Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 21: A 5-year-old boy presents with hepatomegaly, hypoglycemia, and ketosis. What is the most likely diagnosis?

A. Mucopolysaccharidosis
B. Glycogen storage disorder (Correct Answer)
C. Lipopolysaccharidosis
D. Diabetes mellitus

Explanation: ### 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.

Question 22: In the well-fed state, gluconeogenesis in the liver is inhibited by which of the following?

A. Protein breakdown in muscle
B. Alanine content in the liver
C. ADP level (Correct Answer)
D. cGMP

Explanation: ### 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.

Question 23: 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?

A. Benign disease except for cataract formation
B. Chronic emphysema appearing in early adulthood
C. Chronic renal failure appearing in adolescence
D. Death in infancy (Correct Answer)

Explanation: ### Explanation **1. Why "Death in Infancy" is Correct:** The patient has **Classic Galactosemia**, an autosomal recessive disorder caused by a deficiency of **Galactose-1-phosphate uridyltransferase (GALT)**. When the infant consumes milk (containing lactose, which breaks down into glucose and galactose), **Galactose-1-phosphate (Gal-1-P)** and **galactitol** accumulate in tissues. Gal-1-P is a potent cellular toxin. Its accumulation leads to severe liver damage (jaundice, hepatomegaly, cirrhosis), renal tubular dysfunction (Fanconi syndrome), and brain damage. Furthermore, high levels of galactose inhibit the bactericidal activity of neutrophils, making these infants highly susceptible to **E. coli neonatal sepsis**. Without immediate exclusion of dietary galactose (lactose), the combination of liver failure and sepsis typically leads to death in early infancy. **2. Why Other Options are Incorrect:** * **Option A:** This describes **Galactokinase (GALK) deficiency**. In GALK deficiency, Gal-1-P does not accumulate; instead, excess galactose is converted to galactitol, causing cataracts. It is a relatively "benign" condition compared to the classic form. * **Option B:** Chronic emphysema in early adulthood is the hallmark of **Alpha-1 antitrypsin deficiency**, not a carbohydrate metabolism disorder. * **Option C:** While GALT deficiency causes acute renal tubular damage, it does not typically present as isolated chronic renal failure in adolescence; the systemic crisis occurs much earlier. **3. NEET-PG High-Yield Pearls:** * **Enzyme Defect:** GALT (Classic Galactosemia) vs. GALK (Non-classic/Milder). * **Key Presentation:** Vomiting, jaundice, and hepatomegaly immediately after starting milk. * **Classic Association:** Increased risk of **E. coli sepsis** (frequently tested). * **Diagnosis:** Reducing substances in urine (clinitest positive) but glucose oxidase test (dipstick) negative. * **Treatment:** Lifelong soy-based formula (lactose-free diet).

Question 24: Insulin facilitates glucose uptake in all of the following tissues except:

A. Liver
B. Heart (Correct Answer)
C. RBC
D. Kidney

Explanation: **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.

Question 25: Which of the following enzymes is involved in the Calvin cycle?

A. Glucose-6-phosphate dehydrogenase
B. Sedoheptulose-1,7-bisphosphatase (Correct Answer)
C. Glycerol kinase
D. Phosphoribulose kinase

Explanation: **Explanation:** The **Calvin Cycle** (Reductive Pentose Phosphate Cycle) is the primary pathway for carbon fixation in plants. While it is not a human metabolic pathway, it is frequently tested in NEET-PG Biochemistry to contrast with the Pentose Phosphate Pathway (PPP). **1. Why Option B is Correct:** **Sedoheptulose-1,7-bisphosphatase** is a key regulatory enzyme in the regenerative phase of the Calvin cycle. It catalyzes the conversion of Sedoheptulose-1,7-bisphosphate to Sedoheptulose-7-phosphate. This enzyme is unique to the Calvin cycle and is not found in human metabolic pathways, making it a definitive marker for this cycle. **2. Analysis of Incorrect Options:** * **A. Glucose-6-phosphate dehydrogenase (G6PD):** This is the rate-limiting enzyme of the **Pentose Phosphate Pathway (PPP)** in humans. It converts G6P to 6-phosphogluconolactone, generating NADPH. * **C. Glycerol kinase:** This enzyme is involved in **lipid metabolism**, specifically the phosphorylation of glycerol to glycerol-3-phosphate in the liver. It is absent in adipose tissue. * **D. Phosphoribulose kinase:** While this enzyme *is* actually part of the Calvin cycle (converting Ribulose-5-phosphate to Ribulose-1,5-bisphosphate), in the context of standard medical examinations, **Sedoheptulose-1,7-bisphosphatase** is often highlighted as the specific phosphatase distinguishing the cycle's regenerative steps. *(Note: If this were a "Multiple Select" question, D would also be technically correct; however, in single-best-answer formats, B is the classic textbook answer for this specific enzyme class).* **High-Yield Clinical Pearls for NEET-PG:** * **Calvin Cycle vs. PPP:** The Calvin cycle is "reductive" (uses ATP/NADPH to build sugar), whereas the PPP is "oxidative" (breaks sugar to produce NADPH). * **G6PD Deficiency:** The most common enzyme deficiency worldwide, leading to hemolytic anemia due to the inability to maintain reduced glutathione in RBCs. * **Sedoheptulose:** A 7-carbon sugar. In humans, it appears as Sedoheptulose-7-phosphate in the non-oxidative phase of the PPP, catalyzed by **Transaldolase**.

Question 26: Sodium fluoride is known to inhibit which enzyme in glycolysis?

A. Hexokinase
B. Pyruvate Kinase
C. Aconitase
D. Enolase (Correct Answer)

Explanation: **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.

Question 27: What is the end product of anaerobic fermentation?

A. Formic acid
B. Pyruvate
C. Lactate
D. Ethanol (Correct Answer)

Explanation: **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.

Question 28: What is the rate-limiting enzyme in gluconeogenesis?

A. Fructose-1,6-bisphosphatase (Correct Answer)
B. Glucokinase
C. Glycerol kinase
D. Pyruvate dehydrogenase complex (PDH)

Explanation: **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).

Question 29: Which substance can be utilized for gluconeogenesis?

A. Glycogen
B. Acetyl CoA
C. Glycerol (Correct Answer)
D. Leucine

Explanation: **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.

Question 30: Na+ dependent glucose transport is/are inhibited by:

A. Ouabain
B. Na+ azide
C. Phlorizin (Correct Answer)
D. Phloretin

Explanation: **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).

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

Practice Questions

Blood Glucose Regulation

Practice Questions

Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

Practice Questions

Lactose Intolerance and Galactosemia

Practice Questions

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