Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 641: In patients with Retinitis pigmentosa, which of the following substances is known to have decreased levels?

A. Thromboxane
B. Arachidonic acid
C. Docosahexaenoic acid (DHA) (Correct Answer)
D. Linoleic acid

Explanation: ***Docosahexaenoic acid (DHA)*** - **Docosahexaenoic acid (DHA)** is a crucial **omega-3 fatty acid** abundantly found in the **photoreceptor cell membranes**, particularly in the retina. - Reduced levels of DHA are frequently observed in patients with **retinitis pigmentosa**, suggesting its role in disease pathogenesis and retinal health. *Arachidonic acid* - **Arachidonic acid** is an **omega-6 fatty acid** and a precursor to many signaling molecules, but its levels are **not typically decreased** in retinitis pigmentosa. - It plays a **pro-inflammatory role** and is involved in various physiological processes, distinct from the primary metabolic defects in retinitis pigmentosa. *Linoleic acid* - **Linoleic acid** is an essential **omega-6 fatty acid** and a precursor to arachidonic acid, but its deficiencies are not characteristic of retinitis pigmentosa. - It is crucial for **skin barrier function** and overall health, but its metabolic pathways are distinct from those primarily affected in retinal degenerations. *Thromboxane* - **Thromboxane** is a **lipid mediator** primarily involved in **platelet aggregation** and vasoconstriction. - It is not directly associated with the metabolic pathways or structural integrity of the retina, and its levels are not typically altered in retinitis pigmentosa.

Question 642: X-linked adrenoleukodystrophy is a type of:

A. Fatty acid disorder (Correct Answer)
B. Lysosomal storage disorder
C. Glycogen defect disorder
D. Mucopolysaccharidoses

Explanation: ***Fatty acid disorder*** - **X-linked adrenoleukodystrophy (X-ALD)** is characterized by impaired peroxisomal beta-oxidation of **very long-chain fatty acids (VLCFAs)**, leading to their accumulation. - This accumulation primarily affects the **adrenal glands** and **nervous system**, causing progressive demyelination and adrenal insufficiency. *Lysosomal storage disorder* - **Lysosomal storage disorders** involve defects in lysosomal enzymes, leading to the accumulation of specific substrates within lysosomes. - While X-ALD involves fat metabolism, the affected organelles are **peroxisomes**, not lysosomes. *Mucopolysaccharidoses* - **Mucopolysaccharidoses (MPS)** are a group of lysosomal storage disorders characterized by the defective breakdown of **glycosaminoglycans (GAGs)**, also known as mucopolysaccharides. - These disorders present with skeletal abnormalities, intellectual disability, and organomegaly, which are distinct from the primary pathology of X-ALD. *Glycogen defect disorder* - **Glycogen defect disorders** (or glycogen storage diseases) result from mutations in enzymes involved in **glycogen synthesis or breakdown**. - These conditions primarily affect carbohydrate metabolism and can lead to symptoms like hypoglycemia, muscle weakness, and hepatomegaly, distinct from the fatty acid metabolism defect in X-ALD.

Question 643: Which of the following statements about beta oxidation of fatty acids is correct?

A. Fatty acid oxidation defects can lead to hypoglycemia. (Correct Answer)
B. Ketone bodies are formed by complete oxidation of fatty acids during starvation.
C. Odd chain fatty acid oxidation provides only propionyl CoA.
D. Stearic acid on oxidation provides 120 ATPs.

