Carbohydrate Metabolism Practice Questions

Q: Pyruvate can be converted directly into all of the following except?

Phosphoenol pyruvate. **Explanation:** The conversion of **Pyruvate to Phosphoenolpyruvate (PEP)** is a key regulatory step in gluconeogenesis. It is a **two-step process** because the direct conversion of pyruvate to PEP is energetically unfavorable (the reaction catalyzed by Pyruvate Kinase in glycolysis is irreversible). 1. Pyruvate is first carboxylated to **Oxaloacetate** by *Pyruvate Carboxylase* (in the mitochondria). 2. Oxaloacetate is then converted to **PEP** by *PEP Carboxykinase* (PEPCK). **Analysis of Incorrect Options:** * **Alanine:** Pyruvate can be directly transaminated to Alanine via the enzyme **Alanine Transaminase (ALT)**. This is a crucial part of the Cahill cycle (Glucose-Alanine cycle). * **Acetyl CoA:** Pyruvate undergoes oxidative decarboxylation to form Acetyl CoA via the **Pyruvate Dehydrogenase (PDH) complex**. This links glycolysis to the TCA cycle. * **Lactate:** Under anaerobic conditions, Pyruvate is directly reduced to Lactate by **Lactate Dehydrogenase (LDH)**, regenerating $NAD^+$ for glycolysis. **NEET-PG High-Yield Pearls:** * **Pyruvate Carboxylase** requires **Biotin** as a cofactor and is activated by **Acetyl CoA**. * **The "Four Fates of Pyruvate":** 1. Acetyl CoA (Energy), 2. Oxaloacetate (Gluconeogenesis), 3. Alanine (Protein metabolism), 4. Lactate (Anaerobic glycolysis). * **Clinical Correlation:** Deficiency in the PDH complex leads to **Lactic Acidosis** and neurological dysfunction because pyruvate is shunted toward lactate production.

Q: Excess of which of the following can result in cataract?

Sugar alcohol. **Explanation:** The development of cataracts in metabolic disorders is primarily attributed to the **Polyol Pathway**. When blood glucose or galactose levels are chronically elevated, the enzyme **Aldose Reductase** converts these sugars into their corresponding **sugar alcohols** (polyols). 1. **Mechanism (The "Why"):** Glucose is reduced to **Sorbitol**, and Galactose is reduced to **Dulcitol (Galactitol)**. These sugar alcohols are polar and cannot easily diffuse out of the lens cells. Since they accumulate, they create a strong **osmotic gradient**, drawing water into the lens. This causes swelling, lens fiber disruption, and protein denaturation, leading to opacity (cataract). **Analysis of Options:** * **A. Sugar alcohol (Correct):** As explained, Sorbitol and Dulcitol are the direct osmotic agents responsible for lens damage. * **B. Glucose:** While high glucose (Diabetes) triggers the process, it is the *conversion* to sorbitol that causes the cataract, not the glucose molecule itself. * **C. Sugar amines:** These (like glucosamine) are structural components of glycosaminoglycans and are not implicated in osmotic cataract formation. * **D. Galactose:** Similar to glucose, galactose is the precursor. While galactosemia causes cataracts, it does so via its sugar alcohol derivative, Dulcitol. **NEET-PG High-Yield Pearls:** * **Enzyme involved:** Aldose Reductase (uses NADPH). * **Classic Presentation:** "Oil droplet cataract" is seen in Galactosemia (deficiency of GALK or GALT). * **Sorbitol Metabolism:** In most tissues, Sorbitol is converted to Fructose by **Sorbitol Dehydrogenase**. However, the **Lens, Retina, Kidney, and Schwann cells** lack this enzyme, making them highly susceptible to sorbitol-mediated damage (explaining cataracts, retinopathy, and neuropathy in diabetics).

Q: Muscle glycogen stores can NOT be used to provide glucose into the circulation due to lack of which enzyme?

