Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 571: A destitute woman is admitted to the hospital with altered sensorium and dehydration. Urine analysis shows mild proteinuria and no sugar. What other test would be desirable?

A. Fouchet test
B. Rothera test (Correct Answer)
C. Hays test
D. Benedict's test

Explanation: **Explanation:** The clinical presentation of a destitute woman with altered sensorium and dehydration, combined with the absence of urinary sugar, strongly suggests **Starvation Ketosis**. In a state of prolonged fasting or starvation, the body exhausts glycogen stores and shifts to fatty acid oxidation, leading to the production of ketone bodies (acetone, acetoacetate, and β-hydroxybutyrate) to provide energy for the brain. **Why Rothera’s Test is correct:** Rothera’s test is the standard biochemical semi-quantitative test used to detect **acetone and acetoacetate** in the urine. A positive result (development of a permanganate/purple ring) confirms ketonuria. In this patient, it helps differentiate starvation ketosis from other causes of altered sensorium. **Analysis of Incorrect Options:** * **Fouchet’s Test:** Used to detect **Bilirubin** in urine (indicative of jaundice). * **Hay’s Test:** Used to detect **Bile salts** in urine (indicative of obstructive jaundice) based on the principle of surface tension. * **Benedict’s Test:** Used to detect **Reducing sugars** (like glucose). The question specifically mentions "no sugar" in the urine, making this test redundant. **Clinical Pearls for NEET-PG:** * **Ketone Bodies:** Only acetoacetate and acetone react with Rothera’s reagent (sodium nitroprusside). **β-hydroxybutyrate** does not give a positive Rothera’s test. * **Differential Diagnosis:** If the urine showed high sugar AND ketones, the diagnosis would be **Diabetic Ketoacidosis (DKA)**. Since sugar is absent, it is **Starvation Ketosis**. * **Gerhardt’s Test:** An alternative test using Ferric chloride, specifically for acetoacetate.

Question 572: Which apolipoprotein is the main ligand for the LDL receptor?

A. Apolipoprotein A
B. Apolipoprotein B-100 (Correct Answer)
C. Apolipoprotein C-100
D. Apolipoprotein E

Explanation: **Explanation:** The **LDL receptor (LDLR)**, also known as the ApoB/E receptor, is responsible for the cellular uptake of cholesterol-rich lipoproteins via receptor-mediated endocytosis. **Why Apolipoprotein B-100 is correct:** Apolipoprotein B-100 (Apo B-100) is the primary structural protein of VLDL, IDL, and LDL. It serves as the **obligatory ligand** for the LDL receptor. When LDL particles circulate in the plasma, the LDLR recognizes the specific binding domain on Apo B-100, allowing the liver and peripheral tissues to internalize the cholesterol. **Analysis of Incorrect Options:** * **Apolipoprotein A:** Primarily found on HDL (Apo A-I). It acts as an activator of Lecithin-Cholesterol Acyltransferase (LCAT) and is involved in reverse cholesterol transport, not LDL receptor binding. * **Apolipoprotein C-100:** This is a **distractor**. There is no "C-100." Apo C-II is a cofactor for Lipoprotein Lipase (LPL), and Apo C-III inhibits it. * **Apolipoprotein E:** While Apo E is a ligand for the LDL receptor (and the LRP receptor), it is the primary ligand for **Chylomicron remnants** and **IDL**. While it has a higher affinity for the receptor than Apo B-100, Apo B-100 remains the "main" ligand specifically associated with the clearance of **LDL** particles. **High-Yield Clinical Pearls for NEET-PG:** * **Familial Hypercholesterolemia (Type IIa):** Caused by mutations in the LDL receptor or the binding domain of **Apo B-100**, leading to severely elevated LDL levels and early atherosclerosis. * **Apo B-48:** Produced in the intestine (via RNA editing of the Apo B gene); it lacks the LDL receptor-binding domain, which is why chylomicrons are not cleared by the LDLR. * **PCSK9:** A protein that degrades LDL receptors; PCSK9 inhibitors are a modern class of cholesterol-lowering drugs.

