Lipid Metabolism — MCQs
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Which of the following decreases the risk of coronary artery disease?
Free fatty acids are transported by which of the following?
Low-density lipoproteins (LDL) represents a final stage in the catabolism of VLDL. Which of the following receptors are present in the liver for uptake of LDL?
The genetic defect in Refsum's disease lies in?
What is the most important source of reducing equivalents for fatty acid synthesis in the liver?
The reaction Acetoacetyl CoA + succinate → succinyl CoA + acetoacetate occurs in all of the following tissues except?
Which lipoprotein levels are indirectly proportional to the risk of coronary artery disease?
Which of the following is NOT a phospholipid?
What is the function of LCAT?
Cholesterol is transported to peripheral tissues by which lipoprotein?
Lipid Metabolism Indian Medical PG Practice Questions and MCQs
Question 301: Which of the following decreases the risk of coronary artery disease?
- A. Low-density lipoprotein (LDL)
- B. High-density lipoprotein (HDL) (Correct Answer)
- C. Very low-density lipoprotein (VLDL)
- D. Intermediate-density lipoprotein (IDL)
Explanation: **Explanation:** The correct answer is **High-density lipoprotein (HDL)**. HDL is clinically referred to as "good cholesterol" because of its role in **Reverse Cholesterol Transport**. It picks up excess cholesterol from peripheral tissues and blood vessel walls (including coronary arteries) and transports it back to the liver for excretion in bile. This process prevents the formation of atherosclerotic plaques, thereby decreasing the risk of Coronary Artery Disease (CAD). **Analysis of Incorrect Options:** * **Low-density lipoprotein (LDL):** Known as "bad cholesterol," LDL transports cholesterol from the liver to peripheral tissues. High levels lead to cholesterol deposition in arterial walls, forming plaques (atherogenesis). * **Very low-density lipoprotein (VLDL):** Secreted by the liver, VLDL primarily transports endogenous triglycerides. High levels are associated with an increased risk of atherosclerosis. * **Intermediate-density lipoprotein (IDL):** Formed during the conversion of VLDL to LDL, IDL is pro-atherogenic and contributes to plaque buildup. **NEET-PG High-Yield Pearls:** * **ApoA-I:** The major apoprotein associated with HDL (activates LCAT). * **LCAT (Lecithin-Cholesterol Acyltransferase):** The enzyme responsible for esterifying cholesterol within HDL, allowing it to be packed into the core of the lipoprotein. * **CETP (Cholesterol Ester Transfer Protein):** Mediates the exchange of cholesterol esters from HDL for triglycerides from VLDL/LDL. * **Protective Levels:** An HDL level **>60 mg/dL** is considered a "negative" risk factor for CAD (it subtracts from the total risk score).
Question 302: Free fatty acids are transported by which of the following?
- A. Ceruloplasmin
- B. Pre-albumin
- C. Albumin (Correct Answer)
- D. Transthyretin
Explanation: **Explanation:** **1. Why Albumin is the Correct Answer:** Free Fatty Acids (FFAs), also known as non-esterified fatty acids (NEFA), are hydrophobic molecules released from adipose tissue via lipolysis. Because they are insoluble in water, they cannot travel freely in the plasma. **Albumin** serves as the primary transport protein for FFAs in the blood. Each albumin molecule possesses multiple high-affinity binding sites (approximately 7 sites) that sequester the hydrophobic fatty acid chains, allowing them to be transported to tissues like the liver and muscle for β-oxidation. **2. Why the Other Options are Incorrect:** * **Ceruloplasmin (A):** This is an α2-globulin that functions primarily as the major **copper-carrying protein** in the blood and acts as a ferroxidase. * **Pre-albumin (B) & Transthyretin (D):** These are essentially the same protein. Transthyretin (formerly called pre-albumin) is responsible for the transport of **Thyroxine (T4)** and **Retinol** (Vitamin A, via binding with Retinol Binding Protein). It does not transport fatty acids. **3. High-Yield Clinical Pearls for NEET-PG:** * **Lipid Transport Distinction:** While FFAs are carried by Albumin, other lipids (Cholesterol, Triglycerides, Phospholipids) are transported by **Lipoproteins** (Chylomicrons, VLDL, LDL, HDL). * **Albumin Binding:** Albumin also transports bilirubin, calcium, and various drugs (e.g., warfarin, sulfonamides). Competitive binding at these sites can lead to drug interactions or kernicterus in neonates. * **Metabolic State:** FFA levels in the blood rise significantly during **fasting, starvation, and Diabetes Mellitus** due to increased lipolysis.
