Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 451: Which one of the following polyunsaturated fatty acids (PUFAs) is considered to be the main source of eicosanoids in human tissues?

A. Linoleate
B. Linolenate
C. Arachidonate (Correct Answer)
D. Palmitate

Explanation: **Explanation:** **Arachidonate (Arachidonic Acid)** is the correct answer because it serves as the primary precursor for the synthesis of **eicosanoids**, which include prostaglandins, thromboxanes, and leukotrienes. In human tissues, arachidonic acid (a 20-carbon $\omega$-6 PUFA) is typically esterified in membrane phospholipids. Upon physiological or pathological stimuli, it is released by the enzyme **Phospholipase $A_2$**. Once free, it enters either the **Cyclooxygenase (COX)** pathway to form prostanoids or the **Lipoxygenase (LOX)** pathway to form leukotrienes. **Analysis of Incorrect Options:** * **Linoleate (Linoleic Acid):** This is an essential $\omega$-6 fatty acid. While it is the metabolic precursor to arachidonate, it must first undergo desaturation and elongation. It does not serve as the *direct* substrate for eicosanoid synthesis in tissues. * **Linolenate ($\alpha$-Linolenic Acid):** This is an essential $\omega$-3 fatty acid. It is the precursor for EPA and DHA. While $\omega$-3 eicosanoids exist, they are produced in much smaller quantities compared to the arachidonate-derived $\omega$-6 series in humans. * **Palmitate:** This is a 16-carbon **saturated** fatty acid. It is the first fatty acid produced by the fatty acid synthase complex and is not a PUFA, nor is it involved in eicosanoid production. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The release of arachidonate from phospholipids by **Phospholipase $A_2$** is the rate-limiting step in eicosanoid synthesis. * **Steroids' Mechanism:** Glucocorticoids inhibit Phospholipase $A_2$ (via lipocortin/annexin A1), thereby blocking the production of all eicosanoids. * **NSAIDs:** Aspirin and other NSAIDs specifically inhibit the **COX pathway**, preventing prostaglandin synthesis. * **Essential Fatty Acids:** Humans lack $\Delta^{12}$ and $\Delta^{15}$ desaturases, making Linoleic and $\alpha$-Linolenic acids dietary essentials.

Question 452: Which of the following is NOT a feature of Refsum disease?

A. Deficiency of alpha-hydroxylase
B. Defect in beta-oxidation (Correct Answer)
C. Accumulation of phytanic acid
D. Peripheral neuropathy

Explanation: **Explanation:** Refsum disease is a rare autosomal recessive peroxisomal disorder characterized by the inability to degrade **phytanic acid**, a branched-chain fatty acid derived from dietary chlorophyll. **Why Option B is the correct answer:** The fundamental defect in Refsum disease is a deficiency of the enzyme **Phytanoyl-CoA hydroxylase**, which is required for **alpha-oxidation**. Because phytanic acid has a methyl group at the beta-carbon position, it cannot undergo standard beta-oxidation directly. It must first undergo alpha-oxidation to remove one carbon atom. Therefore, the disease is a defect in **alpha-oxidation**, not beta-oxidation. **Analysis of incorrect options:** * **Option A:** Deficiency of alpha-hydroxylase (specifically phytanoyl-CoA hydroxylase) is the primary biochemical cause of the disease. * **Option C:** Due to the enzyme deficiency, **phytanic acid accumulates** in the blood and tissues (especially the nervous system and skin), leading to toxicity. * **Option D:** **Peripheral neuropathy** is a hallmark clinical feature, along with ataxia, retinitis pigmentosa, and sensorineural deafness. **Clinical Pearls for NEET-PG:** * **Dietary Management:** The mainstay of treatment is a diet restricted in phytanic acid (avoiding ruminant fats, dairy, and green leafy vegetables). * **Classic Triad:** Retinitis pigmentosa, peripheral neuropathy, and cerebellar ataxia. * **Ichthyosis:** Patients often present with dry, scaly skin. * **Zellweger Syndrome vs. Refsum:** While both are peroxisomal disorders, Zellweger involves a total failure of peroxisome biogenesis, whereas Refsum is a single enzyme defect.

