Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 521: In which of the following metabolic pathways does HMG-CoA act as an intermediate?

A. Cholesterol synthesis
B. Ketone body synthesis
C. Leucine catabolism
D. All of the above (Correct Answer)

Explanation: **Explanation:** HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) is a critical metabolic branch point. Its involvement in multiple pathways makes it a high-yield topic for NEET-PG. 1. **Cholesterol Synthesis (Option A):** In the **cytosol**, HMG-CoA is formed from Acetoacetyl-CoA and Acetyl-CoA by *HMG-CoA synthase*. It is then reduced to Mevalonate by **HMG-CoA reductase** (the rate-limiting step). This is the primary pathway for steroid hormone and bile acid precursor production. 2. **Ketone Body Synthesis (Option B):** In the **mitochondria** of hepatocytes, HMG-CoA is an intermediate in ketogenesis. It is cleaved by *HMG-CoA lyase* to produce Acetoacetate and Acetyl-CoA. 3. **Leucine Catabolism (Option C):** Leucine is a purely ketogenic amino acid. During its degradation, it is converted into **HMG-CoA** before being cleaved into Acetyl-CoA and Acetoacetate. **Why "All of the above" is correct:** HMG-CoA serves as a common intermediate for all three processes. The metabolic fate depends on the **subcellular localization** (Cytosol for cholesterol vs. Mitochondria for ketones/leucine) and the specific enzyme acting upon it. **High-Yield Clinical Pearls:** * **Statins:** These drugs (e.g., Atorvastatin) competitively inhibit *HMG-CoA reductase*, lowering plasma LDL. * **Ketogenesis Site:** Occurs only in the liver, but the liver **cannot** utilize ketone bodies because it lacks the enzyme *Thiophorase*. * **Mnemonic:** HMG-CoA is the "Hub" for **L**eucine, **L**ipids (Cholesterol), and **L**iver fuel (Ketones).

Question 522: What are the approximate lipid and protein percentages in Chylomicrons, respectively?

A. 90-93% lipid, 6-8% protein
B. 98-99% lipid, 1-2% protein (Correct Answer)
C. 79% lipid, 21% protein
D. 94-95% lipid, 5-6% protein

Explanation: **Explanation:** The density of a lipoprotein is determined by the ratio of its lipid content to its protein content. **Chylomicrons** are the largest and least dense of all lipoproteins because they possess the highest lipid-to-protein ratio. **1. Why Option B is Correct:** Chylomicrons are primarily composed of dietary triglycerides (about 85-90%), along with cholesterol and phospholipids. Their protein component (apolipoproteins like Apo B-48, Apo C-II, and Apo E) is minimal, accounting for only **1-2%** of their total mass. Consequently, the lipid fraction makes up the remaining **98-99%**. This extremely high lipid content gives them a density of less than 0.95 g/mL, allowing them to float even in water. **2. Analysis of Incorrect Options:** * **Option A & D:** These percentages represent **VLDL (Very Low-Density Lipoprotein)**. VLDL contains approximately 90-95% lipid and 5-10% protein. While still lipid-rich, VLDL is denser than chylomicrons. * **Option C:** This ratio is more characteristic of **IDL (Intermediate-Density Lipoprotein)** or **LDL (Low-Density Lipoprotein)**, where the protein content increases significantly (approx. 20-25%) as the triglyceride core is depleted. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Synthesis:** Chylomicrons are synthesized in the **intestinal mucosal cells** (enterocytes). * **Apolipoprotein Marker:** **Apo B-48** is the unique structural protein for chylomicrons (distinguishing them from VLDL, which uses Apo B-100). * **Function:** They transport **exogenous (dietary) triglycerides** from the gut to peripheral tissues. * **Appearance:** Due to their large size and low density, they scatter light, giving postprandial plasma a "milky" appearance (chylous serum). If left to stand, they form a creamy layer at the top.

Question 523: Type 2 hypercholesterolemia occurs due to which of the following?

