Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 571: A pregnant woman with lactase deficiency cannot tolerate milk in her diet and is concerned about producing milk of insufficient caloric value to nourish her baby. What is the best advice to give her?

A. She must consume pure galactose to produce the galactose moiety of lactose.
B. She will be unable to breastfeed her baby because she cannot produce lactose.
C. The production of lactose by the mammary gland does not require the ingestion of milk or milk products. (Correct Answer)
D. She can produce lactose directly by degrading a-lactalbumin.

Explanation: ### Explanation **1. Why Option C is Correct** The production of lactose (milk sugar) in the mammary gland is independent of dietary lactose or milk intake. Lactose is a disaccharide composed of **Glucose and Galactose**. In the mammary gland, the enzyme **Lactose Synthase** (a complex of Galactosyltransferase and $\alpha$-lactalbumin) catalyzes the synthesis of lactose. The glucose required for this process is derived from maternal blood glucose, and the galactose is synthesized endogenously from **Glucose-6-Phosphate** via the hexose monophosphate pathway and the action of **UDP-glucose 4-epimerase**. Therefore, a mother with lactase deficiency can synthesize sufficient lactose for her infant using her own glucose stores. **2. Why Other Options are Incorrect** * **Option A:** Pure galactose consumption is unnecessary because the body can convert glucose to UDP-galactose via the epimerase enzyme. * **Option B:** Lactase deficiency (an intestinal enzyme issue) affects the mother's ability to *digest* lactose, not her ability to *synthesize* it in the mammary glands. Breastfeeding remains perfectly viable. * **Option D:** $\alpha$-lactalbumin is a protein that acts as a regulatory subunit of the lactose synthase enzyme; it is not a precursor that is "degraded" to form lactose. **3. High-Yield Clinical Pearls for NEET-PG** * **Lactose Synthase Complex:** Consists of two parts: **Protein A** (Galactosyltransferase) and **Protein B** ($\alpha$-lactalbumin). * **Hormonal Control:** Prolactin stimulates the synthesis of $\alpha$-lactalbumin, which lowers the $K_m$ of galactosyltransferase for glucose, favoring lactose synthesis during lactation. * **Key Enzyme:** **UDP-glucose 4-epimerase** is the crucial link that allows the conversion of glucose to galactose, ensuring milk production even on a galactose-free diet.

Question 572: Disaccharides are digested mostly in which part of the digestive tract?

A. Oral cavity
B. Stomach
C. Small intestine (Correct Answer)
D. Large intestine

Explanation: **Explanation:** The digestion of carbohydrates is a multi-stage process, but the final breakdown of **disaccharides** (sucrose, lactose, and maltose) occurs almost exclusively in the **small intestine**, specifically at the **brush border** of the enterocytes. 1. **Why the Small Intestine is Correct:** While starch digestion begins in the mouth, disaccharides require specific enzymes called **disaccharidases** (sucrase, lactase, and maltase). These enzymes are located on the brush border membrane of the intestinal villi. Here, disaccharides are hydrolyzed into their constituent monosaccharides (glucose, galactose, and fructose), which are the only forms capable of being absorbed into the bloodstream. 2. **Why Other Options are Incorrect:** * **Oral Cavity:** Salivary amylase begins the breakdown of complex polysaccharides (starch) into maltose, but it cannot digest disaccharides. * **Stomach:** The high acidity (low pH) of the stomach denatures salivary amylase, halting carbohydrate digestion. No specific disaccharidases are secreted in the stomach. * **Large Intestine:** Under normal physiological conditions, all digestible carbohydrates are absorbed before reaching the large intestine. If disaccharides reach the colon (e.g., in lactase deficiency), they are fermented by bacteria, causing osmotic diarrhea and flatulence. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The absorption of monosaccharides is the rate-limiting step in carbohydrate assimilation, except for starch, where digestion is slower. * **Lactase Deficiency:** This is the most common disaccharidase deficiency, leading to lactose intolerance. * **SGLT-1:** Glucose and galactose are absorbed via secondary active transport using the Sodium-Glucose Co-transporter 1. * **GLUT-5:** Fructose is absorbed via facilitated diffusion through the GLUT-5 transporter.

