allosteric inhibitor of the TCA cycle?

Accepted Answer

Isocitrate dehydrogenase

Answer

Pyruvate dehydrogenase

Answer

Alpha-ketoglutarate dehydrogenase

Answer

Malate dehydrogenase

Question 6

A 3-year-old girl presents with a history of fussy eating since weaning, particularly when fruit is part of her diet. She becomes cranky, sweats, and displays dizziness and lethargy after consuming meals containing fruit. Her mother has observed that these symptoms resolve when fruit is excluded from her diet. What is the most likely underlying cause of the problems the girl exhibits when eating fruit?

Accepted Answer

Elevated levels of fructose-1-phosphate in liver cells

Answer

Decreased levels of fructose in the blood

Answer

Elevated levels of glyceraldehyde in liver cells

Answer

High levels of sucrose in the stool

Question 7

What is the first step in the liver's metabolism of fructose?

Accepted Answer

Phosphorylation to fructose 1 phosphate by ATP

Answer

Isomerization of glucose

Answer

Phosphorylation to fructose 1, 6 bisphosphate by ATP

Answer

Phosphorylation to fructose 6 phosphate by ATP

Question 8

The hexose monophosphate shunt occurs in all of the following except?

Accepted Answer

Skin

Answer

Liver

Answer

Adipose tissue

Answer

Mammary gland

Question 9

Which metabolic pathway can utilize propionic acid?

Accepted Answer

Gluconeogenesis

Answer

Glycolysis

Answer

Glycogenolysis

Answer

Glycogenesis

Question 10

Two sisters are diagnosed with hemolytic anemia. Their older brother was previously diagnosed with the same disorder. Two other brothers are asymptomatic. The mother and father are second cousins. Deficiency of which of the following enzymes would be most likely to cause this disorder?

Accepted Answer

Pyruvate kinase

Answer

Debranching enzyme

Answer

Glucose-6-phosphatase

Answer

Glucose-6-phosphate dehydrogenase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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