Carbohydrate Metabolism Practice Questions

Q: Which of the following enzyme defects is the most commonly inherited metabolic disorder of glycolysis?

Pyruvate kinase. **Explanation:** **Pyruvate Kinase (PK) deficiency** is the most common inherited enzyme defect of the **glycolytic pathway** (Embden-Meyerhof pathway) and the second most common cause of enzyme-deficient hemolytic anemia (after G6PD deficiency). **Why Pyruvate Kinase is correct:** Mature erythrocytes lack mitochondria and depend entirely on anaerobic glycolysis for ATP production. PK catalyzes the final step of glycolysis (Phosphoenolpyruvate to Pyruvate), generating ATP. A deficiency leads to ATP depletion, causing failure of the Na⁺/K⁺-ATPase pumps. This results in the loss of intracellular potassium and water, leading to cell dehydration, "echinocyte" (burr cell) formation, and premature destruction in the spleen (extravascular hemolysis). **Why other options are incorrect:** * **Glucokinase:** Defects are rare and typically associated with MODY (Maturity-Onset Diabetes of the Young) type 2, not a primary hemolytic disorder. * **Hexokinase:** Deficiency is extremely rare; it would be more severe as it affects the very first step of glucose utilization. * **Phosphofructokinase (PFK-1):** Deficiency (Tarui disease/GSD Type VII) primarily affects muscles and causes exercise intolerance, though it can cause mild hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Biochemical Hallmark:** Accumulation of **2,3-Bisphosphoglycerate (2,3-BPG)**. This shifts the oxygen dissociation curve to the **right**, facilitating oxygen unloading to tissues (patients often tolerate anemia better than expected). * **Peripheral Smear:** Presence of **Echinocytes** (Burr cells). * **Diagnosis:** Quantitative assay of PK enzyme activity in RBCs.

Q: A 3-day-old infant vomits everything they feed, has a distended abdomen, and diarrhea. The urine is positive for reducing substances by Benedict's test. What is the substance in the urine?

Galactose. ### Explanation The clinical presentation of a 3-day-old neonate with vomiting, abdominal distension, and diarrhea immediately after starting milk feeds, coupled with **reducing substances in the urine**, is a classic description of **Classic Galactosemia** (deficiency of Galactose-1-phosphate uridyltransferase). **1. Why Galactose is correct:** Milk contains lactose, which is disaccharide composed of glucose and galactose. In galactosemia, the infant cannot metabolize galactose. This leads to an accumulation of galactose and galactose-1-phosphate in tissues. Excess galactose is excreted in the urine. Since galactose is a **reducing sugar**, it yields a positive result on **Benedict’s test**. **2. Why other options are incorrect:** * **Sucrose (A):** Sucrose is a **non-reducing sugar** (the only common one) and would give a negative Benedict’s test. Furthermore, sucrose is not found in breast milk. * **Glucose (B):** While glucose is a reducing sugar, isolated glucosuria in a neonate without hyperglycemia is rare and does not typically present with this systemic gastrointestinal distress immediately upon milk ingestion. * **Fructose (D):** Hereditary Fructose Intolerance presents similarly (vomiting, hypoglycemia) but only **after** the introduction of weaning foods (fruit juices/sucrose), not in a 3-day-old on exclusive milk feeds. **3. NEET-PG High-Yield Pearls:** * **Enzyme Deficiency:** Most common is **GALT** (Galactose-1-phosphate uridyltransferase). * **Clinical Triad:** Cataracts (due to **galactitol** accumulation via aldose reductase), hepatosplenomegaly/jaundice, and intellectual disability. * **Infection Risk:** These infants are at high risk for **E. coli sepsis**. * **Diagnosis:** Benedict’s test is positive (nonspecific), but the **Glucose Oxidase test (Dipstick)** is negative (specific for glucose). * **Management:** Immediate withdrawal of milk; switch to soy-based or lactose-free formula.

Q: Which step in glycolysis involves substrate-level phosphorylation?