Question 573: The chyle from the intestine is rich with chylomicrons. Which of the following forms the protein core of chylomicrons?

A. Triglyceride only
B. Triglyceride + cholesterol
C. Triglyceride + cholesterol + phospholipid (Correct Answer)
D. Only cholesterol

Explanation: ### Explanation **Concept Overview:** Chylomicrons are the largest and least dense lipoproteins, synthesized in the intestinal mucosal cells to transport dietary (exogenous) lipids into the circulation. A lipoprotein particle is structured with a **hydrophobic core** and a **hydrophilic shell**. **Why Option C is Correct:** The "protein core" or the internal lipid cargo of a chylomicron consists primarily of **Triglycerides** (about 85-90%) and **Cholesteryl esters**. While phospholipids and free cholesterol are typically found on the surface membrane to provide solubility, the question refers to the overall lipid composition that constitutes the bulk of the particle. In the context of this question, the chylomicron is a complex assembly of **Triglycerides + Cholesterol + Phospholipids** (along with specific apolipoproteins like Apo B-48). **Analysis of Incorrect Options:** * **Options A, B, and D:** These are incomplete. Chylomicrons are never composed of a single lipid type. While triglycerides are the predominant component, they must be packaged with cholesterol and phospholipids to form a stable lipoprotein particle capable of entering the lymphatic system (chyle). **NEET-PG High-Yield Pearls:** * **Apolipoprotein Marker:** **Apo B-48** is the unique structural protein for chylomicrons (synthesized via RNA editing of the Apo B gene in the intestine). * **Route of Transport:** Chylomicrons are too large to enter capillaries directly; they enter **lacteals** (lymphatics), forming **chyle**, and reach the blood via the **thoracic duct**. * **Activation:** They receive **Apo C-II** and **Apo E** from HDL in the plasma. Apo C-II is essential for activating **Lipoprotein Lipase (LPL)** to release free fatty acids. * **Appearance:** Post-prandial plasma appears milky due to high chylomicron content (Chylomicronemia).

Question 574: Which of the following regulates lipolysis in adipocytes?

A. Activation of fatty acid synthesis mediated by cyclic AMP
B. Activation of triglyceride lipase as a result of hormone-stimulated increases in cyclic AMP levels (Correct Answer)
C. Glycerol phosphorylation to prevent futile esterification of fatty acids
D. Activation of cyclic AMP production by insulin

Explanation: **Explanation:** Lipolysis is the process of breaking down stored triacylglycerols (TAGs) into free fatty acids and glycerol. The rate-limiting enzyme for this process is **Hormone-Sensitive Lipase (HSL)**, also known as triglyceride lipase. **Why Option B is Correct:** When the body requires energy (e.g., fasting or exercise), hormones like **Glucagon and Epinephrine** bind to G-protein coupled receptors on adipocytes. This activates Adenylate Cyclase, increasing **cyclic AMP (cAMP)** levels. cAMP activates Protein Kinase A (PKA), which phosphorylates and activates HSL. Once activated, HSL hydrolyzes TAGs, initiating the lipolytic cascade. **Analysis of Incorrect Options:** * **Option A:** Fatty acid synthesis (Lipogenesis) and lipolysis are reciprocal processes. cAMP *inhibits* fatty acid synthesis by inactivating Acetyl-CoA Carboxylase; it does not activate it. * **Option C:** Adipocytes lack the enzyme **Glycerol Kinase**. Therefore, they cannot phosphorylate glycerol to reuse it for re-esterification. Glycerol must be transported to the liver for metabolism. * **Option D:** Insulin is an **anti-lipolytic** hormone. It activates phosphodiesterase, which *breaks down* cAMP, thereby dephosphorylating and inactivating HSL. **High-Yield NEET-PG Pearls:** * **Perilipin:** A protein coating lipid droplets. When phosphorylated by PKA, it changes shape to allow HSL access to the triglycerides. * **Insulin’s Role:** Insulin inhibits lipolysis via two mechanisms: decreasing cAMP levels and activating phosphatases that dephosphorylate HSL. * **Fate of Glycerol:** Since adipocytes cannot use glycerol (due to lack of glycerol kinase), it is a clinical marker of the rate of lipolysis in the blood.