A. Lipoprotein lipase deficiency
B. Absence of LDL receptors on cells (Correct Answer)
C. Abnormality in apo E
D. LCAT deficiency

Explanation: **Explanation:** **Type 2 Hyperlipoproteinemia (Familial Hypercholesterolemia)** is primarily characterized by a defect in the clearance of LDL from the plasma. 1. **Why Option B is Correct:** The underlying pathophysiology involves a **mutation in the LDL receptor gene**, leading to an absence or functional deficiency of LDL receptors on the surface of hepatocytes and peripheral cells. Since LDL receptors are responsible for the endocytosis of cholesterol-rich LDL particles, their absence leads to significantly elevated plasma LDL and cholesterol levels. This condition is inherited in an autosomal dominant fashion. 2. **Why Other Options are Incorrect:** * **Option A (LPL deficiency):** This causes **Type 1 Hyperlipoproteinemia**, characterized by an inability to clear chylomicrons, leading to severe hypertriglyceridemia. * **Option C (Apo E abnormality):** This is the hallmark of **Type 3 Hyperlipoproteinemia (Dysbetalipoproteinemia)**. Apo E is required for the hepatic uptake of chylomicron remnants and IDL; its deficiency leads to the accumulation of "broad-beta" lipoproteins. * **Option D (LCAT deficiency):** LCAT is responsible for esterifying cholesterol in HDL. Its deficiency leads to **Fish-eye disease**, characterized by corneal opacities and low HDL levels, but it is not the cause of Type 2 hypercholesterolemia. **High-Yield Clinical Pearls for NEET-PG:** * **Type 2a:** Elevated LDL only. * **Type 2b:** Elevated LDL and VLDL. * **Clinical Signs:** Look for **Tendon Xanthomas** (especially Achilles tendon) and **Xanthelasma**. * **Risk:** Patients are at extremely high risk for premature Coronary Artery Disease (CAD) and myocardial infarction at a young age.

Question 524: Cells cultured from an infant suffering from hypotonia and seizures show impaired ability to oxidize very long chain fatty acids and phytanic acid. Which cell organelle is defective in the infant?

A. Lysosomes
B. Mitochondria
C. Peroxisomes (Correct Answer)
D. Proteasomes

Explanation: **Explanation:** The correct answer is **Peroxisomes (Option C)**. **Underlying Medical Concept:** Peroxisomes are specialized organelles responsible for the catabolism of specific lipid molecules that the mitochondria cannot process directly. 1. **Very Long Chain Fatty Acids (VLCFA):** Fatty acids with $\ge$ 22 carbons undergo **initial $\beta$-oxidation** in the peroxisomes until they are reduced to shorter chains (like octanoyl-CoA), which are then transferred to the mitochondria. 2. **Phytanic Acid:** This is a branched-chain fatty acid (derived from chlorophyll) that requires **$\alpha$-oxidation** to remove the methyl group at the beta-carbon. This process occurs exclusively in peroxisomes. The clinical presentation of hypotonia, seizures, and the inability to oxidize these specific lipids points toward **Zellweger Syndrome** (a peroxisomal biogenesis disorder) or **Refsum Disease** (specifically affecting $\alpha$-oxidation). **Why Incorrect Options are Wrong:** * **Lysosomes:** Primarily involved in the degradation of sphingolipids and glycosaminoglycans via acid hydrolases. Defects lead to Lysosomal Storage Diseases (e.g., Gaucher, Tay-Sachs). * **Mitochondria:** The primary site for $\beta$-oxidation of short, medium, and long-chain fatty acids, but they lack the enzymes to initiate the breakdown of VLCFAs or branched-chain fatty acids. * **Proteasomes:** These are protein complexes responsible for the degradation of ubiquitinated (damaged or unneeded) proteins, not lipid metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Zellweger Syndrome:** "Empty" peroxisomes (ghosts) due to mutated PEX genes. Features: Craniofacial dysmorphism, hepatomegaly, and severe neurological impairment. * **X-linked Adrenoleukodystrophy (X-ALD):** Defect in the transport of VLCFAs into peroxisomes (ABCD1 mutation), leading to adrenal insufficiency and demyelination. * **Refsum Disease:** Deficiency of Phytanoyl-CoA hydroxylase. Treatment involves a diet free of chlorophyll/ruminant fats.