Question 453: Which of the following is NOT a phospholipid?

A. Plasmalogens
B. Dipalmitoyl lecithin
C. Ceramide (Correct Answer)
D. Cardiolipin

Explanation: **Explanation:** The core concept in this question is the classification of complex lipids. **Phospholipids** must contain a phosphate group in their structure. **Why Ceramide is the correct answer:** Ceramide is a **sphingosine derivative** consisting of a sphingosine backbone attached to a fatty acid via an amide bond. It is the structural precursor for all complex sphingolipids. Crucially, ceramide **does not contain a phosphate group**; it is a simple lipid intermediate. When a phosphate and choline are added to ceramide, it becomes Sphingomyelin (which *is* a phospholipid). **Analysis of Incorrect Options:** * **Plasmalogens:** These are specialized phospholipids where the fatty acid at the C1 position is attached via an **ether linkage** instead of an ester linkage. They are abundant in cardiac tissue and myelin. * **Dipalmitoyl lecithin (DPPC):** This is a major glycerophospholipid. It contains a glycerol backbone, two palmitic acid chains, a phosphate group, and a choline base. * **Cardiolipin (Diphosphatidylglycerol):** This is a unique phospholipid found exclusively in the **inner mitochondrial membrane**. It consists of two molecules of phosphatidic acid connected by a glycerol bridge, containing two phosphate groups. **High-Yield Clinical Pearls for NEET-PG:** * **DPPC Clinical Correlation:** It is the primary component of **Lung Surfactant**. Deficiency in neonates leads to Respiratory Distress Syndrome (RDS). * **Cardiolipin Clinical Correlation:** It is the antigen used in the **VDRL test** for Syphilis. It is also targeted by antibodies in Antiphospholipid Antibody Syndrome (APS). * **Plasmalogen Fact:** Deficiency of plasmalogen synthesis is seen in **Zellweger Syndrome** (a peroxisomal disorder).

Question 454: Which molecule is involved in the transport of long-chain acyl-CoA into the mitochondria?

A. Ornithine
B. Xanthine
C. Carnitine (Correct Answer)
D. Albumin

Explanation: **Explanation:** The transport of long-chain fatty acids into the mitochondria is the rate-limiting step of **beta-oxidation**. While short and medium-chain fatty acids can diffuse freely, long-chain acyl-CoA molecules cannot cross the inner mitochondrial membrane. **1. Why Carnitine is correct:** The **Carnitine Shuttle** facilitates this transport. The enzyme **Carnitine Palmitoyltransferase-I (CPT-I)**, located on the outer mitochondrial membrane, converts acyl-CoA to **acyl-carnitine**. This molecule is then transported across the inner membrane by a translocase. Once inside the matrix, **CPT-II** reconverts it back into acyl-CoA and free carnitine, allowing beta-oxidation to proceed. **2. Why the other options are incorrect:** * **Ornithine:** An amino acid involved in the **Urea Cycle**, acting as a carrier that combines with carbamoyl phosphate to form citrulline. * **Xanthine:** An intermediate in the **Purine degradation pathway**, converted into uric acid by the enzyme xanthine oxidase. * **Albumin:** The primary transport protein in the blood. It carries **free fatty acids (FFAs)** from adipose tissue to peripheral tissues but does not facilitate transport across mitochondrial membranes. **3. High-Yield Clinical Pearls for NEET-PG:** * **Inhibitor:** CPT-I is inhibited by **Malonyl-CoA** (an intermediate of fatty acid synthesis), preventing a futile cycle where synthesis and degradation occur simultaneously. * **Systemic Carnitine Deficiency:** Presents with non-ketotic hypoglycemia, muscle weakness, and liver dysfunction because the body cannot utilize fat for energy during fasting. * **Myopathic CPT-II Deficiency:** The most common disorder of lipid metabolism, characterized by muscle pain and myoglobinuria triggered by prolonged exercise.

Question 455: What is the best predictor for coronary artery disease?

