Carbohydrate Metabolism Practice Questions

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.

Q: Chronic nonspherocytic hemolytic anemia is a manifestation of:

G6PD deficiency. **Explanation:** **Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency** is the most common enzyme deficiency causing **Chronic Nonspherocytic Hemolytic Anemia (CNSHA)**. G6PD is the rate-limiting enzyme of the Hexose Monophosphate (HMP) shunt, responsible for producing **NADPH**. NADPH is essential for maintaining a pool of reduced glutathione, which protects red blood cells (RBCs) from oxidative stress. In G6PD deficiency, oxidative damage leads to the denaturation of hemoglobin (**Heinz bodies**) and subsequent hemolysis. While most variants cause episodic hemolysis triggered by fava beans or infections, certain "Class I" mutations lead to chronic, baseline hemolysis without spherocytes on the peripheral smear. **Analysis of Incorrect Options:** * **Hereditary Elliptocytosis:** This is a structural membrane defect (usually involving spectrin) characterized by elliptical-shaped RBCs, not a primary enzyme deficiency. * **Sickle Cell Anemia:** This is a qualitative hemoglobinopathy (Glu → Val at position 6 of the β-globin chain). While it causes chronic hemolysis, it is characterized by "sickle cells" and vaso-occlusive crises rather than being classified as nonspherocytic enzyme-related anemia. * **Hexokinase Deficiency:** Although it can cause CNSHA, it is extremely rare compared to G6PD deficiency. In the context of standard medical examinations, G6PD is the classic association for this presentation. **High-Yield Pearls for NEET-PG:** * **Inheritance:** G6PD deficiency is **X-linked recessive**. * **Morphology:** Look for **Heinz bodies** (supravital stain) and **Bite cells** (degmacytes) on peripheral smear. * **Trigger factors:** AAA (Antimalarials like Primaquine, Antibiotics like Sulfa drugs, and Aspirin/Acidosis). * **Protection:** G6PD deficiency offers a selective advantage against *Plasmodium falciparum* malaria.

Q: What is invert sugar?

An equimolar mixture of glucose and fructose. **Explanation:** **Invert sugar** is an equimolar mixture of **D-glucose and D-fructose**. It is produced by the hydrolysis of sucrose (table sugar) by the enzyme **sucrase (invertase)** or by dilute acids. **Why the correct answer is right:** The term "invert" refers to the change in the **optical rotation** of the solution. * **Sucrose** is dextrorotatory ($+66.5^\circ$). * Upon hydrolysis, it yields equal parts glucose ($+52.7^\circ$) and fructose ($-92.4^\circ$). * Because the levorotatory (left-turning) power of fructose is much stronger than the dextrorotatory (right-turning) power of glucose, the overall net rotation of the mixture becomes **levorotatory**. This "inversion" from $(+)$ to $(-)$ gives the mixture its name. **Why the incorrect options are wrong:** * **Option B:** Sucrose is the parent disaccharide; it is not a component of the hydrolyzed mixture. * **Option C & D:** Lactose is a disaccharide of glucose and galactose. Hydrolysis of lactose yields these two, but this mixture is not called invert sugar and does not exhibit the specific optical inversion characteristic of sucrose hydrolysis. **High-Yield Facts for NEET-PG:** 1. **Sweetness:** Invert sugar is significantly sweeter than sucrose because free fructose is the sweetest naturally occurring sugar. 2. **Honey:** Honey is a natural source of invert sugar, containing high concentrations of free glucose and fructose. 3. **Specific Rotation:** Remember the values for the exam: Sucrose ($+66.5^\circ$), Glucose ($+52.7^\circ$), and Fructose ($-92.4^\circ$). 4. **Seliwanoff’s Test:** Invert sugar will give a positive result (cherry red color) due to the presence of fructose (a ketose).

Q: Which of the following is NOT a heteropolysaccharide?

Inulin. **Explanation:** The distinction between polysaccharides is based on their monomeric composition. **Inulin** is a **homopolysaccharide**, specifically a polymer of **fructose** (fructosan) linked by β(2→1) glycosidic bonds. Because it consists of only one type of monosaccharide unit, it is not a heteropolysaccharide. **Why the other options are incorrect:** Options A, B, and C are all examples of **Glycosaminoglycans (GAGs)**, which are classic **heteropolysaccharides**. They are composed of repeating disaccharide units consisting of an amino sugar (e.g., glucosamine) and a uronic acid (e.g., glucuronic acid). * **Hyaluronic acid:** Composed of D-glucuronic acid and N-acetyl-D-glucosamine. It is unique among GAGs as it is non-sulfated and not protein-bound. * **Chondroitin sulfate:** The most abundant GAG in the body, found in cartilage and bone. * **Heparan sulfate:** Found on cell surfaces and in the extracellular matrix; it contains higher levels of acetylated glucosamine compared to heparin. **High-Yield Clinical Pearls for NEET-PG:** * **Inulin Clearance:** Inulin is the "Gold Standard" for measuring **Glomerular Filtration Rate (GFR)** because it is freely filtered by the glomeruli but neither reabsorbed nor secreted by the renal tubules. * **Storage forms:** Starch (plants) and Glycogen (animals) are homopolysaccharides of glucose. * **Dextran vs. Dextrin:** Dextran is a branched glucose polymer used as a plasma volume expander; Dextrin is an intermediate product of starch hydrolysis. * **Agar:** A heteropolysaccharide derived from seaweed, used in laboratories as a culture medium.

Q: Keratan sulphate is found in abundance in which of the following locations?

