Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 151: The reducing equivalents for cholesterol biosynthesis are provided by which molecule?

A. FADH2
B. FMNH2
C. NADH
D. NADPH (Correct Answer)

Explanation: ### Explanation **1. Why NADPH is the Correct Answer** Cholesterol biosynthesis is a highly reductive anabolic process occurring primarily in the cytosol and endoplasmic reticulum. The synthesis of one molecule of cholesterol requires **28 molecules of NADPH**. * **Mechanism:** NADPH acts as the essential electron donor (reducing equivalent) at multiple steps, most notably the **rate-limiting step** catalyzed by **HMG-CoA Reductase**, which converts HMG-CoA to Mevalonate. * **Sources of NADPH:** In the liver (the primary site of cholesterol synthesis), NADPH is mainly supplied by the **Pentose Phosphate Pathway (HMP Shunt)** and the **Malic Enzyme** reaction. **2. Why Other Options are Incorrect** * **NADH (Option C):** While NADH is a major electron carrier, it is primarily used in **catabolic** pathways (like Glycolysis and the TCA cycle) to generate ATP via the electron transport chain. It is generally not used for reductive biosynthesis. * **FADH2 & FMNH2 (Options A & B):** These are prosthetic groups involved in redox reactions within the mitochondria (e.g., Beta-oxidation, TCA cycle, and Complex II of ETC). They do not provide the reducing power required for the cytosolic enzymes of cholesterol synthesis. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Rate-Limiting Enzyme:** HMG-CoA Reductase (Target of **Statin** drugs). * **Location:** Occurs in the cytosol and microsomal fraction (ER). * **Precursor:** All 27 carbon atoms of cholesterol are derived from **Acetyl-CoA**. * **Key Intermediate:** **Squalene** is the first 30-carbon hydrocarbon formed in the pathway. * **Rule of Thumb:** Remember that **"NADPH is for Building (Anabolism), NADH is for Burning (Catabolism)."** Other pathways requiring NADPH include Fatty Acid synthesis, Steroidogenesis, and Glutathione reduction.

Question 152: Which receptors are present in the liver for the 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 **LDL receptor (LDLR)**, also known as the **Apo B100/E receptor**, is a cell surface glycoprotein primarily expressed in the liver. It is responsible for the clearance of cholesterol-rich lipoproteins from the plasma. **Why Option C is correct:** The LDL receptor has a high affinity for two specific ligands: 1. **Apolipoprotein B100:** Found on LDL and VLDL. This is the primary ligand for the uptake of LDL particles. 2. **Apolipoprotein E:** Found on IDL (Intermediate-Density Lipoprotein) and Chylomicron remnants. Because the receptor recognizes both proteins to facilitate endocytosis, it is functionally defined by its interaction with both Apo B100 and Apo E. **Analysis of Incorrect Options:** * **Option A:** Apo E alone is the primary ligand for the **LRP (LDL Receptor-Related Protein)**, which clears chylomicron remnants, but it does not account for the uptake of LDL (which lacks Apo E). * **Option B:** Apo A is associated with HDL (High-Density Lipoprotein) and interacts with ABCA1/SR-B1 receptors, not the LDL receptor. * **Option D:** While Apo B100 is the ligand on LDL, the receptor itself is structurally capable of binding both B100 and E; therefore, "B100 and E" is the more complete biochemical description of the receptor's specificity. **High-Yield Clinical Pearls for NEET-PG:** * **Familial Hypercholesterolemia (Type IIa):** Caused by a genetic defect or deficiency in the LDL (B100/E) receptors, leading to severely elevated serum LDL and premature atherosclerosis. * **PCSK9 Inhibitors:** These drugs prevent the degradation of LDL receptors, increasing their recycling to the cell surface and lowering plasma LDL levels. * **Statins:** Work by inhibiting HMG-CoA reductase, which decreases intracellular cholesterol, leading to the **upregulation of LDL receptors** in the liver.

Question 153: What is the true function of apoprotein CII?