Explanation: ***Correct: Fatty acid oxidation defects can lead to hypoglycemia*** - **Fatty acid oxidation** is a crucial energy source during fasting states, providing ATP and supporting **hepatic gluconeogenesis** - **Defects in fatty acid oxidation** (e.g., MCAD, LCAD, VLCAD deficiencies) impair the liver's ability to generate energy from fat breakdown - This forces continued reliance on **glucose** and impairs gluconeogenesis, leading to **hypoglycemia**, especially during fasting or increased metabolic demands - Clinically presents as **hypoketotic hypoglycemia** - a hallmark of fatty acid oxidation disorders *Incorrect: Ketone bodies are formed by complete oxidation of fatty acids during starvation* - **Ketone bodies** (acetoacetate, β-hydroxybutyrate, acetone) are indeed formed during starvation when fatty acids are mobilized - However, they result from **incomplete oxidation** of fatty acids in the liver, specifically from excess acetyl-CoA that cannot enter the TCA cycle - **Complete oxidation** would mean breakdown to CO₂ and H₂O via the TCA cycle and electron transport chain - Ketone bodies serve as alternative fuel for the brain and other tissues during prolonged fasting *Incorrect: Odd chain fatty acid oxidation provides only propionyl CoA* - **Odd-chain fatty acid oxidation** (e.g., C15, C17) yields **multiple acetyl-CoA molecules** plus **one propionyl-CoA** at the end - The word **"only"** makes this statement false - For example, a C17 fatty acid yields **7 acetyl-CoA** molecules and **1 propionyl-CoA** - Propionyl-CoA is then converted to **succinyl-CoA** (via methylmalonyl-CoA), entering the TCA cycle as a gluconeogenic substrate *Incorrect: Stearic acid on oxidation provides 120 ATPs* - **Stearic acid** (C18:0) undergoes 8 cycles of β-oxidation, yielding **9 acetyl-CoA**, **8 FADH₂**, and **8 NADH** - ATP calculation: (8 FADH₂ × 1.5) + (8 NADH × 2.5) + (9 acetyl-CoA × 10) - 2 (activation) = **12 + 20 + 90 - 2 = 120 ATP** - However, the commonly accepted value is **122 ATP** (using modern P/O ratios) or **146 ATP** (using older calculations with P/O ratios of 2 for FADH₂ and 3 for NADH) - The statement claiming exactly 120 ATP is **approximately correct but not the standard teaching value**

Question 644: What is the primary role of scavenger receptors in the development of atherosclerotic plaques?

A. Antibody production
B. Direct pathogen killing
C. Recognition and clearance of modified lipoproteins (Correct Answer)
D. T-cell activation

Explanation: ***Recognition and clearance of modified lipoproteins*** - Scavenger receptors, such as **SR-A** and **CD36**, primarily bind to and internalize **oxidized low-density lipoproteins (oxLDLs)** and other chemically modified lipoproteins - This process is crucial in the formation of **foam cells** within the arterial wall, a hallmark of early atherosclerotic plaque development - Unlike normal LDL receptors, scavenger receptors are not downregulated by intracellular cholesterol, leading to uncontrolled lipid accumulation *Direct pathogen killing* - While macrophages expressing scavenger receptors can participate in innate immunity, this is not their primary role in atherosclerosis - Pathogen recognition involves different receptor systems (TLRs, complement receptors) with distinct signaling pathways *Antibody production* - Antibody production is a function of **B lymphocytes**, not macrophages or their scavenger receptors - Macrophages can present antigens to helper T cells but do not produce antibodies themselves *T-cell activation* - T-cell activation requires antigen presentation via **MHC molecules** by professional antigen-presenting cells - While scavenger receptors facilitate antigen uptake, their primary role in atherosclerosis is lipid accumulation, not immune cell activation

Question 645: Arachidonic acid oxidation involves how many cycles of beta oxidation?

A. 20
B. 9 (Correct Answer)
C. 8
D. 10

Explanation: ***Correct Answer: 9*** - **Arachidonic acid** is a 20-carbon fatty acid (C20:4). For a fatty acid with *n* carbons, the number of beta-oxidation cycles required is **(n/2) - 1**. - Therefore, for **arachidonic acid** (n=20), the number of beta-oxidation cycles is (20/2) - 1, which equals 10 - 1 = **9 cycles**. - Each cycle removes 2 carbons as acetyl-CoA, shortening the fatty acid chain progressively. *Incorrect Option: 10* - This number represents the total number of **acetyl-CoA** molecules generated from a 20-carbon fatty acid, not the number of beta-oxidation cycles. - Each beta-oxidation cycle produces one **acetyl-CoA** and reduces the fatty acid chain by two carbons; the last cycle yields two **acetyl-CoA** molecules directly. *Incorrect Option: 20* - This value corresponds to the total number of carbons in **arachidonic acid**, not the number of beta-oxidation cycles. - The number of beta-oxidation cycles is significantly less than the total carbon count. *Incorrect Option: 8* - This number would be correct for an 18-carbon fatty acid like **stearic acid** (18/2 - 1 = 8), not for **arachidonic acid**. - The calculation explicitly depends on the exact number of carbon atoms in the fatty acid chain.

