Carbohydrate Metabolism Practice Questions

Q: What is the most common enzyme defect in galactosemia?

Uridyl transferase. **Explanation:** Galactosemia is an autosomal recessive disorder of galactose metabolism. The correct answer is **Uridyl transferase** (specifically, Galactose-1-phosphate uridyltransferase or GALT). **1. Why Uridyl Transferase is Correct:** Deficiency of the **GALT** enzyme causes **Classic Galactosemia**, which is the most common and clinically severe form of the disease. In this condition, Galactose-1-phosphate and galactitol accumulate in tissues (liver, brain, and renal tubules), leading to severe manifestations like hepatomegaly, jaundice, and mental retardation shortly after starting milk feeds. **2. Analysis of Incorrect Options:** * **A. Galactokinase (GALK):** Deficiency leads to "Non-classic Galactosemia." It is less common and presents primarily with early-onset cataracts due to galactitol accumulation in the lens, but lacks the severe systemic toxicity seen in the GALT deficiency. * **C. 4-epimerase (GALE):** UDP-galactose-4-epimerase deficiency is the rarest form. It can exist in a benign peripheral form (limited to RBCs) or a rare systemic form resembling classic galactosemia. **3. NEET-PG High-Yield Pearls:** * **Screening Test:** Benedict’s test is positive (reducing sugar in urine), but the Glucose Oxidase (dipstick) test is negative. * **Cataracts:** Formed due to the conversion of excess galactose to **Galactitol** by the enzyme *Aldose Reductase*. * **Key Association:** Infants with classic galactosemia are at a significantly increased risk of **E. coli neonatal sepsis**. * **Management:** Immediate exclusion of lactose and galactose from the diet (stop breastfeeding, switch to soy milk).

Q: Which of the following metabolic disorders causes post-prandial hypoglycemia?

Hereditary fructose intolerance. **Explanation:** **Hereditary Fructose Intolerance (HFI)** is caused by a deficiency of **Aldolase B**. When a patient ingests fructose (or sucrose), fructose-1-phosphate (F1P) accumulates in the liver. This accumulation "traps" intracellular inorganic phosphate, leading to ATP depletion. High levels of F1P and low phosphate levels **allosterically inhibit glycogen phosphorylase** (preventing glycogenolysis) and interfere with **gluconeogenesis**. Consequently, after a meal containing fructose, the liver cannot release glucose, leading to severe **post-prandial hypoglycemia**. **Analysis of Incorrect Options:** * **GSD Type I (Von Gierke Disease):** Characterized by **fasting hypoglycemia** because the deficiency of Glucose-6-Phosphatase prevents the liver from releasing glucose derived from both glycogenolysis and gluconeogenesis during periods of starvation. * **GSD Type III (Cori Disease):** Caused by debranching enzyme deficiency. It presents with **fasting hypoglycemia**, though typically milder than Type I, as gluconeogenesis remains intact. * **Fanconi-Bickel Syndrome (GSD Type XI):** Caused by a **GLUT-2 mutation**. While it involves glycogen accumulation, it typically presents with fasting hypoglycemia and post-prandial hyperglycemia (due to impaired glucose uptake/transport). **High-Yield Clinical Pearls for NEET-PG:** * **HFI Presentation:** Symptoms (vomiting, jaundice, hypoglycemia) appear only **after weaning** from breast milk (when fruits/sucrose are introduced). * **Urine Test:** Patients with HFI show a **positive Benedict’s test** (reducing sugar) but a **negative glucose oxidase test** (dipstick), indicating a non-glucose reducing sugar (fructose). * **Treatment:** Strict avoidance of fructose, sucrose, and sorbitol.

Q: After overnight fasting, in which of the following cells are glucose transporter levels reduced?