Question 303: Low-density lipoproteins (LDL) represents a final stage in the catabolism of VLDL. Which of the following receptors are present in the liver for uptake of LDL?
- A. Apolipoprotein E
- B. Apolipoprotein A and Apolipoprotein E
- C. Apolipoprotein E and Apolipoprotein B100 (Correct Answer)
- D. Apolipoprotein B100
Explanation: **Explanation:** The uptake of Low-Density Lipoprotein (LDL) by the liver is mediated by the **LDL Receptor (LDLR)**, also known as the **Apo B100/E receptor**. This receptor is highly specific and recognizes two primary ligands: **Apolipoprotein B100** (the structural protein of VLDL, IDL, and LDL) and **Apolipoprotein E** (found on VLDL, IDL, and Chylomicron remnants). While LDL contains only Apo B100, the receptor itself is structurally designed to bind both proteins, facilitating the clearance of both LDL and IDL (which contains both B100 and E) from the circulation. **Analysis of Options:** * **Option C (Correct):** The LDL receptor is dual-specific. It binds Apo B100 on LDL particles and Apo E on Remnant particles (IDL). * **Option A:** Apo E is the primary ligand for the **LRP (LDL Receptor-related Protein)** and is used for chylomicron remnant uptake, but it is only one half of the LDLR's binding capability. * **Option B:** Apo A is associated with HDL (High-Density Lipoprotein) and is involved in reverse cholesterol transport via the ABCA1 transporter and SR-B1 receptor, not the LDL receptor. * **Option D:** While LDL particles only carry Apo B100, the hepatic receptor that clears them is physiologically characterized as the Apo B100/E receptor. **High-Yield Clinical Pearls for NEET-PG:** 1. **Familial Hypercholesterolemia (Type IIa):** Caused by a genetic defect or absence of the LDL (B100/E) receptor, leading to severely elevated serum LDL and premature atherosclerosis. 2. **PCSK9 Inhibitors:** These drugs prevent the degradation of LDL receptors, increasing their recycling to the hepatocyte surface to lower blood cholesterol. 3. **Wolman Disease:** A lysosomal storage disorder where a deficiency in acid lipase prevents the breakdown of cholesteryl esters after LDL is internalized.
Question 304: The genetic defect in Refsum's disease lies in?
- A. Alpha oxidation of fatty acids (Correct Answer)
- B. LDL receptors
- C. Fatty acid synthase complex
- D. Recognition of apo A1
Explanation: **Explanation:** **Refsum’s Disease** is a rare autosomal recessive neurological disorder caused by a deficiency in the enzyme **Phytanoyl-CoA hydroxylase**. This enzyme is essential for **Alpha-oxidation**, a metabolic pathway occurring in peroxisomes. 1. **Why Option A is Correct:** Dietary chlorophyll contains **Phytanic acid**, a branched-chain fatty acid with a methyl group at the beta-carbon. This methyl group blocks Beta-oxidation. To bypass this, the body uses Alpha-oxidation to remove the terminal carboxyl carbon as $CO_2$. In Refsum’s disease, this pathway fails, leading to the toxic accumulation of phytanic acid in the blood and tissues (especially the brain and skin). 2. **Why Other Options are Incorrect:** * **Option B (LDL receptors):** Defects here lead to **Familial Hypercholesterolemia**, characterized by high LDL levels and xanthomas. * **Option C (Fatty acid synthase):** This complex is responsible for *de novo* lipogenesis (synthesis of palmitate); defects are not associated with Refsum’s disease. * **Option D (Apo A1):** Apo A1 is the primary protein in HDL. Defects lead to **Tangier disease** or familial hypoalphalipoproteinemia. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Tetrad:** Retinitis pigmentosa, Peripheral neuropathy, Cerebellar ataxia, and Nerve deafness. * **Ichthyosis:** Patients often present with dry, scaly skin. * **Management:** Strict dietary restriction of chlorophyll-containing foods (green leafy vegetables) and ruminant fats (milk, beef) which contain phytanic acid. * **Zellweger Syndrome:** Another peroxisomal disorder, but it involves a total failure of peroxisome biogenesis, unlike the single-enzyme defect in Refsum’s.