Glucose-6-phosphatase. **Explanation:** The correct answer is **Glucose-6-phosphatase**. **1. Why Glucose-6-phosphatase is the correct answer:** Glycogenolysis in both the liver and muscle produces **Glucose-6-phosphate (G6P)**. In the liver, the enzyme **Glucose-6-phosphatase** cleaves the phosphate group from G6P to form free glucose, which can then be transported out of the cell into the bloodstream to maintain blood glucose levels. However, **skeletal muscle lacks Glucose-6-phosphatase**. Consequently, the G6P produced in muscles is trapped within the myocytes and must enter the glycolytic pathway to generate ATP for local muscular contraction rather than contributing to systemic blood glucose. **2. Analysis of Incorrect Options:** * **A. Glucose-6-phosphate dehydrogenase (G6PD):** This is the rate-limiting enzyme of the Hexose Monophosphate (HMP) Shunt, responsible for generating NADPH and ribose-5-phosphate. It is not involved in releasing free glucose. * **B. Glutamate dehydrogenase:** This enzyme is involved in amino acid metabolism (oxidative deamination), converting glutamate to alpha-ketoglutarate. * **D. Glucokinase:** This enzyme (found in the liver and pancreatic beta cells) catalyzes the phosphorylation of glucose to G6P. It is the functional opposite of Glucose-6-phosphatase. **3. NEET-PG High-Yield Pearls:** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. It presents with severe fasting hypoglycemia, hepatomegaly, and hyperlactatemia. * **Muscle Glycogen:** Serves as a fuel reserve for synthesis of ATP during muscle contraction. * **Liver Glycogen:** Serves as a glucose reserve for the maintenance of blood glucose during fasting. * **Cori Cycle:** Muscle glycogen can indirectly contribute to blood glucose by being converted to lactate, which travels to the liver to undergo gluconeogenesis.

Q: Overingestion of fructose leads to which of the following conditions?

Hypertriglyceridemia. **Explanation:** The correct answer is **Hypertriglyceridemia**. Unlike glucose, fructose metabolism in the liver bypasses the major rate-limiting step of glycolysis—the enzyme **Phosphofructokinase-1 (PFK-1)**. Fructose is rapidly phosphorylated by **fructokinase** to fructose-1-phosphate and subsequently cleaved into trioses (DHAP and glyceraldehyde). This unregulated flux floods the liver with acetyl-CoA, which serves as a substrate for **de novo lipogenesis**. This leads to increased synthesis of fatty acids and VLDL, ultimately resulting in hypertriglyceridemia and non-alcoholic fatty liver disease (NAFLD). **Analysis of Incorrect Options:** * **B. Hypouricemia:** Incorrect. Fructose ingestion causes **Hyperuricemia**. Rapid phosphorylation of fructose depletes intracellular ATP and inorganic phosphate, stimulating the purine degradation pathway, which increases uric acid production. * **C. Hyperphosphatemia:** Incorrect. It causes **Hypophosphatemia**. The "trapping" of inorganic phosphate in the form of fructose-1-phosphate reduces serum phosphate levels. * **D. Hypoglycemia:** Incorrect. While hereditary fructose intolerance (aldolase B deficiency) causes severe hypoglycemia, simple overingestion in a healthy individual typically leads to metabolic derangements like insulin resistance rather than acute hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **Essential Fructosuria:** Deficiency of **Fructokinase**; a benign, asymptomatic condition where fructose appears in the urine (reducing sugar positive). * **Hereditary Fructose Intolerance (HFI):** Deficiency of **Aldolase B**; characterized by hypoglycemia, jaundice, and vomiting after ingesting fruit/sucrose. * **Key Enzyme:** Fructokinase has a much higher $V_{max}$ than glucokinase, explaining why the liver clears fructose so rapidly.

Q: In the conversion of lactic acid to glucose (gluconeogenesis), three reactions of the glycolytic pathway are circumvented. Which of the following enzymes do NOT participate in this process?

Pyruvate kinase. **Explanation:** Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors (like lactate). While it shares many enzymes with glycolysis, it must bypass three **irreversible** steps of glycolysis to proceed energetically. These "bottleneck" steps are catalyzed by Hexokinase, Phosphofructokinase-1, and Pyruvate kinase. **Why Pyruvate Kinase is the Correct Answer:** Pyruvate kinase is a **glycolytic enzyme** that converts Phosphoenolpyruvate (PEP) to Pyruvate. In gluconeogenesis, this reaction cannot be reversed directly. Instead, the body bypasses it using a two-step process involving **Pyruvate carboxylase** and **PEP carboxykinase (PEPCK)**. Therefore, Pyruvate kinase does not participate in gluconeogenesis; it is actually inhibited during this process to prevent a futile cycle. **Analysis of Incorrect Options:** * **A. Pyruvate carboxylase:** A key gluconeogenic enzyme that converts pyruvate to oxaloacetate in the mitochondria. It requires **Biotin** as a cofactor. * **B. Phosphoenolpyruvate carboxykinase (PEPCK):** Converts oxaloacetate to PEP, completing the bypass of the pyruvate kinase step. * **D. Glucose-6-phosphatase:** The final bypass enzyme found in the ER of the liver and kidney. It converts Glucose-6-phosphate to free glucose, allowing it to enter the bloodstream. **High-Yield Clinical Pearls for NEET-PG:** * **The Four Key Gluconeogenic Enzymes:** Pyruvate carboxylase, PEPCK, Fructose-1,6-bisphosphatase (the rate-limiting step), and Glucose-6-phosphatase. * **Acetyl-CoA Connection:** Pyruvate carboxylase is **obligatorily activated** by Acetyl-CoA. * **Location:** Gluconeogenesis occurs primarily in the **Liver** (90%) and Kidney (10%). It does not occur in the muscle due to the absence of Glucose-6-phosphatase. * **Cori Cycle:** Lactate produced in muscles travels to the liver to be converted back to glucose via these bypass enzymes.