Adipocytes. **Explanation:** The regulation of glucose uptake depends on the specific type of glucose transporter (GLUT) expressed on the cell membrane. **1. Why Adipocytes are correct:** Adipocytes (and skeletal muscle) primarily express **GLUT-4**, which is the only **insulin-dependent** glucose transporter. In a fasting state, insulin levels are low. This causes GLUT-4 transporters to be sequestered from the cell surface and internalized into intracellular vesicles, significantly reducing their levels on the plasma membrane. This mechanism conserves glucose for glucose-dependent tissues like the brain. **2. Why the other options are incorrect:** * **Brain cells (A):** Express **GLUT-1 and GLUT-3**. These are insulin-independent and have a low Km (high affinity), ensuring constant glucose uptake even during hypoglycemia. * **Hepatocytes (B):** Express **GLUT-2**. This is an insulin-independent, high-capacity transporter that allows bidirectional glucose flux. Its presence on the membrane does not decrease during fasting; rather, it facilitates glycogenolysis and gluconeogenesis to release glucose into the blood. * **RBCs (D):** Express **GLUT-1**. RBCs lack mitochondria and rely solely on anaerobic glycolysis; therefore, they require constant, insulin-independent glucose access. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT-4** is the "Insulin-Responsive" transporter found in **Adipose tissue and Striated muscle** (Skeletal & Cardiac). * **Exercise** can trigger GLUT-4 translocation to the membrane in muscles independent of insulin (important for managing Diabetes). * **GLUT-2** acts as a "Glucose Sensor" in Pancreatic Beta-cells. * **GLUT-5** is unique as it primarily transports **Fructose** (found in the small intestine and spermatozoa).

Q: Which GLUT transporter is responsible for the secretion of insulin from beta cells of the pancreas?

GLUT2. **Explanation:** The correct answer is **GLUT2**. In pancreatic beta cells, GLUT2 acts as a "glucose sensor." It is a high-capacity, high-Km (low affinity) transporter, meaning its rate of glucose transport is proportional to blood glucose levels. When blood glucose rises after a meal, GLUT2 facilitates the rapid entry of glucose into the beta cells. This glucose is then phosphorylated by **Glucokinase**, leading to ATP production, closure of ATP-sensitive K+ channels, depolarization, and the subsequent release of insulin. **Analysis of Incorrect Options:** * **GLUT1:** Found in most tissues, including RBCs and the blood-brain barrier. It provides basal glucose uptake but is not the primary trigger for insulin secretion. * **GLUT3:** Primarily found in neurons. It has a very low Km (high affinity), ensuring the brain receives glucose even during hypoglycemia. * **GLUT4:** The only **insulin-dependent** glucose transporter. It is sequestered in intracellular vesicles and translocates to the cell membrane of skeletal muscle and adipose tissue only in the presence of insulin. * **Note:** While humans primarily use GLUT1/3 for beta-cell sensing, GLUT2 remains the classic textbook and exam answer for the "glucose sensor" mechanism in medical boards. **High-Yield Clinical Pearls for NEET-PG:** * **Bidirectional Transport:** GLUT2 is unique because it allows bidirectional transport, crucial for glucose release from the liver during gluconeogenesis. * **Fanconi-Bickel Syndrome:** A rare glycogen storage disease caused by a mutation in the GLUT2 gene. * **SGLT vs. GLUT:** Remember that SGLTs (Sodium-Glucose Linked Transporters) use active transport (secondary), while GLUTs use **facilitated diffusion**.

Q: The metabolic defect in hereditary fructose intolerance is due to deficiency of which enzyme?

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 deficient, **Fructose-1-Phosphate accumulates** intracellularly. This "traps" inorganic phosphate, leading to ATP depletion. The lack of ATP inhibits gluconeogenesis and glycogenolysis, resulting in severe postprandial hypoglycemia, jaundice, and vomiting following the ingestion of fructose, sucrose, or sorbitol. **Analysis of Incorrect Options:** * **A. Fructokinase:** Deficiency causes **Essential Fructosuria**. This is a benign, asymptomatic condition where fructose is excreted in the urine because it cannot be "trapped" in cells. * **C. Xylitol dehydrogenase:** Deficiency of L-xylulose reductase (part of the uronic acid pathway) leads to **Essential Pentosuria**, characterized by the excretion of L-xylulose in urine. * **D. Phosphofructokinase:** This is the rate-limiting enzyme of glycolysis. Its deficiency (Tarui disease) leads to Glycogen Storage Disease Type VII, characterized by exercise-induced muscle cramps and hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **The "Trapping" Mechanism:** Accumulation of F1P is the toxic event in HFI, similar to Galactose-1-Phosphate accumulation in Classic Galactosemia. * **Dietary Management:** Treatment involves strict avoidance of **fructose, sucrose** (glucose + fructose), and **sorbitol** (which converts to fructose). * **Clinical Presentation:** Symptoms typically appear when an infant is weaned from breast milk and introduced to fruit juices or formulas containing sucrose.

