Carbohydrate Metabolism Practice Questions

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: Sodium fluoride is known to inhibit which enzyme in glycolysis?

Enolase. **Explanation:** **Correct Answer: D. Enolase** Sodium fluoride (NaF) is a potent inhibitor of **Enolase**, the enzyme responsible for the dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP) in glycolysis. The inhibition occurs because fluoride ions, in the presence of inorganic phosphate, form a complex with magnesium ions (**Fluorophosphate-Magnesium complex**). Since Enolase requires $Mg^{2+}$ as a cofactor, this complex displaces the magnesium, effectively inactivating the enzyme and halting the glycolytic pathway. **Analysis of Incorrect Options:** * **A. Hexokinase:** This is the first regulatory enzyme of glycolysis. It is inhibited by its product, Glucose-6-Phosphate, but not by fluoride. * **B. Pyruvate Kinase:** This catalyzes the final step of glycolysis. It is inhibited by ATP and Alanine and activated by Fructose-1,6-bisphosphate (feed-forward activation). * **C. Aconitase:** This is an enzyme of the **TCA cycle** (not glycolysis). It is inhibited by **Fluoroacetate** (rat poison), which is converted to fluorocitrate, not by sodium fluoride. **Clinical Pearls for NEET-PG:** * **Blood Glucose Estimation:** NaF is used in "Grey-top" vacutainers for blood sugar estimation. It prevents "in vitro" glycolysis by RBCs, ensuring the glucose level measured reflects the patient's actual blood sugar at the time of collection. * **Anticoagulant Pairing:** NaF is usually combined with **Potassium Oxalate**. While NaF inhibits glycolysis, Potassium Oxalate acts as the anticoagulant by chelating calcium. * **Fluoride in Dentistry:** Fluoride also inhibits the metabolism of oral bacteria (like *S. mutans*) via the same enolase-inhibition mechanism, helping prevent dental caries.

Q: What is the end product of anaerobic fermentation?

Ethanol. **Explanation:** The core concept here is the fate of pyruvate under anaerobic conditions. In humans, anaerobic glycolysis leads to lactate; however, the term **"fermentation"** specifically refers to the process occurring in microorganisms like yeast. **1. Why Ethanol is Correct:** In yeast and some bacteria, anaerobic fermentation involves the conversion of pyruvate into ethanol. This occurs in two steps: * **Pyruvate Decarboxylase:** Converts pyruvate into acetaldehyde, releasing $CO_2$. (Requires TPP as a cofactor). * **Alcohol Dehydrogenase:** Reduces acetaldehyde to **ethanol**. This step is crucial because it regenerates $NAD^+$ from $NADH$, allowing glycolysis to continue in the absence of oxygen. **2. Analysis of Incorrect Options:** * **Lactate:** This is the end product of anaerobic glycolysis in **mammalian cells** (e.g., exercising muscle, RBCs). While similar in purpose (regenerating $NAD^+$), it is not the product of "fermentation" in the context of this question's specific terminology. * **Pyruvate:** This is the end product of **aerobic** glycolysis. In anaerobic conditions, pyruvate must be further reduced to regenerate $NAD^+$. * **Formic Acid:** This is a byproduct of specific bacterial pathways (like mixed acid fermentation) but is not the standard end product of the classic fermentation pathway taught in medical biochemistry. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Cofactor Alert:** Pyruvate decarboxylase (used in fermentation) requires **Thiamine Pyrophosphate (TPP/Vitamin B1)**. * **RBC Metabolism:** Mature RBCs lack mitochondria; therefore, they *always* produce lactate as an end product, even in the presence of oxygen. * **The "Why":** The primary goal of converting pyruvate to lactate or ethanol is not energy production, but the **regeneration of $NAD^+$** to keep the Glyceraldehyde-3-phosphate dehydrogenase reaction of glycolysis running.

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).

Question 1

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 2

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 3

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 4

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

Accepted Answer

Heart

Answer

Liver

Answer

RBC

Answer

Kidney

Question 5

Which of the following enzymes is involved in the Calvin cycle?

Accepted Answer

Sedoheptulose-1,7-bisphosphatase

Answer

Glucose-6-phosphate dehydrogenase

Answer

Glycerol kinase

Answer

Phosphoribulose kinase

Question 6

Sodium fluoride is known to inhibit which enzyme in glycolysis?

Accepted Answer

Enolase

Answer

Hexokinase

Answer

Pyruvate Kinase

Answer

Aconitase

Question 7

What is the end product of anaerobic fermentation?

Accepted Answer

Ethanol

Answer

Formic acid

Answer

Pyruvate

Answer

Lactate

Question 8

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 9

Which substance can be utilized for gluconeogenesis?

Accepted Answer

Glycerol

Answer

Glycogen

Answer

Acetyl CoA

Answer

Leucine

Question 10

Na+ dependent glucose transport is/are inhibited by:

Accepted Answer

Phlorizin

Answer

Ouabain

Answer

Na+ azide

Answer

Phloretin

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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