Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 491: Which of the following is cardio-protective?

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

Explanation: **Explanation:** **HDL (High-Density Lipoprotein)** is known as the "Good Cholesterol" because of its primary role in **Reverse Cholesterol Transport**. It picks up excess cholesterol from peripheral tissues and atherosclerotic plaques and transports it back to the liver for excretion in bile. This process prevents lipid accumulation in arterial walls, making it highly **cardio-protective**. Additionally, HDL possesses anti-inflammatory, anti-thrombotic, and antioxidant properties that further protect the vascular endothelium. **Why other options are incorrect:** * **LDL (Low-Density Lipoprotein):** Known as "Bad Cholesterol," it transports cholesterol from the liver to peripheral tissues. High levels lead to cholesterol deposition in sub-endothelial spaces, leading to atherosclerosis and Coronary Artery Disease (CAD). * **VLDL (Very Low-Density Lipoprotein):** Produced by the liver to transport endogenous triglycerides. High levels are associated with an increased risk of metabolic syndrome and cardiovascular events. * **IDL (Intermediate-Density Lipoprotein):** A transient remnant formed during the conversion of VLDL to LDL. Like LDL, it is pro-atherogenic. **High-Yield NEET-PG Pearls:** * **ApoA-I:** The major apoprotein associated with HDL (activates LCAT). * **LCAT (Lecithin-Cholesterol Acyltransferase):** The enzyme responsible for esterifying cholesterol within HDL, converting "discoid HDL" into "spherical HDL." * **CETP (Cholesterol Ester Transfer Protein):** Mediates the exchange of cholesterol esters from HDL for triglycerides from VLDL/LDL. * **Friedewald Equation:** LDL = Total Cholesterol – [HDL + (Triglycerides/5)]. (Note: Not applicable if TG >400 mg/dL).

Question 492: Esters of fat-soluble vitamins are digested by:

A. Pancreatic lipase
B. Cholesterol esterase (Correct Answer)
C. Colipase
D. Carboxypeptidase

Explanation: ### Explanation **Correct Option: B. Cholesterol esterase** **Concept:** Fat-soluble vitamins (A, D, E, and K) are often consumed as esters (e.g., retinyl esters). To be absorbed by the intestinal mucosa, these must be hydrolyzed into free vitamins and fatty acids. **Cholesterol esterase** (also known as **Nonspecific Lipid Esterase** or Bile Salt-Activated Lipase) is the primary enzyme responsible for this process. It has broad specificity and hydrolyzes not only cholesterol esters but also esters of fat-soluble vitamins, monoglycerides, and phospholipids. **Analysis of Incorrect Options:** * **A. Pancreatic lipase:** This is the primary enzyme for the digestion of dietary **triacylglycerols (TAGs)**. It specifically targets the 1 and 3 positions of the glycerol backbone but does not efficiently hydrolyze vitamin esters. * **C. Colipase:** This is a protein co-factor secreted by the pancreas. It does not have enzymatic activity itself; rather, it displaces bile salts from the oil-water interface to allow pancreatic lipase to bind to its substrate. * **D. Carboxypeptidase:** This is a **proteolytic enzyme** (exopeptidase) involved in protein digestion, specifically cleaving amino acids from the C-terminal end of peptides. It has no role in lipid or vitamin metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Absorption Requirement:** Fat-soluble vitamin absorption is entirely dependent on **micelle formation** and adequate bile salt concentration. * **Steatorrhea:** Conditions causing fat malabsorption (e.g., Chronic Pancreatitis, Celiac disease) lead to secondary deficiencies of fat-soluble vitamins. * **Vitamin A Storage:** Once absorbed, Vitamin A is re-esterified and transported in chylomicrons to the liver, where it is stored in **Ito cells** (Stellate cells) as retinyl palmitate.

Question 493: Which among the following is not a saturated fatty acid?

