Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 741: The fatty acid present in breast milk which is important for growth and CNS development is:

A. Docosahexaenoic acid (Correct Answer)
B. Palmitic acid
C. Linolenic acid
D. Linoleic acid

Explanation: ***Docosahexaenoic acid*** - **Docosahexaenoic acid (DHA)** is a crucial **omega-3 fatty acid** abundantly found in breast milk. - It plays a vital role in the **development of the brain** and **retina**, particularly during infancy. *Palmitic acid* - **Palmitic acid** is a **saturated fatty acid** and a major component of fats in both plants and animals, including breast milk. - While it serves as an **energy source**, it is not primarily recognized for its specific role in CNS development like DHA. *Linolenic acid* - **Alpha-linolenic acid (ALA)** is an **omega-3 essential fatty acid** that is a precursor to DHA and EPA, meaning the body can convert it into these longer-chain fatty acids. - While important as a precursor, **ALA itself is not the direct fatty acid** primarily responsible for brain and CNS development in the same way DHA is. *Linoleic acid* - **Linoleic acid** is an **omega-6 essential fatty acid**, meaning the body cannot produce it and it must be obtained from the diet. - It is important for overall growth and development, but it is not specifically highlighted for its direct role in **CNS development** in the same way DHA is for breast milk.

Question 742: Dietary cholesterol is transported from the intestine to the liver by:

A. Apo-E
B. Apo-A
C. Apo-C
D. Apo-B (Correct Answer)

Explanation: ***Apo-B*** - **Apolipoprotein B-48** (Apo-B48) is the key structural protein of **chylomicrons**, which are large lipoprotein particles formed in intestinal enterocytes. - Chylomicrons are responsible for transporting **dietary triglycerides** and **cholesterol** from the intestine via the lymphatic system into the bloodstream and ultimately to the liver. - Apo-B48 is essential for chylomicron assembly and secretion from the intestine. *Apo-A* - **Apolipoprotein A-I** (ApoA-I) is the primary apolipoprotein of **high-density lipoprotein (HDL)**. - HDL is mainly involved in **reverse cholesterol transport**, moving cholesterol from peripheral tissues back to the liver. *Apo-C* - **Apolipoprotein C-II** (ApoC-II) is an activator of **lipoprotein lipase (LPL)**, which metabolizes triglycerides in chylomicrons and VLDL. - **Apolipoprotein C-III** (ApoC-III) inhibits LPL activity and hepatic uptake of triglyceride-rich lipoproteins. *Apo-E* - **Apolipoprotein E** (ApoE) is crucial for the receptor-mediated uptake of **chylomicron remnants** and **VLDL remnants** by the liver. - While involved in remnant clearance, it is not the primary apolipoprotein for the initial transport of dietary cholesterol from the intestine within intact chylomicrons.

Question 743: What is the primary glycerol-containing absorption product of dietary triglycerides in the gut?

A. Fatty acids
B. Mono acyl glycerol (Correct Answer)
C. Diacyl glycerol
D. Triglycerides

Explanation: ***Monoacylglycerol (2-MAG)*** - **Pancreatic lipase** hydrolyzes dietary triglycerides at positions 1 and 3, producing **2-monoacylglycerol** and **free fatty acids** as the two major products. - As the **primary glycerol-containing product**, 2-monoacylglycerol constitutes approximately **25% of digested lipid mass** and is essential for efficient **triglyceride resynthesis** in enterocytes. - Both 2-MAG and fatty acids are packaged into **mixed micelles** and co-absorbed, but 2-MAG is the predominant form containing the **intact glycerol backbone**. *Fatty acids* - **Free fatty acids** are co-absorbed with 2-monoacylglycerol in approximately a **2:1 molar ratio** (2 fatty acids per 1 monoacylglycerol). - While quantitatively significant, fatty acids do **not contain the glycerol backbone** structure. - Both products are required for efficient absorption and subsequent **triglyceride resynthesis** within enterocytes. *Diacylglycerol* - **Diacylglycerol** is a minor intermediate product during triglyceride hydrolysis. - It is rapidly hydrolyzed further by **pancreatic lipase** to yield **monoacylglycerol** and free fatty acids. - Not a significant absorption product in the intestinal lumen. *Triglycerides* - **Intact triglycerides** are too large and hydrophobic to be absorbed directly across the intestinal brush border. - They must first undergo **enzymatic hydrolysis** by pancreatic lipase into smaller absorbable units (2-MAG and fatty acids).

Question 744: Acetyl CoA carboxylase is stimulated by all except which of the following?

