Metabolism of Triacylglycerols Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Metabolism of Triacylglycerols. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Metabolism of Triacylglycerols Indian Medical PG Question 1: A person switches from a high-fat diet to a low-fat diet with a compensatory increase in carbohydrates to maintain the same caloric intake. Which lipoprotein is likely to increase?
- A. Chylomicron
- B. IDL
- C. HDL
- D. VLDL (Correct Answer)
Metabolism of Triacylglycerols Explanation: ***VLDL***
- A low-fat diet with increased **carbohydrates** can lead to increased hepatic synthesis of triglycerides, which are then packaged into **VLDL** particles for transport from the liver. This is because excess carbohydrates can be converted to fatty acids and then to triglycerides in the liver.
- The liver's increased triglyceride production, driven by abundant **glucose** from carbohydrates, directly corresponds to a rise in **VLDL** secretion to export these lipids.
*Chylomicron*
- **Chylomicrons** primarily transport **dietary fats** (exogenous triglycerides) absorbed from the intestine.
- Switching to a low-fat diet would typically lead to a *decrease* in chylomicron production, as less dietary fat is available for absorption.
*IDL*
- **IDL** (Intermediate-Density Lipoprotein) is a remnant of **VLDL** metabolism, formed after VLDL loses some triglycerides.
- While VLDL may increase, leading to *more* IDL formation, IDL itself is not the primary component that *increases* directly due to high carbohydrate intake; rather, the precursor **VLDL** is directly affected.
*HDL*
- **HDL** (High-Density Lipoprotein) is involved in **reverse cholesterol transport**, picking up excess cholesterol from peripheral tissues and returning it to the liver.
- High carbohydrate intake, especially refined carbohydrates, can sometimes lead to a *decrease* in HDL levels, not an increase.
Metabolism of Triacylglycerols Indian Medical PG Question 2: Which of the following actions of GH is mediated by IGF-1?
- A. Na+ retention
- B. decreases insulin
- C. Antilipolysis (Correct Answer)
- D. Lipolysis
Metabolism of Triacylglycerols Explanation: ***Antilipolysis***
* **Insulin-like growth factor 1 (IGF-1)**, stimulated by GH, plays a role in reducing **lipolysis** indirectly.
* IGF-1 promotes **anabolic processes** and nutrient storage, which can lead to decreased fat breakdown.
*Na+ retention*
* **Na+ retention** is more directly influenced by hormones like **aldosterone** and **ADH**, not IGF-1.
* While GH can exert some influence on fluid and electrolyte balance, this specific action is not primarily mediated by IGF-1.
*decreases insulin*
* IGF-1 and GH generally tend to **increase insulin sensitivity** in some tissues or antagonize insulin effects indirectly.
* IGF-1's primary metabolic role is not to decrease insulin itself directly.
*Lipolysis*
* **Growth hormone (GH)** directly promotes **lipolysis**, breaking down fat for energy.
* However, the question specifically asks for actions mediated by **IGF-1**, which has an opposite, antilipolytic effect.
Metabolism of Triacylglycerols Indian Medical PG Question 3: In case of LPL deficiency, which of the following will increase after a fat rich diet?
- A. LDL
- B. HDL
- C. Lipoprotein (a)
- D. Chylomicron (Correct Answer)
Metabolism of Triacylglycerols Explanation: ***Chylomicron***
- **LPL (lipoprotein lipase)** is crucial for the breakdown of **chylomicrons** and VLDL. A deficiency leads to an accumulation of undigested chylomicrons in the bloodstream after a fat-rich meal.
- **Chylomicrons** transport dietary triglycerides from the intestines to tissues. Without LPL, these triglycerides remain packaged in chylomicrons.
*LDL*
- **LDL (low-density lipoprotein)** levels are not directly increased by a short-term fat-rich diet in the context of LPL deficiency. LDL primarily carries cholesterol and is formed from VLDL remnants, a process that is also impaired by LPL deficiency indirectly.
- While chronic LPL deficiency can affect overall lipid metabolism, the immediate post-meal increase is not in LDL but in triglyceride-rich lipoproteins.
