Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 721: What is the primary product of fatty acid oxidation (β-oxidation)?

A. Acetyl CoA (Correct Answer)
B. Malonyl CoA
C. Ketone bodies
D. Cholesterol

Explanation: ***Acetyl CoA*** - Beta-oxidation of fatty acids involves a series of reactions that cleave two-carbon units from the fatty acyl chain, forming **acetyl CoA**. - **Acetyl CoA** is the direct product of each cycle of β-oxidation and then enters the **citric acid cycle** to generate ATP or serves as a precursor for other anabolic pathways. *Malonyl CoA* - **Malonyl CoA** is a key intermediate in **fatty acid synthesis**, not degradation. - It's formed from acetyl CoA by acetyl-CoA carboxylase and acts as a substrate for **fatty acid synthase**, and also as a physiological inhibitor of carnitine palmitoyltransferase I (CPT-I), thereby regulating β-oxidation. *Ketone bodies* - **Ketone bodies** (**acetoacetate** and **β-hydroxybutyrate**) are produced from acetyl CoA in the liver during conditions of low glucose availability or prolonged fasting. - They serve as an alternative fuel source for tissues like the brain and muscles, but are secondary products derived from the condensation of acetyl CoA molecules, not the primary direct product of fatty acid breakdown itself. *Cholesterol* - **Cholesterol** is a steroid lipid synthesized from **acetyl CoA** through a complex multi-step pathway (via HMG-CoA reductase pathway). - It is an important structural component of cell membranes and a precursor for steroid hormones and bile acids, but not a direct product of fatty acid catabolism.

Question 722: What is the characteristic nitrogenous product of lecithin hydrolysis?

A. Fatty acids
B. Choline (Correct Answer)
C. Glucose
D. Phosphoric acid

Explanation: ***Choline*** - Lecithin is a type of **phospholipid** called **phosphatidylcholine**, meaning its head group contains choline. - Therefore, during hydrolysis, the **choline** component is released as the characteristic nitrogenous product. *Glucose* - **Glucose** is a simple sugar and a carbohydrate, not a component of lecithin. - It is a primary source of **energy** for cells but is not released during lipid hydrolysis. *Fatty acids* - While **fatty acids** are indeed components of lecithin (two fatty acid chains are attached to the glycerol backbone), they are not nitrogenous. - Fatty acids are **hydrophobic hydrocarbon chains** that make up a significant part of the lipid structure. *Phosphoric acid* - **Phosphoric acid** (or phosphate) is also a component of lecithin, connecting the glycerol backbone to the choline group. - However, it is an **inorganic acid** and does not contain nitrogen.

Question 723: Which hormone inhibits hormone-sensitive lipase?

A. Insulin (Correct Answer)
B. GH
C. ACTH
D. Thyroid hormone

Explanation: ***Insulin*** - **Insulin** is a key anabolic hormone that promotes energy storage and inhibits catabolic processes, including the breakdown of triglycerides. - It directly inhibits **hormone-sensitive lipase (HSL)** activity, thus reducing the release of free fatty acids from adipose tissue. *Thyroid hormone* - **Thyroid hormones** (T3 and T4) generally promote catabolism and increase metabolic rate, including the mobilization of lipids. - They tend to **stimulate rather than inhibit** hormone-sensitive lipase expression and activity. *GH* - **Growth hormone (GH)** has lipolytic effects, meaning it promotes the breakdown of fats to provide energy. - GH typically **stimulates HSL activity** and increases the release of free fatty acids from adipocytes. *ACTH* - **Adrenocorticotropic hormone (ACTH)** primarily stimulates the adrenal cortex to produce cortisol. - **Cortisol** can have lipolytic effects in certain contexts and does not directly inhibit HSL; instead, catecholamines act as direct stimulators of HSL.

Question 724: Which of the following is an ω-6 fatty acid?