Cornea. **Explanation:** **Keratan Sulphate (KS)** is a unique Glycosaminoglycan (GAG) because it is the only one that does not contain uronic acid (it contains galactose instead). It exists in two forms: KS I and KS II. **Why Cornea is the Correct Answer:** **Keratan Sulphate I** is found predominantly in the **cornea**. Its primary physiological role is to maintain corneal transparency. The specific arrangement and hydration levels of KS within the corneal stroma minimize light scattering, ensuring a clear path for vision. Deficiency or abnormalities in KS metabolism (as seen in Macular Corneal Dystrophy) lead to corneal opacification. **Analysis of Incorrect Options:** * **Heart Muscle:** While the heart valves contain GAGs like Dermatan sulphate and Hyaluronic acid to maintain structural integrity, Keratan sulphate is not a major constituent of the myocardium (heart muscle). * **Liver:** The liver is the site of GAG degradation, but it does not store Keratan sulphate. Heparan sulphate is the more relevant GAG associated with cell surfaces and basement membranes in systemic organs. * **Adrenal Cortex:** This is an endocrine tissue primarily involved in steroidogenesis; it does not contain significant amounts of specialized structural GAGs like Keratan sulphate. **High-Yield NEET-PG Pearls:** * **KS I vs. KS II:** KS I is found in the **cornea**, while KS II is found in **cartilage and bone**. * **Unique Structure:** Remember, KS is the "odd one out" among GAGs—it contains **Galactose** instead of Uronic acid. * **Clinical Correlation:** **Morquio Syndrome (MPS IV)** is characterized by the inability to degrade Keratan sulphate, leading to skeletal deformities and corneal clouding. * **Most Abundant GAG:** Chondroitin sulphate (found in cartilage/bone). * **Only Non-Sulphated GAG:** Hyaluronic acid.

Q: During a hunger strike with a liquid diet and minimal calories, which of the following hormonal and metabolic changes would occur after 4 to 5 hours?

Increased cyclic AMP and increased liver glycogenolysis. **Explanation:** The core concept here is the body's response to the **post-absorptive state** (early fasting). Approximately 4–5 hours after a meal, blood glucose levels begin to decline. This triggers the pancreas to secrete **glucagon** and the adrenal medulla to release **epinephrine**. **1. Why Option B is Correct:** Glucagon and epinephrine bind to G-protein coupled receptors (GPCRs) on hepatocytes. This activates **adenylyl cyclase**, which converts ATP into **cyclic AMP (cAMP)**. Elevated cAMP activates Protein Kinase A (PKA), which phosphorylates and activates **Glycogen Phosphorylase**. This leads to **liver glycogenolysis**, the primary mechanism for maintaining blood glucose levels during short-term fasting. **2. Why Other Options are Incorrect:** * **Option A:** Decreased cAMP and increased glycogen synthesis occur in the **fed state** under the influence of insulin. * **Option C:** Epinephrine levels **increase** (not decrease) during fasting or stress to stimulate glucose mobilization. * **Option D:** While Ca²⁺ does increase in muscle during contraction to stimulate glycogenolysis, the question focuses on the systemic hormonal response to a "hunger strike" (fasting), where liver glycogenolysis is the priority for blood glucose maintenance. Furthermore, increased Ca²⁺ would **increase**, not decrease, glycogenolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glycogen Phosphorylase is the rate-limiting enzyme for glycogenolysis; it is activated by phosphorylation. * **Liver vs. Muscle:** Liver glycogen maintains **blood glucose**; muscle glycogen is used only for **local energy** (muscle lacks Glucose-6-Phosphatase). * **Timeline:** Liver glycogen stores are typically exhausted after **12–24 hours** of fasting, after which gluconeogenesis becomes the sole source of blood glucose.

Question 1

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 2

GLP-1 is broken down by which enzyme?

Accepted Answer

Dipeptidyl peptidase

Answer

α-glucosidase

Answer

Rasburicase

Answer

Glucose 1 phosphatase

Question 3

In muscle, phosphorylase b is inactivated by:

Accepted Answer

ATP

Answer

cAMP

Answer

Ca ions

Answer

Glucose

Question 4

Chronic nonspherocytic hemolytic anemia is a manifestation of:

Accepted Answer

G6PD deficiency

Answer

Hereditary elliptocytosis

Answer

Sickle cell anemia

Answer

Hexokinase deficiency

Question 5

What is invert sugar?

Accepted Answer

An equimolar mixture of glucose and fructose

Answer

An equimolar mixture of sucrose and glucose

Answer

An equimolar mixture of glucose and lactose

Answer

An equimolar mixture of glucose and galactose

Question 6

All of the following are true regarding G6PD deficiency except?

Accepted Answer

Females are commonly affected

Answer

A recessive X-linked trait

Answer

Oxidative stress causes hemolysis

Answer

Protective against Plasmodium falciparum malaria

Question 7

Which of the following is NOT a heteropolysaccharide?

Accepted Answer

Inulin

Answer

Hyaluronic acid

Answer

Chondroitin sulfate

Answer

Heparan sulfate

Question 8

Alcohol causes hypoglycemia due to:

Accepted Answer

Decreased gluconeogenesis

Answer

Increased NADH

Answer

Decreased lipogenesis

Answer

Decreased glycogenesis

Question 9

Keratan sulphate is found in abundance in which of the following locations?

Accepted Answer

Cornea

Answer

Heart muscle

Answer

Liver

Answer

Adrenal cortex

Question 10

During a hunger strike with a liquid diet and minimal calories, which of the following hormonal and metabolic changes would occur after 4 to 5 hours?

Accepted Answer

Increased cyclic AMP and increased liver glycogenolysis

Answer

Decreased cyclic AMP and increased liver glycogen synthesis

Answer

Decreased epinephrine levels and increased liver glycogenolysis

Answer

Increased Ca2+ in muscle and decreased glycogenolysis

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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