Carbohydrate Metabolism Practice Questions

Q: Which glycolytic enzyme(s) are inhibited by fluoride?

Enolase. **Explanation:** **Enolase** is the correct answer because it is specifically inhibited by fluoride ions. In the glycolytic pathway, Enolase catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP). This enzyme requires magnesium ions ($Mg^{2+}$) as a cofactor. Fluoride binds with phosphate and magnesium to form a **magnesium-fluorophosphate complex**, which competitively displaces $Mg^{2+}$ from the enzyme’s active site, thereby halting glycolysis. **Analysis of Incorrect Options:** * **Hexokinase (A):** This is the first regulatory step of glycolysis (Glucose to Glucose-6-P). It is inhibited by its product, Glucose-6-phosphate, but not by fluoride. * **Aldolase (B):** This enzyme cleaves Fructose-1,6-bisphosphate into DHAP and Glyceraldehyde-3-phosphate. It is not a target for fluoride inhibition. * **Pyruvate Kinase (D):** This is the final irreversible step of glycolysis. It is regulated by allosteric effectors (like Fructose-1,6-bisphosphate) and covalent modification, but it is not inhibited by fluoride. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Blood Glucose Estimation:** In clinical practice, blood samples for glucose estimation are collected in **grey-topped vials** containing **Sodium Fluoride (NaF)**. * **Preservation:** NaF acts as a glycolytic inhibitor, preventing RBCs from consuming the glucose in the sample, ensuring an accurate measurement of the patient's blood sugar levels. * **Anticoagulant Pair:** NaF is usually paired with **Potassium Oxalate**, which acts as the anticoagulant by chelating calcium. * **Note:** Fluoride inhibition is reversible if more magnesium is added, but in laboratory settings, it effectively "fixes" the glucose concentration for up to 48–72 hours.

Q: The pyruvate dehydrogenase multienzyme complex is responsible for all the following reactions EXCEPT:

Production of a molecule of ATP. **Explanation:** The **Pyruvate Dehydrogenase (PDH) Complex** is a mitochondrial multienzyme system that serves as the bridge between glycolysis and the TCA cycle. It converts pyruvate into Acetyl-CoA through a process known as **oxidative decarboxylation**. **Why Option B is correct:** The PDH complex does **not** produce ATP directly. Instead, it reduces $NAD^+$ to **NADH**. This NADH later enters the Electron Transport Chain (ETC) to generate ATP via oxidative phosphorylation. Direct ATP (or GTP) production in the carbohydrate metabolic pathway occurs via substrate-level phosphorylation in glycolysis and the TCA cycle (Succinate thiokinase reaction), but not in the PDH reaction. **Analysis of Incorrect Options:** * **Option A:** PDH catalyzes the removal of a carboxyl group from pyruvate as $CO_2$. This is the "decarboxylation" step performed by the $E_1$ subunit (Pyruvate dehydrogenase). * **Option C:** The remaining two-carbon unit after decarboxylation is oxidized to form an **acetyl group** (hydroxyethyl group initially). * **Option D:** The acetyl group is eventually transferred to **Coenzyme A (CoA-SH)** to form Acetyl-CoA, which is the final product that enters the TCA cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Coenzymes (The "Tender Loving Care For Nancy" Mnemonic):** PDH requires five cofactors: **T**hiamine pyrophosphate ($B_1$), **L**ipoic acid, **C**oenzyme A ($B_5$), **F**AD ($B_2$), and **N**AD ($B_3$). * **Deficiency:** Thiamine deficiency (Beriberi/Wernicke-Korsakoff) leads to PDH failure, causing lactic acidosis and neurological symptoms. * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the -SH groups of **Lipoic acid**, leading to similar clinical features as thiamine deficiency.

Q: In hereditary fructose intolerance, which enzyme is defective?