Pyruvate kinase. **Explanation:** **Substrate-level phosphorylation** is the direct synthesis of ATP (or GTP) from ADP (or GDP) by the transfer of a high-energy phosphate group from a phosphorylated intermediate, independent of the electron transport chain. In glycolysis, there are **two steps** where this occurs: 1. **Phosphoglycerate Kinase:** Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate. 2. **Pyruvate Kinase (Option B):** This is the final irreversible step of glycolysis where Phosphoenolpyruvate (PEP) is converted to Pyruvate. PEP contains a high-energy phosphate bond; its cleavage provides sufficient energy to phosphorylate ADP to ATP. **Analysis of Incorrect Options:** * **A. Glyceraldehyde 3-phosphate dehydrogenase:** This step involves the oxidation of the substrate and the addition of inorganic phosphate to form 1,3-BPG. It produces **NADH**, not ATP. * **C. Phosphofructokinase (PFK-1):** This is the rate-limiting step of glycolysis. It **consumes** one molecule of ATP to phosphorylate Fructose-6-phosphate; it does not generate it. * **D. Enolase:** This enzyme catalyzes a dehydration reaction (2-phosphoglycerate to PEP). While it creates a high-energy bond, it does not involve phosphate transfer to ADP. **NEET-PG High-Yield Pearls:** * **Net ATP Yield:** Aerobic glycolysis yields 5 or 7 ATP (depending on the shuttle), while anaerobic glycolysis yields a net of **2 ATP**. * **Clinical Correlation:** **Pyruvate Kinase deficiency** is the second most common cause of enzyme-deficient hemolytic anemia (after G6PD deficiency). Since RBCs lack mitochondria, they rely entirely on glycolysis for ATP; a deficiency leads to ATP depletion, causing rigid cells and premature destruction (hemolysis). * **Arsenate Poisoning:** Arsenate competes with inorganic phosphate in the GAPDH step, bypassing the first substrate-level phosphorylation and resulting in **zero net ATP** production.

Q: Which Glycogen Storage Disease does not affect muscles?

Type 1. **Explanation:** The correct answer is **Type 1 (von Gierke disease)**. This is because the deficient enzyme, **Glucose-6-Phosphatase**, is primarily expressed in the liver and kidneys. Muscle tissue lacks this enzyme naturally; instead, muscles utilize Glucose-6-Phosphate directly for glycolysis to generate ATP rather than releasing free glucose into the bloodstream. Consequently, Type 1 GSD presents with severe fasting hypoglycemia and hepatomegaly, but **no muscle symptoms** (no weakness or cramping). **Analysis of Incorrect Options:** * **Type 2 (Pompe disease):** Caused by a deficiency in **Lysosomal α-1,4-glucosidase (Acid Maltase)**. Since lysosomes are present in all cells, this disease affects the heart and skeletal muscles severely, leading to hypertrophic cardiomyopathy and hypotonia. * **Type 3 (Cori disease):** Caused by a deficiency in the **Debranching enzyme**. Unlike Type 1, this affects both the liver and muscles, often presenting with hepatomegaly along with progressive distal muscle weakness. * **Type 4 (Andersen disease):** Caused by a deficiency in the **Branching enzyme**, leading to the accumulation of abnormal glycogen (polyglucosan). This affects the liver, heart, and skeletal muscles, often resulting in cirrhosis and muscular hypotonia. **High-Yield Clinical Pearls for NEET-PG:** * **Type 1 (von Gierke):** Look for the "Doll-like face," hyperuricemia (gout), lactic acidosis, and hyperlipidemia. * **Type 5 (McArdle):** Affects **only** muscles (Myophosphorylase deficiency). Look for exercise-induced cramps and myoglobinuria. * **Mnemonic:** Remember that Type 1 and Type 6 (Hers) are primarily hepatic, while Type 5 is purely muscular. Type 2, 3, and 4 have overlapping systemic/muscular involvement.

Q: A 6-month-old infant, exclusively fed infant formula, develops jaundice, hepatomegaly, vomiting, lethargy, irritability, and seizures upon introduction of fruit juices. Urine-reducing substances are positive. Which of the following is the likely diagnosis explaining this child's condition?

