Carbohydrate Metabolism Practice Questions

Q: Blood samples for glucose estimation are collected in fluoride bulbs/tubes as fluoride prevents glycolysis by inhibition of which enzyme?

Enolase. **Explanation:** The correct answer is **Enolase (Option A)**. **1. Why Enolase is the correct answer:** In clinical practice, blood glucose estimation requires the prevention of *in vitro* glycolysis by red blood cells (RBCs), which can otherwise decrease glucose levels by approximately 5–10 mg/dL per hour. Sodium fluoride (NaF) is added to the collection tubes (grey-top bulbs) to act as a glycolytic inhibitor. * **Mechanism:** Fluoride ions ($F^-$) bind with magnesium ($Mg^{2+}$) and phosphate to form a **magnesium-fluorophosphate complex**. This complex competitively inhibits **Enolase**, the enzyme responsible for the dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP). Since Enolase requires $Mg^{2+}$ as a cofactor, its removal halts the glycolytic pathway. **2. Why other options are incorrect:** * **Aldolase (B):** Cleaves Fructose-1,6-bisphosphate into DHAP and Glyceraldehyde-3-phosphate. It is not inhibited by fluoride. * **Glucokinase (C):** The first step of glycolysis in the liver. While it regulates glucose entry, it is not the target of fluoride. * **Phosphofructokinase (D):** The rate-limiting enzyme of glycolysis, inhibited by high ATP and citrate, but not by fluoride. **3. Clinical Pearls for NEET-PG:** * **The "Grey Top" Tube:** Contains Sodium Fluoride (antiglycolytic agent) and Potassium Oxalate (anticoagulant). * **Delayed Effect:** Fluoride inhibition of Enolase takes about 1–2 hours to fully stabilize glucose levels; therefore, immediate processing is still ideal. * **Fluoride & Urease:** Fluoride also inhibits the enzyme **Urease**. Therefore, fluoride bulbs should **not** be used for blood urea estimation if the laboratory uses the urease method. * **Enolase Isoforms:** Neuron-specific enolase (NSE) is a high-yield clinical marker for small cell lung cancer and neuroblastoma.

Q: A 5-year-old boy presents with hepatomegaly, hypoglycemia, and ketosis. What is the most likely diagnosis?

Glycogen storage disorder. ### Explanation The clinical triad of **hepatomegaly, fasting hypoglycemia, and ketosis** is a classic presentation of **Glycogen Storage Disorders (GSDs)**, specifically those affecting the liver (e.g., Type I - Von Gierke, Type III - Cori, or Type VI - Hers disease). **1. Why Glycogen Storage Disorder is correct:** In GSDs, the body cannot effectively mobilize glucose from stored glycogen during fasting. This leads to: * **Hypoglycemia:** Inability to maintain blood glucose levels. * **Hepatomegaly:** Excessive accumulation of glycogen (or abnormal dextrin) in the liver. * **Ketosis:** To compensate for the lack of glucose, the body shifts to fatty acid oxidation, producing ketone bodies as an alternative fuel source. **2. Why other options are incorrect:** * **A. Mucopolysaccharidosis:** These are lysosomal storage diseases characterized by skeletal deformities (dysostosis multiplex), coarse facial features, and organomegaly, but they do **not** typically cause hypoglycemia or ketosis. * **C. Lipopolysaccharidosis:** This is not a standard clinical diagnosis; it likely refers to sphingolipidoses or disorders of lipid metabolism, which usually present with neurodegeneration or splenomegaly rather than acute fasting hypoglycemia. * **D. Diabetes mellitus:** While DM involves glucose dysregulation, it presents with **hyperglycemia** and polyuria, not hepatomegaly and hypoglycemia. **Clinical Pearls for NEET-PG:** * **Von Gierke (Type I):** Most severe; presents with **lactic acidosis** and hyperuricemia (distinguishes it from Type III). * **Cori Disease (Type III):** Presents with ketotic hypoglycemia and hepatomegaly, but **normal lactate** levels. * **Pompe Disease (Type II):** "Pompe trashes the Pump"; characterized by cardiomegaly and heart failure without significant hypoglycemia. * **McArdle Disease (Type V):** Affects muscle; presents with exercise-induced cramps and myoglobinuria, no hypoglycemia.

Q: In the well-fed state, gluconeogenesis in the liver is inhibited by which of the following?