Aldolase B. **Explanation:** **Hereditary Fructose Intolerance (HFI)** is an autosomal recessive disorder caused by a deficiency of **Aldolase B**. In the liver, fructose is first converted to Fructose-1-Phosphate (F1P) by fructokinase. Aldolase B is responsible for cleaving F1P into dihydroxyacetone phosphate (DHAP) and glyceraldehyde. When Aldolase B is defective, **Fructose-1-Phosphate accumulates** intracellularly. This "traps" inorganic phosphate, leading to ATP depletion, inhibition of glycogenolysis, and gluconeogenesis, which clinically manifests as severe postprandial hypoglycemia, vomiting, and jaundice following the introduction of fruit or formula. **Analysis of Incorrect Options:** * **A. Phosphofructokinase:** This is the rate-limiting enzyme of glycolysis. Deficiency (Tarui disease) leads to glycogen storage disease type VII, characterized by exercise intolerance and muscle cramps. * **B. Fructose 2,6-bisphosphatase:** This is a regulatory bifunctional enzyme that controls the levels of Fructose 2,6-bisphosphate, thereby regulating glycolysis and gluconeogenesis; it is not primary to HFI. * **C. Fructokinase:** Deficiency causes **Essential Fructosuria**. This is a benign condition because fructose is not "trapped" in cells; it simply spills into the urine (fructosemia and fructosuria) without causing metabolic crisis. **High-Yield Clinical Pearls for NEET-PG:** * **The "Trapping" Mechanism:** ATP depletion in HFI leads to liver failure and proximal renal tubular acidosis (Fanconi syndrome). * **Dietary Management:** Treatment requires strict avoidance of **fructose, sucrose (glucose + fructose), and sorbitol**. * **Clinical Presentation:** Symptoms typically appear when an infant is weaned from breast milk and introduced to fruits or juices. * **Reducing Sugars:** In HFI, urine dipstick for glucose is negative, but a test for reducing sugars (Benedict’s test) is positive.

Q: Which of the following does not depend on GLUT4 for glucose uptake?

Brain. **Explanation:** The core concept tested here is the distinction between **insulin-dependent** and **insulin-independent** glucose transport. **Why the Brain is the Correct Answer:** The brain is a vital organ that requires a continuous supply of glucose regardless of insulin levels. Glucose uptake in the brain (specifically across the blood-brain barrier and into neurons) is mediated primarily by **GLUT1** and **GLUT3**. These transporters are insulin-independent and have a low $K_m$ (high affinity), ensuring that the brain can sequester glucose even during fasting or hypoglycemic states. Therefore, the brain does not depend on GLUT4. **Why the Other Options are Incorrect:** * **Skeletal Muscle, Cardiac Muscle, and Adipose Tissue:** These tissues are the primary sites for **GLUT4** expression. GLUT4 is the only insulin-responsive glucose transporter. In the resting state, GLUT4 is sequestered in intracellular vesicles. Upon insulin signaling, these vesicles translocate to the plasma membrane to allow glucose entry. Consequently, these tissues are highly dependent on GLUT4 for post-prandial glucose disposal. **High-Yield NEET-PG Pearls:** * **GLUT4 Location:** Remember the mnemonic **"Muscle and Fat need 4"** (Skeletal, Cardiac, and Adipose). * **Exercise Exception:** In skeletal muscle, exercise/muscle contraction can trigger GLUT4 translocation to the membrane via an **AMPK-mediated pathway**, independent of insulin. This is why exercise helps manage blood sugar in Type 2 Diabetes. * **GLUT2:** Found in the Liver, Pancreas ($\beta$-cells), and Kidney. It has a high $K_m$ (low affinity) and acts as a glucose sensor. * **SGLT1/2:** These are active transporters (sodium-glucose co-transporters) found in the small intestine and renal tubules, unlike the GLUT family which facilitates passive diffusion.

Q: Which mucopolysaccharide/proteoglycans are present in the eye?

Keratan sulfate and chondroitin sulfate. **Explanation:** The correct answer is **D. Keratan sulfate and chondroitin sulfate**. Mucopolysaccharides (Glycosaminoglycans or GAGs) are essential components of the extracellular matrix. In the eye, specifically within the **cornea**, the transparency and structural integrity are maintained by a precise arrangement of collagen fibrils and specific GAGs. 1. **Keratan Sulfate (KS I):** This is the most abundant GAG in the cornea. It plays a critical role in maintaining corneal transparency by regulating the spacing between collagen fibrils. 2. **Chondroitin Sulfate:** Along with its isomer, Dermatan sulfate, it is found in the corneal stroma and the vitreous humor. In the cornea, Chondroitin-4-sulfate and Chondroitin-6-sulfate contribute to the hydration and osmotic pressure of the tissue. **Analysis of Incorrect Options:** * **Options A, B, & C:** While **Dermatan sulfate** is present in the cornea (often as a hybrid with chondroitin), it is primarily associated with the skin, blood vessels, and heart valves. **Heparan sulfate** is a component of cell surfaces and basement membranes (like the glomerular basement membrane) but is not a primary structural GAG of the ocular media. **NEET-PG High-Yield Pearls:** * **Keratan Sulfate I** is corneal; **Keratan Sulfate II** is found in cartilage. * **Hyaluronic acid** is the primary GAG found in the **vitreous humor** (it is the only GAG that is non-sulfated and not covalently bound to a protein). * **Macular Corneal Dystrophy** is a clinical condition caused by a defect in the sulfation of Keratan Sulfate, leading to corneal opacity. * **Heparin** has the highest negative charge density of any biological molecule.