Hereditary fructose intolerance. **Explanation:** The clinical presentation points toward **Hereditary Fructose Intolerance (HFI)**. The key diagnostic clue is the onset of symptoms (vomiting, jaundice, hepatomegaly, and seizures) specifically following the **introduction of fruit juices** (which contain fructose and sucrose) in an infant previously asymptomatic on formula or breast milk. **1. Why Hereditary Fructose Intolerance is correct:** HFI is caused by a deficiency of **Aldolase B**. When fructose is ingested, it is phosphorylated to **Fructose-1-Phosphate (F1P)** by fructokinase. Due to the lack of Aldolase B, F1P accumulates in hepatocytes, trapping intracellular inorganic phosphate. This depletion of ATP and Pi inhibits both **glycogenolysis and gluconeogenesis**, leading to profound postprandial hypoglycemia (causing seizures/lethargy) and liver damage (jaundice/hepatomegaly). The presence of **urine-reducing substances** (fructose) with a negative glucose dipstick is a classic finding. **2. Why other options are incorrect:** * **Galactosemia:** Symptoms appear much earlier (within days of birth) as soon as the infant starts breastfeeding or taking lactose-containing formula. * **Tyrosinemia:** While it causes hepatomegaly and jaundice, it is not specifically triggered by the introduction of fruit juices. * **α1-Antitrypsin deficiency:** Presents with neonatal cholestasis or chronic liver disease, but does not cause acute hypoglycemia or positive urine-reducing substances upon sugar ingestion. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficient:** Aldolase B (Chromosome 9q). * **The "Trapping" Mechanism:** Accumulation of F1P is the toxic event. * **Dietary Management:** Total avoidance of Fructose, Sucrose (Glucose + Fructose), and Sorbitol. * **Distinction:** Essential Fructosuria (Fructokinase deficiency) is a benign, asymptomatic condition, unlike HFI.

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.

Question 1

What is the most common enzyme deficiency in humans?

Accepted Answer

Glucose-6-phosphate dehydrogenase

Answer

Glucose-6-phosphatase

Answer

Hexokinase

Answer

Glucose-1,6-diphosphatase

Question 2

A 2-month-old boy is evaluated for failure to thrive. The child experiences a seizure during examination. Physical examination reveals hepatomegaly, confirmed by CT scan, which also shows renomegaly. Serum chemistries demonstrate severe hypoglycemia, hyperlipidemia, lactic acidosis, and ketosis. Which of the following diseases best accounts for this presentation?

Accepted Answer

Von Gierke's disease

Answer

Gaucher's disease

Answer

McArdle's disease

Answer

Niemann-Pick disease

Question 3

Which of the following enzyme defects is the most commonly inherited metabolic disorder of glycolysis?

Accepted Answer

Pyruvate kinase

Answer

Glucokinase

Answer

Hexokinase

Answer

Phosphofructokinase

Question 4

A 3-day-old infant vomits everything they feed, has a distended abdomen, and diarrhea. The urine is positive for reducing substances by Benedict's test. What is the substance in the urine?

Accepted Answer

Galactose

Answer

Sucrose

Answer

Glucose

Answer

Fructose

Question 5

Which step in glycolysis involves substrate-level phosphorylation?

Accepted Answer

Pyruvate kinase

Answer

Glyceraldehyde 3-phosphate dehydrogenase

Answer

Phosphofructokinase

Answer

Enolase

Question 6

Which Glycogen Storage Disease does not affect muscles?

Accepted Answer

Type 1

Answer

Type 2

Answer

Type 3

Answer

Type 4

Question 7

A 6-month-old infant, exclusively fed infant formula, develops jaundice, hepatomegaly, vomiting, lethargy, irritability, and seizures upon introduction of fruit juices. Urine-reducing substances are positive. Which of the following is the likely diagnosis explaining this child's condition?

Accepted Answer

Hereditary fructose intolerance

Answer

Tyrosinemia

Answer

Galactosemia

Answer

a1-Antitrypsin deficiency

Question 8

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

Accepted Answer

Glycogen

Answer

Amylase

Answer

Maltose

Answer

Fructose

Question 9

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 10

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

Accepted Answer

3 months

Answer

15 days

Answer

1 month

Answer

6 months

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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