Q: Which of the following carbohydrates has no asymmetric carbon atom?

Dihydroxyacetone. ### Explanation **Core Concept: Chirality in Carbohydrates** An asymmetric carbon atom (chiral center) is a carbon atom attached to four different groups. In carbohydrate chemistry, the presence of these centers determines optical activity and the existence of stereoisomers. **Why Dihydroxyacetone is the Correct Answer:** Dihydroxyacetone is a **ketotriose** (the simplest ketose). Its chemical structure is $CH_2OH-CO-CH_2OH$. * Carbon 1 and Carbon 3 are attached to two hydrogen atoms each (not four different groups). * Carbon 2 is part of a carbonyl group ($C=O$) and is $sp^2$ hybridized, meaning it cannot be asymmetric. Consequently, dihydroxyacetone is the **only monosaccharide that is optically inactive** because it lacks a chiral center. **Analysis of Incorrect Options:** * **Glyceraldehyde (Option B):** The simplest aldose. Its central carbon (C2) is attached to $-H$, $-OH$, $-CHO$, and $-CH_2OH$. It has **one** asymmetric carbon. * **Glucose (Option A):** An aldohexose. In its open-chain form, it contains **four** asymmetric carbon atoms (C2, C3, C4, and C5). * **Fructose (Option D):** A ketohexose. In its open-chain form, it contains **three** asymmetric carbon atoms (C3, C4, and C5). **NEET-PG High-Yield Pearls:** 1. **Formula for Isomers:** The number of possible stereoisomers for a sugar is $2^n$, where $n$ is the number of asymmetric carbon atoms. Since $n=0$ for dihydroxyacetone, it has only $1$ form ($2^0=1$). 2. **Reference Sugar:** Glyceraldehyde is the reference sugar used to designate the **D and L notations** of all other sugars. 3. **Epimers:** Glucose and Galactose are C4 epimers; Glucose and Mannose are C2 epimers. 4. **Clinical Link:** Dihydroxyacetone phosphate (DHAP) is a crucial intermediate in **Glycolysis** and **Gluconeogenesis**, acting as a bridge to lipid metabolism via glycerol-3-phosphate.

Q: Cori's cycle is concerned with the transport of which substance?

Lactate. **Explanation:** The **Cori Cycle** (also known as the Lactic Acid Cycle) is a metabolic pathway that describes the metabolic cooperation between skeletal muscle and the liver. **Why Lactate is Correct:** During vigorous exercise, muscular demand for ATP exceeds the oxygen supply, leading to **anaerobic glycolysis**. In this process, pyruvate is converted into **lactate** by the enzyme *Lactate Dehydrogenase (LDH)* to regenerate NAD+. Since the muscle cannot convert lactate back to glucose, the lactate is released into the bloodstream and transported to the **liver**. In the liver, lactate is converted back to pyruvate and then to glucose via **gluconeogenesis**. This glucose is then released back into the blood to be used by the muscle again, completing the cycle. **Why Other Options are Incorrect:** * **Option A (Alanine):** Alanine is the primary transport molecule in the **Cahill Cycle** (Glucose-Alanine Cycle). This cycle is used to transport amino groups from muscle to the liver for urea synthesis while providing glucose back to the muscle. * **Option B (Glutamate):** While glutamate is a key intermediate in amino acid metabolism and nitrogen transport, it is not the specific substance cycled between muscle and liver in the Cori cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Net Energy Cost:** The Cori cycle is energy-expensive; it consumes **6 ATP** in the liver (gluconeogenesis) while producing only **2 ATP** in the muscle (glycolysis). * **Enzyme:** *Lactate Dehydrogenase* is the key enzyme; LDH-5 (M4) isoenzyme favors the conversion of pyruvate to lactate in the muscle. * **Clinical Relevance:** Failure of the Cori cycle (e.g., in severe liver disease or hypoxia) leads to **Lactic Acidosis**.