Question 573: The polyol pathway is responsible for the formation of which of the following?

A. Fructose from glucose (Correct Answer)
B. Galactose from fructose
C. Galactose from glucose
D. Glucose from fructose

Explanation: The **Polyol Pathway** (also known as the Sorbitol pathway) is a two-step metabolic process that bypasses glycolysis to convert Glucose into Fructose. ### 1. Why Option A is Correct The pathway occurs in two distinct enzymatic steps: * **Step 1:** Glucose is reduced to **Sorbitol** (a polyol/sugar alcohol) by the enzyme **Aldose Reductase**, using NADPH as a cofactor. * **Step 2:** Sorbitol is oxidized to **Fructose** by the enzyme **Sorbitol Dehydrogenase**, using NAD+ as a cofactor. This pathway is primarily active in the seminal vesicles (fructose is the main energy source for sperm) and the liver. ### 2. Why Other Options are Incorrect * **Options B & C:** Galactose metabolism involves the **Leloir pathway** (Galactokinase and GALT enzymes). While Aldose Reductase can convert Galactose to Galactitol (dulcitol), it does not produce Fructose from Galactose. * **Option D:** The conversion of Fructose to Glucose occurs via **Gluconeogenesis** (after fructose enters glycolysis as DHAP/Glyceraldehyde-3-P), not the polyol pathway. ### 3. NEET-PG High-Yield Clinical Pearls * **Osmotic Damage:** In **Diabetes Mellitus**, hyperglycemia leads to excess glucose entering the polyol pathway. Sorbitol is polar and cannot easily cross cell membranes. * **Tissue Vulnerability:** Tissues like the **Lens, Retina, and Schwann cells** have high Aldose Reductase but **lack Sorbitol Dehydrogenase**. Consequently, sorbitol accumulates, causing osmotic swelling. * **Complications:** This accumulation is a major mechanism behind **Diabetic Cataracts** and **Diabetic Neuropathy**. * **Cofactor Depletion:** Excessive use of NADPH by Aldose Reductase depletes the pool available for Glutathione Reductase, increasing **oxidative stress** in the cell.

Question 574: Dextrose is:

A. D (+) glucose (Correct Answer)
B. D (-) glucose
C. L (+) glucose
D. L (-) glucose

Explanation: **Explanation:** **1. Why A is Correct:** Dextrose is the common name for **D-glucose**. The "D" prefix refers to the **configurational isomerism** (the orientation of the -OH group on the penultimate carbon, which matches D-glyceraldehyde). The "(+)" sign refers to its **optical activity**, specifically its ability to rotate plane-polarized light to the right (**dextrorotatory**). Because naturally occurring glucose is dextrorotatory, it is clinically and commercially referred to as Dextrose. **2. Why the Other Options are Incorrect:** * **B (D (-) glucose):** While the "D" configuration is correct, glucose is never levorotatory (-). Fructose is an example of a sugar that is D-configured but levorotatory (hence called Levulose). * **C & D (L-glucose):** L-isomers are the mirror images of D-isomers. L-glucose is a synthetic sugar that does not occur naturally in biological systems and cannot be phosphorylated by hexokinase; therefore, it cannot be used as an energy source by the human body. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Isomerism:** Most monosaccharides in the human body are in the **D-form**, whereas amino acids are predominantly in the **L-form**. * **Specific Rotation:** The specific rotation of equilibrium D-glucose is **+52.7°**. * **Mutarotation:** When D-glucose is dissolved in water, it exists in an equilibrium between α-D-glucose (+112°) and β-D-glucose (+19°). This change in optical rotation over time is called mutarotation. * **Clinical Use:** Dextrose (5%, 10%, 25%, 50%) is used intravenously to treat hypoglycemia and provide calories. Note that 1 gram of dextrose provides **3.4 kcal** (slightly less than the 4 kcal/g in dietary carbohydrates due to hydration).