A. Myristic acid
B. Stearic acid
C. Palmitic acid
D. Linoleic acid (Correct Answer)

Explanation: ### Explanation The correct answer is **Linoleic acid**. **1. Why Linoleic acid is the correct answer:** Fatty acids are classified based on the presence of double bonds. **Saturated fatty acids (SFAs)** have no double bonds, while **Unsaturated fatty acids** contain one or more double bonds. Linoleic acid is a **Polyunsaturated Fatty Acid (PUFA)** containing 18 carbon atoms and **two double bonds** (18:2; Δ9,12). It is an essential fatty acid belonging to the Omega-6 (ω-6) family, meaning the body cannot synthesize it and it must be obtained through the diet. **2. Why the other options are incorrect:** * **Myristic acid (A):** A saturated fatty acid with 14 carbon atoms (14:0). * **Palmitic acid (C):** The most common saturated fatty acid in the human body, containing 16 carbon atoms (16:0). It is the primary product of the Fatty Acid Synthase (FAS) complex. * **Stearic acid (B):** A saturated fatty acid with 18 carbon atoms (18:0). **3. High-Yield Clinical Pearls for NEET-PG:** * **Essential Fatty Acids:** There are two—Linoleic acid (ω-6) and Linolenic acid (ω-3). Arachidonic acid becomes essential only if Linoleic acid is deficient. * **Mnemonic for Saturated FAs:** "**L**ittle **M**ice **P**lay **S**tealthily" (**L**auric 12C, **M**yristic 14C, **P**almitic 16C, **S**tearic 18C). * **Prostaglandin Precursor:** Arachidonic acid (20:4; Δ5,8,11,14) is the direct precursor for the synthesis of eicosanoids (prostaglandins, thromboxanes, and leukotrienes). * **Refsum Disease:** A metabolic disorder caused by a deficiency in the α-oxidation of **Phytanic acid** (a branched-chain fatty acid).

Question 494: Which fatty acid contains the maximum amount of polyunsaturated fatty acids (PUFA)?

A. Palmitic acid
B. Stearic acid
C. Oleic acid
D. Linoleic acid (Correct Answer)

Explanation: ### Explanation **1. Why Linoleic Acid is Correct:** Fatty acids are classified based on the number of double bonds in their hydrocarbon chain. **Polyunsaturated fatty acids (PUFA)** are those containing two or more double bonds. * **Linoleic acid** is an 18-carbon fatty acid with **two double bonds** (18:2; Δ9,12). * It is an **essential fatty acid** (Omega-6 family) because the human body lacks the enzymes (desaturases) to introduce double bonds beyond the Δ9 position. **2. Analysis of Incorrect Options:** * **A. Palmitic acid:** This is a **saturated fatty acid (SFA)** with 16 carbons and zero double bonds (16:0). It is the most common SFA in the human body. * **B. Stearic acid:** This is a **saturated fatty acid (SFA)** with 18 carbons and zero double bonds (18:0). It is commonly found in animal fats and cocoa butter. * **C. Oleic acid:** This is a **monounsaturated fatty acid (MUFA)** with 18 carbons and one double bond (18:1; Δ9). It is the primary component of olive oil. **3. High-Yield Clinical Pearls for NEET-PG:** * **Essential Fatty Acids (EFA):** Only two are strictly essential—**Linoleic acid** (Omega-6) and **Alpha-linolenic acid** (Omega-3). Arachidonic acid becomes essential only if Linoleic acid is deficient. * **PUFA Functions:** They are precursors for eicosanoids (prostaglandins, leukotrienes) and are vital for maintaining cell membrane fluidity. * **Clinical Deficiency:** Deficiency of EFAs leads to **Phrynoderma** (toad skin), characterized by follicular hyperkeratosis on the extensor surfaces of extremities. * **Order of Unsaturation:** Stearic (0) < Oleic (1) < Linoleic (2) < Linolenic (3) < Arachidonic (4).

Question 495: What is the role of carnitine in lipid metabolism?

