Carbohydrate Metabolism Practice Questions

Q: Classic Galactosemia is due to deficiency of which enzyme?

Galactose-1-Phosphate-Uridyl-Transferase. **Explanation:** **Classic Galactosemia** (Type 1) is an autosomal recessive disorder caused by a deficiency of the enzyme **Galactose-1-Phosphate Uridyl Transferase (GALT)**. In the normal metabolic pathway (Leloir pathway), GALT is responsible for converting Galactose-1-phosphate and UDP-glucose into UDP-galactose and Glucose-1-phosphate. Its deficiency leads to the toxic accumulation of Galactose-1-phosphate and galactitol in tissues like the liver, brain, and lens of the eye. **Analysis of Incorrect Options:** * **Hexosaminidase A:** Deficiency leads to **Tay-Sachs Disease**, a lysosomal storage disorder characterized by GM2 ganglioside accumulation and a "cherry-red spot" on the macula. * **Glucocerebrosidase:** Deficiency causes **Gaucher Disease**, the most common lysosomal storage disease, characterized by hepatosplenomegaly and "wrinkled tissue paper" appearance of macrophages. * **Sphingomyelinase:** Deficiency results in **Niemann-Pick Disease**, leading to sphingomyelin accumulation and foam cells. **Clinical Pearls for NEET-PG:** * **Presentation:** Symptoms appear shortly after starting milk feeds (lactose = glucose + galactose). Look for jaundice, hepatomegaly, infantile cataracts (due to galactitol), and failure to thrive. * **Sepsis Risk:** These infants have a significantly increased predisposition to **E. coli neonatal sepsis**. * **Diagnosis:** Reducing substances in urine (Clinitest positive) but a negative glucose oxidase test (Dipstick). * **Treatment:** Immediate and lifelong elimination of galactose and lactose from the diet.

Q: Which enzyme of the citric acid cycle catalyzes a reaction involving substrate-level phosphorylation?

Succinate thiokinase. **Explanation:** **1. Why Succinate Thiokinase is Correct:** Succinate thiokinase (also known as **Succinyl-CoA synthetase**) is the only enzyme in the Citric Acid Cycle (TCA cycle) that performs **substrate-level phosphorylation (SLP)**. In this reaction, the high-energy thioester bond of Succinyl-CoA is cleaved to form Succinate. The energy released is used to phosphorylate GDP to **GTP** (which is later converted to ATP). This is a crucial step because it generates high-energy phosphate directly without the requirement of the Electron Transport Chain or Oxygen. **2. Why the Other Options are Incorrect:** * **Pyruvate Kinase (Option A):** While this enzyme does perform substrate-level phosphorylation (converting Phosphoenolpyruvate to Pyruvate), it is a part of **Glycolysis**, not the Citric Acid Cycle. * **Phosphoglycerate Kinase (Option C):** Similarly, this enzyme performs SLP (converting 1,3-bisphosphoglycerate to 3-phosphoglycerate), but it is also an enzyme of the **Glycolytic pathway**. * **Option D:** Incorrect because only Succinate thiokinase fits both criteria: being an SLP enzyme and belonging to the TCA cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Definition of SLP:** The synthesis of ATP/GTP from ADP/GDP coupled directly to the dephosphorylation of a high-energy intermediate. * **Total SLP sites:** In one complete turn of the aerobic oxidation of glucose, there are **3 SLP sites**: two in Glycolysis (Phosphoglycerate kinase, Pyruvate kinase) and one in the TCA cycle (Succinate thiokinase). * **TCA Yield:** One turn of the TCA cycle produces **1 GTP (via SLP)**, 3 NADH, and 1 FADH2, totaling 10 ATP equivalents. * **Isoenzymes:** Succinate thiokinase has two isoforms; one is specific for GDP (liver/kidney) and one for ADP (skeletal/heart muscle).

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

Debranching enzyme. **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.

Q: An infant presents with hepatomegaly, hypoglycemia, hyperlipidemia, and acidosis. What is the diagnosis?

Von Gierke's disease. **Explanation:** The clinical presentation of hepatomegaly, hypoglycemia, hyperlipidemia, and acidosis is the classic "quadrad" of **Von Gierke’s Disease (GSD Type I)**. **1. Why Option A is Correct:** Von Gierke’s disease is caused by a deficiency of **Glucose-6-Phosphatase**. This enzyme is essential for the final step of both glycogenolysis and gluconeogenesis. Its absence leads to: * **Severe Fasting Hypoglycemia:** The liver cannot release free glucose into the blood. * **Hepatomegaly:** Excessive accumulation of glycogen (normal structure) in the liver. * **Lactic Acidosis:** Glucose-6-phosphate shunts into the glycolytic pathway, increasing lactate. * **Hyperlipidemia & Hyperuricemia:** Diverted metabolites lead to increased VLDL/triglycerides and purine degradation (Gout). **2. Why Other Options are Incorrect:** * **B. Cori’s Disease (GSD Type III):** Caused by **Debranching enzyme** deficiency. While it presents with hepatomegaly and hypoglycemia, the symptoms are generally milder, and **lactate levels are typically normal** because gluconeogenesis remains intact. * **C. Pompe’s Disease (GSD Type II):** Caused by **Lysosomal α-1,4-glucosidase** deficiency. It primarily affects the heart and muscles (**Cardiomegaly**), and notably, it does **not** present with hypoglycemia as glycogen mobilization in the liver is unaffected. **Clinical Pearls for NEET-PG:** * **"Doll-like facies"** (fatty cheeks) is a characteristic physical finding in Von Gierke’s. * **Hyperuricemia** in GSD Type I can lead to "Gouty arthritis" later in life. * **Treatment:** Frequent oral cornstarch (slow-release glucose) and avoidance of fructose/galactose.