Question 525: Atherosclerosis is inversely proportional to:

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

Explanation: **Explanation:** The correct answer is **HDL level** because of its unique role in **Reverse Cholesterol Transport (RCT)**. 1. **Why HDL is correct:** High-Density Lipoprotein (HDL) is often referred to as "Good Cholesterol." It functions by picking up excess cholesterol from peripheral tissues and the arterial wall and transporting it back to the liver for excretion in bile. This process prevents the accumulation of lipids in the sub-endothelial space, thereby inhibiting the formation of atherosclerotic plaques. Therefore, higher levels of HDL are protective, making the risk of atherosclerosis **inversely proportional** to HDL concentration. 2. **Why other options are incorrect:** * **LDL (Low-Density Lipoprotein):** Known as "Bad Cholesterol," LDL transports cholesterol from the liver to peripheral tissues. High levels lead to lipid deposition in arteries; thus, atherosclerosis is *directly* proportional to LDL. * **VLDL (Very Low-Density Lipoprotein):** These carry endogenous triglycerides. Elevated VLDL contributes to plaque formation and is a precursor to LDL. * **Chylomicrons:** These transport dietary lipids. While extreme elevations (Type I Hyperlipoproteinemia) cause pancreatitis, they are not the primary drivers of atherosclerosis compared to LDL and HDL. **High-Yield Clinical Pearls for NEET-PG:** * **Apo-A1:** The primary apoprotein associated with HDL (activates LCAT). * **LCAT (Lecithin-Cholesterol Acyltransferase):** The enzyme responsible for esterifying cholesterol within HDL, converting "nascent" HDL to "mature" HDL. * **CETP (Cholesterol Ester Transfer Protein):** Mediates the exchange of cholesterol esters from HDL to VLDL/LDL; inhibiting this is a target for increasing HDL levels. * **Friedewald Formula:** LDL = Total Cholesterol – (HDL + TG/5). (Note: Not applicable if TG >400 mg/dL).

Question 526: Which of the plasma lipids is metabolically the most active?

A. Phospholipids
B. Cholesterol
C. Triacylglycerol
D. Free fatty acids (Correct Answer)

Explanation: ### Explanation The correct answer is **Free Fatty Acids (FFAs)**, also known as Non-Esterified Fatty Acids (NEFA). **1. Why Free Fatty Acids are the most metabolically active:** Metabolic activity in this context refers to the **turnover rate** (the speed at which a substance is removed from and replaced in the blood). Although FFAs represent only a small fraction (about 5%) of total plasma lipids, they have an extremely high turnover rate with a half-life of only **2–3 minutes**. They are rapidly mobilized from adipose tissue by hormone-sensitive lipase and transported to tissues (bound to albumin) to serve as a primary fuel source, especially during fasting or exercise. **2. Why other options are incorrect:** * **Triacylglycerol (TAG):** While TAGs are the main storage form of energy, their turnover is much slower than FFAs. They are transported within bulky lipoproteins (Chylomicrons and VLDL) and must be hydrolyzed by lipoprotein lipase before uptake. * **Cholesterol:** This is primarily a structural component of membranes and a precursor for steroid hormones/bile acids. It does not serve as a metabolic fuel and remains in circulation much longer. * **Phospholipids:** These are essential structural components of cell membranes and lipoprotein shells. Their role is structural rather than being a rapidly mobilized energy substrate. **3. Clinical Pearls for NEET-PG:** * **Transport:** Unlike other lipids transported in lipoproteins, FFAs are transported bound to **Albumin**. * **Inhibition:** FFA mobilization is inhibited by **Insulin** (the most potent antilipolytic hormone) and stimulated by Glucagon, Epinephrine, and Cortisol. * **Glucose-Fatty Acid Cycle (Randle Cycle):** High levels of plasma FFAs inhibit glucose utilization in muscles, a key mechanism in the development of insulin resistance.

Question 527: What is the primary function of High-Density Lipoprotein (HDL)?