Question 575: The muscle shown in the illustration predominantly uses which glucose transporter?

A. GLUT 1
B. GLUT 2
C. GLUT 3
D. GLUT 4 (Correct Answer)

Explanation: ***GLUT 4*** - **GLUT 4** is the predominant glucose transporter in **skeletal muscle** and **adipose tissue**, requiring **insulin** for translocation to the cell membrane. - It has **high affinity** for glucose (low Km) and is crucial for **postprandial glucose uptake**, making it clinically relevant in **insulin resistance** and **type 2 diabetes**. *GLUT 1* - **GLUT 1** is found in **brain**, **red blood cells**, and **placenta**, providing **basal glucose uptake** independent of insulin. - It has **high affinity** for glucose but is **constitutively expressed** at the cell membrane, not translocated like GLUT 4. *GLUT 2* - **GLUT 2** is predominantly found in **liver**, **pancreatic beta cells**, **kidney**, and **small intestine** with **low affinity** (high Km) for glucose. - It functions as a **glucose sensor** in pancreatic beta cells and allows **bidirectional glucose transport** in hepatocytes, unlike muscle tissue. *GLUT 3* - **GLUT 3** is primarily expressed in **brain tissue** and **neurons**, providing **high-affinity glucose transport** for neural metabolism. - It is **insulin-independent** and constitutively active, unlike the **insulin-dependent GLUT 4** required for muscle glucose uptake.

Question 576: Which molecule, known to initiate cataract formation in the eye lens, also has a 1-phosphate derivative responsible for liver failure?

A. Sorbitol
B. Mannitol
C. Inositol
D. Galactitol (Correct Answer)

Explanation: **Explanation:** The correct answer is **Galactitol** (also known as dulcitol). This question tests the integration of the polyol pathway and galactose metabolism. **1. Why Galactitol is correct:** In patients with **Galactosemia** (due to Galactose-1-phosphate uridyltransferase or Galactokinase deficiency), excess galactose is diverted into the polyol pathway. The enzyme **Aldose Reductase** reduces galactose into **Galactitol**. Unlike sorbitol, galactitol cannot be further metabolized and accumulates in the lens. It is osmotically active, drawing water into the lens, causing swelling and **cataract formation**. The second part of the question refers to **Classic Galactosemia** (GALT deficiency), where **Galactose-1-phosphate** accumulates in the liver. This metabolite is toxic, leading to ATP depletion and phosphate sequestration, resulting in **liver failure**, jaundice, and hepatomegaly. **2. Why other options are incorrect:** * **Sorbitol:** While sorbitol causes cataracts in diabetic patients (via glucose reduction), its 1-phosphate derivative is not associated with liver failure. Fructose-1-phosphate (from fructose metabolism) causes liver damage in Hereditary Fructose Intolerance. * **Mannitol:** It is a sugar alcohol used clinically as an osmotic diuretic to reduce intracranial pressure; it is not a primary cause of metabolic cataracts or liver failure. * **Inositol:** A precursor for signaling molecules (IP3/DAG); it does not accumulate to cause cataracts or liver toxicity in this metabolic context. **High-Yield NEET-PG Pearls:** * **Enzyme Deficiency:** Classic Galactosemia is due to **GALT** deficiency (more severe); Galactokinase deficiency causes cataracts but *not* liver failure. * **Cataract Type:** Galactosemia typically presents with "Oil-drop cataracts." * **Dietary Management:** Immediate removal of lactose and galactose from the diet is life-saving.

Question 577: Which of the following is also known as milk sugar?