A. Acyl CoA (Correct Answer)
B. ATP
C. Insulin
D. Citrate

Explanation: ***Acyl CoA*** - **Acyl CoA** (specifically long-chain fatty acyl CoAs) is an **inhibitor** of acetyl CoA carboxylase (ACC), signifying an abundance of fatty acids and a need to reduce further synthesis. - This feedback inhibition helps regulate **fatty acid synthesis**, ensuring that the pathway is downregulated when sufficient fatty acids are present. *Citrate* - **Citrate** is a potent **allosteric activator** of acetyl CoA carboxylase, indicating a high energy state and excess mitochondrial acetyl CoA, which can be channeled into fatty acid synthesis. - Its presence promotes the polymerization of ACC monomers into active polymers, enhancing enzyme activity. *ATP* - **ATP** is required as a substrate for the carboxylation reaction catalyzed by ACC, providing the energy for the formation of **malonyl CoA**. - High levels of ATP indirectly signal a state of energy abundance, which favors anabolic processes like fatty acid synthesis. *Insulin* - **Insulin** is a hormonal activator of acetyl CoA carboxylase, promoting its dephosphorylation via **protein phosphatase 2A**. - This dephosphorylation leads to increased enzyme activity, stimulating **fatty acid synthesis** in response to high blood glucose after a meal.

Question 745: Insulin promotes lipogenesis by all except?

A. Increases the transport of glucose in to the cell
B. Inhibits PDH (Correct Answer)
C. Increasing acetyl co A carboxylase activity
D. Decreases intracellular cAMP level

Explanation: ***Inhibits PDH*** - Insulin **activates pyruvate dehydrogenase (PDH)**, not inhibits it, converting pyruvate to acetyl-CoA for fatty acid synthesis. - Since insulin **does not inhibit PDH**, this is the correct answer to this EXCEPT question. - Inhibiting PDH would decrease lipogenesis, which is opposite to insulin's effect. *Increasing acetyl CoA carboxylase activity* - **Acetyl-CoA carboxylase (ACC)** is the rate-limiting enzyme in fatty acid synthesis. - Insulin **activates ACC** (by dephosphorylation), which is a key mechanism for promoting lipogenesis. *Increases the transport of glucose into the cell* - Insulin enhances glucose uptake into adipose tissue and muscle cells by promoting **GLUT4 transporter** translocation to the cell membrane. - This increased glucose provides substrate (via glycolysis to acetyl-CoA) for fatty acid synthesis. *Decreases intracellular cAMP level* - Insulin **lowers intracellular cAMP levels** by activating phosphodiesterase, which degrades cAMP. - Lower cAMP reduces **hormone-sensitive lipase (HSL)** activity, leading to decreased lipolysis and thereby favoring lipogenesis.

Question 746: Apolipoprotein A-I is found in which of the following lipoproteins?

A. HDL (Correct Answer)
B. VLDL
C. LDL
D. Chylomicrons

Explanation: ***HDL*** - **Apolipoprotein A-I (ApoA-I)** is the primary protein component of **high-density lipoprotein (HDL)**. - Its main roles are the activation of **lecithin-cholesterol acyltransferase (LCAT)** and to facilitate the transport of **cholesterol** from peripheral tissues back to the liver (**reverse cholesterol transport**). *VLDL* - **Very-low-density lipoprotein (VLDL)** primarily contains **apolipoprotein B-100 (ApoB-100)**, **ApoC-II**, and **ApoE**. - Its main function is the transport of **triglycerides** synthesized in the liver to peripheral tissues. *LDL* - **Low-density lipoprotein (LDL)** contains a single molecule of **apolipoprotein B-100 (ApoB-100)**. - LDL is responsible for delivering **cholesterol** from the liver to peripheral cells. *Chylomicrons* - **Chylomicrons** primarily contain **apolipoprotein B-48 (ApoB-48)**, **ApoC-II**, and **ApoE**. - They are responsible for transporting **dietary triglycerides** and **cholesterol** from the intestines to peripheral tissues and the liver.