Q: Which of the following factors does NOT reduce postprandial glycemia?

Small particle size of food. **Explanation:** The core concept behind postprandial glycemia is the **Glycemic Index (GI)**, which measures how quickly a carbohydrate-containing food raises blood glucose levels. Factors that slow down digestion or absorption reduce postprandial glycemia. **Why Option A is correct:** **Small particle size** (e.g., finely ground flour vs. whole grains) increases the surface area available for digestive enzymes like salivary and pancreatic amylase. This leads to **rapid digestion and faster glucose absorption**, thereby **increasing** postprandial glycemia rather than reducing it. **Why the other options are incorrect:** * **B. Enzyme inhibitors:** Naturally occurring inhibitors (like amylase inhibitors in some legumes) or pharmacological agents (like Acarbose) slow the breakdown of complex carbohydrates, reducing the rate of glucose entry into the blood. * **C. Inadequate cooking:** Raw or undercooked starch exists in a semi-crystalline form (resistant starch) that is difficult for enzymes to penetrate. Cooking causes "gelatinization," making starch digestible; hence, inadequate cooking keeps the glycemic response low. * **D. Presence of protein and fat:** These macronutrients delay **gastric emptying** and stimulate the release of incretins (like GIP and GLP-1), which slows the rise of blood glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Soluble Fiber:** (e.g., Pectin, Gums) increases the viscosity of intestinal contents, slowing glucose absorption and lowering postprandial peaks. * **Resistant Starch:** Acts similarly to dietary fiber and is fermented in the colon rather than absorbed in the small intestine. * **Acarbose/Miglitol:** These are competitive inhibitors of **$\alpha$-glucosidase**, used clinically to specifically target and reduce postprandial hyperglycemia in Type 2 Diabetes.

Question 1

An infant appears normal at birth following an uncomplicated pregnancy and begins to nurse. Within 3 days the infant exhibits vomiting and diarrhea. A week later jaundice is evident, and the infant becomes febrile. On physical examination hepatomegaly is present. Laboratory studies show abnormal reducing substances in the urine, and Escherichia coli is cultured from blood. This infant most likely has a deficiency in which of the following enzymes?

Accepted Answer

Galactose-1-phosphate uridyltransferase

Answer

Glucocerebrosidase

Answer

Phenylalanine hydroxylase

Answer

Sphingomyelinase

Question 2

Pyruvate can be converted directly into all of the following except?

Accepted Answer

Phosphoenol pyruvate

Answer

Alanine

Answer

Acetyl CoA

Answer

Lactate

Question 3

Excess of which of the following can result in cataract?

Accepted Answer

Sugar alcohol

Answer

Glucose

Answer

Sugar amines

Answer

Galactose

Question 4

Muscle glycogen stores can NOT be used to provide glucose into the circulation due to lack of which enzyme?

Accepted Answer

Glucose-6-phosphatase

Answer

Glucose-6-phosphate dehydrogenase

Answer

Glutamate dehydrogenase

Answer

Glucokinase

Question 5

Overingestion of fructose leads to which of the following conditions?

Accepted Answer

Hypertriglyceridemia

Answer

Hypouricemia

Answer

Hyperphosphatemia

Answer

Hypoglycemia

Question 6

Which of the following is NOT a product of the Pentose Phosphate Pathway?

Accepted Answer

CO2

Answer

Glyceraldehyde 3-phosphate

Answer

NADPH

Answer

Sedoheptulose phosphate

Question 7

In the conversion of lactic acid to glucose (gluconeogenesis), three reactions of the glycolytic pathway are circumvented. Which of the following enzymes do NOT participate in this process?

Accepted Answer

Pyruvate kinase

Answer

Pyruvate carboxylase

Answer

Phosphoenol pyruvate carboxy kinase

Answer

Glucose-6-phosphatase

Question 8

Which of the following carbohydrates has no asymmetric carbon atom?

Accepted Answer

Dihydroxyacetone

Answer

Glucose

Answer

Glyceraldehyde

Answer

Fructose

Question 9

Cori's cycle is concerned with the transport of which substance?

Accepted Answer

Lactate

Answer

Alanine

Answer

Glutamate

Answer

None of the above

Question 10

Which of the following factors does NOT reduce postprandial glycemia?

Accepted Answer

Small particle size of food

Answer

Presence of enzyme inhibitors in food

Answer

Inadequate cooking of starch

Answer

Presence of protein and fat in association with carbohydrate

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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