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

Explanation: ### Explanation **Lactose** is known as **milk sugar** because it is the primary carbohydrate found in the milk of mammals (comprising approximately 2–8% of milk by weight). It is a disaccharide composed of one molecule of **D-galactose** and one molecule of **D-glucose**, joined by a **β(1→4) glycosidic linkage**. It is synthesized in the mammary glands and serves as a vital energy source for newborns. #### Analysis of Options: * **Mannose (A):** An epimer of glucose at the C-2 position. It is found in many fruits and is a key component of glycoproteins, but it is not the primary sugar in milk. * **Glucose (B):** Known as **grape sugar** or **blood sugar**. It is the primary metabolic fuel for the body and a monosaccharide component of lactose, but it does not exist independently as "milk sugar." * **Galactose (C):** Known as **brain sugar** because it is a constituent of glycolipids (cerebrosides) in the brain and myelin sheath. While it is a component of lactose, the disaccharide itself is the milk sugar. #### High-Yield Clinical Pearls for NEET-PG: 1. **Lactose Intolerance:** Caused by a deficiency of the enzyme **lactase** (brush border disaccharidase), leading to osmotic diarrhea, bloating, and flatulence upon milk consumption. 2. **Galactosemia:** A deficiency in enzymes like **GALT** (Galactose-1-phosphate uridyltransferase) prevents the metabolism of galactose (derived from lactose), leading to cataracts, liver damage, and intellectual disability. 3. **Reducing Sugar:** Lactose is a reducing sugar because it possesses a free anomeric carbon on the glucose residue. 4. **Source:** It is the only carbohydrate of animal origin in the human diet (except for small amounts of glycogen).

Question 578: In which of the following steps is ATP released?

A. Phosphoenolpyruvate to pyruvate (Correct Answer)
B. Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate
C. Fructose-6-phosphate to fructose-1,6-bisphosphate
D. Glucose to glucose-6-phosphate

Explanation: ### Explanation In glycolysis, ATP is generated via **substrate-level phosphorylation**. This process involves the direct transfer of a high-energy phosphate group from a metabolic intermediate to ADP. **1. Why Option A is Correct:** The conversion of **Phosphoenolpyruvate (PEP) to Pyruvate** is the final step of glycolysis, catalyzed by the enzyme **Pyruvate Kinase**. PEP contains a high-energy enol-phosphate bond. When this bond is cleaved, the energy released is sufficient to phosphorylate ADP to ATP. Since one glucose molecule produces two PEP molecules, this step yields **2 ATP** per glucose. This is an irreversible, rate-limiting step. **2. Why the Other Options are Incorrect:** * **Option B:** The conversion of Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate (catalyzed by GAPDH) is an oxidation-reduction reaction. It generates **NADH**, not ATP. * **Option C & D:** These steps (catalyzed by **Phosphofructokinase-1** and **Hexokinase/Glucokinase** respectively) are the "investment phase" of glycolysis. They **consume ATP** rather than releasing it. **High-Yield NEET-PG Pearls:** * **Substrate-Level Phosphorylation (SLP):** In glycolysis, SLP occurs at two steps: 1,3-BPG to 3-Phosphoglycerate (Phosphoglycerate Kinase) and PEP to Pyruvate (Pyruvate Kinase). * **Pyruvate Kinase Deficiency:** This is the second most common cause of enzyme-deficient **hemolytic anemia** (after G6PD deficiency). Without ATP from this step, RBCs cannot maintain their Na+/K+ ATPase pumps, leading to swelling and lysis. * **Arsenic Poisoning:** Arsenate competes with inorganic phosphate in Step B, bypassing the first SLP step and resulting in **zero net ATP** production from glycolysis.

Question 579: Oxidation of galactose with a strong oxidizing agent produces which of the following?