Question 747: All of the following are products of the cyclooxygenase pathway from arachidonic acid, except for:

A. PGE2
B. PGD2
C. PGF2α
D. LTD4 (Correct Answer)

Explanation: ***LTD4*** - Leukotriene D4 (**LTD4**) is a product of the **lipoxygenase pathway**, not the cyclooxygenase pathway, from arachidonic acid. - It plays a significant role in **bronchoconstriction** and inflammation, especially in asthma. *PGE2* - **Prostaglandin E2 (PGE2)** is a major product of the **cyclooxygenase (COX) pathway**. - It is involved in mediating **fever**, pain, and inflammation. *PGD2* - **Prostaglandin D2 (PGD2)** is also produced via the **cyclooxygenase (COX) pathway**. - It is primarily associated with allergic reactions, **bronchoconstriction**, and sleep regulation. *PGF2α* - **Prostaglandin F2 alpha (PGF2α)** is a product derived from arachidonic acid through the **cyclooxygenase (COX) pathway**. - It plays a role in **uterine contractions**, fertility, and vascular tone.

Question 748: Enzymes involved in polyunsaturated fatty acid synthesis are

A. Acetyltransferase
B. Desaturase (Correct Answer)
C. Pyruvate carboxylase
D. Acyltransferase

Explanation: ***Correct: Desaturase*** - **Desaturases** introduce double bonds into fatty acid chains, a key step in synthesizing **polyunsaturated fatty acids (PUFAs)**. - This process is essential for converting saturated and monounsaturated fatty acids into increasingly unsaturated forms. *Incorrect: Pyruvate carboxylase* - **Pyruvate carboxylase** is involved in **gluconeogenesis** and **anaplerosis** by converting pyruvate to oxaloacetate. - It does not play a direct role in the synthesis of **fatty acids**, especially polyunsaturated ones. *Incorrect: Acetyltransferase* - **Acetyltransferases** catalyze the transfer of an acetyl group from **acetyl-CoA** to another molecule. - While acetyl-CoA is a precursor for fatty acid synthesis, acetyltransferases themselves are not the enzymes responsible for introducing **double bonds** into fatty acid chains to make them polyunsaturated. *Incorrect: Acyltransferase* - **Acyltransferases** transfer an acyl group (a fatty acid chain) from one molecule to another, often involved in **lipid synthesis** and modification. - They are crucial for forming esters like triglycerides and phospholipids but do not introduce **double bonds** to create polyunsaturated fatty acids.

Question 749: Which cooking oil is known to have the highest percentage of polyunsaturated fatty acids?

A. Safflower oil (Correct Answer)
B. Coconut oil
C. Butter
D. Vanaspati

Explanation: ***Safflower oil*** - Safflower oil contains one of the **highest concentrations of polyunsaturated fatty acids**, particularly **linoleic acid**, among commonly used cooking oils. - This composition makes it a popular choice for health-conscious consumers looking to increase their intake of beneficial fats. *Coconut oil* - Coconut oil is predominantly composed of **saturated fatty acids**, notably **lauric acid**, which makes it solid at room temperature. - Its high saturated fat content distinguishes it significantly from oils rich in polyunsaturated fats. *Butter* - Butter is a dairy product that is very high in **saturated animal fats** and **cholesterol**. - Its fatty acid profile is dominated by short- and medium-chain saturated fatty acids, making it distinct from oils rich in polyunsaturated fats. *Vanaspati* - Vanaspati is a **partially hydrogenated vegetable oil** that is high in **trans fatty acids** and saturated fats due to its processing. - While it originates from vegetable oils, hydrogenation alters its fatty acid structure, reducing polyunsaturated content and increasing less healthy fats.

Question 750: The human plasma lipoprotein containing the highest percentage of triacylglycerol by weight is which one?

A. VLDL
B. Chylomicrons (Correct Answer)
C. HDL
D. LDL

Explanation: ***Chylomicrons*** - Chylomicrons are primarily responsible for transporting **exogenous (dietary) triglycerides** from the intestines to peripheral tissues. - They are the **largest and least dense** lipoproteins, composed of approximately 85-90% triacylglycerol by weight. *VLDL* - **Very Low-Density Lipoproteins (VLDL)** transport **endogenous (liver-synthesized) triglycerides** to peripheral tissues. - While rich in triglycerides (about 50% by weight), their percentage is significantly lower than that of chylomicrons. *HDL* - **High-Density Lipoproteins (HDL)** are primarily involved in **reverse cholesterol transport**, moving cholesterol from peripheral tissues back to the liver. - They have the **highest percentage of protein** and the lowest percentage of triacylglycerol among the major lipoproteins. *LDL* - **Low-Density Lipoproteins (LDL)** are the primary transporters of **cholesterol** to peripheral tissues. - They are derived from VLDL and contain a high percentage of cholesterol esters, with a relatively low percentage of triacylglycerol.

Practice by Chapter

Lipid Classification and Chemistry

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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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