Q: A 3-month-old infant presents with hepatosplenomegaly and failure to thrive. A liver biopsy reveals glycogen with an abnormal, amylopectin-like structure with long outer chains and missing branches. Which of the following enzymes would most likely be deficient?

Branching enzyme. ### Explanation The clinical presentation and biopsy findings point towards **Glycogen Storage Disease Type IV (Andersen Disease)**. **1. Why the Correct Answer is Right:** The **Branching Enzyme** (α-1,4 → α-1,6 transglucosidase) is responsible for creating α-1,6-glycosidic bonds by transferring a string of glucose residues to a side chain. A deficiency in this enzyme leads to the accumulation of an abnormal form of glycogen called **polyglucosan** (or amylopectin-like glycogen). Because the enzyme cannot create branches, the glycogen molecules have **fewer branch points and very long outer chains**. These insoluble, straight-chain structures trigger an immune response, leading to progressive liver cirrhosis, hepatosplenomegaly, and failure to thrive. **2. Why Incorrect Options are Wrong:** * **Alpha-amylase:** This is a digestive enzyme found in saliva and pancreatic secretions that breaks down dietary starch into maltose; it is not involved in intracellular glycogen synthesis. * **Debranching enzyme (GSD Type III / Cori Disease):** Deficiency leads to the accumulation of **Limit Dextrins** (glycogen with very short outer branches). Patients typically present with hypoglycemia, which is not the primary feature here. * **Glucose-6-phosphatase (GSD Type I / Von Gierke Disease):** This results in the accumulation of **normal-structured glycogen** in the liver and kidneys, presenting with severe fasting hypoglycemia, lactic acidosis, and hyperuricemia. **3. NEET-PG High-Yield Pearls:** * **Andersen Disease (Type IV):** Think "Long chains, No branches." It is the most "fatal" early-onset GSD due to liver cirrhosis. * **Cori Disease (Type III):** Think "Short branches" (Limit Dextrins). * **Mnemonic for GSDs:** **V**on Gierke (I), **P**ompe (II), **C**ori (III), **A**ndersen (IV), **M**cArdle (V), **H**ers (VI) → "**V**ery **P**oor **C**arbohydrate **A**nalysis **M**akes **H**umans" sick.

Q: Phosphorylase b is maintained in an inactivated state by?

Insulin. **Explanation:** The regulation of glycogenolysis centers on the enzyme **Glycogen Phosphorylase**, which exists in two forms: **Phosphorylase *a*** (phosphorylated, active) and **Phosphorylase *b*** (dephosphorylated, inactive). **Why Insulin is correct:** Insulin is an anabolic hormone that promotes glycogen synthesis and inhibits glycogen breakdown. It triggers **Protein Phosphatase-1 (PP1)**, which removes the phosphate group from Phosphorylase *a*, converting it back into the inactive **Phosphorylase *b***. Furthermore, insulin activates phosphodiesterase to lower cAMP levels, effectively maintaining the enzyme in its inactive state to prevent glucose release. **Analysis of Incorrect Options:** * **ATP:** In the muscle, ATP acts as an allosteric inhibitor of Phosphorylase *b*, but it does not "maintain" the state via hormonal signaling like insulin. High ATP signals high energy, reducing the *activity* of the enzyme rather than its phosphorylation state. * **cAMP:** This is a second messenger for Glucagon and Epinephrine. Increased cAMP activates Protein Kinase A (PKA), which leads to the activation of Phosphorylase Kinase, ultimately **activating** Phosphorylase *b* into *a*. * **Calcium:** In contracting muscles, Calcium binds to the calmodulin subunit of Phosphorylase Kinase, **activating** it. This ensures glycogen is broken down to provide energy for contraction. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** Glycogen Phosphorylase is the rate-limiting step of glycogenolysis. * **Covalent Modification:** The conversion between *a* and *b* forms is the classic example of regulation via phosphorylation/dephosphorylation. * **Muscle vs. Liver:** Muscle phosphorylase is sensitive to **AMP** (allosteric activator), whereas liver phosphorylase is not; liver phosphorylase is primarily regulated by **Glucose**.

