Carbohydrate Metabolism Practice Questions

Q: For glucose estimation in blood, what is the appropriate mode of transport from a primary health center to a laboratory?

Sodium fluoride. **Explanation:** The estimation of blood glucose requires the prevention of **in vitro glycolysis**. Even after blood is drawn, RBCs and WBCs continue to consume glucose at a rate of approximately 5–7% per hour. **Why Sodium Fluoride (NaF) is the correct choice:** Sodium fluoride acts as a **glycolytic inhibitor**. It works by inhibiting the enzyme **Enolase** in the glycolytic pathway (specifically by forming a complex with magnesium and phosphate, depriving the enzyme of its cofactor). This "locks" the glucose level at the time of collection, making it the ideal preservative for transport. It is typically used in a **Grey-top vacutainer**, often combined with Potassium Oxalate (an anticoagulant). **Why the other options are incorrect:** * **EDTA (Ethylenediaminetetraacetic acid):** Primarily used for Hematology (CBC) as it chelates calcium. It does not inhibit glycolysis; thus, glucose levels will falsely decrease during transport. * **Citrate:** Used for coagulation studies (PT/APTT) and ESR. Like EDTA, it has no effect on the glycolytic enzymes. * **0.9% Saline:** This is an isotonic crystalloid used for fluid resuscitation or as a diluent. It has no preservative properties for glucose. **High-Yield NEET-PG Pearls:** 1. **Enzyme Inhibition:** NaF inhibits **Enolase**. 2. **The "1-hour" Rule:** If blood is not collected in NaF, plasma must be separated from cells within 1 hour to prevent significant glucose drop. 3. **Grey Top Tube:** Contains NaF (antiglycolytic) and Potassium Oxalate (anticoagulant). 4. **Clinical Caveat:** NaF inhibition of enolase is delayed for the first 1–2 hours; therefore, some initial glycolysis still occurs.

Q: All of the following are true about Galactosemia EXCEPT?

Patients are symptomatic immediately after birth. **Explanation:** **Why Option B is the Correct Answer (The "EXCEPT"):** Classic Galactosemia (deficiency of **Galactose-1-phosphate uridyltransferase/GALT**) does not present *immediately* at birth because the fetus is protected by maternal metabolism in utero. Symptoms only manifest **after the initiation of milk feeding** (breast milk or cow’s milk formula), which contains lactose. Lactose is broken down into glucose and galactose; once galactose enters the system and cannot be metabolized, toxic metabolites begin to accumulate. Symptoms typically appear within the first few days to a week of life. **Analysis of Other Options:** * **Option A:** True. The deficiency leads to the accumulation of **Galactose-1-phosphate** (toxic to liver, kidneys, and brain) and **Galactitol** (produced via aldose reductase). Galactitol is osmotically active and is the primary cause of **cataracts**. * **Option C:** True. Accumulation of metabolites in the liver leads to hepatomegaly, jaundice (conjugated or unconjugated), and eventually cirrhosis if untreated. * **Option D:** True. There is a well-documented, high-yield association between Galactosemia and **Neonatal E. coli sepsis**. It is believed that inhibited leucocyte bactericidal activity or increased galactose levels enhance bacterial virulence. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficiency:** Most common is GALT (Classic Galactosemia). * **Screening:** Reducing substances in urine (Benedict's test positive) but a negative glucose oxidase test (Dipstick). * **Clinical Triad:** Oil-drop cataracts, Liver failure, and Intellectual disability. * **Management:** Immediate withdrawal of milk; switch to **Soy-based formula** or lactose-free formula. * **Note:** Even with treatment, many patients develop long-term complications like ovarian failure and speech deficits.

Q: Which form of carbohydrate is present in proteoglycans?