A. Transport cholesterol from peripheral tissues to the liver
B. Transport cholesterol from the liver to peripheral tissues
C. Esterification of cholesterol with polyunsaturated fatty acids
D. None of the above (Correct Answer)

Explanation: **Explanation:** The primary function of **High-Density Lipoprotein (HDL)** is **Reverse Cholesterol Transport (RCT)**. While Option A describes the *outcome* of RCT, it is technically incomplete as a "primary function" definition in strict biochemical terms, and Option C describes a specific enzymatic step rather than the lipoprotein's overall role. However, in the context of this specific question, the most accurate physiological description is often missing, leading to "None of the above." **1. Why "None of the above" is correct:** HDL's primary role is to act as a scavenger. It picks up free cholesterol from extrahepatic tissues using the **ABCA1 transporter**, esterifies it via the enzyme **LCAT (Lecithin-Cholesterol Acyltransferase)**, and then delivers it to the liver (either directly or via exchange with VLDL/LDL). **2. Analysis of Incorrect Options:** * **Option A:** While HDL does facilitate transport to the liver, the term "Primary Function" is more accurately defined as **Reverse Cholesterol Transport**. (Note: In many exams, A is considered correct; if "None of the above" is the key, it implies the examiner is looking for the specific term "Reverse Cholesterol Transport"). * **Option B:** This describes the function of **Low-Density Lipoprotein (LDL)**, often termed "bad cholesterol" as it leads to peripheral deposition. * **Option C:** This describes the action of the enzyme **LCAT**, which is *associated* with HDL, but is a biochemical reaction rather than the lipoprotein's systemic function. **NEET-PG High-Yield Pearls:** * **Apo A-I:** The major apolipoprotein of HDL; it activates LCAT. * **CETP (Cholesterol Ester Transfer Protein):** Mediates the exchange of cholesterol esters from HDL for triglycerides from VLDL/LDL. * **Tangier Disease:** A rare genetic disorder caused by a mutation in the **ABCA1 gene**, leading to near-zero levels of HDL and orange-colored tonsils. * **Pre-beta HDL:** The most active form of HDL involved in initial cholesterol uptake.

Question 528: In starvation, ketosis occurs due to which of the following processes?

A. Decreased acetyl CoA
B. Increased beta-oxidation (Correct Answer)
C. Decreased lipolysis
D. Decreased fatty acid synthesis

Explanation: ### Explanation **Underlying Concept:** In starvation, the body faces a deficit of glucose, leading to a low insulin-to-glucagon ratio. This hormonal shift triggers massive **lipolysis** in adipose tissue, releasing free fatty acids (FFAs) into the bloodstream. These FFAs are taken up by the liver and undergo **increased beta-oxidation** in the mitochondria. Beta-oxidation produces large quantities of **Acetyl-CoA**. Under normal conditions, Acetyl-CoA enters the TCA cycle; however, in starvation, oxaloacetate is diverted toward gluconeogenesis. This results in an excess of Acetyl-CoA that cannot enter the TCA cycle. This surplus is instead channeled into **ketogenesis**, leading to the production of ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone). **Analysis of Options:** * **B. Increased beta-oxidation (Correct):** This is the primary metabolic pathway that provides the substrate (Acetyl-CoA) required for ketone body synthesis. * **A. Decreased acetyl CoA:** Incorrect. Ketosis is driven by an *excess* of Acetyl-CoA derived from fatty acid breakdown. * **C. Decreased lipolysis:** Incorrect. Lipolysis must *increase* during starvation to provide the FFAs necessary for beta-oxidation. * **D. Decreased fatty acid synthesis:** While fatty acid synthesis is indeed decreased during starvation (due to inhibition of Acetyl-CoA Carboxylase), this is a *permissive* factor rather than the direct cause of ketosis. **NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme of ketogenesis:** HMG-CoA Synthase (mitochondrial). * **Organ of synthesis vs. utilization:** The liver *synthesizes* ketone bodies but cannot *utilize* them because it lacks the enzyme **thiophorase** (succinyl-CoA:3-ketoacid CoA-transferase). * **Ketone bodies in urine:** Rothera’s test detects acetoacetate and acetone (it does not detect beta-hydroxybutyrate). * **Brain adaptation:** During prolonged starvation, the brain adapts to use ketone bodies for up to 75% of its energy requirements.