A. Catalyzation of the cyclization sequence
B. Essential for extracellular transfer of fatty acids
C. Essential for biosynthesis of fatty acids
D. Transfer of activated long-chain free fatty acids into mitochondria (Correct Answer)

Explanation: **Explanation:** The correct answer is **D: Transfer of activated long-chain free fatty acids into mitochondria.** **Why it is correct:** The inner mitochondrial membrane is impermeable to long-chain fatty acids (LCFA). To undergo **beta-oxidation**, these fatty acids must enter the mitochondrial matrix. This is achieved via the **Carnitine Shuttle**. 1. Fatty acids are first activated to Fatty Acyl-CoA in the cytosol. 2. **Carnitine Palmitoyltransferase-I (CPT-I)**, the rate-limiting enzyme of beta-oxidation, converts Acyl-CoA to Acyl-carnitine. 3. Acyl-carnitine is transported across the inner membrane by a translocase. 4. **CPT-II** then reconverts it back to Acyl-CoA inside the matrix for oxidation. **Why other options are incorrect:** * **A:** Cyclization sequences are characteristic of cholesterol synthesis (e.g., squalene to lanosterol), not carnitine function. * **B:** Extracellular transfer of fatty acids is primarily handled by **Albumin** (for free fatty acids) or **Lipoproteins** (for esterified fats), not carnitine. * **C:** Fatty acid biosynthesis occurs in the **cytosol** and requires NADPH and Acetyl-CoA carboxylase; carnitine is involved in catabolism (breakdown), not synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Inhibitor:** Malonyl-CoA (the first intermediate of fatty acid synthesis) inhibits CPT-I, preventing a futile cycle where synthesis and breakdown occur simultaneously. * **Carnitine Deficiency:** Presents with **non-ketotic hypoglycemia** (due to impaired gluconeogenesis and lack of ketone bodies) and muscle weakness during fasting or exercise. * **Sources:** Carnitine is derived from **Lysine and Methionine**; synthesis requires **Vitamin C**. * **Systemic Primary Carnitine Deficiency:** Caused by a defect in the OCTN2 carnitine transporter.

Question 496: What dietary component should be administered to manage chyluria?

A. Short-chain fatty acids
B. Medium-chain fatty acids (Correct Answer)
C. Long-chain fatty acids
D. Omega-3 unsaturated fatty acids

Explanation: ### Explanation **Core Concept: Lymphatic Bypass of MCFAs** Chyluria is the presence of chyle (lymphatic fluid containing emulsified fats) in the urine, typically caused by a communication between the intestinal lymphatics and the urinary tract (often due to filariasis). The goal of dietary management is to reduce the flow of chyle through the lymphatic system. **Why Medium-Chain Fatty Acids (MCFAs) are correct:** MCFAs (6–12 carbons) are unique because they are **water-soluble**. Unlike long-chain fats, they do not require bile salts for micelle formation or incorporation into **chylomicrogens**. Instead, MCFAs are absorbed directly into the **portal venous blood** and transported to the liver bound to albumin. By bypassing the lymphatic system, MCFAs provide a necessary energy source without increasing lymphatic pressure or leakage into the urine. **Analysis of Incorrect Options:** * **Long-chain fatty acids (LCFAs):** These are the primary cause of symptoms. LCFAs must be re-esterified into triglycerides and packaged into chylomicrogens, which travel through the **thoracic duct**. This increases lymphatic flow, worsening chyluria. * **Short-chain fatty acids (SCFAs):** While they also enter the portal blood, they are primarily produced by colonic fermentation of fiber and are not a significant dietary source of calories needed for managing malabsorption or chyluria. * **Omega-3 unsaturated fatty acids:** These are a subset of LCFAs. Despite their anti-inflammatory benefits, they still require lymphatic transport via chylomicrogens and would exacerbate chyluria. **High-Yield Clinical Pearls for NEET-PG:** * **Chyluria Triad:** Milky white urine, hemato-chyluria (occasionally), and presence of fat globules/chyle. * **Diagnostic Test:** The urine turns clear when shaken with **ether** (which dissolves the fat). * **MCFA Absorption:** They do not require **pancreatic lipase** or **bile salts** for absorption, making them also ideal for patients with chronic pancreatitis or biliary obstruction. * **Key Enzyme:** MCFAs bypass the need for **Acyl-CoA synthetase** and **CPT-1** for entry into the mitochondria for beta-oxidation.