Polysaccharide. **Explanation:** **1. Why Polysaccharide is Correct:** Proteoglycans are complex macromolecules consisting of a core protein covalently attached to one or more **glycosaminoglycan (GAG)** chains. GAGs are long, unbranched **polysaccharides** composed of repeating disaccharide units (usually an amino sugar and a uronic acid). Because these GAG chains consist of hundreds of sugar units, they are classified as polysaccharides. Their high negative charge (due to sulfate and carboxyl groups) allows them to attract water, providing the "cushioning" effect essential for the extracellular matrix and cartilage. **2. Why Other Options are Incorrect:** * **Monosaccharide (A):** These are single sugar units (e.g., glucose). While they are the building blocks of GAGs, the functional carbohydrate unit in a proteoglycan is the long chain, not the individual monomer. * **Disaccharide (B):** GAGs are made of *repeating* disaccharide units, but the final structure is a long-chain polymer. A single disaccharide does not constitute a proteoglycan. * **Oligosaccharide (C):** These are short chains (typically 3–10 units). Oligosaccharides are the carbohydrate component of **Glycoproteins**, not Proteoglycans. This is a common point of confusion in exams. **3. NEET-PG High-Yield Clinical Pearls:** * **Proteoglycan vs. Glycoprotein:** In Proteoglycans, the carbohydrate content is dominant (up to 95%), whereas, in Glycoproteins, the protein content is dominant. * **Hyaluronic Acid:** The only GAG that is **not sulfated** and not covalently attached to a protein core. * **Mucopolysaccharidoses (MPS):** These are lysosomal storage disorders (e.g., Hurler and Hunter syndromes) caused by the deficiency of enzymes required to degrade these GAG polysaccharides. * **Heparin:** A naturally occurring anticoagulant GAG found in mast cells.

Q: Why should individuals with hyperlipidemia and/or gout minimize their intake of sucrose and high fructose syrups?

Fructose is initially phosphorylated by liver fructokinase.. **Explanation:** The metabolic handling of fructose in the liver is the key to understanding its link to hyperlipidemia and gout. Unlike glucose, fructose metabolism bypasses the major rate-limiting step of glycolysis (phosphofructokinase-1). **1. Why Option A is Correct:** Fructose is rapidly phosphorylated by **fructokinase** (ketohexokinase) to Fructose-1-Phosphate (F1P). This reaction is extremely fast and unregulated. * **Hyperlipidemia:** The rapid influx of fructose leads to an overproduction of pyruvate and acetyl-CoA, which serves as a substrate for *de novo* lipogenesis, increasing VLDL and triglyceride levels. * **Gout:** The rapid phosphorylation by fructokinase causes **ATP depletion** and an accumulation of inorganic phosphate. This triggers the purine degradation pathway, leading to increased production of **uric acid**, which exacerbates gout. **2. Why Other Options are Incorrect:** * **Option B:** Fructose-1-Phosphate is cleaved by **Aldolase B** (not enolase) into dihydroxyacetone phosphate (DHAP) and glyceraldehyde. * **Option C:** Fructose is not converted to UDP-fructose; UDP-sugars (like UDP-glucose/galactose) are primarily involved in glycogen synthesis and galactose metabolism. * **Option D:** Fructose is converted into glycolytic intermediates (DHAP/Glyceraldehyde-3-P), not galactose. **Clinical Pearls for NEET-PG:** * **Essential Fructosuria:** Deficiency of Fructokinase (asymptomatic, fructose in urine). * **Hereditary Fructose Intolerance (HFI):** Deficiency of **Aldolase B**. It causes severe hypoglycemia and liver damage because F1P traps intracellular phosphate. * **High-Yield Fact:** Fructose is the only sugar that can bypass the PFK-1 "bottleneck," making it highly lipogenic.

Q: As part of a research experiment, a person undergoes two fine needle muscle biopsies to obtain small amounts of tissue for biochemical analysis. The first is taken at the beginning of the experiment while the subject is at rest. The second is taken at the end of 30 minutes of aerobic exercise on a stationary bicycle. The activity of the muscle pyruvate dehydrogenase complexes is found to be much higher in the second measurement. Which of the following biochemical changes would be most likely to produce this effect?