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

Explanation: **Explanation:** **Why LDL is the correct answer:** Low-Density Lipoprotein (LDL) is the primary carrier of cholesterol from the liver to peripheral tissues. It is highly susceptible to oxidation; **oxidized LDL** is taken up by macrophages via scavenger receptors, leading to the formation of **foam cells**, which are the hallmark of atherosclerotic plaques. Because LDL levels correlate most directly with the initiation and progression of atherosclerosis, it is clinically regarded as the "Bad Cholesterol" and the most significant predictor for Coronary Artery Disease (CAD) among the given options. **Why the other options are incorrect:** * **HDL (High-Density Lipoprotein):** Known as "Good Cholesterol," it mediates **reverse cholesterol transport** (carrying cholesterol from tissues back to the liver). High levels are cardioprotective, while low levels are a risk factor, but it is not a primary driver of plaque formation like LDL. * **VLDL (Very Low-Density Lipoprotein):** Primarily transports endogenous triglycerides. While elevated VLDL contributes to metabolic syndrome, it is a precursor to LDL and not as direct a predictor of CAD as LDL itself. * **Chylomicrons:** These transport exogenous (dietary) triglycerides. They are rapidly cleared from the blood postprandially and are not typically associated with the chronic process of atherosclerosis. **NEET-PG High-Yield Pearls:** * **Friedewald Equation:** $LDL = \text{Total Cholesterol} - [HDL + (TG/5)]$. Note: This is invalid if Triglycerides (TG) are $>400\text{ mg/dL}$. * **Apolipoproteins:** **Apo B-100** is the primary protein component of LDL and VLDL (atherogenic), while **Apo A-1** is found in HDL (protective). * **Lp(a):** Lipoprotein (a) is an independent, genetically determined risk factor for CAD that competes with plasminogen, potentially inhibiting fibrinolysis.

Question 456: Which of the following is a catabolic pathway?

A. Cholesterol synthesis
B. Glycogenesis
C. Fatty acid synthesis
D. Ketone body synthesis (Correct Answer)

Explanation: ### Explanation In biochemistry, metabolism is divided into **anabolic** (synthetic) and **catabolic** (breakdown) pathways. **Why Ketone Body Synthesis (Ketogenesis) is the Correct Answer:** While the term "synthesis" often implies anabolism, **Ketogenesis** is functionally a catabolic process. It occurs primarily in the liver mitochondria during fasting, starvation, or uncontrolled diabetes. It involves the breakdown of fatty acids via $\beta$-oxidation to produce **Acetyl-CoA**, which is then converted into ketone bodies (Acetoacetate, $\beta$-hydroxybutyrate, and Acetone). These serve as a crucial energy source for peripheral tissues (brain, heart, muscle) when glucose is scarce. Therefore, it is part of the body's energy-releasing, "breakdown" response. **Analysis of Incorrect Options:** * **A. Cholesterol synthesis:** This is a purely anabolic pathway occurring in the cytosol and ER, requiring ATP and NADPH to build complex steroid rings from Acetyl-CoA units. * **B. Glycogenesis:** This is the synthesis of glycogen from glucose molecules for storage. It is an anabolic process stimulated by insulin in the fed state. * **C. Fatty acid synthesis (Lipogenesis):** This is an anabolic pathway occurring in the cytosol, where Acetyl-CoA is converted into long-chain fatty acids (e.g., Palmitate) for energy storage. **NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme of Ketogenesis:** HMG-CoA Synthase (Mitochondrial). * **Rate-limiting enzyme of Cholesterol synthesis:** HMG-CoA Reductase (Cytosolic). * **Ketone bodies** are water-soluble and do not require albumin for transport, unlike free fatty acids. * **The Liver cannot utilize ketone bodies** because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase).

Question 457: Which of the following is NOT a ketone body?