Question 646: All are true about beta oxidation of fatty acids except which of the following?

A. Carnitine Acyl transferase I defect causes a decrease in acylcarnitine levels
B. Carnitine Acyl transferase I is stimulated by malonyl CoA (Correct Answer)
C. Carnitine acyl transferase I is the rate limiting enzyme of fatty acid oxidation
D. Carnitine acyl transferase I is inhibited by malonyl CoA

Explanation: ***Carnitine Acyl transferase I is stimulated by malonyl CoA*** - Malonyl CoA is a key intermediate in **fatty acid synthesis** and acts as an **inhibitor** of carnitine acyltransferase I (CAT1), not a stimulator. - This inhibition ensures that when fatty acid synthesis is active, fatty acid oxidation is suppressed, preventing a futile cycle. *Carnitine acyl transferase I is the rate limiting enzyme of fatty acid oxidation* - **Carnitine palmitoyltransferase 1 (CPT1)**, also known as carnitine acyltransferase I, is indeed the **rate-limiting step** for long-chain fatty acid entry into the mitochondrial matrix for beta-oxidation. - This enzyme controls the transport of fatty acyl-CoA into the mitochondria, which is essential for its subsequent breakdown. *Carnitine acyl transferase I is inhibited by malonyl CoA.* - **Malonyl CoA**, a precursor in fatty acid synthesis, serves as an allosteric inhibitor of **CAT1**. - This mechanism ensures that when the cell is actively synthesizing fatty acids, it simultaneously prevents their breakdown, helping to regulate overall energy metabolism. *Carnitine Acyl transferase I defect causes a decrease in acylcarnitine levels* - A defect in **CAT1** means fatty acids cannot be efficiently transported into the mitochondria to be converted into acylcarnitine for transport. - This leads to an accumulation of **free fatty acids** in the cytoplasm and a **decrease in acylcarnitine levels** in the blood and tissues, which can be used diagnostically.

Question 647: Which of the following statements about reverse cholesterol transport is false?

A. Transport of cholesterol from extrahepatic tissues to liver.
B. Cholesterol Ester Transfer Protein reduces HDL levels.
C. ATP Binding Cassette Transporter protein is involved in the conversion of HDL3 to HDL2.
D. Lecithin Cholesterol Acyl Transferase helps in the conversion of Spheroidal HDL to Discoidal HDL. (Correct Answer)

Explanation: ***Lecithin Cholesterol Acyl Transferase helps in the conversion of Spheroidal HDL to Discoidal HDL*** - **Lecithin-cholesterol acyltransferase (LCAT)** catalyzes the esterification of cholesterol within HDL particles, transforming **discoidal HDL** into **spheroidal HDL**. - This process traps cholesterol esters inside the HDL core, promoting the maturation of HDL and its capacity to accept more cholesterol. *Transport of cholesterol from extrahepatic tissues to liver is true* - This statement is **true** and describes the primary function of **reverse cholesterol transport**, where excess cholesterol from peripheral cells is returned to the liver for excretion or recycling. - **High-density lipoprotein (HDL)** plays a crucial role in mediating this transport. *Cholesterol Ester Transfer Protein reduces HDL levels* - This statement is generally considered **true** because **cholesterol ester transfer protein (CETP)** facilitates the exchange of cholesterol esters from HDL to VLDL/LDL in exchange for triglycerides, which can lead to a reduction in HDL cholesterol. - This exchange process makes HDL particles smaller and more susceptible to catabolism, thus lowering **HDL levels**. *ATP Binding Cassette Transporter protein is involved in the conversion of HDL3 to HDL2* - The **ATP-binding cassette transporter A1 (ABCA1)** is primarily involved in the initial efflux of **unesterified cholesterol** and **phospholipids** from cells to lipid-poor apoA-I, forming nascent, discoidal HDL, not in the conversion of HDL3 to HDL2. - The conversion of **HDL3 to HDL2** largely depends on the accumulation of cholesterol esters within the HDL particle, which is mediated by **LCAT**, not directly by ABCA1.

