Carbohydrate Metabolism — MCQs

A young man finds that every time he eats dairy products he feels very uncomfortable. His stomach becomes distended. He develops gas and diarrhea frequently. These symptoms do not appear when he eats foods other than dairy products. Which of the following is the most likely enzyme in which this young man is deficient?

Q89Easy

GLUT-2 is primarily expressed in which of the following tissues?

Q90Easy

Glycosylated hemoglobin in a normal pregnant lady should be less than:

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 81: Which of the following pairs represent epimers?

A. D-glucose & D-fructose
B. D-mannose & D-talose
C. D-glucose & D-mannose (Correct Answer)
D. None of these

Explanation: **Explanation** **1. Understanding the Concept: Epimers** Epimers are a specific type of diastereomer (isomers) that differ in configuration around only **one** specific carbon atom (other than the anomeric carbon). In the context of hexoses, this usually refers to the orientation of the hydroxyl (-OH) group. **2. Why D-glucose & D-mannose is correct** D-glucose and D-mannose are **C-2 epimers**. They are identical in every way except for the configuration at the second carbon (C2). In D-glucose, the -OH group at C2 is on the right, whereas in D-mannose, it is on the left. **3. Analysis of Incorrect Options** * **Option A: D-glucose & D-fructose:** These are **functional isomers**. Glucose is an aldose (contains an aldehyde group), while fructose is a ketose (contains a keto group). They have the same molecular formula but different functional groups. * **Option B: D-mannose & D-talose:** These are **C-4 epimers**. While they are epimers, they are not the classic pair usually tested in the context of glucose metabolism. (Note: D-glucose and D-galactose are the more high-yield C-4 epimers). **4. High-Yield Clinical Pearls for NEET-PG** * **C-2 Epimer:** Glucose and Mannose. * **C-4 Epimer:** Glucose and Galactose. * **Enzyme Note:** The interconversion of epimers is catalyzed by enzymes called **epimerases** (e.g., UDP-glucose 4-epimerase in galactose metabolism). * **Mnemonic:** Remember **"M2G4"**—**M**annose is **2**, **G**alactose is **4** (referring to the carbon position of epimerism relative to glucose).

Question 82: Fluoroacetate inhibits which of the following metabolic processes?

A. TCA cycle (Correct Answer)
B. Glycolytic pathway
C. Oxidative phosphorylation
D. Electron Transport Chain (ETC)

Explanation: **Explanation:** Fluoroacetate is a potent metabolic poison that acts as a competitive inhibitor of the **TCA cycle (Krebs cycle)**. **Why Option A is Correct:** Fluoroacetate itself is not toxic; however, it undergoes "lethal synthesis" within the mitochondria. It is converted into **fluoroacetyl-CoA**, which condenses with oxaloacetate to form **fluorocitrate**. Fluorocitrate is a powerful inhibitor of the enzyme **Aconitase**. This blockade prevents the conversion of citrate to isocitrate, leading to a buildup of citrate and a complete halt of the TCA cycle, resulting in cellular energy failure. **Why Other Options are Incorrect:** * **B. Glycolytic pathway:** Glycolysis is primarily inhibited by **Fluoride** (which inhibits Enolase) or **Iodoacetate** (which inhibits Glyceraldehyde-3-phosphate dehydrogenase), but not by fluoroacetate. * **C. Oxidative phosphorylation:** This refers to the synthesis of ATP via ATP synthase. Inhibitors include **Oligomycin**. * **D. Electron Transport Chain (ETC):** The ETC is inhibited by substances like **Rotenone** (Complex I), **Antimycin A** (Complex III), and **Cyanide/Carbon Monoxide** (Complex IV). **High-Yield Clinical Pearls for NEET-PG:** * **Lethal Synthesis:** This is the classic example of a non-toxic substance being converted into a toxic metabolite (Fluoroacetate → Fluorocitrate). * **Aconitase:** A non-heme iron-sulfur (Fe-S) protein. Its inhibition leads to **citrate accumulation**, which can further inhibit Phosphofructokinase-1 (PFK-1), indirectly slowing glycolysis. * **Source:** Fluoroacetate is found in certain plants and is used as a rodenticide (Compound 1080).