Question 305: What is the most important source of reducing equivalents for fatty acid synthesis in the liver?
- A. Glycolysis
- B. TCA cycle
- C. Uronic acid pathway
- D. HMP pathway (Correct Answer)
Explanation: ### Explanation **Correct Option: D. HMP Pathway (Hexose Monophosphate Shunt)** Fatty acid synthesis is a reductive process that occurs in the cytosol and requires **NADPH** as a source of reducing equivalents. The **HMP pathway** (specifically the oxidative phase catalyzed by Glucose-6-Phosphate Dehydrogenase) is the primary source of NADPH in the liver, lactating mammary glands, and adipose tissue. * **Note:** Another significant source is the **Malic Enzyme** reaction, which converts malate to pyruvate, but the HMP shunt remains the most important contributor. **Why other options are incorrect:** * **A. Glycolysis:** This pathway produces **NADH**, not NADPH. While glycolysis provides the substrate (Pyruvate) which eventually becomes Acetyl-CoA for lipogenesis, it does not provide the reducing power required for the Fatty Acid Synthase complex. * **B. TCA Cycle:** This cycle occurs in the mitochondria and primarily generates **NADH and FADH₂** for the electron transport chain to produce ATP. It does not directly supply NADPH to the cytosol for lipid synthesis. * **C. Uronic Acid Pathway:** This pathway is involved in the synthesis of glucuronic acid (for conjugation/detoxification) and pentoses. While it is an alternative route for glucose metabolism, it is not a major source of NADPH for fatty acid synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of HMP Shunt:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **Rate-limiting enzyme of Fatty Acid Synthesis:** Acetyl-CoA Carboxylase (requires Biotin). * **Key Tissues for HMP Shunt:** Liver, Adrenal cortex, Erythrocytes (for glutathione reduction), and Lactating mammary glands. * **Subcellular site:** Both HMP shunt and Fatty Acid synthesis occur in the **Cytosol**.
Question 306: The reaction Acetoacetyl CoA + succinate → succinyl CoA + acetoacetate occurs in all of the following tissues except?
- A. Brain
- B. Striated muscle
- C. Liver (Correct Answer)
- D. Cardiac muscle
Explanation: **Explanation:** The question tests the concept of **Ketolysis** (the utilization of ketone bodies). The reaction described is the activation of acetoacetate into acetoacetyl CoA, which is the rate-limiting step of ketone body utilization. **1. Why Liver is the Correct Answer:** The enzyme responsible for this reaction is **Thiophorase** (also known as Succinyl CoA-Acetoacetate CoA Transferase). While the liver is the primary site for **Ketogenesis** (production of ketone bodies), it lacks the enzyme Thiophorase. This is a crucial physiological adaptation that prevents the liver from consuming the ketone bodies it produces, ensuring they are exported to peripheral tissues for energy during fasting or starvation. **2. Why Other Options are Incorrect:** * **Brain (A):** During prolonged starvation, the brain adapts to use ketone bodies as its primary energy source. It contains high levels of Thiophorase. * **Striated/Skeletal Muscle (B) & Cardiac Muscle (D):** These are extrahepatic tissues that readily utilize ketone bodies for energy, especially when glucose availability is low. They possess the necessary Thiophorase enzyme to convert acetoacetate back into acetyl-CoA for the TCA cycle. **Clinical Pearls & High-Yield Facts:** * **Rate-limiting enzyme of Ketogenesis:** HMG-CoA Synthase (Mitochondrial). * **Rate-limiting enzyme of Ketolysis:** Thiophorase (absent in Liver). * **Ketone Bodies:** Include Acetone (non-metabolizable, exhaled), Acetoacetate, and β-Hydroxybutyrate. * **Organ preference:** The heart actually prefers fatty acids and ketone bodies over glucose under normal physiological conditions.