Question 497: Fatty acids cannot be utilized by which of the following?

A. Muscles
B. Heart
C. Liver
D. RBC (Correct Answer)

Explanation: **Explanation:** The correct answer is **RBC (Red Blood Cells)**. **1. Why RBCs cannot utilize Fatty Acids:** The utilization of fatty acids for energy occurs through **$\beta$-oxidation**, a process that takes place exclusively within the **mitochondria**. Mature RBCs lack mitochondria (as well as a nucleus and other organelles) to maximize space for hemoglobin and prevent the consumption of the oxygen they transport. Consequently, RBCs are entirely dependent on **anaerobic glycolysis** for their energy needs and cannot oxidize fatty acids or ketone bodies. **2. Why other options are incorrect:** * **Muscles (Skeletal):** At rest and during low-to-moderate intensity exercise, long-chain fatty acids are the preferred fuel source for skeletal muscle. * **Heart:** The myocardium is metabolically demanding and highly aerobic. Under normal physiological conditions, **60–80%** of the heart's energy is derived from fatty acid oxidation. * **Liver:** The liver is the primary site for fatty acid metabolism. It oxidizes fatty acids to provide energy for gluconeogenesis and converts excess acetyl-CoA into ketone bodies during fasting. **High-Yield Clinical Pearls for NEET-PG:** * **Brain Paradox:** Although the brain has mitochondria, it cannot utilize fatty acids because they are bound to albumin and cannot cross the **Blood-Brain Barrier (BBB)**. The brain uses glucose or ketone bodies (during starvation). * **Essential Enzyme:** The "rate-limiting" step of fatty acid oxidation is the transport of fatty acids into the mitochondria via the **Carnitine Shuttle** (Enzyme: CPT-1). * **Energy Yield:** The complete oxidation of one molecule of Palmitic acid (16 carbons) yields **106 ATP**.

Question 498: What is the major fuel source for the brain after several weeks of starvation?

A. Glucose
B. Fatty acid
C. beta-Hydroxybutyrate (Correct Answer)
D. Glycerol

Explanation: **Explanation:** The brain typically relies on glucose as its primary energy source. However, during prolonged starvation (beyond 2–3 days), the body undergoes metabolic adaptation to preserve muscle mass and maintain blood glucose levels for essential functions. **Why beta-Hydroxybutyrate is correct:** During starvation, the liver undergoes intense **ketogenesis**, converting fatty acids into ketone bodies: acetoacetate and **beta-hydroxybutyrate**. Unlike fatty acids, these ketone bodies are water-soluble and can cross the **blood-brain barrier (BBB)**. After several weeks of starvation, the brain adapts to utilize ketone bodies for up to **60–70%** of its energy requirements, with beta-hydroxybutyrate being the most abundant and efficient fuel source. This shift reduces the need for gluconeogenesis, thereby sparing skeletal muscle protein from breakdown. **Why other options are incorrect:** * **A. Glucose:** While the brain always requires a basal amount of glucose, its consumption drops significantly during starvation to conserve the body’s limited glucose stores. * **B. Fatty acids:** Although plasma levels are high, long-chain fatty acids **cannot cross the BBB** and therefore cannot be used by the brain for energy. * **D. Glycerol:** Glycerol is released from adipose tissue during lipolysis and can be used by the liver for gluconeogenesis, but it is not a direct fuel source for the brain. **NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme of ketogenesis:** HMG-CoA Synthase (Mitochondrial). * **Organ that cannot use ketones:** The **Liver** (lacks the enzyme Thiophorase/Succinyl-CoA:3-ketoacid CoA transferase). * **Ketone body detection:** The Rothera’s test detects acetoacetate and acetone, but **not** beta-hydroxybutyrate. * **Energy yield:** Beta-hydroxybutyrate provides more ATP than acetoacetate because it is more reduced.

Question 499: The liver removes low-density lipoproteins (LDLs) from the blood by the LDLs binding to which of the following?