Question 648: Which of the following statements about the properties of VLDL and LDL is true?

A. LDL is formed from VLDL. (Correct Answer)
B. VLDL remnants are primarily taken up by the liver.
C. LDL is formed in the liver.
D. In electrophoresis, VLDL migrates less cathodal than LDL.

Explanation: ***LDL is formed from VLDL.*** - **Very low-density lipoprotein (VLDL)** is secreted by the liver and transports **triglycerides** to peripheral tissues. As most of the triglycerides are removed by **lipoprotein lipase**, VLDL is converted into **intermediate-density lipoprotein (IDL)** and then further to **low-density lipoprotein (LDL)**. - This conversion primarily occurs in the bloodstream as VLDL loses its triglyceride content. - This statement is **unambiguously true** and represents the established metabolic pathway. *VLDL remnants are primarily taken up by the liver.* - While this statement has some truth, **VLDL remnants (IDL)** have **two major fates**: approximately **50% are taken up by the liver** via apoE-mediated endocytosis through the **LDL receptor** and **LRP**, while the remaining **50% are converted to LDL** by hepatic lipase. - The term "primarily" (meaning mostly or mainly) is thus **not entirely accurate** since both pathways are equally significant. - In contrast, **chylomicron remnants** are almost exclusively (>90%) taken up by the liver, making this statement more applicable to them. *LDL is formed in the liver.* - The liver primarily produces and secretes **VLDL**, not LDL directly. - LDL is a product of the **catabolism** of VLDL in the circulation, not formed de novo in the liver. *In electrophoresis, VLDL migrates less cathodal than LDL.* - In standard **agarose gel electrophoresis**, **VLDL** migrates in the **pre-beta** region, which is **more cathodal** (less anodic) than **LDL** which migrates in the **beta** region. - This means VLDL is more cathodal than LDL, making this statement **incorrect** (it states the opposite).

Question 649: Which of the following is not an essential fatty acid?

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

Explanation: ***Palmitic acid*** - **Palmitic acid** is a **saturated fatty acid** that can be synthesized by the human body and is therefore not considered essential. - It is one of the most common fatty acids in animals and plants and is a major component of membrane lipids. *Linoleic acid* - **Linoleic acid** (an **omega-6 fatty acid**) is an essential fatty acid because the human body cannot synthesize it and it must be obtained from the diet. - It serves as a precursor for other fatty acids, including **arachidonic acid**. *Linolenic acid* - **Linolenic acid** (an **omega-3 fatty acid**) is an essential fatty acid that the human body cannot produce. - It is critical for cell membrane structure and as a precursor for other important fatty acids like **EPA** and **DHA**. *Arachidonic acid* - While important for various biological processes, **arachidonic acid** can be synthesized in the body from the essential fatty acid **linoleic acid**. - Therefore, it is considered conditionally essential, as its essentiality depends on adequate intake of its precursor.

Question 650: Which of the following is a transfatty acid?

A. Oleic acid
B. Elaidic acid (Correct Answer)
C. Stearic acid
D. Arachidonic acid

Explanation: ***Elaidic acid*** - **Elaidic acid** is a common **trans-fatty acid** found in partially hydrogenated vegetable oils. - Its chemical structure includes a **trans double bond**, which gives it properties distinct from cis-fatty acids. *Oleic acid* - **Oleic acid** is a **monounsaturated fatty acid** commonly found in olive oil and other plant fats. - It has a **cis double bond**, which causes a bend in its molecular structure. *Stearic acid* - **Stearic acid** is a **saturated fatty acid** with no double bonds in its carbon chain. - It is found in animal fats and some plant oils, and its straight chain allows for tight packing. *Arachidonic acid* - **Arachidonic acid** is a **polyunsaturated omega-6 fatty acid** with multiple cis double bonds. - It is involved in inflammation and is a precursor to eicosanoids.

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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