A. Cervonic acid
B. Linoleic acid (Correct Answer)
C. Alpha linolenic acid
D. Elaidic acid

Explanation: ***Linoleic acid*** - **Linoleic acid** (LA), an 18-carbon fatty acid with two double bonds (18:2), is classified as an **ω-6 fatty acid** because its first double bond is located at the sixth carbon atom from the methyl end of the fatty acid chain. - It is an **essential fatty acid** that must be obtained through diet, serving as a precursor for other ω-6 fatty acids like arachidonic acid. *Cervonic acid* - **Cervonic acid** is another name for **docosahexaenoic acid (DHA)**, which is an **ω-3 fatty acid** (22:6). - Its first double bond is located at the third carbon from the methyl end. *Alpha linolenic acid* - **Alpha-linolenic acid** (ALA) is an **ω-3 fatty acid** (18:3). - Its first double bond is located at the third carbon atom from the methyl end. *Elaidic acid* - **Elaidic acid** is a **trans fatty acid** (18:1 trans-9). - It is classified as an **ω-9 fatty acid** due to the position of its double bond, but its trans configuration is the primary distinguishing feature.

Question 725: Which of the following is a lipotropic factor?

A. Sphingomyelin
B. Histidine
C. Bilirubin
D. Methionine (Correct Answer)

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**.

Question 726: Why is oxidized LDL considered more atherogenic?

A. Is not recognized by LDL receptors
B. Is taken up by scavenger receptors (Correct Answer)
C. Promotes inflammation in arterial walls
D. Accumulates in macrophages

Explanation: ***Is taken up by scavenger receptors*** - **Oxidized LDL (oxLDL)** is taken up by **scavenger receptors (CD36, SR-A)** on macrophages, which have **no feedback regulation**. - Unlike native LDL receptors that downregulate when cells have sufficient cholesterol, **scavenger receptors continue unlimited uptake**, leading to foam cell formation. - This **unregulated uptake mechanism** is the key reason why oxLDL is **more atherogenic** than native LDL. - The result is lipid-laden macrophages forming **fatty streaks**, the initial lesions of **atherosclerosis**. *Is not recognized by LDL receptors* - While true that oxLDL has **reduced affinity** for native LDL receptors due to oxidative modification, this alone doesn't explain increased atherogenicity. - The critical factor is what happens instead—its uptake via an **alternative, unregulated pathway**. *Accumulates in macrophages* - This is a **consequence** of scavenger receptor uptake, not the primary mechanism. - Foam cell formation occurs **because** of unregulated scavenger receptor uptake, making this a downstream effect. *Promotes inflammation in arterial walls* - OxLDL does promote inflammation through multiple mechanisms (cytokine release, endothelial dysfunction). - However, this is a **secondary effect** that occurs after uptake and accumulation—not the primary reason for atherogenicity.

Question 727: Which of the following is monoenoic acid ?

A. Linoleic acid
B. Oleic acid (Correct Answer)
C. Linolenic acid
D. Arachidonic acid

Explanation: ***Oleic acid*** - **Oleic acid** is a **monounsaturated fatty acid** (MUFA), meaning it has **one double bond** in its hydrocarbon chain. - Its presence in many natural fats and oils makes it a significant component of the human diet. *Arachidonic acid* - **Arachidonic acid** is a **polyunsaturated fatty acid** (PUFA) containing **four double bonds**. - It is a precursor for **eicosanoids**, including prostaglandins and leukotrienes, involved in inflammation and other physiological processes. *Linoleic acid* - **Linoleic acid** is an **essential omega-6 polyunsaturated fatty acid** with **two double bonds**. - It is crucial for human health and serves as a precursor for other fatty acids like arachidonic acid. *Linolenic acid* - **Linolenic acid** refers to two essential fatty acids: **alpha-linolenic acid (ALA)**, an omega-3 fatty acid with **three double bonds**, and **gamma-linolenic acid (GLA)**, an omega-6 fatty acid also with three double bonds. - Both are **polyunsaturated fatty acids** with multiple double bonds.

Question 728: What is the role of Apoprotein C-II in lipid metabolism?