Increased pyruvate concentration. **Explanation:** The **Pyruvate Dehydrogenase (PDH) complex** is the key regulatory enzyme that converts pyruvate into acetyl CoA, linking glycolysis to the TCA cycle. Its activity is tightly regulated by a phosphorylation-dephosphorylation cycle. **Why Option D is Correct:** During aerobic exercise, the rate of glycolysis increases to meet energy demands, leading to an **increase in pyruvate concentration**. Pyruvate acts as a potent **inhibitor of PDH Kinase**. Since PDH Kinase normally phosphorylates (inactivates) the PDH complex, inhibiting the kinase keeps the PDH complex in its **active (dephosphorylated) state**. Thus, increased pyruvate directly promotes PDH activity to ensure efficient aerobic metabolism. **Analysis of Incorrect Options:** * **A. Decreased ADP:** Exercise *increases* ADP levels. High ADP inhibits PDH Kinase, thereby activating PDH. Decreased ADP would have the opposite effect. * **B. Decreased intracellular Ca²⁺:** Exercise *increases* cytosolic Ca²⁺ (released during muscle contraction). Ca²⁺ is a powerful **activator of PDH Phosphatase**, which dephosphorylates and activates the PDH complex. * **C. Increased acetyl CoA:** Acetyl CoA is a product of the PDH reaction. High levels exert **feedback inhibition** by activating PDH Kinase, which shuts down the enzyme complex. **High-Yield NEET-PG Pearls:** * **PDH Regulation:** Active = Dephosphorylated (promoted by PDH Phosphatase); Inactive = Phosphorylated (promoted by PDH Kinase). * **Activators of PDH:** Pyruvate, NAD+, ADP, and **Calcium** (the most important link between muscle contraction and energy production). * **Inhibitors of PDH:** Acetyl CoA, NADH, and ATP (indicators of high energy status). * **Clinical Correlation:** PDH deficiency is a common cause of congenital lactic acidosis, as pyruvate is shunted to lactate when the PDH pathway is blocked.

Q: What is the purpose of Lactate dehydrogenase in anaerobic glycolysis?

Replenishment of NAD+. **Explanation:** In anaerobic glycolysis, the primary objective is to maintain a continuous flow of glucose through the glycolytic pathway to generate ATP in the absence of oxygen. **1. Why Option C is Correct:** The enzyme **Glyceraldehyde-3-phosphate dehydrogenase (G3PDH)** requires **NAD+** as a co-factor to convert Glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate. In aerobic conditions, NADH is oxidized back to NAD+ via the electron transport chain (ETC). However, in anaerobic conditions (like exercising muscle or in RBCs lacking mitochondria), the ETC cannot function. **Lactate Dehydrogenase (LDH)** solves this by reducing Pyruvate to Lactate, simultaneously oxidizing NADH back to **NAD+**. This replenishment of NAD+ ensures that glycolysis can continue. **2. Why other options are incorrect:** * **Option A:** While lactate is produced, it is a metabolic "end-product" or byproduct. The *purpose* of the reaction is not to make lactate, but to recycle the co-enzyme. * **Option B:** LDH does not directly produce ATP. ATP is produced by Phosphoglycerate kinase and Pyruvate kinase via substrate-level phosphorylation. * **Option D:** LDH *consumes* NADH; it does not replenish it. NADH is generated by G3PDH. **Clinical Pearls & High-Yield Facts:** * **RBCs:** Since RBCs lack mitochondria, they rely exclusively on LDH to regenerate NAD+ for energy. * **LDH Isoenzymes:** LDH is a tetramer. **LDH-1 (H4)** is predominant in the heart, while **LDH-5 (M4)** is predominant in the liver and skeletal muscle. * **Myocardial Infarction:** Historically, an elevation in LDH-1 (causing an "LDH flip" where LDH-1 > LDH-2) was used as a diagnostic marker for MI. * **Warburg Effect:** Cancer cells often utilize anaerobic glycolysis (producing lactate) even in the presence of oxygen to support rapid growth.

Q: A child presents with massive hepatomegaly and hypoglycemia. There is no improvement in blood glucose on the administration of glucagon. What is the probable diagnosis?