A. Cholesterol synthesis
B. Triglyceride transport
C. Increases the activity of lipoprotein lipase (Correct Answer)
D. Fatty acid synthesis

Explanation: **Explanation:** **Apoprotein CII (Apo C-II)** is a crucial cofactor in lipid metabolism. It is primarily found on the surface of **Chylomicrons** and **Very Low-Density Lipoproteins (VLDL)**. 1. **Why Option C is Correct:** The primary physiological role of Apo C-II is to act as a potent **activator of Lipoprotein Lipase (LPL)**. LPL is an enzyme located on the capillary endothelial walls of adipose tissue and muscle. When Apo C-II binds to LPL, it triggers the hydrolysis of triglycerides within the core of chylomicrons and VLDL into free fatty acids and glycerol, allowing tissues to utilize or store the fat. 2. **Why Other Options are Incorrect:** * **Option A & D:** Cholesterol and fatty acid synthesis are intracellular enzymatic processes (primarily in the liver and cytoplasm) regulated by enzymes like HMG-CoA reductase and Fatty Acid Synthase, respectively. Apoproteins are structural or regulatory components of circulating lipoproteins, not enzymes for synthesis. * **Option B:** While Apo C-II is *carried* on transport particles, the specific function of "transport" is attributed to the lipoprotein particle as a whole (and structural proteins like Apo B-48 or B-100), not specifically to the C-II cofactor. **High-Yield Clinical Pearls for NEET-PG:** * **Deficiency:** A genetic deficiency of either Apo C-II or LPL leads to **Type I Hyperlipoproteinemia (Familial Chylomicronemia Syndrome)**, characterized by severe hypertriglyceridemia, eruptive xanthomas, and recurrent pancreatitis. * **Source:** Apo C-II is donated to nascent chylomicrons and VLDL by **HDL** in the circulation. * **Antagonist:** **Apo C-III** acts as an inhibitor of LPL, opposing the action of Apo C-II.

Question 154: Which cholesterol is designated as "Good Cholesterol"?

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

Explanation: **Explanation:** **HDL (High-Density Lipoprotein)** is designated as "Good Cholesterol" because of its role in **Reverse Cholesterol Transport**. It picks up excess cholesterol from peripheral tissues and blood vessel walls (including atherosclerotic plaques) and transports it back to the liver for excretion in bile. This process prevents lipid accumulation in the arteries, thereby reducing the risk of atherosclerosis and coronary artery disease. **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 arterial walls, forming plaques. * **VLDL (Very Low-Density Lipoprotein):** Primarily carries endogenous triglycerides from the liver to peripheral tissues. It is a precursor to LDL and is considered pro-atherogenic. * **IDL (Intermediate-Density Lipoprotein):** Formed during the degradation of VLDL. It is transient and eventually converted into LDL; elevated levels also contribute to cardiovascular risk. **High-Yield Clinical Pearls for NEET-PG:** * **ApoA-I:** The major apoprotein associated with HDL (activates LCAT). * **LCAT (Lecithin-Cholesterol Acyltransferase):** The enzyme responsible for esterifying cholesterol within HDL, converting "nascent" discoid HDL into "mature" spherical HDL. * **CETP (Cholesterol Ester Transfer Protein):** Mediates the exchange of cholesterol esters from HDL for triglycerides from VLDL/LDL. * **Protective Levels:** HDL levels >60 mg/dL are considered cardio-protective, while <40 mg/dL is a major risk factor for heart disease.

Question 155: Increased formation of ketone bodies during fasting is a result of?

A. Decreased level of circulating glucagon
B. Decreased formation of Acetyl CoA in the liver
C. Increased levels of free fatty acids in blood (Correct Answer)
D. Inhibition of beta-oxidation of fatty acids in the liver

Explanation: ### Explanation **Correct Option: C. Increased levels of free fatty acids in blood** Ketogenesis is primarily regulated by the availability of its substrate. During fasting, the insulin-to-glucagon ratio decreases, leading to the activation of **Hormone-Sensitive Lipase (HSL)** in adipose tissue. This triggers lipolysis, releasing large amounts of **Free Fatty Acids (FFAs)** into the bloodstream. These FFAs are taken up by the liver and undergo $\beta$-oxidation to produce **Acetyl CoA**. When the rate of Acetyl CoA production exceeds the capacity of the TCA cycle (due to the diversion of oxaloacetate for gluconeogenesis), the excess Acetyl CoA is shunted into the ketogenic pathway. Therefore, the increased flux of FFAs from adipose tissue is the "rate-limiting" physiological step for ketogenesis. **Why other options are incorrect:** * **A. Decreased level of circulating glucagon:** In fasting, glucagon levels **increase**, not decrease. Glucagon promotes lipolysis and inhibits Malonyl-CoA production, which facilitates fatty acid entry into the mitochondria. * **B. Decreased formation of Acetyl CoA:** Ketone bodies are synthesized *from* Acetyl CoA. An increase in Acetyl CoA (derived from $\beta$-oxidation) is a prerequisite for ketogenesis. * **D. Inhibition of beta-oxidation:** Ketogenesis requires **active $\beta$-oxidation**. Inhibition of this process would decrease the supply of Acetyl CoA, thereby stopping ketone body formation. --- ### High-Yield NEET-PG Pearls * **Rate-limiting enzyme of Ketogenesis:** HMG-CoA Synthase (Mitochondrial). * **Primary Ketone Bodies:** Acetoacetate and $\beta$-hydroxybutyrate (Acetone is a non-metabolizable byproduct excreted via lungs). * **Organ Specificity:** The liver **produces** ketone bodies but cannot **utilize** them because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). * **The "Malonyl-CoA" Switch:** During fasting, low Malonyl-CoA levels disinhibit **CPT-1**, allowing FFAs to enter the mitochondria for $\beta$-oxidation and subsequent ketogenesis.