A. None of the options
B. Inhibits lipoprotein lipase
C. Facilitates triglyceride transport
D. Activates lipoprotein lipase (Correct Answer)

Explanation: ***Activates lipoprotein lipase*** - **Apoprotein C-II (ApoC-II)** is a crucial **activator** of **lipoprotein lipase (LPL)**. - LPL is an enzyme responsible for **hydrolyzing triglycerides** from chylomicrons and VLDL, allowing fatty acids to be taken up by tissues. - **Deficiency of ApoC-II** leads to severe hypertriglyceridemia due to inability to activate LPL. *Inhibits lipoprotein lipase* - This is the function of **ApoC-III**, not ApoC-II. - ApoC-III **inhibits LPL activity**, which is the opposite role of ApoC-II. *Facilitates triglyceride transport* - While apoproteins are essential for **assembly and transport of lipoproteins** that carry triglycerides, this is not the specific primary role of ApoC-II. - ApoC-II's primary function is **regulating LPL enzyme activity**, not direct transport facilitation. *None of the options* - This is incorrect because ApoC-II clearly **activates lipoprotein lipase**, which is one of the given options.

Question 729: In a person fasting overnight with carnitine deficiency, which of the following chemicals increase in quantity in blood?

A. Ketone body levels
B. Fatty acid levels (Correct Answer)
C. Glucose levels
D. Amino acid levels

Explanation: ***Fatty acid levels*** - **Carnitine deficiency** impairs the transport of **long-chain fatty acids** into the mitochondria for beta-oxidation. - This leads to an accumulation of **fatty acids** in the blood as they cannot be efficiently metabolized for energy during fasting. - Therefore, **fatty acid levels increase** in the blood. *Ketone body levels* - **Ketone bodies** are produced from the **beta-oxidation of fatty acids** in the liver. - With **carnitine deficiency**, fatty acid oxidation is impaired, thus **reducing** the production of ketone bodies, not increasing them. *Glucose levels* - During **fasting**, the body relies on **gluconeogenesis** and **glycogenolysis** to maintain glucose levels. - With carnitine deficiency primarily affecting fat metabolism and preventing fatty acid utilization, the body cannot spare glucose effectively. - This leads to **hypoglycemia** (decreased glucose), not increased glucose levels. *Amino acid levels* - **Amino acid metabolism** is largely independent of **carnitine**. - While amino acids can be used for gluconeogenesis during fasting, carnitine deficiency does not directly cause an increase in circulating amino acid levels.

Question 730: Which of the following fatty acids has the maximum number of carbon atoms?

A. Oleic acid
B. Linolenic acid
C. Arachidonic acid
D. Cervonic acid (Correct Answer)

Explanation: **Cervonic acid** - **Cervonic acid**, also known as **docosahexaenoic acid (DHA)**, is a long-chain omega-3 fatty acid with **22 carbon atoms** and 6 double bonds (22:6). - It is a primary structural component of the brain and retina and is the longest fatty acid among the options provided. *Oleic acid* - **Oleic acid** is a monounsaturated fatty acid with **18 carbon atoms** and one double bond (18:1). - It is a common fatty acid found in many animal fats and vegetable oils, but it has fewer carbon atoms than cervonic acid. *Linolenic acid* - **Linolenic acid** refers to two essential fatty acids: alpha-linolenic acid (ALA) and gamma-linolenic acid (GLA). Both have **18 carbon atoms**. - Alpha-linolenic acid (ALA) is an omega-3 fatty acid with 3 double bonds (18:3), while gamma-linolenic acid (GLA) is an omega-6 fatty acid with 3 double bonds (18:3), neither of which has more carbon atoms than cervonic acid. *Arachidonic acid* - **Arachidonic acid** is an omega-6 fatty acid with **20 carbon atoms** and four double bonds (20:4). - It is a precursor to eicosanoids and is longer than oleic and linolenic acids but shorter than cervonic acid.

Practice by Chapter

Lipid Classification and Chemistry

Practice Questions

Fatty Acid Oxidation

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Ketone Body Metabolism

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Fatty Acid Synthesis

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Metabolism of Triacylglycerols

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Phospholipid Metabolism

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Cholesterol Metabolism and Biosynthesis

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Bile Acids and Bile Salts

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Lipoprotein Metabolism and Transport

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Dyslipidemias and Atherosclerosis

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Prostaglandins and Eicosanoids

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Fatty Liver and Lipotropic Factors

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