Question 575: What is the major source of extracellular cholesterol for human tissues?

A. Very-low-density lipoproteins (VLDLs)
B. Low-density lipoproteins (LDLs) (Correct Answer)
C. High-density lipoproteins (HDLs)
D. Albumin

Explanation: **Explanation:** The correct answer is **Low-density lipoproteins (LDLs)**. LDL is the primary vehicle for transporting cholesterol from the liver to peripheral (extrahepatic) tissues. It is formed from VLDL via IDL in the circulation. LDL contains the highest concentration of cholesterol and cholesterol esters (approximately 50% of its mass). Tissues take up this cholesterol through **LDL receptor-mediated endocytosis**, which recognizes the **Apo B-100** lipoprotein present on the LDL surface. **Why other options are incorrect:** * **VLDLs:** These are primarily responsible for transporting **endogenous triglycerides** from the liver to peripheral tissues. While they contain some cholesterol, their main cargo is lipid fuel (TG). * **HDLs:** These are involved in **Reverse Cholesterol Transport**. They collect excess cholesterol from peripheral tissues and transport it back to the liver for excretion or recycling. Thus, they are a "sink" rather than a "source" for peripheral tissues. * **Albumin:** This protein transports **Free Fatty Acids (FFAs)**, bilirubin, and various drugs in the blood, but it does not transport significant amounts of cholesterol. **High-Yield Clinical Pearls for NEET-PG:** * **Apo B-100** is the structural protein for VLDL, IDL, and LDL. * **Rate-limiting step of cholesterol synthesis:** HMG-CoA Reductase (inhibited by Statins). * **Friedewald Equation:** LDL Cholesterol = Total Cholesterol – [HDL Cholesterol + (Triglycerides/5)]. (Note: Not valid if TG >400 mg/dL). * **Wolman Disease:** A lysosomal storage disease caused by a deficiency in cholesteryl ester hydrolase, preventing the release of free cholesterol from internalized LDL.

Question 576: Insulin inhibits ketogenesis by all except:

A. Inhibiting lipolysis
B. Increased esterification of fatty acids
C. Directing acetyl-CoA to the TCA cycle
D. Increasing b-oxidation (Correct Answer)

Explanation: **Explanation:** Ketogenesis occurs primarily in the liver during states of low insulin and high glucagon (starvation or uncontrolled diabetes). Insulin is a potent **anti-ketogenic** hormone. **Why Option D is the correct answer:** Insulin **inhibits** beta-oxidation; it does not increase it. Insulin stimulates the synthesis of **Malonyl-CoA** (via Acetyl-CoA Carboxylase). Malonyl-CoA inhibits **Carnitine Palmitoyltransferase-I (CPT-I)**, the rate-limiting enzyme that transports fatty acids into the mitochondria. By blocking this transport, insulin prevents beta-oxidation, thereby depriving the ketogenesis pathway of its substrate (Acetyl-CoA). **Analysis of Incorrect Options:** * **A. Inhibiting lipolysis:** Insulin inhibits Hormone-Sensitive Lipase (HSL) in adipose tissue. This reduces the release of free fatty acids (FFAs) into the blood, leaving the liver with no raw material for ketone bodies. * **B. Increased esterification:** Insulin promotes the conversion of FFAs into triglycerides (esterification) rather than allowing them to enter the ketogenic pathway. * **C. Directing acetyl-CoA to TCA cycle:** By promoting the fed state, insulin ensures that oxaloacetate is available to condense with Acetyl-CoA to enter the TCA cycle, rather than diverting Acetyl-CoA toward HMG-CoA synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of Ketogenesis:** HMG-CoA Synthase (Mitochondrial). * **Ketone bodies:** Acetoacetate, 3-hydroxybutyrate, and Acetone (non-metabolizable). * **Key Regulator:** Malonyl-CoA is the "gatekeeper," preventing the simultaneous occurrence of fatty acid synthesis and beta-oxidation (Reciprocal Regulation). * **Organ utilization:** The brain, heart, and skeletal muscle can use ketones, but the **liver cannot** (due to lack of Thiophorase/succinyl-CoA:3-ketoacid CoA-transferase).