Q: MODY 1 is caused by mutations in which of the following genes?

HNF-4 alpha. Maturity-Onset Diabetes of the Young (MODY) is a group of monogenic disorders characterized by non-insulin-dependent diabetes occurring typically before age 25, inherited in an autosomal dominant pattern [1]. **Correct Option: A (HNF-4 alpha)** MODY 1 is caused by a mutation in the **Hepatocyte Nuclear Factor-4 alpha (HNF-4α)** gene. This transcription factor is essential for the expression of genes involved in glucose transport and metabolism in pancreatic beta cells. Mutations lead to progressive beta-cell dysfunction and impaired insulin secretion. **Incorrect Options:** * **B. HNF-1 alpha:** This mutation causes **MODY 3**, which is the most common form of MODY worldwide. It is characterized by a low renal threshold for glucose (glycosuria). * **C. HNF-1 beta:** This mutation causes **MODY 5**, which is uniquely associated with renal cysts and genitourinary abnormalities (Renal Cysts and Diabetes Syndrome). * **D. Glucokinase (GCK):** This mutation causes **MODY 2**. It results in a "resetting" of the glucose sensor, leading to mild, stable fasting hyperglycemia that often does not require pharmacological treatment [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Most Common MODY:** MODY 3 (HNF-1α). * **Most Common in Pregnancy:** MODY 2 (GCK). * **Treatment Sensitivity:** Patients with MODY 1 and MODY 3 are exquisitely sensitive to **Sulfonylureas**, which are the first-line treatment, often allowing patients to discontinue insulin. * **Key Diagnostic Clue:** Absence of beta-cell antibodies (GAD, IA-2) and low/normal insulin levels in a young, non-obese patient with a strong family history of diabetes.

Q: A few days after birth, a child becomes lethargic, refuses feeds, and vomits during breastfeeding. On examination, he is pot-bellied. Benedict's test is positive in urine. What is the likely reducing substance present in urine?

Galactose. ### Explanation The clinical presentation describes a classic case of **Classic Galactosemia**, typically caused by a deficiency of the enzyme **Galactose-1-phosphate uridyltransferase (GALT)**. **1. Why Galactose is the correct answer:** Symptoms of galactosemia appear shortly after birth once the infant begins breastfeeding (milk contains **lactose**, which is hydrolyzed into glucose and **galactose**). The accumulation of galactose and its metabolites leads to liver dysfunction (hepatomegaly/pot-bellied appearance), vomiting, and lethargy. Because galactose is a **reducing sugar**, it reacts positively with **Benedict’s reagent**. However, since the defect is in galactose metabolism and not glucose regulation, the urine dipstick (which uses glucose oxidase) would be negative, while the Benedict's test is positive. **2. Why the other options are incorrect:** * **Glucose:** While glucose is a reducing sugar, it is typically associated with Diabetes Mellitus. In this neonatal context, the specific trigger of breastfeeding points toward milk sugar (galactose). * **Fructose:** Fructosemia (Hereditary Fructose Intolerance) presents only after the introduction of weaning foods (fruit juices/sucrose), not during exclusive breastfeeding. * **Sucrose:** Sucrose is a **non-reducing sugar** and would yield a negative Benedict's test. **3. High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficiency:** Most common is GALT (Classic Galactosemia). * **Key Findings:** Hepatomegaly, **Oil-drop cataracts** (due to galactitol accumulation in the lens), and intellectual disability if untreated. * **Infection Risk:** These infants are at a significantly increased risk of **E. coli neonatal sepsis**. * **Diagnosis:** Positive Benedict's test (reducing sugar) + Negative Glucose oxidase test (dipstick). * **Management:** Immediate cessation of breast milk; switch to soy-based or lactose-free formula.

Q: Substrate level phosphorylation in glycolysis is seen in which enzyme?

