Carbohydrate Metabolism Practice Questions

Q: Which of the following reactions in the TCA cycle is an example of substrate-level phosphorylation?

Succinyl CoA to succinate. ### Explanation **Correct Answer: C. Succinyl CoA to succinate** In the TCA cycle, the conversion of **Succinyl CoA to Succinate** is the only step that generates a high-energy phosphate bond directly without the involvement of the electron transport chain. This process is called **Substrate-Level Phosphorylation (SLP)**. The enzyme **Succinate thiokinase** (also known as Succinyl CoA synthetase) cleaves the high-energy thioester bond of Succinyl CoA. The energy released is used to phosphorylate GDP to **GTP** (in the liver and kidneys) or ADP to **ATP** (in heart and muscle). This is a high-yield fact because it represents the only "direct" energy gain within the cycle itself. #### Analysis of Incorrect Options: * **Option A (Isocitrate to oxalosuccinate):** This is part of the reaction catalyzed by *Isocitrate dehydrogenase*. It involves the reduction of NAD+ to **NADH**, not the direct formation of ATP/GTP. * **Option B (Alpha-ketoglutarate to succinyl CoA):** Catalyzed by the *α-ketoglutarate dehydrogenase complex*, this oxidative decarboxylation produces **NADH** and CO₂. * **Option D (Succinate to fumarate):** Catalyzed by *Succinate dehydrogenase* (Complex II of ETC), this reaction involves the reduction of FAD to **FADH₂**. #### NEET-PG High-Yield Pearls: 1. **Location:** The TCA cycle occurs in the **mitochondrial matrix**, but Succinate dehydrogenase is the only enzyme attached to the **inner mitochondrial membrane**. 2. **Arsenite Poisoning:** The α-ketoglutarate dehydrogenase complex is inhibited by Arsenite. 3. **ATP Yield:** One turn of the TCA cycle yields **10 ATP** (3 NADH = 7.5, 1 FADH₂ = 1.5, 1 GTP = 1). 4. **Regulatory Step:** The conversion of Isocitrate to α-ketoglutarate is the **rate-limiting step** of the cycle.

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

Fructose from glucose. 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.

Q: Diabetes mellitus is associated with which type of lactic acidosis?

Type B lactic acidosis. ### Explanation Lactic acidosis is classified into two primary types based on the presence or absence of tissue hypoxia. **Why Type B is Correct:** **Type B lactic acidosis** occurs in the **absence of clinical evidence of tissue hypoxia** or hypoperfusion. In Diabetes Mellitus (DM), lactic acidosis occurs primarily due to metabolic derangements rather than oxygen debt. 1. **Metabolic Dysfunction:** In DM, there is an impairment of the enzyme **pyruvate dehydrogenase (PDH)**, leading to the diversion of pyruvate to lactate. 2. **Drug-Induced:** A classic high-yield association is the use of **Metformin** (a Biguanide), which inhibits mitochondrial respiration and gluconeogenesis, leading to lactate accumulation. 3. **Ketoacidosis:** Severe DKA can impair lactate clearance in the liver. **Analysis of Incorrect Options:** * **Type A Lactic Acidosis:** This is caused by **hypoxia or hypoperfusion** (e.g., shock, septicemia, severe hemorrhage, or heart failure). While a diabetic patient *can* develop shock, the primary association of the disease process itself is Type B. * **Type D Lactic Acidosis:** This is caused by **D-lactate** produced by abnormal gut bacteria, typically seen in **Short Bowel Syndrome**. Standard laboratory tests for lactate only measure the L-isomer, making this a distinct clinical entity unrelated to DM. **High-Yield Clinical Pearls for NEET-PG:** * **Normal Lactate Levels:** 5 mmol/L with a pH <7.35. * **Metformin Warning:** Metformin is contraindicated if the eGFR is <30 mL/min/1.73 m² due to the high risk of Type B lactic acidosis. * **Enzyme Link:** Deficiency of **Thiamine (B1)** can also cause Type B lactic acidosis because it is a cofactor for PDH.

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

GLUT 4. ***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.

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

Galactitol. **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.

Q: Hers disease is due to deficiency of which enzyme?