Question 307: Which lipoprotein levels are indirectly proportional to the risk of coronary artery disease?
- A. HDL (Correct Answer)
- B. LDL
- C. Cholesterol
- D. TG
Explanation: **Explanation:** The correct answer is **HDL (High-Density Lipoprotein)**. This is because HDL is known as the **"Good Cholesterol"** due to its role in **Reverse Cholesterol Transport**. HDL picks up excess cholesterol from peripheral tissues and atherosclerotic plaques in the arterial walls and transports it back to the liver for excretion in bile. High levels of HDL are cardioprotective, meaning as HDL levels increase, the risk of Coronary Artery Disease (CAD) decreases (**inverse/indirect relationship**). **Analysis of Incorrect Options:** * **LDL (Low-Density Lipoprotein):** Known as "Bad Cholesterol," it transports cholesterol from the liver to peripheral tissues. High levels lead to cholesterol deposition in arteries (atherogenesis), showing a **direct** correlation with CAD risk. * **Cholesterol:** Total serum cholesterol includes LDL, HDL, and VLDL. Elevated total cholesterol is a primary risk factor for atherosclerosis and has a **direct** relationship with CAD. * **TG (Triglycerides):** Hypertriglyceridemia is an independent risk factor for cardiovascular disease and is often associated with low HDL and high VLDL. It has a **direct** correlation with CAD risk. **High-Yield Facts for NEET-PG:** * **ApoA-1** is the major apoprotein associated with HDL (activates LCAT). * **LCAT (Lecithin-Cholesterol Acyltransferase)** is the enzyme responsible for esterifying cholesterol within HDL, converting discoid HDL into spherical mature HDL. * **CETP (Cholesterol Ester Transfer Protein)** facilitates the exchange of TG from VLDL for cholesterol esters from HDL. * **Friedewald Equation:** LDL = [Total Cholesterol] – [HDL] – [TG/5]. (Note: This is invalid if TG >400 mg/dL).
Question 308: Which of the following is NOT a phospholipid?
- A. Sphingomyelin
- B. Cephalin
- C. Cerebroside (Correct Answer)
- D. Cardiolipin
Explanation: ### Explanation The classification of lipids is based on their chemical composition. **Phospholipids** must contain a phosphate group as part of their structure, whereas **Glycolipids** contain a carbohydrate moiety instead of phosphate. **Why Cerebroside is the correct answer:** Cerebrosides are **Glycosphingolipids**. They consist of a ceramide backbone (sphingosine + fatty acid) attached to a single sugar unit (usually glucose or galactose). Because they lack a phosphate group, they are classified as glycolipids, not phospholipids. **Analysis of Incorrect Options:** * **Sphingomyelin:** This is the only **phospholipid** that does not have a glycerol backbone; it uses sphingosine instead. However, because it contains a phosphate group (phosphorylcholine), it is classified as a sphingophospholipid. * **Cephalin (Phosphatidylethanolamine):** A major glycerophospholipid found in cell membranes, particularly in nervous tissue. It contains glycerol, fatty acids, phosphate, and ethanolamine. * **Cardiolipin (Diphosphatidylglycerol):** An important glycerophospholipid found exclusively in the **inner mitochondrial membrane**. It is essential for the optimal function of the electron transport chain. **High-Yield Clinical Pearls for NEET-PG:** 1. **Cardiolipin:** Decreased levels are associated with **Barth Syndrome**, and it is the antigen used in the **VDRL test** for Syphilis. 2. **Sphingomyelin:** Accumulates in **Niemann-Pick Disease** due to a deficiency of the enzyme sphingomyelinase. 3. **Lecithin (Phosphatidylcholine):** The most abundant phospholipid in the cell membrane; its ratio to sphingomyelin (L:S ratio) in amniotic fluid is a marker for **fetal lung maturity**. 4. **Phosphatidylinositol:** Acts as a precursor for second messengers like $IP_3$ and $DAG$.