*HDL*
- **HDL (high-density lipoprotein)** is involved in reverse cholesterol transport and is generally not directly increased after a fat-rich diet, especially in LPL deficiency.
- In fact, severe hypertriglyceridemia, often seen in LPL deficiency, can sometimes lead to lower HDL levels due to altered lipid exchange.
*Lipoprotein (a)*
- **Lipoprotein (a)**, or Lp(a), is a genetically determined lipoprotein similar to LDL but with an added apolipoprotein (a) and its levels are not acutely affected by dietary fat intake or LPL deficiency.
- Lp(a) levels are determined primarily by genetic factors and do not participate in the post-prandial handling of dietary fats.
Metabolism of Triacylglycerols Indian Medical PG Question 4: A patient came to the emergency room with severe abdominal pain. The serum triglyceride level was $1500 \mathrm{mg} / \mathrm{dL}$. What is the most likely defect?
- A. Apo B-48
- B. Apo B-100
- C. Apo C-II (Correct Answer)
- D. LDL receptor
- E. Lipoprotein lipase
Metabolism of Triacylglycerols Explanation: ***Apo C-II***
- **Apo C-II** is an essential cofactor for **lipoprotein lipase (LPL)**, which is responsible for hydrolyzing triglycerides from chylomicrons and VLDL.
- A defect in Apo C-II leads to severely impaired triglyceride clearance, resulting in **chylomicronemia** and extremely high serum triglyceride levels (e.g., 1500 mg/dL), which can cause acute pancreatitis.
- Both Apo C-II deficiency and LPL deficiency present similarly, but Apo C-II deficiency is the more specific answer when considering the **"defect"** terminology, as it represents the regulatory cofactor rather than the enzyme itself.
*Apo B-48*
- **Apo B-48** is a structural protein uniquely found on **chylomicrons**, synthesized in the intestine, and is essential for their formation and secretion.
- A defect in Apo B-48 (e.g., in abetalipoproteinemia) would lead to the **absence of chylomicrons**, resulting in very low or undetectable triglyceride levels after a fat-containing meal, not high levels.
*Apo B-100*
- **Apo B-100** is the primary apolipoprotein of **VLDL, IDL, and LDL**, and it is crucial for VLDL assembly in the liver and for LDL receptor binding.
- Defects in Apo B-100 leading to hyperlipidemia typically cause elevated LDL cholesterol (e.g., familial defective Apo B-100), rather than severe hypertriglyceridemia associated with chylomicronemia.
*LDL receptor*
- The **LDL receptor** is responsible for the uptake of **LDL particles** from the bloodstream, primarily in the liver.
- A defect in the LDL receptor (e.g., in familial hypercholesterolemia) primarily causes **elevated LDL cholesterol** levels, but typically does not lead to the extreme hypertriglyceridemia seen in this patient.
*Lipoprotein lipase*
- **Lipoprotein lipase (LPL)** is the enzyme that hydrolyzes triglycerides in chylomicrons and VLDL particles.
- A primary deficiency of LPL itself (Type I familial chylomicronemia) would also cause severe hypertriglyceridemia similar to Apo C-II deficiency.
- However, Apo C-II deficiency is the more specific answer as it represents the **cofactor defect** that impairs LPL function, while direct LPL deficiency is a separate genetic entity.
Metabolism of Triacylglycerols Indian Medical PG Question 5: All are activated by insulin except?
- A. Lipoprotein lipase
- B. Pyruvate kinase
- C. Acetyl-CoA carboxylase
- D. Hormone sensitive lipase (Correct Answer)
Metabolism of Triacylglycerols Explanation: ***Hormone sensitive lipase***
- **Insulin** is an **anabolic hormone** that promotes energy storage; it **inhibits** hormone-sensitive lipase (HSL) activity which is responsible for **fat breakdown (lipolysis)**.
- When insulin levels are high, the body stores fat rather than breaks it down, thus **decreasing** HSL activity.
*Lipoprotein lipase*
- **Insulin activates lipoprotein lipase (LPL)**, an enzyme that breaks down triglycerides in **chylomicrons** and **VLDL** into fatty acids for storage in adipose tissue.