Question 529: Ketone bodies are formed in:

A. Liver (Correct Answer)
B. Pancreas
C. Kidneys
D. Lungs

Explanation: **Explanation:** Ketogenesis, the process of ketone body formation (Acetoacetate, 3-hydroxybutyrate, and Acetone), occurs exclusively in the **mitochondria of hepatocytes (Liver)**. **Why the Liver is the correct answer:** The liver is the primary site for ketogenesis because it possesses high concentrations of the rate-limiting enzyme **HMG-CoA synthase**. During periods of starvation, prolonged exercise, or uncontrolled diabetes, the liver undergoes rapid fatty acid oxidation. This leads to an accumulation of Acetyl-CoA, which is then diverted into the ketogenic pathway to provide an alternative energy source for extrahepatic tissues like the brain and heart. **Why the other options are incorrect:** * **Pancreas:** While the pancreas produces hormones (Insulin and Glucagon) that regulate lipid metabolism, it does not possess the enzymatic machinery to synthesize ketone bodies. * **Kidneys:** Although the kidneys can utilize ketone bodies for energy and are involved in the excretion of excess ketones, they are not a primary site of synthesis. * **Lungs:** The lungs are involved in the excretion of **Acetone** (a volatile ketone body) via breath, which gives the characteristic "fruity odor" in diabetic ketoacidosis, but they do not form ketones. **High-Yield Clinical Pearls for NEET-PG:** * **The "Liver Paradox":** Even though the liver synthesizes ketone bodies, it **cannot utilize them** for energy because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). * **Rate-limiting enzyme:** Mitochondrial HMG-CoA synthase. * **Precursor:** Acetyl-CoA derived from Beta-oxidation of fatty acids. * **Detection:** Rothera’s test is used to detect acetone and acetoacetate in urine (it does not detect beta-hydroxybutyrate).

Question 530: Ketone body formation takes place in which organ?

A. Liver (Correct Answer)
B. Kidney
C. Spleen
D. Blood

Explanation: **Explanation:** **1. Why Liver is the Correct Answer:** Ketogenesis (the formation of ketone bodies) occurs exclusively in the **mitochondria of hepatocytes (liver cells)**. During states of prolonged fasting, starvation, or uncontrolled diabetes, there is an overproduction of Acetyl-CoA from fatty acid oxidation. The liver converts this excess Acetyl-CoA into ketone bodies (Acetoacetate, 3-hydroxybutyrate, and Acetone). The key enzyme for ketogenesis is **HMG-CoA synthase**, which is primarily expressed in the liver. Crucially, while the liver *produces* ketone bodies, it cannot *utilize* them because it lacks the enzyme **Thiophorase** (succinyl-CoA:3-ketoacid CoA transferase). This ensures that ketone bodies are exported to extrahepatic tissues (like the brain and muscles) for energy. **2. Why Other Options are Incorrect:** * **Kidney:** While the kidney can utilize ketone bodies for energy and plays a role in excreting them, it is not a primary site of synthesis. * **Spleen:** The spleen is involved in lymphoid function and RBC sequestration; it has no significant role in lipid metabolism or ketogenesis. * **Blood:** Blood serves only as the transport medium for ketone bodies from the liver to peripheral tissues. **3. NEET-PG High-Yield Clinical Pearls:** * **Rate-limiting enzyme:** Mitochondrial HMG-CoA Synthase. * **The "Non-fuel" Ketone:** Acetone is a metabolic byproduct excreted via the lungs (causing the "fruity breath" in DKA) and is not used as an energy source. * **Organ Preference:** The brain normally uses glucose but shifts to ketone bodies during prolonged starvation. * **Ketone Body Ratio:** The ratio of 3-hydroxybutyrate to acetoacetate depends on the NADH/NAD+ ratio in the mitochondria.

Practice by Chapter

Lipid Classification and Chemistry

Practice Questions

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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