Q: A boy presents with vomiting, a bloated abdomen, and abdominal pain. He attended an ice-cream eating competition last night and has a past history of similar episodes following the ingestion of milk and milk products. What is the likely cause?

Lactase deficiency. **Explanation:** The clinical presentation of abdominal bloating, pain, and vomiting following the ingestion of milk products (ice cream) is a classic manifestation of **Lactose Intolerance**, caused by **Lactase deficiency**. **Why the correct answer is right:** Lactase is a brush-border enzyme in the small intestine that hydrolyzes lactose into glucose and galactose. In its absence, undigested lactose passes into the colon. Here, it acts osmotically, drawing water into the lumen (causing diarrhea and bloating), and is fermented by colonic bacteria into hydrogen gas, methane, and lactic acid. This fermentation leads to flatulence, abdominal distension, and pain. **Why incorrect options are wrong:** * **Pancreatic/Salivary Amylase deficiency:** Amylase breaks down complex starches (polysaccharides) into maltose. Deficiency would lead to malabsorption of starches, not specifically milk-based sugars. * **Food poisoning:** While it causes vomiting and pain, the recurrent history ("past history of similar episodes") specifically linked to milk products points toward a metabolic/enzymatic defect rather than an acute infection. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** The **Hydrogen Breath Test** is the gold standard (increased H₂ due to bacterial fermentation). Stool analysis shows **low pH** (acidic) and the presence of **reducing sugars**. * **Types:** Primary (age-related decline), Secondary (due to mucosal damage like Celiac or Rotavirus), and Congenital (rare). * **Biochemical Note:** Lactose is a disaccharide with a **β-1,4 glycosidic linkage**.

Q: Which of the following statements is true regarding GLUT-5?

Sodium independent transport. **Explanation:** Glucose transporters (GLUTs) are a family of membrane proteins that facilitate the transport of glucose and other sugars across the cell membrane via **facilitated diffusion**. This process is **sodium-independent**, unlike the SGLT (Sodium-Glucose Linked Transporters) found in the kidneys and intestines, which require a sodium gradient. **Why Option D is correct:** GLUT-5 is unique among the GLUT family because its primary substrate is **fructose**, not glucose. Like all members of the GLUT family (GLUT 1-14), it operates via facilitated diffusion, meaning it moves solutes down their concentration gradient without requiring energy or sodium ions. **Analysis of Incorrect Options:** * **Option A:** GLUT-1 and GLUT-3 are the primary transporters in the **brain**, ensuring a constant glucose supply regardless of blood sugar levels. * **Option B:** GLUT-4 is the specific transporter found in **adipose tissue and skeletal muscle**. * **Option C:** Only **GLUT-4** is insulin-dependent. GLUT-5 is insulin-independent and is primarily expressed in the small intestine (apical membrane of enterocytes), spermatozoa, and kidneys. **High-Yield NEET-PG Pearls:** * **GLUT-1:** Blood-brain barrier, RBCs (Basal uptake). * **GLUT-2:** Bidirectional; found in Liver, Pancreas (B-cells), and Kidney. It has a high $K_m$ (low affinity). * **GLUT-3:** Neurons (Highest affinity). * **GLUT-4:** Insulin-responsive (Muscle/Fat). * **GLUT-5:** Fructose specific. * **SGLT-1/2:** Secondary active transport (Sodium-dependent); found in the small intestine and proximal convoluted tubule of the kidney.

Q: Which metabolic pathway provides the reducing agent primarily used in lipogenesis?