Question 156: All of the following statements are true regarding lipoproteins except:

A. VLDL transports endogenous lipids.
B. LDL transports lipids to the tissues.
C. Increased blood cholesterol is associated with increased LDL receptors. (Correct Answer)
D. Increased HDL is associated with decreased risk of coronary disease.

Explanation: ### Explanation The correct answer is **C** because it describes an inverse physiological relationship. **1. Why Option C is the correct (false) statement:** The regulation of cholesterol is governed by a negative feedback mechanism. When intracellular cholesterol levels are high, the cell downregulates the synthesis of **LDL receptors** (via the SREBP pathway) to prevent further uptake. Conversely, when blood cholesterol is high due to familial hypercholesterolemia, it is often because LDL receptors are **defective or decreased** in number, leading to poor clearance of LDL from the plasma. Therefore, increased blood cholesterol is associated with a **decrease** (not increase) in functional LDL receptors. **2. Analysis of other options:** * **Option A (True):** VLDL (Very Low-Density Lipoprotein) is synthesized in the liver to transport **endogenous triglycerides** to peripheral tissues. * **Option B (True):** LDL (Low-Density Lipoprotein) is the primary carrier of cholesterol in the blood, delivering it to peripheral tissues for membrane synthesis and steroidogenesis. It is often termed "bad cholesterol." * **Option C (True):** HDL (High-Density Lipoprotein) mediates **reverse cholesterol transport**, carrying excess cholesterol from tissues back to the liver. High levels are cardioprotective. **3. NEET-PG High-Yield Pearls:** * **Apolipoprotein B-100** is the structural protein for VLDL, IDL, and LDL; it acts as the ligand for the LDL receptor. * **Apolipoprotein A-I** is the major protein in HDL and activates LCAT (Lecithin-Cholesterol Acyltransferase). * **Rate-limiting enzyme** of cholesterol synthesis: HMG-CoA Reductase (inhibited by Statins). * **Friedewald Equation:** LDL = Total Cholesterol – (HDL + TG/5). (Note: Not valid if TG > 400 mg/dL).

Question 157: What is the major lipid component of lipoproteins?

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

Explanation: **Explanation:** Lipoproteins (Chylomicrons, VLDL, LDL, and HDL) are complex aggregates of lipids and proteins that transport hydrophobic lipids through the aqueous environment of the plasma. While the core of these particles consists of Triacylglycerols (TAG) and Cholesteryl Esters (CE), the specific fatty acid composition within these lipids is highly regulated. **Why Arachidonic Acid is the Correct Answer:** In the context of human plasma lipoproteins, **Arachidonic acid (C20:4, ω-6)** is a major polyunsaturated fatty acid (PUFA) found predominantly in the **phospholipid** shell and **cholesteryl esters** of the lipoprotein particles. It serves as a critical precursor for the synthesis of eicosanoids (prostaglandins, leukotrienes). Studies on human plasma profiles indicate that arachidonic acid and linoleic acid constitute a significant portion of the fatty acid chains in the lipid components of circulating lipoproteins. **Analysis of Incorrect Options:** * **A. Oleic acid (C18:1):** A monounsaturated fatty acid (MUFA). While common in the diet (olive oil), it is not the predominant structural component compared to essential PUFAs in human lipoprotein esters. * **B. Palmitic acid (C16:0):** The most common saturated fatty acid. While it is a major component of membrane phospholipids, it is often replaced by unsaturated chains in the transport forms of lipids to maintain fluidity. * **C. Linoleic acid (C18:2):** An essential fatty acid and a precursor to arachidonic acid. While highly prevalent, arachidonic acid is often highlighted in biochemical exams due to its physiological significance in signaling. **High-Yield Clinical Pearls for NEET-PG:** * **Apolipoproteins:** Know the markers (B-48 for Chylomicrons, B-100 for LDL/VLDL, A-I for HDL). * **Lecithin-Cholesterol Acyltransferase (LCAT):** This enzyme transfers a fatty acid (often linoleic or arachidonic acid) from lecithin to cholesterol to form cholesteryl esters in HDL. * **Essential Fatty Acids:** Linoleic and Linolenic acids must be obtained from the diet; Arachidonic acid becomes essential only if Linoleic acid is deficient.