Von Gierke disease. ### Explanation **Correct Answer: A. Von Gierke disease (GSD Type I)** The clinical presentation of **massive hepatomegaly** and **fasting hypoglycemia** that is **unresponsive to glucagon** is classic for Von Gierke disease. **Underlying Concept:** Von Gierke disease is caused by a deficiency of **Glucose-6-Phosphatase**. This enzyme is the final common step for both **glycogenolysis** (breakdown of glycogen) and **gluconeogenesis** (production of glucose from non-carbohydrate sources). Because this enzyme is missing, the liver cannot release free glucose into the blood. When glucagon is administered, it attempts to stimulate these pathways, but since the "exit gate" (Glucose-6-Phosphatase) is blocked, blood glucose levels fail to rise. **Why incorrect options are wrong:** * **B. McArdle disease (GSD Type V):** This is a muscle phosphorylase deficiency. It presents with muscle cramps and myoglobinuria after exercise; it does **not** cause hypoglycemia or hepatomegaly. * **C & D. Cori/Forbes disease (GSD Type III):** This is a deficiency of the **Debranching enzyme**. While it causes hepatomegaly and hypoglycemia, the hypoglycemia is usually milder because **gluconeogenesis remains intact**. Therefore, patients typically show a **partial glycemic response to glucagon** (especially in the post-prandial state). **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Hallmarks:** Look for the "4 Hypers": **Hyper**uricemia (leading to gout), **Hyper**lipidemia, **Hyper**lactatemia (lactic acidosis), and **Hyper**ketonemia. * **Physical Exam:** "Doll-like facies" (fatty cheeks) due to adipose deposition. * **Diagnostic Clue:** If the question mentions "lactic acidosis after glucagon administration," it strongly points to Type I, as the liver diverts glucose-6-phosphate into the glycolytic pathway instead of releasing glucose.

Q: GLP-1 is broken down by which enzyme?

Dipeptidyl peptidase. **Explanation:** **Correct Answer: C. Dipeptidyl peptidase (DPP-4)** GLP-1 (Glucagon-like peptide-1) is an **incretin hormone** secreted by the L-cells of the intestine in response to food intake. It stimulates insulin secretion, inhibits glucagon release, and slows gastric emptying. However, endogenous GLP-1 has a very short half-life (1–2 minutes) because it is rapidly degraded by the enzyme **Dipeptidyl peptidase-4 (DPP-4)**. DPP-4 cleaves the two N-terminal amino acids from the GLP-1 peptide, rendering it inactive. This physiological process is the basis for **DPP-4 inhibitors** (e.g., Sitagliptin, Vildagliptin) used in Type 2 Diabetes to prolong GLP-1 action. **Analysis of Incorrect Options:** * **A. α-glucosidase:** This enzyme is located in the intestinal brush border and breaks down complex carbohydrates into glucose. Inhibitors like Acarbose act here, not on GLP-1. * **B. Rasburicase:** This is a recombinant urate oxidase enzyme used to treat hyperuricemia (Tumor Lysis Syndrome) by converting uric acid to allantoin. * **D. Glucose 1 phosphatase:** This enzyme is involved in glycogen metabolism (converting Glucose-1-P to Glucose), not in the degradation of peptide hormones. **High-Yield Clinical Pearls for NEET-PG:** * **Incretin Effect:** Oral glucose causes a much higher insulin response than intravenous glucose due to GLP-1 and GIP release. * **GLP-1 Agonists (Exenatide, Liraglutide):** These are "DPP-4 resistant" synthetic analogs used for weight loss and diabetes. * **DPP-4 Inhibitors (Gliptins):** These are weight-neutral drugs that increase endogenous GLP-1 levels. * **Key GLP-1 Action:** It acts via G-protein coupled receptors (GPCR) to increase cAMP in pancreatic beta cells.