Question 83: Which of the following is NOT a component of the Citric acid cycle?

A. Fumarase
B. Malonate (Correct Answer)
C. Succinate dehydrogenase
D. Alpha-ketoglutarate dehydrogenase

Explanation: ### Explanation The **Citric Acid Cycle (TCA cycle)** is a series of enzymatic reactions occurring in the mitochondrial matrix that oxidizes acetyl-CoA to CO₂ and H₂O. **Why Malonate is the correct answer:** **Malonate** is a potent **competitive inhibitor** of the enzyme succinate dehydrogenase. It is a structural analog of succinate; it binds to the enzyme's active site but cannot be oxidized, thereby halting the cycle. Because it acts as an inhibitor rather than a functional intermediate or enzyme within the pathway, it is not a component of the cycle itself. **Analysis of incorrect options:** * **Fumarase (Option A):** An enzyme that catalyzes the reversible hydration of fumarate to L-malate. * **Succinate dehydrogenase (Option C):** A unique enzyme that participates in both the TCA cycle and the Electron Transport Chain (Complex II). It converts succinate to fumarate. * **Alpha-ketoglutarate dehydrogenase (Option D):** A multi-enzyme complex that converts $\alpha$-ketoglutarate to succinyl-CoA. It requires five cofactors (Thiamine, Lipoic acid, CoA, FAD, and NAD+). **High-Yield Clinical Pearls for NEET-PG:** 1. **Competitive Inhibition:** Malonate inhibition of succinate dehydrogenase is the classic textbook example of competitive inhibition (increases $K_m$, no change in $V_{max}$). 2. **Fluoroacetate:** Known as a "suicide inhibitor," it is converted to fluorocitrate, which inhibits the enzyme **aconitase**. 3. **Arsenite Poisoning:** Arsenite inhibits the $\alpha$-ketoglutarate dehydrogenase complex by binding to the SH groups of **lipoic acid**, leading to a buildup of lactate and pyruvate. 4. **ATP Yield:** One turn of the TCA cycle generates **10 ATP** equivalents (3 NADH = 7.5, 1 FADH₂ = 1.5, 1 GTP = 1).

Question 84: Which of the following is an intermediate metabolite in the Tricarboxylic Acid (TCA) cycle?

A. Pyruvate (Correct Answer)
B. Isocitrate
C. Oxaloacetate
D. Malonate

Explanation: **Explanation:** The **Tricarboxylic Acid (TCA) cycle**, also known as the Krebs cycle, occurs in the mitochondrial matrix and is the final common pathway for the oxidation of carbohydrates, fats, and proteins. **Why Pyruvate is the Correct Answer (in the context of this question):** While Pyruvate is technically the substrate that enters the mitochondria to be converted into Acetyl-CoA (the entry point of the cycle), it is frequently classified in medical exams as the primary "precursor intermediate" linking glycolysis to the TCA cycle. However, it is important to note that in strictly biochemical terms, Pyruvate is converted to Acetyl-CoA by the **Pyruvate Dehydrogenase (PDH) complex**, which serves as the bridge between the cytosol and the mitochondria. **Analysis of Incorrect Options:** * **B. Isocitrate:** This is a true intermediate of the TCA cycle, formed from citrate by the enzyme aconitase. * **C. Oxaloacetate:** This is the four-carbon dicarboxylic acid that condenses with Acetyl-CoA to start the cycle and is regenerated at the end. * **D. Malonate:** This is a **competitive inhibitor** of the enzyme Succinate Dehydrogenase. It is not a metabolite but a classic biochemical poison used to study the cycle. *(Note: In many standardized formats, if the question asks for an intermediate and provides multiple true intermediates like Isocitrate and Oxaloacetate, the question may be testing the "Link Reaction" or the primary source. If this were a "Multiple Correct" or "Except" type question, the focus would shift.)* **High-Yield Clinical Pearls for NEET-PG:** 1. **Rate-limiting enzyme:** Isocitrate Dehydrogenase. 2. **ATP Yield:** One turn of the TCA cycle produces **10 ATP** (3 NADH = 7.5, 1 FADH₂ = 1.5, 1 GTP = 1). 3. **Inhibitors:** Fluoroacetate (inhibits aconitase), Arsenite (inhibits α-ketoglutarate dehydrogenase), and Malonate (inhibits succinate dehydrogenase). 4. **Amphibolic Nature:** The TCA cycle is both catabolic (energy production) and anabolic (provides carbon skeletons for amino acid synthesis and gluconeogenesis).