A. Acetoacetate
B. Acetone
C. beta-hydroxybutyrate
D. Acetyl-CoA (Correct Answer)

Explanation: **Explanation:** Ketone bodies are water-soluble molecules produced by the liver from fatty acids during periods of low glucose availability (starvation, fasting, or untreated diabetes mellitus). **Why Acetyl-CoA is the correct answer:** Acetyl-CoA is the **precursor** for ketone body synthesis (ketogenesis), but it is not a ketone body itself. It is a central metabolic intermediate that enters the TCA cycle for energy production. In the liver, when Acetyl-CoA levels exceed the capacity of the TCA cycle, it is diverted into the HMG-CoA pathway to form ketone bodies. **Why the other options are incorrect:** * **Acetoacetate:** This is the primary ketone body formed in the liver. It can be converted into the other two forms. * **Beta-hydroxybutyrate:** Formed by the reduction of acetoacetate. Although technically a carboxylic acid and not a "ketone" by chemical structure, it is biologically classified as a ketone body and is the predominant form found in the blood during ketosis. * **Acetone:** Produced by the spontaneous non-enzymatic decarboxylation of acetoacetate. It is highly volatile and excreted via the lungs, giving the characteristic "fruity odor" to the breath in ketoacidosis. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Ketogenesis:** Liver mitochondria (but the liver **cannot** utilize ketone bodies because it lacks the enzyme **Thiophorase** / Succinyl-CoA:3-ketoacid CoA transferase). * **Rate-limiting enzyme:** HMG-CoA Synthase. * **Detection:** The **Rothera’s test** detects Acetoacetate and Acetone, but **not** beta-hydroxybutyrate. * **Utilization:** Ketone bodies are the preferred fuel for the heart and renal cortex; the brain uses them only during prolonged starvation.

Question 458: Which apolipoprotein is considered the primary apoprotein of cholesterol?

A. Apolipoprotein A1
B. Apolipoprotein A2
C. Apolipoprotein C1
D. Apolipoprotein E (Correct Answer)

Explanation: **Explanation:** The correct answer is **Apolipoprotein E (Apo E)**. In the context of lipid metabolism, Apo E is recognized as the "primary apoprotein of cholesterol" because of its critical role in the hepatic uptake of cholesterol-rich particles. It serves as the high-affinity ligand for the **LDL receptor (Apo B100/E receptor)** and the **LRP (LDL Receptor-Related Protein)**. This interaction allows the liver to clear chylomicron remnants and VLDL remnants (IDL), which are highly enriched in cholesterol esters, from the circulation. **Analysis of Options:** * **Apolipoprotein A1 (A):** The primary structural protein of **HDL**. Its main role is the activation of LCAT (Lecithin-Cholesterol Acyltransferase) for reverse cholesterol transport. * **Apolipoprotein A2 (B):** A secondary protein found in HDL; its physiological function is less defined but it is not the primary mediator of cholesterol particle clearance. * **Apolipoprotein C1 (C):** Primarily involved in the activation of LCAT and inhibition of CETP; it does not serve as the primary cholesterol-clearing ligand. **High-Yield Clinical Pearls for NEET-PG:** * **Type III Hyperlipoproteinemia (Dysbetalipoproteinemia):** Caused by a deficiency or polymorphism of **Apo E (specifically E2/E2 isoform)**, leading to the accumulation of cholesterol-rich remnants and xanthomas. * **Alzheimer’s Disease:** The **Apo E4** isoform is a significant genetic risk factor for late-onset Alzheimer’s. * **Apo B-100** is the structural protein for VLDL/LDL, while **Apo B-48** is unique to chylomicrons (lacks the LDL receptor-binding domain).