Pentose phosphate pathway. **Explanation:** The correct answer is **A. Pentose Phosphate Pathway (PPP)**. **1. Why it is correct:** Lipogenesis (fatty acid synthesis) occurs in the cytosol and requires **NADPH** as a reducing agent to reduce keto groups to hydroxyl groups and double bonds to single bonds. The primary source of this NADPH is the **Pentose Phosphate Pathway** (specifically the oxidative phase via Glucose-6-phosphate dehydrogenase). Another secondary source is the activity of the **Malic enzyme**, which converts malate to pyruvate. **2. Why other options are incorrect:** * **B. Glycolysis:** This pathway occurs in the cytosol but produces **NADH**, not NADPH. NADH is primarily used for ATP production in the electron transport chain, not for reductive biosynthesis. * **C. TCA Cycle:** This mitochondrial pathway produces **NADH and FADH₂**, which are used for oxidative phosphorylation. While it provides the substrate (Citrate) for lipogenesis, it does not provide the reducing power. * **D. Gluconeogenesis:** This is an anabolic pathway that consumes energy (ATP/GTP) and reducing equivalents (NADH) to synthesize glucose; it does not generate NADPH. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of PPP:** Glucose-6-phosphate dehydrogenase (G6PD). * **Tissues with high PPP activity:** Adrenal cortex, liver, lactating mammary glands, and RBCs (all require NADPH for either steroid/fatty acid synthesis or maintaining reduced glutathione). * **The "Citrate Shuttle":** Since Acetyl-CoA cannot cross the mitochondrial membrane, it exits as **Citrate**. In the cytosol, Citrate is cleaved by *ATP Citrate Lyase* to provide Acetyl-CoA for lipogenesis. * **Key Regulatory Enzyme:** Acetyl-CoA Carboxylase (inhibited by Glucagon/Palmitoyl-CoA; activated by Insulin/Citrate).

Question 1

What is the most common enzyme defect in galactosemia?

Accepted Answer

Uridyl transferase

Answer

Galactokinase

Answer

4-epimerase

Answer

None of the above

Question 2

Which of the following metabolic disorders causes post-prandial hypoglycemia?

Accepted Answer

Hereditary fructose intolerance

Answer

Glycogen storage disease type I

Answer

Glycogen storage disease type III

Answer

Fanconi-Bickel syndrome

Question 3

After overnight fasting, in which of the following cells are glucose transporter levels reduced?

Accepted Answer

Adipocytes

Answer

Brain cells

Answer

Hepatocytes

Answer

RBCs

Question 4

Which GLUT transporter is responsible for the secretion of insulin from beta cells of the pancreas?

Accepted Answer

GLUT2

Answer

GLUT1

Answer

GLUT3

Answer

GLUT4

Question 5

The metabolic defect in hereditary fructose intolerance is due to deficiency of which enzyme?

Accepted Answer

Aldolase-B

Answer

Fructokinase

Answer

Xylitol dehydrogenase

Answer

Phosphofructokinase

Question 6

A 3-month-old infant presents with hepatosplenomegaly and failure to thrive. A liver biopsy reveals glycogen with an abnormal, amylopectin-like structure with long outer chains and missing branches. Which of the following enzymes would most likely be deficient?

Accepted Answer

Branching enzyme

Answer

Alpha amylase

Answer

Debranching enzyme

Answer

Glucose-6-phosphatase

Question 7

Phosphorylase b is maintained in an inactivated state by?

Accepted Answer

Insulin

Answer

ATP

Answer

cAMP

Answer

Calcium

Question 8

A boy presents with vomiting, a bloated abdomen, and abdominal pain. He attended an ice-cream eating competition last night and has a past history of similar episodes following the ingestion of milk and milk products. What is the likely cause?

Accepted Answer

Lactase deficiency

Answer

Pancreatic amylase deficiency

Answer

Salivary amylase deficiency

Answer

Food poisoning

Question 9

Which of the following statements is true regarding GLUT-5?

Accepted Answer

Sodium independent transport

Answer

Present in the brain

Answer

Present in adipose tissue, skeletal muscle, and skin

Answer

Mediated by insulin transport

Question 10

Which metabolic pathway provides the reducing agent primarily used in lipogenesis?

Accepted Answer

Pentose phosphate pathway

Answer

Glycolysis

Answer

TCA cycle

Answer

Gluconeogenesis

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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