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

Lactic acid. **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.

Q: A 6-month-old baby presents to the emergency department with features of cardiac failure. On examination, developmental delay and a large tongue are noted. Echocardiography reveals severe concentric left ventricular hypertrophy. What is the most probable diagnosis?

Pompe disease. **Explanation:** The clinical triad of **infantile-onset heart failure**, **severe concentric left ventricular hypertrophy (LVH)**, and **macroglossia** (large tongue) in a 6-month-old is classic for **Pompe Disease (Glycogen Storage Disease Type II)**. **Why Pompe Disease is Correct:** Pompe disease is caused by a deficiency of the lysosomal enzyme **acid alpha-glucosidase (acid maltase)**. Unlike other GSDs, glycogen accumulates within lysosomes in various tissues, most notably the **cardiac and skeletal muscles**. This leads to massive cardiomegaly (often described as a "globular heart" on X-ray), hypertrophic cardiomyopathy, and profound hypotonia ("floppy baby"). The macroglossia and developmental delay further support this diagnosis. **Why Other Options are Incorrect:** * **Von Gierke’s Disease (GSD Type I):** Primarily affects the liver and kidneys. It presents with severe hypoglycemia, hepatomegaly, and "doll-like" facies, but **never involves the heart**. * **Forbes/Cori Disease (GSD Type III):** Caused by debranching enzyme deficiency. While it can occasionally involve the heart, it typically presents with hepatomegaly and growth retardation; the cardiac involvement is never as severe or early-onset as in Pompe. * **McArdle’s Disease (GSD Type V):** A muscle phosphorylase deficiency that presents in **adolescence or adulthood** with exercise-induced cramps and myoglobinuria. It does not cause infantile heart failure. **High-Yield Clinical Pearls for NEET-PG:** * **ECG Finding:** Pompe disease characteristically shows **short PR intervals** and **giant QRS complexes** (due to massive hypertrophy). * **Enzyme:** Acid Alpha-Glucosidase (Acid Maltase). * **Stain:** Periodic Acid-Schiff (PAS) positive vacuoles in muscle biopsy. * **Treatment:** Enzyme Replacement Therapy (ERT) with Alglucosidase alfa.

Q: Which enzyme aids in the production of more glucose?

Pyruvate carboxylase. ### 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.

Q: What is the approximate carbohydrate reserve of the human body?

350 gm. **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).

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

Alanine. **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.

Question 1

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?

Accepted Answer

Increased sorbitol in the lens

Answer

Decreased fructose in the lens

Answer

Increased oxidative phosphorylation in the lens

Answer

Macular degeneration

Question 2

Classic Galactosemia is due to deficiency of which enzyme?

Accepted Answer

Galactose-1-Phosphate-Uridyl-Transferase

Answer

Hexosaminidase

Answer

Glucocerebroside

Answer

Sphingomyelinase

Question 3

Which enzyme of the citric acid cycle catalyzes a reaction involving substrate-level phosphorylation?

Accepted Answer

Succinate thiokinase

Answer

Pyruvate kinase

Answer

Phosphoglycerate kinase

Answer

All of the above

Question 4

Forbes' disease is due to deficiency of which enzyme?

Accepted Answer

Debranching enzyme

Answer

Branching enzyme

Answer

Myophosphorylase

Answer

Hepatic phosphorylase

Question 5

An infant presents with hepatomegaly, hypoglycemia, hyperlipidemia, and acidosis. What is the diagnosis?

Accepted Answer

Von Gierke's disease

Answer

Cori's disease

Answer

Pompe's disease

Answer

All of the above

Question 6

What is the end product of glycolysis under anaerobic conditions?

Accepted Answer

Lactic acid

Answer

Pyruvic acid

Answer

Acetoacetic acid

Answer

Oxaloacetic acid

Question 7

A 6-month-old baby presents to the emergency department with features of cardiac failure. On examination, developmental delay and a large tongue are noted. Echocardiography reveals severe concentric left ventricular hypertrophy. What is the most probable diagnosis?

Accepted Answer

Pompe disease

Answer

Von Gierke's disease

Answer

Forbes disease

Answer

McArdle's disease

Question 8

Which enzyme aids in the production of more glucose?

Accepted Answer

Pyruvate carboxylase

Answer

Pyruvate kinase

Answer

Pyruvate dehydrogenase (PDH)

Answer

Pyruvate decarboxylase

Question 9

What is the approximate carbohydrate reserve of the human body?

Accepted Answer

350 gm

Answer

600 gm

Answer

950 gm

Answer

1500 gm

Question 10

What is the most important amino acid substrate for gluconeogenesis?

Accepted Answer

Alanine

Answer

Leucine

Answer

Lysine

Answer

Histidine

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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