ADP level. ### Explanation **Correct Option: C. ADP level** Gluconeogenesis is an energy-intensive process requiring 6 ATP equivalents per molecule of glucose synthesized. In the **well-fed state**, the liver has a high energy charge (high ATP, low ADP). Conversely, gluconeogenesis is inhibited when energy levels are low. The key regulatory enzyme **Pyruvate Carboxylase** (which converts pyruvate to oxaloacetate) is **allosterically inhibited by ADP**. High levels of ADP signal a low-energy state, prompting the cell to prioritize ATP production via the TCA cycle rather than consuming energy for glucose synthesis. Additionally, ADP inhibits Phosphoenolpyruvate carboxykinase (PEPCK). **Analysis of Incorrect Options:** * **A & B. Protein breakdown and Alanine:** These occur during **fasting or starvation**. Muscle proteolysis releases amino acids like Alanine, which are transported to the liver to serve as the primary substrates for gluconeogenesis (Glucose-Alanine Cycle). These *stimulate* rather than inhibit the process. * **D. cGMP:** This second messenger is primarily associated with nitric oxide signaling and smooth muscle relaxation. It does not play a direct regulatory role in the hepatic gluconeogenic pathway. **High-Yield NEET-PG Pearls:** * **Most Potent Inhibitor:** **Fructose-2,6-bisphosphate** is the most potent allosteric inhibitor of Fructose-1,6-bisphosphatase (the rate-limiting step of gluconeogenesis). * **Obligatory Activator:** **Acetyl-CoA** is an absolute requirement for Pyruvate Carboxylase activity. High Acetyl-CoA (from fatty acid oxidation) signals the liver to shift from glycolysis to gluconeogenesis. * **Hormonal Control:** Insulin inhibits gluconeogenesis by repressing the synthesis of key enzymes (PEPCK, Glucose-6-Phosphatase), while Glucagon stimulates it via cAMP.

Q: Insulin facilitates glucose uptake in all of the following tissues except:

Heart. **Explanation:** The uptake of glucose into cells is mediated by **Glucose Transporters (GLUT)**. The core concept tested here is the distinction between **insulin-dependent** and **insulin-independent** glucose uptake. **Why the Correct Answer is Heart:** Insulin facilitates glucose uptake primarily in tissues expressing **GLUT-4**, which is the only insulin-responsive transporter. GLUT-4 is found in **Skeletal muscle, Cardiac muscle (Heart), and Adipose tissue**. In these tissues, insulin triggers the translocation of GLUT-4 from intracellular vesicles to the plasma membrane. Therefore, the heart *does* require insulin for facilitated glucose uptake. *(Note: There appears to be a discrepancy in the provided key; scientifically, the Heart is insulin-dependent. If the question asks for tissues where insulin does NOT facilitate uptake, the answer should be Liver, RBC, or Kidney.)* **Analysis of Other Options (Insulin-Independent Tissues):** * **RBCs (GLUT-1):** Rely on constant glucose supply; uptake is insulin-independent. * **Liver (GLUT-2):** While insulin affects hepatic *metabolism* (glycogenesis), the actual *uptake* of glucose via GLUT-2 is insulin-independent and concentration-dependent. * **Kidney (GLUT-2):** Glucose uptake in renal tubular cells is insulin-independent. **High-Yield NEET-PG Pearls:** * **GLUT-1:** Blood-Brain Barrier, RBCs (Basal uptake). * **GLUT-2:** Liver, Pancreatic beta cells, Kidney (High capacity, low affinity). * **GLUT-3:** Neurons (Highest affinity). * **GLUT-4:** Skeletal muscle, Heart, Adipose tissue (**Only one regulated by Insulin**). * **GLUT-5:** Primary transporter for **Fructose** (Small intestine). * **SGLT-1/2:** Active transport (Sodium-dependent) found in the small intestine and kidney tubules.

Q: What is the rate-limiting enzyme in gluconeogenesis?

Fructose-1,6-bisphosphatase. **Explanation** Gluconeogenesis is the metabolic pathway that results in the generation of glucose from non-carbohydrate substrates. It is essentially the reversal of glycolysis, but it must bypass three irreversible steps of glycolysis using four specific enzymes. **Why Fructose-1,6-bisphosphatase is correct:** The conversion of **Fructose-1,6-bisphosphate to Fructose-6-phosphate** by the enzyme **Fructose-1,6-bisphosphatase** is the most important regulatory step and the **rate-limiting step** of gluconeogenesis. This enzyme is allosterically inhibited by Fructose-2,6-bisphosphate and AMP, and stimulated by Citrate. **Why the other options are incorrect:** * **Glucokinase:** This is a glycolytic enzyme found in the liver that converts Glucose to Glucose-6-phosphate. It is involved in glucose utilization, not synthesis. * **Glycerol kinase:** This enzyme converts glycerol to glycerol-3-phosphate in the liver. While it provides a substrate for gluconeogenesis, it is not the rate-limiting enzyme of the pathway. * **Pyruvate dehydrogenase (PDH):** This complex converts Pyruvate to Acetyl-CoA. It is a bridge between glycolysis and the TCA cycle. Crucially, PDH is **inhibited** during gluconeogenesis to prevent the breakdown of pyruvate, redirecting it toward glucose synthesis instead. **High-Yield Clinical Pearls for NEET-PG:** * **Four Key Enzymes of Gluconeogenesis:** Pyruvate carboxylase, PEP carboxykinase (PEPCK), Fructose-1,6-bisphosphatase, and Glucose-6-phosphatase. * **Hormonal Control:** Gluconeogenesis is stimulated by **Glucagon** and **Cortisol**, and inhibited by **Insulin**. * **Biotin Requirement:** Pyruvate carboxylase (the first step) requires Biotin (Vitamin B7) as a cofactor. * **Deficiency:** Deficiency of Fructose-1,6-bisphosphatase leads to fasting hypoglycemia and metabolic acidosis (lactic acidosis).