Question 459: Fatty acids of cholesterol are mainly

A. Oleic acid
B. Linoleic acid
C. Palmitic acid
D. All of these (Correct Answer)

Explanation: ### Explanation **1. Understanding the Concept** Cholesterol exists in the body in two forms: **Free cholesterol** and **Esterified cholesterol (Cholesteryl esters)**. In plasma, approximately 70% of cholesterol is esterified. This process involves attaching a fatty acid to the hydroxyl group at the C3 position of the cholesterol molecule. The fatty acids found in cholesteryl esters are derived from the plasma pool or intracellular synthesis. While specific tissues may favor certain fatty acids, the overall composition in human plasma and tissues includes a variety of saturated and unsaturated fatty acids. **2. Analysis of Options** * **Oleic Acid (A):** This is a common monounsaturated fatty acid (18:1) frequently found in cholesterol esters, particularly those synthesized by the enzyme **ACAT** (Acyl-CoA:cholesterol acyltransferase) within cells. * **Linoleic Acid (B):** This is a polyunsaturated essential fatty acid (18:2). It is the predominant fatty acid found in plasma cholesteryl esters because the enzyme **LCAT** (Lecithin-cholesterol acyltransferase) preferentially transfers linoleic acid from phosphatidylcholine (lecithin) to cholesterol. * **Palmitic Acid (C):** This is a common 16-carbon saturated fatty acid that is also frequently incorporated into cholesterol esters during de novo synthesis and remodeling. Since all three fatty acids are significant components of the cholesterol ester pool in the human body, **Option D (All of these)** is the correct answer. **3. Clinical Pearls & High-Yield Facts** * **LCAT vs. ACAT:** LCAT (activated by **Apo A-I**) functions in the plasma (HDL), while ACAT functions intracellularly. * **Reverse Cholesterol Transport:** LCAT plays a crucial role in this process by creating a concentration gradient that allows HDL to "mop up" free cholesterol from peripheral tissues. * **Fish Eye Disease:** A rare genetic condition caused by partial LCAT deficiency, leading to corneal opacities. * **Primary LCAT Deficiency:** Characterized by the "triad" of corneal opacities, hemolytic anemia, and proteinuria (renal failure).

Question 460: Which enzyme is involved in the activation step for fatty acid beta-oxidation?

A. Thiophorase
B. Thiokinase (Correct Answer)
C. Thiolase
D. Thioesterase

Explanation: **Explanation:** **1. Why Thiokinase is Correct:** Fatty acid oxidation occurs in the mitochondria, but fatty acids must first be "activated" in the cytosol to enter the metabolic pathway. **Thiokinase** (also known as **Acyl-CoA Synthetase**) catalyzes the conversion of a free fatty acid into an **Active Fatty Acid (Acyl-CoA)**. This reaction requires ATP (which is hydrolyzed to AMP and PPi) and Coenzyme A. This activation is the essential first step before the fatty acid can be transported across the mitochondrial membrane via the Carnitine shuttle. **2. Analysis of Incorrect Options:** * **Thiophorase (Succinyl-CoA:3-ketoacid CoA transferase):** This enzyme is involved in **ketolysis** (breakdown of ketone bodies). It is notably absent in the liver, which is why the liver cannot utilize the ketone bodies it produces. * **Thiolase:** This enzyme acts in the **final step** of each beta-oxidation cycle, where it cleaves the 3-ketoacyl-CoA to release Acetyl-CoA and a shortened Acyl-CoA chain. * **Thioesterase:** These enzymes catalyze the hydrolysis of thioester bonds (e.g., releasing a fatty acid chain from the Fatty Acid Synthase complex during lipogenesis). **3. High-Yield Clinical Pearls for NEET-PG:** * **Energetics:** The activation step consumes **two high-energy phosphate bonds** (ATP → AMP) because the pyrophosphate (PPi) produced is further hydrolyzed to 2Pi. * **Location:** While activation occurs in the cytosol/outer mitochondrial membrane, beta-oxidation occurs in the **mitochondrial matrix**. * **Rate-Limiting Step:** The activation is not the rate-limiting step; the transport of Acyl-CoA into the mitochondria via **Carnitine Palmitoyltransferase-I (CPT-I)** is the primary regulatory point.

Practice by Chapter

Lipid Classification and Chemistry

Practice Questions

Fatty Acid Oxidation

Practice Questions

Ketone Body Metabolism

Practice Questions

Fatty Acid Synthesis

Practice Questions

Metabolism of Triacylglycerols

Practice Questions

Phospholipid Metabolism

Practice Questions

Cholesterol Metabolism and Biosynthesis

Practice Questions

Bile Acids and Bile Salts

Practice Questions

Lipoprotein Metabolism and Transport

Practice Questions

Dyslipidemias and Atherosclerosis

Practice Questions

Prostaglandins and Eicosanoids

Practice Questions

Fatty Liver and Lipotropic Factors

Practice Questions

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