- This activation promotes the uptake of fatty acids into fat cells, aligning with insulin's role in **energy storage**.
*Pyruvate kinase*
- **Insulin activates pyruvate kinase** in glycolysis, promoting the conversion of **phosphoenolpyruvate to pyruvate**.
- This enzyme's activation enhances glucose utilization and energy production following a meal when insulin levels are high.
*Acetyl-CoA carboxylase*
- **Insulin activates acetyl-CoA carboxylase (ACC)**, the **rate-limiting enzyme in fatty acid synthesis**.
- Activation of ACC leads to the production of **malonyl-CoA**, which commits acetyl-CoA to fatty acid synthesis, storing excess energy as fat.
Metabolism of Triacylglycerols Indian Medical PG Question 6: Which of the following is not a substrate for glucose formation?
- A. Lactate
- B. Glycerol
- C. Alanine
- D. Acetyl coenzyme A (Correct Answer)
Metabolism of Triacylglycerols Explanation: ***Acetyl coenzyme A***
- **Acetyl CoA** cannot be converted to glucose because the two carbons from the acetyl group are lost as carbon dioxide in the **Krebs cycle**, making a net synthesis of glucose impossible.
- The irreversible nature of the **pyruvate dehydrogenase complex** prevents the conversion of Acetyl CoA back to **pyruvate**, which is a crucial step for gluconeogenesis.
*Lactate*
- **Lactate** is a major substrate for gluconeogenesis, particularly during exercise and fasting, via the **Cori cycle**.
- **Lactate dehydrogenase** converts lactate to **pyruvate**, which can then enter the gluconeogenic pathway.
*Glycerol*
- **Glycerol**, derived from triglyceride breakdown, enters gluconeogenesis by being converted to **glycerol-3-phosphate** and then to **dihydroxyacetone phosphate (DHAP)**.
- DHAP is an intermediate in glycolysis and gluconeogenesis, allowing for its conversion to glucose.
*Alanine*
- **Alanine** is a **glucogenic amino acid** that can be transaminated to **pyruvate**.
- **Pyruvate** can then proceed through the gluconeogenic pathway to synthesize glucose, especially during prolonged fasting.
Metabolism of Triacylglycerols Indian Medical PG Question 7: Active metabolite in the synthesis of fatty acids is:
- A. Malonyl CoA (Correct Answer)
- B. Stearate
- C. Acetyl CoA
- D. Palmitate
Metabolism of Triacylglycerols Explanation: ***Malonyl CoA***
- **Malonyl CoA** is the immediate **two-carbon donor** in fatty acid synthesis, formed from acetyl CoA and bicarbonate.
- It adds **two-carbon units** to the growing fatty acid chain during each cycle of synthesis, making it the primary active metabolic form in this process.
*Stearate (an end product of fatty acid synthesis)*
- **Stearate** is a **saturated fatty acid end product** (C18:0) of fatty acid synthesis, not an active metabolite that directly participates in the elongation process.
- While it is a result of fatty acid synthesis, it does not serve as a building block for further elongation in the manner of malonyl CoA.
*Acetyl CoA (a precursor in fatty acid synthesis)*
- **Acetyl CoA** is the **initial precursor** for fatty acid synthesis, which is then carboxylated to form malonyl CoA.
- It is not the *active* two-carbon donor during the elongation steps of fatty acid synthesis itself, but rather the substrate for malonyl CoA synthesis.
*Palmitate (an end product of fatty acid synthesis)*
- **Palmitate** is the **primary 16-carbon saturated fatty acid** and is the usual end product of *de novo* fatty acid synthesis in humans.
- Like stearate, it is an end product and does not serve as an active metabolic intermediate for chain elongation during the synthesis process itself.
Metabolism of Triacylglycerols Indian Medical PG Question 8: What is the primary mechanism of action of 5-α reductase?
- A. Reduction of C4-C5 double bond (Correct Answer)
- B. Breakage of amide bond
- C. Breakage of C-N bond
- D. Breakage of N-N bond
Metabolism of Triacylglycerols Explanation: ***Reduction of C4-C5 double bond***
- 5-α reductase is a **NADPH-dependent reductase enzyme** that catalyzes the **reduction (saturation) of the C4-C5 double bond** in the A-ring of testosterone to form **dihydrotestosterone (DHT)**.