A. LDL receptors and then internalizing them (Correct Answer)
B. HDL receptors and then internalizing them
C. The albumin present on LDLs and then internalizing them
D. The transferrin present on LDL and then internalizing them

Explanation: The liver is the primary organ responsible for clearing Low-Density Lipoprotein (LDL) from systemic circulation, a process critical for maintaining cholesterol homeostasis. ### **Explanation of the Correct Answer** **Option A** is correct because the clearance of LDL is mediated by **LDL receptors (LDLR)** located on the surface of hepatocytes. These receptors specifically recognize **Apolipoprotein B-100**, which is the primary structural protein of LDL. Once the LDL particle binds to the receptor, the entire complex is internalized via **clathrin-mediated endocytosis**. Inside the cell, the LDL is degraded in lysosomes to release free cholesterol, while the LDL receptors are typically recycled back to the cell surface. ### **Why Other Options are Incorrect** * **Option B:** HDL receptors (such as **SR-BI**) are involved in "Reverse Cholesterol Transport," allowing the liver to take up cholesterol esters from High-Density Lipoprotein (HDL), not LDL. * **Option C:** Albumin is a transport protein for free fatty acids and bilirubin, but it is not a structural component of LDL nor a ligand for its clearance. * **Option D:** Transferrin is the transport protein for **iron**. It binds to transferrin receptors, not LDL receptors, and has no role in lipid metabolism. ### **High-Yield Clinical Pearls for NEET-PG** * **Familial Hypercholesterolemia (Type IIa):** Caused by a genetic deficiency or defect in **LDL receptors**, leading to severely elevated serum LDL and premature atherosclerosis. * **PCSK9 Inhibitors:** A modern class of drugs (e.g., Alirocumab) that prevents the degradation of LDL receptors, thereby increasing their density on the liver surface and lowering blood LDL levels. * **Statins:** These drugs inhibit HMG-CoA reductase, which decreases intracellular cholesterol, leading to an **upregulation of LDL receptors** and increased clearance of LDL from the blood.

Question 500: Carnitine is required for which process in fatty acid metabolism?

A. Fatty acid synthesis
B. Fatty acid oxidation (Correct Answer)
C. Fatty acid storage
D. Ketone body synthesis

Explanation: **Explanation:** **Why Fatty Acid Oxidation is Correct:** Long-chain fatty acids (LCFAs) cannot freely cross the inner mitochondrial membrane to undergo **Beta-oxidation**. Carnitine acts as a specialized "shuttle" system. The process involves three key steps: 1. **CPT-I (Carnitine Palmitoyltransferase-I):** Located on the outer mitochondrial membrane, it converts Fatty Acyl-CoA to Acyl-carnitine. 2. **Translocase:** Transports Acyl-carnitine into the mitochondrial matrix. 3. **CPT-II:** Located on the inner membrane, it converts Acyl-carnitine back into Fatty Acyl-CoA, releasing carnitine to be reused. Without carnitine, LCFAs remain trapped in the cytosol, preventing ATP production via oxidation. **Why Other Options are Incorrect:** * **A. Fatty acid synthesis:** This occurs in the **cytosol**. It requires Citrate (to move Acetyl-CoA out of the mitochondria) and NADPH, but not carnitine. * **C. Fatty acid storage:** Storage involves the esterification of fatty acids into Triacylglycerols (TAGs) within adipose tissue, which does not involve the carnitine shuttle. * **D. Ketone body synthesis:** While ketogenesis uses Acetyl-CoA derived from oxidation, the specific requirement for carnitine is at the transport stage of oxidation, not the enzymatic synthesis of ketone bodies in the liver. **High-Yield Clinical Pearls for NEET-PG:** * **Inhibitor:** Malonyl-CoA (the first intermediate of FA synthesis) inhibits **CPT-I**, preventing a futile cycle where synthesis and oxidation happen simultaneously. * **Systemic Carnitine Deficiency:** Presents with **non-ketotic hypoglycemia** (due to impaired gluconeogenesis and lack of acetyl-CoA) and muscle weakness. * **Sources:** Carnitine is derived from Lysine and Methionine; meat is the primary dietary source.

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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