Question 577: Which lipoprotein has the highest cholesterol content?

A. LDL (Low-density lipoprotein) (Correct Answer)
B. VLDL (Very low-density lipoprotein)
C. Chylomicrons
D. IDL (Intermediate-density lipoprotein)

Explanation: **Explanation:** Lipoproteins are classified based on their density and relative composition of lipids (triglycerides, cholesterol, phospholipids) and proteins. **Why LDL is the correct answer:** Low-density lipoprotein (LDL) is the primary carrier of cholesterol in the blood. It contains the highest percentage of **cholesterol and cholesterol esters** (approximately 50% of its total weight). LDL is formed from the metabolism of VLDL and IDL; as triglycerides are removed by lipoprotein lipase, the particle becomes enriched with cholesterol. Its primary function is to transport cholesterol from the liver to peripheral tissues. **Why the other options are incorrect:** * **Chylomicrons:** These have the lowest density and the largest size. They are composed predominantly of **triglycerides (85-90%)** derived from dietary intake. * **VLDL:** These are synthesized in the liver and primarily transport **endogenous triglycerides (55-65%)**. * **IDL:** This is a transient intermediate formed during the conversion of VLDL to LDL. While it contains more cholesterol than VLDL, it still has a higher triglyceride-to-cholesterol ratio compared to LDL. **High-Yield NEET-PG Clinical Pearls:** * **"Bad Cholesterol":** LDL is termed "bad" because high levels are strongly associated with atherosclerosis and coronary artery disease. * **Apolipoprotein B-100:** This is the characteristic structural protein found in VLDL, IDL, and LDL. * **Friedewald Equation:** Used to calculate LDL cholesterol: $LDL = Total\ Cholesterol - HDL - (Triglycerides/5)$. (Note: This is invalid if TG >400 mg/dL). * **HDL:** Known as "Good Cholesterol," it has the highest **protein** content and is involved in reverse cholesterol transport.

Question 578: Acyl carnitine functions in:

A. Transport of long-chain fatty acids (Correct Answer)
B. Transport of short-chain fatty acids
C. Transport of NADH
D. Transport of FADH

Explanation: ### Explanation **Correct Option: A. Transport of long-chain fatty acids** The primary site for fatty acid oxidation (Beta-oxidation) is the mitochondrial matrix. However, the inner mitochondrial membrane is impermeable to **Long-Chain Fatty Acids (LCFA)**. To overcome this, the **Carnitine Shuttle** is utilized. 1. LCFAs are first activated to Acyl-CoA in the cytosol. 2. The enzyme **Carnitine Palmitoyltransferase-I (CPT-I)** converts Acyl-CoA into **Acyl-carnitine**. 3. Acyl-carnitine is then transported across the inner membrane via a translocase. 4. Once inside, **CPT-II** reconverts it back to Acyl-CoA for oxidation. Thus, Acyl-carnitine is the essential transport form of LCFAs. **Incorrect Options:** * **B. Short-chain fatty acids:** Unlike LCFAs, short-chain (C2–C4) and medium-chain (C6–C12) fatty acids are water-soluble and can diffuse freely into the mitochondrial matrix without the need for the carnitine shuttle. * **C & D. NADH and FADH:** These reducing equivalents are transported from the cytosol to the mitochondria via the **Malate-Aspartate shuttle** or the **Glycerol 3-phosphate shuttle**, not the carnitine system. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Step:** CPT-I is the rate-limiting enzyme of beta-oxidation. * **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 (due to impaired gluconeogenesis) and muscle weakness. * **Location:** Carnitine is primarily stored in skeletal muscle.

Question 579: Which of the following does NOT predispose to atherosclerosis?