- This reduction involves **adding two hydrogen atoms** across the double bond, converting it to a single bond with **5-α stereochemistry**.
- DHT is a more potent androgen crucial for **prostate development, external genitalia formation, and male pattern baldness**, making 5-α reductase inhibitors (like finasteride) clinically important for treating benign prostatic hyperplasia and androgenetic alopecia.
*Breakage of amide bond*
- Breaking **amide bonds (C-N bonds with a carbonyl)** is the function of **proteases and amidases**, not reductases.
- This process involves **hydrolysis** and is fundamental to protein degradation and peptide metabolism.
*Breakage of C-N bond*
- **Carbon-nitrogen bond cleavage** occurs in reactions like **deamination** (catalyzed by deaminases) or metabolism of nitrogenous compounds.
- Reductases perform **electron transfer reactions**, not bond cleavage reactions.
*Breakage of N-N bond*
- **Nitrogen-nitrogen bond** cleavage is rare in human biochemistry and may occur in hydrazine metabolism or by specialized enzymes.
- Steroid hormones do not contain N-N bonds, making this mechanism irrelevant to 5-α reductase function.
Metabolism of Triacylglycerols Indian Medical PG Question 9: What is the primary receptor for High-Density Lipoprotein (HDL) in cholesterol metabolism?
- A. SR-BI (Correct Answer)
- B. LDLR
- C. HDLR
- D. SR-82
Metabolism of Triacylglycerols Explanation: ***SR-BI***
- **Scavenger Receptor class B type 1 (SR-BI)** is the primary receptor responsible for selective uptake of **cholesteryl esters** from HDL into cells, particularly the liver and steroidogenic tissues.
- Unlike other lipoprotein receptors, SR-BI mediates the **selective transfer** of cholesterol without internalizing the entire HDL particle.
*LDLR*
- The **Low-Density Lipoprotein Receptor (LDLR)** is the primary receptor for **LDL** and very low-density lipoprotein (VLDL) remnants, mediating their endocytosis and degradation.
- While it plays a crucial role in cholesterol metabolism, its main function is related to the uptake of **LDL cholesterol**, not HDL.
*HDLR*
- **HDLR** is not a recognized receptor in cholesterol metabolism.
- This term may be a distracter created by combining HDL with the common receptor nomenclature.
*SR-82*
- **SR-82** is not a recognized receptor involved in cholesterol metabolism.
- Similar to HDLR, this is a distracter term.
Metabolism of Triacylglycerols Indian Medical PG Question 10: Which of the following is a lipotropic factor?
- A. Sphingomyelin
- B. Histidine
- C. Bilirubin
- D. Methionine (Correct Answer)
Metabolism of Triacylglycerols Explanation: ***Methionine***
- **Methionine** is an essential amino acid that serves as a precursor for **choline** and **creatine**, both of which play crucial roles in lipid metabolism and transport.
- Lipotropic factors prevent or reverse the accumulation of **fat in the liver** by promoting the synthesis of **lipoproteins**, which package and transport fats from the liver to other tissues.
*Sphingomyelin*
- **Sphingomyelin** is a type of **sphingolipid**, a component of cell membranes and myelin sheaths, but it does not directly act as a lipotropic factor to prevent fatty liver.
- While it's involved in cellular signaling and membrane structure, it does not directly facilitate the metabolism or transport of **hepatic triglycerides** in the same way as lipotropic agents.
*Histidine*
- **Histidine** is an essential amino acid involved in protein synthesis and the production of **histamine**, but it is not considered a primary lipotropic factor.
- Its main roles are in **immune response** and **neurotransmission**, not in preventing fat accumulation in the liver.
*Bilirubin*
- **Bilirubin** is a waste product from the breakdown of **heme**, primarily from red blood cells. It is excreted by the liver.
- It is known for its **antioxidant properties** but does not play a direct role as a lipotropic factor in lipid metabolism or in preventing **fatty liver**.
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