Pyruvate kinase. **Explanation:** **Substrate-level phosphorylation (SLP)** 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, without the involvement of the electron transport chain or oxygen. **1. Why Pyruvate Kinase is correct:** In the final step of glycolysis, **Pyruvate Kinase** catalyzes the conversion of Phosphoenolpyruvate (PEP) to Pyruvate. PEP contains a high-energy phosphate bond; its hydrolysis releases enough energy to drive the phosphorylation of ADP to **ATP**. This is one of the two SLP steps in glycolysis (the other being Phosphoglycerate kinase). **2. Why the other options are incorrect:** * **Succinate thiokinase (Succinyl-CoA Synthetase):** While this enzyme *does* perform substrate-level phosphorylation (converting Succinyl-CoA to Succinate and generating GTP), it occurs in the **TCA Cycle**, not glycolysis. * **Enolase:** This enzyme catalyzes the dehydration of 2-phosphoglycerate to PEP. It creates a high-energy bond but does not synthesize ATP. * **Pyruvate dehydrogenase (PDH):** This is a multi-enzyme complex that converts pyruvate to Acetyl-CoA (oxidative decarboxylation). It produces NADH but does not perform SLP. **High-Yield Clinical Pearls for NEET-PG:** * **Total SLP in Glycolysis:** 4 ATP are generated per glucose molecule (via Phosphoglycerate kinase and Pyruvate kinase). The *net* gain is 2 ATP. * **Arsenite Poisoning:** Arsenite inhibits the PDH complex. However, **Arsenate** competes with inorganic phosphate in glycolysis, bypassing the SLP step of 1,3-BPG, resulting in zero net ATP production. * **Rapoport-Luebering Cycle:** In RBCs, 1,3-BPG can be converted to 2,3-BPG. This bypasses an SLP step, meaning the RBC sacrifices ATP production to facilitate oxygen delivery to tissues.

Question 1

A 10-month-old male child presents with vomiting, lethargy, and severe jaundice, which began when weaning was initiated with fruit juice. Laboratory investigations reveal prolonged clotting time, hypoalbuminemia, and elevated bilirubin and transaminase levels. What is the diagnosis?

Accepted Answer

Hereditary fructose intolerance

Answer

Von Gierke's disease

Answer

Benign fructosuria

Answer

Lactose intolerance

Question 2

Which glycolytic enzyme(s) are inhibited by fluoride?

Accepted Answer

Enolase

Answer

Hexokinase

Answer

Aldolase

Answer

Pyruvate Kinase

Question 3

The pyruvate dehydrogenase multienzyme complex is responsible for all the following reactions EXCEPT:

Accepted Answer

Production of a molecule of ATP

Answer

A decarboxylation reaction

Answer

Formation of an acetyl group from pyruvate

Answer

Combination of the acetyl group with a cofactor

Question 4

In hereditary fructose intolerance, which enzyme is defective?

Accepted Answer

Aldolase B

Answer

Phosphofructokinase

Answer

Fructose 2, 6-biphosphatase

Answer

Fructokinase

Question 5

Which of the following does not depend on GLUT4 for glucose uptake?

Accepted Answer

Brain

Answer

Skeletal muscles

Answer

Cardiac muscle

Answer

Adipose tissue

Question 6

Which mucopolysaccharide/proteoglycans are present in the eye?

Accepted Answer

Keratan sulfate and chondroitin sulfate

Answer

Keratan sulfate & dermatan sulfate

Answer

Dermatan sulfate & heparan sulfate

Answer

Dermatan sulfate & chondroitin sulfate

Question 7

MODY 1 is caused by mutations in which of the following genes?

Accepted Answer

HNF-4 alpha

Answer

HNF-1 alpha

Answer

HNF-1 beta

Answer

Glucokinase

Question 8

A few days after birth, a child becomes lethargic, refuses feeds, and vomits during breastfeeding. On examination, he is pot-bellied. Benedict's test is positive in urine. What is the likely reducing substance present in urine?

Accepted Answer

Galactose

Answer

Glucose

Answer

Fructose

Answer

Sucrose

Question 9

What is considered the most severe form of G-6-PD deficiency?

Accepted Answer

Type 3

Answer

Typed

Answer

Type 2

Answer

None of the above

Question 10

Substrate level phosphorylation in glycolysis is seen in which enzyme?

Accepted Answer

Pyruvate kinase

Answer

Succinate thiokinase

Answer

Enolase

Answer

Pyruvate dehydrogenase (PDH)

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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