A. Homocysteinemia
B. Fibrinogen
C. Calcium (Correct Answer)
D. Lipoprotein A

Explanation: **Explanation:** The correct answer is **Calcium**. In the context of atherosclerosis, calcium is a **marker** of the disease process rather than a **predisposing risk factor**. Vascular calcification occurs as a late-stage phenomenon where calcium hydroxyapatite deposits in the necrotic core of an existing atherosclerotic plaque. While a high "Calcium Score" on a CT scan indicates the presence and extent of coronary artery disease, dietary or serum calcium levels do not initiate or predispose an individual to the formation of plaques. **Analysis of Incorrect Options:** * **Homocysteinemia:** High levels of homocysteine cause endothelial dysfunction and oxidative stress, promoting lipid peroxidation and plaque formation. It is a well-recognized independent risk factor. * **Fibrinogen:** As a key coagulation factor, elevated fibrinogen increases blood viscosity and promotes platelet aggregation and thrombus formation over atherosclerotic lesions. * **Lipoprotein (a) [Lp(a)]:** This is a low-density lipoprotein variant containing apolipoprotein(a). It is highly atherogenic because it is pro-inflammatory and structurally resembles plasminogen, thereby inhibiting fibrinolysis and promoting thrombosis. **Clinical Pearls for NEET-PG:** * **Lp(a):** Known as the "independent genetic risk factor" for premature coronary artery disease. * **Homocysteine:** Metabolism requires Vitamin B12, B6, and Folate; deficiencies in these vitamins lead to hyperhomocysteinemia. * **Coronary Artery Calcium (CAC) Score:** Used for risk stratification in asymptomatic patients, but remember: *Calcification stabilizes the plaque; it does not cause it.*

Question 580: A patient presents with palmar xanthomas, indicating an increased risk of atherosclerosis and coronary artery disease. Lipid profile reveals elevated triacylglycerols and cholesterol, with increased levels of IDL and chylomicrons. What is the pathophysiology of this condition?

A. LDL deficiency
B. VLDL overproduction
C. Apo C-2 deficiency
D. Apo E deficiency (Correct Answer)

Explanation: ### Explanation The clinical presentation of **Palmar Xanthomas** (xanthoma striatum palmare) combined with elevated IDL and chylomicron remnants is pathognomonic for **Type III Hyperlipoproteinemia** (also known as Dysbetalipoproteinemia or Broad Beta Disease). **1. Why Apo E deficiency is correct:** Apolipoprotein E (Apo E) serves as the essential ligand for the hepatic uptake of **chylomicron remnants** and **IDL** (VLDL remnants) via the LDL-receptor-related protein (LRP) and the LDL receptor. In Apo E deficiency (specifically the E2/E2 homozygous isoform), these remnants cannot be cleared by the liver. They accumulate in the blood, leading to elevated cholesterol and triglycerides, and deposit in the palmar creases, causing characteristic xanthomas. **2. Why other options are incorrect:** * **LDL deficiency:** This would lead to Abetalipoproteinemia, characterized by malabsorption and low cholesterol, not xanthomas or elevated lipids. * **VLDL overproduction:** This is seen in Type IV Hypertriglyceridemia. While it increases triglycerides, it does not typically cause palmar xanthomas or significant IDL accumulation. * **Apo C-2 deficiency:** Apo C-2 is a cofactor for Lipoprotein Lipase (LPL). Deficiency leads to Type I Hyperlipoproteinemia (Chylomicronemia syndrome), characterized by eruptive xanthomas and pancreatitis, but not typically palmar xanthomas or premature atherosclerosis. **3. NEET-PG High-Yield Pearls:** * **Pathognomonic Sign:** Palmar xanthomas = Type III Hyperlipoproteinemia. * **Electrophoresis:** Shows a "Broad Beta Band" due to the overlap of IDL and VLDL. * **Genetics:** Most commonly associated with the **Apo E2 isoform** (which has low affinity for receptors), while Apo E4 is associated with Alzheimer’s disease. * **Risk:** High risk for both Coronary Artery Disease (CAD) and Peripheral Vascular Disease.

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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