A. Mucic acid (Correct Answer)
B. Gluconic acid
C. Galacturonic acid
D. Saccharic acid

Explanation: **Explanation:** The oxidation of monosaccharides depends on the strength of the oxidizing agent used. When a sugar is treated with a **strong oxidizing agent** (like concentrated Nitric Acid, $HNO_3$), both the aldehyde group at $C_1$ and the primary alcohol group at $C_6$ are oxidized to carboxylic acid groups. This results in the formation of a **dicarboxylic acid** (also known as an aldaric acid). 1. **Why Mucic Acid is correct:** When **Galactose** undergoes oxidation with concentrated $HNO_3$, it forms **Mucic acid** (Galactaric acid). Unlike other aldaric acids, Mucic acid is clinically significant because it is **insoluble in water** and forms crystals. This property is the basis of the "Mucic Acid Test" used to identify galactose in urine. 2. **Why other options are incorrect:** * **Gluconic acid:** This is a monocarboxylic acid formed by the oxidation of only the $C_1$ aldehyde group of Glucose (using a mild oxidizing agent). * **Galacturonic acid:** This is a uronic acid formed when only the $C_6$ primary alcohol of Galactose is oxidized (usually via enzymatic pathways). * **Saccharic acid (Glucaric acid):** This is the dicarboxylic acid produced when **Glucose** is treated with a strong oxidizing agent ($HNO_3$). **High-Yield Clinical Pearls for NEET-PG:** * **Mucic Acid Test:** Used to differentiate galactose from other sugars. The formation of insoluble, glass-like crystals confirms the presence of galactose or lactose. * **Galactosemia:** A deficiency in GALT (Galactose-1-phosphate uridyltransferase) leads to the accumulation of galactose, which can be detected in urine via the Mucic acid test. * **Sorbitol vs. Dulcitol:** Reduction of Glucose yields Sorbitol; reduction of Galactose yields **Dulcitol** (Galactitol), which is implicated in cataract formation in galactosemic patients.

Question 580: Insulin is required for glucose transport in all of the following tissues except:

A. RBC (Correct Answer)
B. Skeletal muscles
C. Adipose tissue
D. Heart muscles

Explanation: **Explanation:** The transport of glucose into cells is mediated by a family of glucose transporters (GLUT). The requirement for insulin depends on the specific GLUT isoform expressed on the cell membrane. **1. Why RBC is the correct answer:** Red Blood Cells (RBCs) express **GLUT-1**, which is an insulin-independent transporter. This ensures a continuous supply of glucose to the RBCs, which rely solely on glycolysis for energy as they lack mitochondria. Other tissues that do not require insulin for glucose uptake include the brain (GLUT-3), liver (GLUT-2), and kidneys. **2. Why the other options are incorrect:** * **Skeletal Muscle, Adipose Tissue, and Heart Muscle:** These tissues primarily express **GLUT-4**. GLUT-4 is the only insulin-responsive glucose transporter. In the resting state, GLUT-4 is sequestered in intracellular vesicles. Upon insulin binding to its receptor, these vesicles translocate and fuse with the plasma membrane, allowing glucose entry. Therefore, these tissues are highly dependent on insulin for glucose uptake. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **GLUT-1:** Found in RBCs, Blood-Brain Barrier, and kidneys. Responsible for basal glucose uptake. * **GLUT-2:** A high-capacity, low-affinity (high Km) transporter found in the **Liver, Pancreatic beta-cells, and Small Intestine**. It acts as a "glucose sensor." * **GLUT-3:** Found in **Neurons** (highest affinity for glucose to protect the brain during hypoglycemia). * **GLUT-4:** The only **insulin-dependent** transporter; found in skeletal muscle, cardiac muscle, and adipose tissue. * **GLUT-5:** Primarily a **fructose** transporter found in the small intestine and spermatozoa. * **SGLT-1/2:** Sodium-dependent glucose co-transporters (active transport) found in the intestinal mucosa and renal tubules.

Practice by Chapter

Carbohydrate Chemistry and Classification

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Glycolysis: Reactions and Regulation

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Gluconeogenesis: Reactions and Regulation

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Glycogen Metabolism: Synthesis and Breakdown

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Glycogen Storage Diseases

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Pentose Phosphate Pathway

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Metabolism of Fructose and Galactose

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Disorders of Fructose and Galactose Metabolism

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

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Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

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