Q: In muscle, phosphorylase b is inactivated by:

ATP. **Explanation:** Glycogen phosphorylase is the rate-limiting enzyme of glycogenolysis. In muscle, it exists in two forms: **Phosphorylase *a*** (active, phosphorylated) and **Phosphorylase *b*** (less active, dephosphorylated). **Why ATP is the correct answer:** Muscle glycogenolysis is primarily regulated by the energy status of the cell. Phosphorylase *b* is subject to **allosteric regulation**. When the cell has high energy levels, **ATP** and **Glucose-6-Phosphate** act as allosteric inhibitors, binding to the enzyme and stabilizing its inactive state. This prevents unnecessary breakdown of glycogen when energy is abundant. **Analysis of Incorrect Options:** * **A. cAMP:** This acts as a second messenger that activates Protein Kinase A (PKA), which eventually leads to the *activation* (phosphorylation) of phosphorylase *b* into phosphorylase *a*. * **B. Ca ions:** During muscle contraction, Ca²⁺ levels rise. Calcium binds to the calmodulin subunit of Phosphorylase Kinase, *activating* it, which in turn activates phosphorylase. * **C. Glucose:** While glucose is a potent allosteric inhibitor of glycogen phosphorylase in the **liver**, it is not a significant regulator in the **muscle**. Muscle cells lack Glucose-6-Phosphatase and do not regulate blood glucose levels. **High-Yield Facts for NEET-PG:** 1. **AMP** is the potent allosteric **activator** of muscle phosphorylase *b* (signaling low energy). 2. **Covalent modification:** Phosphorylation (via Phosphorylase Kinase) converts the *b* form to the *a* form. 3. **McArdle Disease (GSD Type V):** Caused by a deficiency of muscle glycogen phosphorylase, leading to exercise intolerance and myoglobinuria.

Question 1

For glucose estimation in blood, what is the appropriate mode of transport from a primary health center to a laboratory?

Accepted Answer

Sodium fluoride

Answer

EDTA

Answer

Citrate

Answer

0.9% saline

Question 2

All of the following are true about Galactosemia EXCEPT?

Accepted Answer

Patients are symptomatic immediately after birth

Answer

Galactose 1 phosphate and galactitol have toxic effects on various organs

Answer

Jaundice and liver dysfunction commonly seen

Answer

E.coli sepsis is an important cause of mortality

Question 3

Which form of carbohydrate is present in proteoglycans?

Accepted Answer

Polysaccharide

Answer

Monosaccharide

Answer

Disaccharide

Answer

Oligosaccharide

Question 4

Why should individuals with hyperlipidemia and/or gout minimize their intake of sucrose and high fructose syrups?

Accepted Answer

Fructose is initially phosphorylated by liver fructokinase.

Answer

After initial modification, fructose is cleaved by a specific enolase.

Answer

Fructose is converted to UDP-fructose.

Answer

Fructose is ultimately converted to galactose.

Question 5

As part of a research experiment, a person undergoes two fine needle muscle biopsies to obtain small amounts of tissue for biochemical analysis. The first is taken at the beginning of the experiment while the subject is at rest. The second is taken at the end of 30 minutes of aerobic exercise on a stationary bicycle. The activity of the muscle pyruvate dehydrogenase complexes is found to be much higher in the second measurement. Which of the following biochemical changes would be most likely to produce this effect?

Accepted Answer

Increased pyruvate concentration

Answer

Decreased ADP

Answer

Decreased intracellular Ca2+

Answer

Increased acetyl CoA

Question 6

What is the purpose of Lactate dehydrogenase in anaerobic glycolysis?

Accepted Answer

Replenishment of NAD+

Answer

Production of Lactate

Answer

Production of ATP

Answer

Replenishment of NADH

Question 7

Synthesis of 1 molecule of glucose from 2 molecules of lactate requires how many ATP molecules?

Accepted Answer

6

Answer

2

Answer

4

Answer

8

Question 8

A child presents with massive hepatomegaly and hypoglycemia. There is no improvement in blood glucose on the administration of glucagon. What is the probable diagnosis?

Accepted Answer

Von Gierke disease

Answer

McArdle disease

Answer

Cori disease

Answer

Forbes disease

Question 9

GLP-1 is broken down by which enzyme?

Accepted Answer

Dipeptidyl peptidase

Answer

α-glucosidase

Answer

Rasburicase

Answer

Glucose 1 phosphatase

Question 10

In muscle, phosphorylase b is inactivated by:

Accepted Answer

ATP

Answer

cAMP

Answer

Ca ions

Answer

Glucose

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?