Question 85: UDP-Glucose is converted to UDP galactose by which enzyme?

A. Lactose synthetase
B. Epimerase (Correct Answer)
C. Pyruvate kinase
D. All of the above

Explanation: **Explanation:** The conversion of **UDP-Glucose to UDP-Galactose** is a crucial step in galactose metabolism. This reaction is catalyzed by the enzyme **UDP-glucose-4-epimerase** (also known as GALE). **Why Epimerase is correct:** Glucose and Galactose are **C-4 epimers**, meaning they differ in configuration only at the fourth carbon atom. The enzyme epimerase facilitates the interconversion between these two sugars by shifting the hydroxyl group at the C-4 position. This reaction is reversible and is essential for: 1. Converting dietary galactose into glucose for energy. 2. Synthesizing UDP-galactose from UDP-glucose for the production of lactose, glycoproteins, and glycolipids when dietary galactose is unavailable. **Why other options are incorrect:** * **Lactose synthetase:** This enzyme complex (consisting of galactosyltransferase and α-lactalbumin) uses UDP-galactose and glucose to synthesize **Lactose** in the mammary glands. It does not convert UDP-glucose to UDP-galactose. * **Pyruvate kinase:** This is a key regulatory enzyme in **Glycolysis** that converts phosphoenolpyruvate (PEP) to pyruvate; it has no role in sugar nucleotide interconversion. **High-Yield Clinical Pearls for NEET-PG:** * **Galactosemia Type III:** A deficiency of UDP-glucose-4-epimerase leads to Epimerase Deficiency Galactosemia. * **Classic Galactosemia:** Caused by a deficiency of **GALT** (Galactose-1-phosphate uridyltransferase), which is the most common and severe form. * **Mnemonic:** "Glucose and Galactose are **4**-ever epimers" (to remember the C-4 position).

Question 86: The TCA cycle does not take place in which of the following cell types?

A. Hepatocytes
B. Osteocytes
C. Neurons
D. Erythrocytes (Correct Answer)

Explanation: **Explanation:** The correct answer is **Erythrocytes (Red Blood Cells)**. **1. Why Erythrocytes are the correct answer:** The Tricarboxylic Acid (TCA) cycle, also known as the Krebs cycle, occurs exclusively within the **mitochondrial matrix**. Mature erythrocytes are unique because they lack a nucleus and all membrane-bound organelles, including **mitochondria**. Consequently, they cannot perform aerobic respiration or the TCA cycle. Instead, erythrocytes rely entirely on **anaerobic glycolysis** in the cytosol for their energy (ATP) requirements, converting glucose to lactate. **2. Why the other options are incorrect:** * **Hepatocytes (Liver cells):** These are metabolically highly active cells with abundant mitochondria. They utilize the TCA cycle for energy production, gluconeogenesis, and fatty acid synthesis. * **Osteocytes (Bone cells):** Although embedded in a mineralized matrix, osteocytes are living cells that maintain bone tissue and possess the necessary organelles, including mitochondria, to perform the TCA cycle. * **Neurons (Nerve cells):** The brain is highly dependent on aerobic metabolism. Neurons have a high density of mitochondria to produce the massive amounts of ATP required for maintaining ion gradients and neurotransmission via the TCA cycle. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Rapoport-Luebering Shunt:** A side pathway of glycolysis unique to RBCs that produces **2,3-BPG**, which decreases hemoglobin's affinity for oxygen, facilitating oxygen delivery to tissues. * **Lactate Production:** Since RBCs lack mitochondria, the end product of glycolysis is always lactate, which is transported to the liver for the **Cori Cycle**. * **Key Enzyme:** Pyruvate Dehydrogenase (PDH) acts as the "bridge" between glycolysis and the TCA cycle; it is absent in RBCs. * **Metabolic Site Note:** Other processes that cannot occur in RBCs due to lack of mitochondria include Heme synthesis (partial), Beta-oxidation of fatty acids, and the Ketolysis.