Question 309: What is the function of LCAT?
- A. Helps in cholesterol synthesis
- B. Converts cholesterol to cholesterol ester (Correct Answer)
- C. Helps in the formation of chylomicrons
- D. All of the above
Explanation: **Explanation:** **LCAT (Lecithin-Cholesterol Acyltransferase)** is a plasma enzyme synthesized by the liver and associated primarily with **High-Density Lipoprotein (HDL)**. 1. **Why Option B is Correct:** LCAT catalyzes the transfer of a fatty acid from the C2 position of lecithin (phosphatidylcholine) to the free hydroxyl group of cholesterol. This reaction produces **cholesterol esters** and lysolecithin. Because cholesterol esters are more hydrophobic than free cholesterol, they move from the surface of the HDL particle into its core. This process is the fundamental step in **Reverse Cholesterol Transport**, allowing HDL to "trap" cholesterol from peripheral tissues and transport it back to the liver. 2. **Why Other Options are Incorrect:** * **Option A:** Cholesterol synthesis occurs intracellularly (primarily in the liver and intestines) via the HMG-CoA reductase pathway, not by LCAT. * **Option C:** Chylomicron formation occurs in the intestinal mucosal cells and requires Apo B-48 and MTP (Microsomal Triglyceride Transfer Protein), not LCAT. **High-Yield Clinical Pearls for NEET-PG:** * **Activator:** LCAT is specifically activated by **Apo A-I** (the major apoprotein of HDL). * **Intracellular Counterpart:** While LCAT esterifies cholesterol in the *plasma*, the enzyme **ACAT** (Acyl-CoA:cholesterol acyltransferase) performs this function *inside cells*. * **Clinical Correlation:** Deficiency of LCAT leads to **Fish-eye disease** or Familial LCAT deficiency, characterized by corneal opacities, hemolytic anemia, and renal failure due to the accumulation of free cholesterol in tissues.
Question 310: Cholesterol is transported to peripheral tissues by which lipoprotein?
- A. High-density lipoprotein (HDL)
- B. Very-low-density lipoprotein (VLDL)
- C. Low-density lipoprotein (LDL) (Correct Answer)
- D. Intermediate-density lipoprotein (IDL)
Explanation: **Explanation:** The transport of cholesterol in the blood is mediated by specific lipoproteins, each serving a distinct physiological role based on its composition and apolipoprotein content. **Why LDL is the Correct Answer:** **Low-density lipoprotein (LDL)** is the primary carrier of cholesterol in the systemic circulation. It is formed from the metabolism of VLDL and IDL. LDL contains a high concentration of cholesterol esters and expresses **Apo B-100**, which acts as a ligand for LDL receptors on peripheral tissues. This allows LDL to deliver cholesterol to extrahepatic cells for membrane synthesis and steroidogenesis. Because it distributes cholesterol to the body (and can deposit it in arterial walls), it is clinically termed "Bad Cholesterol." **Analysis of Incorrect Options:** * **HDL (High-density lipoprotein):** Known as "Good Cholesterol," HDL mediates **reverse cholesterol transport**. It picks up excess cholesterol from peripheral tissues and returns it to the liver via the SR-B1 receptor. * **VLDL (Very-low-density lipoprotein):** Produced by the liver, its primary role is the transport of **endogenous triglycerides** to peripheral tissues, not cholesterol. * **IDL (Intermediate-density lipoprotein):** A transient metabolic intermediate formed during the conversion of VLDL to LDL. While it contains cholesterol, its main fate is either uptake by the liver or further conversion into LDL. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme of cholesterol synthesis:** HMG-CoA Reductase (inhibited by Statins). * **Apolipoprotein for LDL:** Apo B-100. * **Friedewald Equation:** LDL = [Total Cholesterol] – [HDL] – [Triglycerides/5]. (Note: This is invalid if TG >400 mg/dL). * **Wolman Disease:** A lysosomal storage disorder caused by a deficiency in acid lipase, preventing the breakdown of cholesterol esters delivered by LDL.
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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