Liver phosphorylase. **Explanation:** **Hers Disease (GSD Type VI)** is a glycogen storage disease caused by a deficiency of **Liver Phosphorylase**. This enzyme is responsible for the rate-limiting step of glycogenolysis in the liver, breaking down glycogen into glucose-1-phosphate. When deficient, glycogen accumulates in the liver, leading to hepatomegaly and mild fasting hypoglycemia (milder than Von Gierke’s because gluconeogenesis remains intact). **Analysis of Options:** * **Option A (Correct):** Liver phosphorylase deficiency leads to Hers disease. It presents with growth retardation and hepatomegaly in early childhood. * **Option B (Incorrect):** Deficiency of **Muscle Phosphorylase** causes **McArdle Disease (GSD Type V)**, characterized by exercise-induced muscle cramps and myoglobinuria. * **Option C (Incorrect):** Deficiency of the **Branching Enzyme** (α-1,4 → α-1,6 transglucosidase) causes **Andersen Disease (GSD Type IV)**, which typically leads to liver cirrhosis and early infantile death. * **Option D (Incorrect):** Deficiency of **Phosphofructokinase (PFK-1)** in muscles and RBCs causes **Tarui Disease (GSD Type VII)**, presenting similarly to McArdle disease but with associated hemolytic anemia. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Remember **"H"** for **H**ers and **H**epatic phosphorylase; **"M"** for **M**cArdle and **M**uscle phosphorylase. * Unlike Type I (Von Gierke), Hers disease typically has **normal** levels of lactate and uric acid. * Ketosis is a common finding in Hers disease during fasting. * **GSD Type VI and IX** (Phosphorylase kinase deficiency) are clinically similar and often grouped together as they both affect the activation of liver phosphorylase.

Q: In which of the following steps is ATP released?

Phosphoenolpyruvate to pyruvate. ### 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.

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

Mucic acid. **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.

Q: Cataract in galactosemia is caused by the accumulation of which substance?

Galactitol. **Explanation:** The correct answer is **Galactitol (Dulcitol)**. **1. Why Galactitol is correct:** In patients with Galactosemia (due to deficiency of GALT, GALK, or GALE enzymes), galactose levels rise in the blood and tissues. In the lens of the eye, the enzyme **Aldose Reductase** reduces excess galactose into its sugar alcohol form, **Galactitol**. Unlike galactose, galactitol cannot be further metabolized or easily diffuse out of the lens. It acts as an osmotically active substance, drawing water into the lens fibers. This leads to swelling, denaturation of lens proteins, and subsequent opacity, forming a **cataract**. **2. Why other options are incorrect:** * **Sorbitol:** While sorbitol also causes cataracts via the same osmotic mechanism, it is derived from **Glucose** (seen in Diabetes Mellitus), not galactose. * **Galactose:** While galactose is the substrate that accumulates systemically, it is not the direct cause of the cataract; its metabolic byproduct (galactitol) is the culprit due to its osmotic effect. **3. High-Yield Clinical Pearls for NEET-PG:** * **Classic Galactosemia:** Deficiency of **Galactose-1-phosphate uridyltransferase (GALT)**. It presents with "Oil-drop cataracts," hepatosplenomegaly, and jaundice. * **Galactokinase Deficiency (GALK):** A milder form where the primary clinical manifestation is early-onset cataracts without the severe systemic features of GALT deficiency. * **The Polyol Pathway:** This pathway (Aldose Reductase) is responsible for both diabetic cataracts (Glucose $\rightarrow$ Sorbitol) and galactosemic cataracts (Galactose $\rightarrow$ Galactitol). * **Key Enzyme:** Aldose Reductase has a high $K_m$ for glucose/galactose, meaning it only becomes active when sugar levels are significantly elevated.

Question 1

Which of the following reactions in the TCA cycle is an example of substrate-level phosphorylation?

Accepted Answer

Succinyl CoA to succinate

Answer

Isocitrate to oxalosuccinate

Answer

Alpha-ketoglutarate to succinyl CoA

Answer

Succinate to fumarate

Question 2

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?

Accepted Answer

The production of lactose by the mammary gland does not require the ingestion of milk or milk products.

Answer

She must consume pure galactose to produce the galactose moiety of lactose.

Answer

She will be unable to breastfeed her baby because she cannot produce lactose.

Answer

She can produce lactose directly by degrading a-lactalbumin.

Question 3

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

Accepted Answer

Fructose from glucose

Answer

Galactose from fructose

Answer

Galactose from glucose

Answer

Glucose from fructose

Question 4

Diabetes mellitus is associated with which type of lactic acidosis?

Accepted Answer

Type B lactic acidosis

Answer

Type A lactic acidosis

Answer

Type D lactic acidosis

Answer

All of the above

Question 5

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

Accepted Answer

GLUT 4

Answer

GLUT 1

Answer

GLUT 2

Answer

GLUT 3

Question 6

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

Accepted Answer

Galactitol

Answer

Sorbitol

Answer

Mannitol

Answer

Inositol

Question 7

Hers disease is due to deficiency of which enzyme?

Accepted Answer

Liver phosphorylase

Answer

Muscle phosphorylase

Answer

Branching enzyme

Answer

RBC phosphofructokinase

Question 8

In which of the following steps is ATP released?

Accepted Answer

Phosphoenolpyruvate to pyruvate

Answer

Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate

Answer

Fructose-6-phosphate to fructose-1,6-bisphosphate

Answer

Glucose to glucose-6-phosphate

Question 9

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

Accepted Answer

Mucic acid

Answer

Gluconic acid

Answer

Galacturonic acid

Answer

Saccharic acid

Question 10

Cataract in galactosemia is caused by the accumulation of which substance?

Accepted Answer

Galactitol

Answer

Sorbitol

Answer

Galactose

Answer

None of the above

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?