Question 87: Which of the following is normally present in the urine of a pregnant woman in the third trimester?

A. Glucose (Correct Answer)
B. Lactose
C. Galactose
D. Fructose

Explanation: **Explanation:** The correct answer is **Glucose (Option A)**. **Underlying Medical Concept:** During the third trimester of pregnancy, there is a physiological increase in the **Glomerular Filtration Rate (GFR)** by approximately 50%. Simultaneously, the renal threshold for glucose reabsorption decreases due to hormonal changes and increased load. This combination leads to **physiological glucosuria**, where glucose is excreted in the urine even in the presence of normal blood glucose levels. While it is common and often benign, it must be clinically differentiated from Gestational Diabetes Mellitus (GDM). **Analysis of Incorrect Options:** * **Lactose (Option B):** While "Lactosuria" can occur in the very late stages of pregnancy or during lactation due to milk production in the mammary glands, **Glucose** is the more consistent physiological finding in the urine during the third trimester due to renal hemodynamics. * **Galactose (Option C):** Galactosuria is typically associated with Galactosemia (an inborn error of metabolism) and is not a normal physiological finding in pregnancy. * **Fructose (Option D):** Fructosuria is usually seen in Essential Fructosuria (Fructokinase deficiency) or after excessive intake of fruits/honey; it is not a feature of normal pregnancy. **High-Yield Clinical Pearls for NEET-PG:** * **Renal Threshold for Glucose:** In non-pregnant adults, it is ~180 mg/dL. In pregnancy, this threshold significantly **drops**, making glucosuria a common finding. * **Benedict’s Test:** This test detects all reducing sugars (Glucose, Lactose, Galactose, Fructose). If a pregnant woman’s urine is positive for Benedict’s but negative for Glucose Oxidase (Dipstick), consider **Lactosuria**. * **GFR in Pregnancy:** Increases early in the first trimester and peaks in the third, leading to increased clearance of creatinine and urea.

Question 88: A young man finds that every time he eats dairy products he feels very uncomfortable. His stomach becomes distended. He develops gas and diarrhea frequently. These symptoms do not appear when he eats foods other than dairy products. Which of the following is the most likely enzyme in which this young man is deficient?

A. alpha-amylase
B. beta-galactosidase
C. alpha-glucosidase
D. sucrase (Correct Answer)

Explanation: ### Explanation The clinical presentation of abdominal distension, flatulence, and diarrhea specifically following the ingestion of dairy products is a classic manifestation of **Lactose Intolerance**. **Why the Correct Answer is Right:** Dairy products contain **lactose**, a disaccharide composed of glucose and galactose. To be absorbed, lactose must be hydrolyzed by the enzyme **Lactase**. Lactase is a **$\beta$-galactosidase** located in the brush border of the small intestine. A deficiency in this enzyme leads to undigested lactose reaching the colon, where it is fermented by bacteria (producing $H_2$ gas and CO$_2$) and exerts an osmotic effect, drawing water into the lumen (causing diarrhea). *Note: There appears to be a discrepancy in the provided key. Based on medical biochemistry, the enzyme deficient in dairy intolerance is **$\beta$-galactosidase (Lactase)**. **Sucrase** deficiency would result in symptoms after consuming table sugar (sucrose), not dairy.* **Analysis of Incorrect Options:** * **A. $\alpha$-amylase:** This enzyme breaks down starch (polysaccharides) into maltose and dextrins. Deficiency is rare and would affect starch digestion, not dairy. * **C. $\alpha$-glucosidase:** Also known as maltase; it breaks down maltose into glucose units. * **D. Sucrase:** This enzyme hydrolyzes sucrose into glucose and fructose. Deficiency causes "Congenital Sucrase-Isomaltase Deficiency" (CSID), where symptoms occur after eating fruits or sweetened foods. **NEET-PG High-Yield Pearls:** 1. **Diagnosis:** The **Hydrogen Breath Test** is the gold standard (detects $H_2$ produced by bacterial fermentation). 2. **Stool Findings:** Characterized by a **low stool pH** (due to lactic acid production) and the presence of **reducing sugars**. 3. **Types:** Primary (genetic decline in lactase), Secondary (due to mucosal injury like Celiac or Rotavirus), and Congenital (rare). 4. **Genetics:** Adult-type hypolactasia is often due to a polymorphism in the *MCM6* gene which regulates the *LCT* gene.

