Carbohydrate Metabolism Practice Questions

Q: Na+ dependent glucose transport is/are inhibited by:

Phlorizin. **Explanation:** Glucose transport across cell membranes occurs via two primary mechanisms: **Facilitated Diffusion (GLUT)** and **Secondary Active Transport (SGLT)**. **1. Why Phlorizin is Correct:** Sodium-dependent glucose transporters (**SGLT-1** in the small intestine and **SGLT-2** in the renal proximal tubules) transport glucose against its concentration gradient by coupling it with the downhill movement of Na+. **Phlorizin** is a competitive inhibitor of SGLT-1 and SGLT-2. It prevents the binding of glucose to the transporter, thereby inhibiting Na+-dependent glucose absorption in the gut and reabsorption in the kidneys. **2. Analysis of Incorrect Options:** * **Ouabain (A):** This is a cardiac glycoside that inhibits the **Na+-K+ ATPase pump**. While it indirectly affects SGLT by dissipating the Na+ gradient, it is not a direct inhibitor of the glucose transporter itself. * **Na+ Azide (B):** This is a metabolic poison that inhibits the **Electron Transport Chain (Complex IV)**. It stops ATP production, affecting all active transport processes generally, but is not specific to glucose transport. * **Phloretin (D):** This is a derivative of phlorizin that specifically inhibits **GLUT** (facilitated diffusion) transporters, particularly GLUT2, rather than the Na+-dependent SGLT transporters. **Clinical Pearls for NEET-PG:** * **SGLT-2 Inhibitors (Gliflozins):** Drugs like Dapagliflozin (derived from the concept of phlorizin) are used in Type 2 Diabetes to induce glucosuria. * **Oral Rehydration Therapy (ORT):** Based on the principle of SGLT-1, where Na+ and Glucose are co-transported, facilitating water absorption. * **GLUT vs. SGLT:** Remember, GLUTs are **uniporters** (passive), while SGLTs are **symporters** (secondary active).

Q: Which of the following is affected by gluconeogenesis?

Growth hormone. ### Explanation The core of this question lies in distinguishing between the **substrates** (raw materials) of gluconeogenesis and the **hormonal regulators** that influence the process. **Why Growth Hormone (GH) is the correct answer:** Gluconeogenesis is a metabolic pathway that generates glucose from non-carbohydrate precursors during fasting or stress. This process is tightly regulated by hormones. **Growth Hormone (GH)** is a counter-regulatory (diabetogenic) hormone. It **stimulates gluconeogenesis** in the liver and decreases peripheral glucose uptake to maintain blood glucose levels. Therefore, the activity and rate of gluconeogenesis are directly "affected" (stimulated) by the presence of Growth Hormone. **Why the other options are incorrect:** * **A, B, and C (Lactate, Glycerol, Alanine):** These are the primary **substrates** for gluconeogenesis. While they are *consumed* during the process, they are the building blocks rather than the factors that regulate or are "affected by" the pathway's induction in a physiological sense. * **Lactate** enters via the Cori Cycle. * **Glycerol** enters via phosphorylation to DHAP. * **Alanine** is the primary glucogenic amino acid (Cahill Cycle). **NEET-PG High-Yield Pearls:** 1. **Hormonal Control:** Gluconeogenesis is stimulated by **Glucagon, Epinephrine, Cortisol, and Growth Hormone**. It is inhibited by **Insulin**. 2. **Key Regulatory Enzyme:** The most important rate-limiting step is **Fructose-1,6-bisphosphatase**. 3. **Energy Requirement:** Gluconeogenesis is an endergonic process; it requires **6 ATP/GTP** molecules to produce one molecule of glucose from two molecules of pyruvate. 4. **Location:** It occurs primarily in the **Liver** (90%) and **Kidney** (10%), specifically within the mitochondria and cytosol.

Q: A 10-month-old male child presents with vomiting upon ingesting fruits, despite maintaining normal weight. He was exclusively breastfed until this point. The physician suspects fructose intolerance. Which enzyme would be deficient in this child?

Aldolase B. ### Explanation **Correct Option: A. Aldolase B** The clinical presentation describes **Hereditary Fructose Intolerance (HFI)**. This autosomal recessive disorder is caused by a deficiency of **Aldolase B**, an enzyme primarily found in the liver, kidney, and small intestine. * **Pathophysiology:** Aldolase B cleaves Fructose-1-Phosphate (F1P) into DHAP and Glyceraldehyde. In its absence, **F1P accumulates** intracellularly, sequestering inorganic phosphate ($P_i$). This depletion of $P_i$ inhibits glycogenolysis and gluconeogenesis, leading to severe postprandial hypoglycemia and vomiting. * **Clinical Correlation:** Symptoms typically appear when the infant is weaned from breast milk (which contains lactose) and introduced to fruits, juices, or honey (which contain **fructose** and **sucrose**). **Analysis of Incorrect Options:** * **B. Hexokinase:** This enzyme has a low affinity for fructose and is not the primary pathway for fructose metabolism in the liver. * **C. Fructokinase:** Deficiency causes **Essential Fructosuria**. This is a benign, asymptomatic condition where fructose is excreted in the urine (reducing sugar positive) because F1P does not accumulate to trap phosphate. * **D. Glucose-6-phosphatase:** Deficiency leads to **Von Gierke Disease (GSD Type I)**. While it causes hypoglycemia and hepatomegaly, it is not specifically triggered by fruit ingestion and presents with lactic acidosis and hyperuricemia. **High-Yield NEET-PG Pearls:** * **The "Trap":** Fructokinase deficiency is *asymptomatic*; Aldolase B deficiency is *symptomatic*. * **Reducing Sugars:** In HFI, urine dipstick for glucose is negative, but the **Clinitest (Benedict’s test)** is positive for reducing sugars. * **Management:** Strict avoidance of fructose, sucrose (fructose + glucose), and sorbitol.

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.

Question 1

Na+ dependent glucose transport is/are inhibited by:

Accepted Answer

Phlorizin

Answer

Ouabain

Answer

Na+ azide

Answer

Phloretin

Question 2

Which of the following is affected by gluconeogenesis?

Accepted Answer

Growth hormone

Answer

Lactate

Answer

Glycerol

Answer

Alanine

Question 3

A 10-month-old male child presents with vomiting upon ingesting fruits, despite maintaining normal weight. He was exclusively breastfed until this point. The physician suspects fructose intolerance. Which enzyme would be deficient in this child?

Accepted Answer

Aldolase B

Answer

Hexokinase

Answer

Fructokinase

Answer

Glucose-6-phosphatase

Question 4

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 5

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 6

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 7

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 8

Which step in glycolysis involves substrate-level phosphorylation?

Accepted Answer

Pyruvate kinase

Answer

Glyceraldehyde 3-phosphate dehydrogenase

Answer

Phosphofructokinase

Answer

Enolase

Question 9

Which Glycogen Storage Disease does not affect muscles?

Accepted Answer

Type 1

Answer

Type 2

Answer

Type 3

Answer

Type 4

Question 10

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

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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