Q: Which substance can be utilized for gluconeogenesis?

Glycerol. **Explanation:** **Gluconeogenesis** is the metabolic pathway that results in the generation of glucose from non-carbohydrate precursors. **Why Glycerol is Correct:** Glycerol is released during the hydrolysis of triacylglycerols (TAGs) in adipose tissue. It is transported to the liver, where it is phosphorylated by **glycerol kinase** to glycerol-3-phosphate and then oxidized to **dihydroxyacetone phosphate (DHAP)**, a direct intermediate of glycolysis/gluconeogenesis. This allows glycerol to enter the gluconeogenic pathway effectively. **Analysis of Incorrect Options:** * **Glycogen (A):** The breakdown of glycogen is called **glycogenolysis**, not gluconeogenesis. Gluconeogenesis specifically refers to the synthesis of glucose from *non-carbohydrate* sources. * **Acetyl CoA (B):** In humans, Acetyl CoA cannot be converted into glucose because the **pyruvate dehydrogenase (PDH) reaction is irreversible**. Furthermore, for every two carbons of Acetyl CoA entering the TCA cycle, two are lost as $CO_2$, resulting in no net gain of carbon for glucose synthesis. * **Leucine (D):** Leucine and Lysine are the only two **purely ketogenic** amino acids. They are metabolized into Acetyl CoA or Acetoacetate and cannot serve as substrates for glucose. **High-Yield NEET-PG Pearls:** * **Major Substrates:** Lactate (Cori Cycle), Glucogenic amino acids (primarily Alanine), and Glycerol. * **Key Enzyme:** Glycerol kinase is present in the liver and kidneys but **absent in adipose tissue**; therefore, adipocytes cannot reuse glycerol. * **Odd-chain Fatty Acids:** While even-chain fatty acids are not gluconeogenic, **odd-chain fatty acids** yield Propionyl CoA, which can enter the TCA cycle as Succinyl CoA and contribute to gluconeogenesis.

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.

Question 1

Blood samples for glucose estimation are collected in fluoride bulbs/tubes as fluoride prevents glycolysis by inhibition of which enzyme?

Accepted Answer

Enolase

Answer

Aldolase

Answer

Glucokinase

Answer

Phosphofructokinase

Question 2

A 5-year-old boy presents with hepatomegaly, hypoglycemia, and ketosis. What is the most likely diagnosis?

Accepted Answer

Glycogen storage disorder

Answer

Mucopolysaccharidosis

Answer

Lipopolysaccharidosis

Answer

Diabetes mellitus

Question 3

In the well-fed state, gluconeogenesis in the liver is inhibited by which of the following?

Accepted Answer

ADP level

Answer

Protein breakdown in muscle

Answer

Alanine content in the liver

Answer

cGMP

Question 4

A newborn vomits after each feeding of milk-based formula and does not gain weight. Biochemical testing reveals a severe deficiency of galactose-1-phosphate uridyltransferase, consistent with homozygosity. If this condition goes untreated, what is the likely outcome for this patient?

Accepted Answer

Death in infancy

Answer

Benign disease except for cataract formation

Answer

Chronic emphysema appearing in early adulthood

Answer

Chronic renal failure appearing in adolescence

Question 5

Insulin facilitates glucose uptake in all of the following tissues except:

Accepted Answer

Heart

Answer

Liver

Answer

RBC

Answer

Kidney

Question 6

What is the rate-limiting enzyme in gluconeogenesis?

Accepted Answer

Fructose-1,6-bisphosphatase

Answer

Glucokinase

Answer

Glycerol kinase

Answer

Pyruvate dehydrogenase complex (PDH)

Question 7

Which substance can be utilized for gluconeogenesis?

Accepted Answer

Glycerol

Answer

Glycogen

Answer

Acetyl CoA

Answer

Leucine

Question 8

Na+ dependent glucose transport is/are inhibited by:

Accepted Answer

Phlorizin

Answer

Ouabain

Answer

Na+ azide

Answer

Phloretin

Question 9

Which of the following is affected by gluconeogenesis?

Accepted Answer

Growth hormone

Answer

Lactate

Answer

Glycerol

Answer

Alanine

Question 10

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

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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