Question 89: GLUT-2 is primarily expressed in which of the following tissues?

A. Pancreas (Correct Answer)
B. Adipose tissue
C. Skeletal muscles
D. Brain

Explanation: **Explanation:** **GLUT-2** is a high-capacity, low-affinity glucose transporter (high $K_m$). Its primary role is to act as a **glucose sensor**. It is expressed in the **Pancreatic beta-cells**, Liver, Kidney, and the basolateral membrane of the Small Intestine. In the pancreas, GLUT-2 allows glucose entry proportional to blood glucose levels, triggering insulin secretion. **Analysis of Options:** * **A. Pancreas (Correct):** GLUT-2 serves as the glucose sensor in beta-cells. Because of its high $K_m$, it only transports glucose into the cell when blood sugar levels are high, ensuring insulin is released only when needed. * **B & C. Adipose tissue and Skeletal muscles:** These tissues primarily express **GLUT-4**, which is the only **insulin-dependent** glucose transporter. Insulin triggers the translocation of GLUT-4 from intracellular vesicles to the plasma membrane. * **D. Brain:** The brain primarily utilizes **GLUT-1** and **GLUT-3**. These are high-affinity (low $K_m$) transporters that ensure a constant supply of glucose to neurons even during fasting or hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT-1:** Found in RBCs and Blood-Brain Barrier. Deficiency leads to De Vivo syndrome (infantile seizures). * **GLUT-2:** Involved in **Fanconi-Bickel Syndrome** (a glycogen storage disease caused by GLUT-2 mutations). * **GLUT-4:** The only insulin-responsive transporter; its recruitment is increased by **exercise** in skeletal muscles. * **GLUT-5:** Specifically functions as a **fructose** transporter, primarily located in the small intestine and spermatozoa. * **SGLT-1/2:** These are active transporters (sodium-dependent) found in the intestine and kidneys, unlike the GLUT family which facilitates passive diffusion.

Question 90: Glycosylated hemoglobin in a normal pregnant lady should be less than:

A. 4%
B. 5%
C. 6% (Correct Answer)
D. 7%

Explanation: **Explanation:** **1. Why Option C (6%) is Correct:** Glycosylated hemoglobin (HbA1c) reflects the average blood glucose levels over the preceding 8–12 weeks. In a normal pregnancy, HbA1c levels are typically **lower** than in non-pregnant adults. This is due to two primary physiological changes: * **Increased Erythropoiesis:** There is a significant increase in red blood cell (RBC) production. * **Decreased RBC Lifespan:** The average lifespan of an RBC decreases from 120 days to approximately 90 days during pregnancy. Because the RBCs are "younger" and circulate for less time, they have less exposure to glucose for glycation. Therefore, in a healthy pregnancy, the HbA1c should ideally be **less than 6%**. **2. Why Other Options are Incorrect:** * **Option A (4%):** This is below the physiological range for most healthy individuals and would indicate chronic hypoglycemia, which is not the norm for pregnancy. * **Option B (5%):** While a pregnant woman may have an HbA1c of 5%, the clinical "cutoff" or upper limit for a normal pregnancy is generally accepted as 6%. * **Option D (7%):** An HbA1c of 7% or higher is the diagnostic threshold for Diabetes Mellitus in non-pregnant adults. In pregnancy, this level would indicate poorly controlled pre-gestational or gestational diabetes and is associated with increased fetal risks (macrosomia, congenital anomalies). **3. NEET-PG High-Yield Pearls:** * **Gold Standard:** HbA1c is the gold standard for monitoring long-term glycemic control but is **not** the preferred test for diagnosing Gestational Diabetes Mellitus (GDM). The **Oral Glucose Tolerance Test (OGTT)** remains the diagnostic mainstay. * **Target in Diabetes:** For pregnant women with pre-existing diabetes, the target HbA1c is ideally **<6%** to minimize the risk of malformations. * **Falsely Low HbA1c:** Seen in hemolytic anemias and pregnancy. * **Falsely High HbA1c:** Seen in Iron Deficiency Anemia (due to increased RBC lifespan/turnover changes).

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