Question 158: Which of the following is NOT a constituent of glycosphingolipid?

A. Sphingosine
B. Fatty acid
C. Glycerol (Correct Answer)
D. Carbohydrate

Explanation: **Explanation:** The core concept tested here is the structural classification of lipids. Lipids are broadly divided into **glycerophospholipids** (glycerol-based) and **sphingolipids** (sphingosine-based). **Why Glycerol is the correct answer:** Glycosphingolipids belong to the sphingolipid family. Their structural backbone is **Ceramide**, which consists of a long-chain amino alcohol called **sphingosine** attached to a **fatty acid** via an amide bond. In glycosphingolipids, a **carbohydrate** (monosaccharide or oligosaccharide) is attached to the primary hydroxyl group of ceramide. **Glycerol** is the backbone for triglycerides and phospholipids (like lecithin), but it is entirely absent in sphingolipids. **Analysis of Incorrect Options:** * **A. Sphingosine:** This is the 18-carbon amino alcohol that serves as the fundamental structural framework for all sphingolipids. * **B. Fatty Acid:** A long-chain fatty acid is attached to the amino group of sphingosine to form Ceramide, the parent compound of glycosphingolipids. * **D. Carbohydrate:** By definition, "glyco-" refers to the sugar component (e.g., glucose in glucocerebroside or complex oligosaccharides in gangliosides) attached to the ceramide. **High-Yield Clinical Pearls for NEET-PG:** * **Ceramide** = Sphingosine + Fatty acid. * **Cerebroside** = Ceramide + Single sugar (Glucose or Galactose). * **Ganglioside** = Ceramide + Oligosaccharide + **Sialic acid (NANA)**. * **Sphingomyelin** is the only sphingolipid that is a phospholipid (contains phosphate) but NOT a glycolipid. * **Clinical Correlation:** Deficiencies in lysosomal enzymes that degrade glycosphingolipids lead to **Sphingolipidoses** (e.g., Gaucher’s, Niemann-Pick, and Tay-Sachs disease).

Question 159: Phospholipids are classified as which type of lipid?

A. Simple lipids
B. Derived lipids
C. Compound lipids (Correct Answer)
D. None of the above

Explanation: ### Explanation **Correct Answer: C. Compound lipids** **Understanding the Concept:** Lipids are classified based on their chemical composition. **Compound (or complex) lipids** are esters of fatty acids with alcohols that contain **additional groups** such as phosphate, nitrogenous bases, carbohydrates, or proteins. Phospholipids are the most important members of this group. They consist of a glycerol or sphingosine backbone, two fatty acids, and a phosphoric acid residue often linked to a nitrogenous base (like choline in Lecithin). Because they contain this additional phosphoric acid group, they are categorized as compound lipids. **Analysis of Incorrect Options:** * **A. Simple lipids:** These are esters of fatty acids with various alcohols but contain **no other groups**. Examples include Triacylglycerols (fats and oils) and Waxes. * **B. Derived lipids:** These are substances produced by the **hydrolysis** of simple and compound lipids. They include fatty acids, glycerol, steroids (like cholesterol), lipid-soluble vitamins, and hormones. * **D. None of the above:** Incorrect, as phospholipids fit the specific definition of compound lipids. **NEET-PG High-Yield Pearls:** * **Amphipathic Nature:** Phospholipids have both a hydrophilic (polar) head and a hydrophobic (non-polar) tail, making them the structural backbone of biological membranes. * **Lecithin (Phosphatidylcholine):** The most abundant phospholipid in the cell membrane. * **Dipalmitoyl Lecithin:** Acts as a **lung surfactant**. Deficiency in premature infants leads to Respiratory Distress Syndrome (RDS). * **Sphingomyelin:** The only phospholipid that does not contain glycerol (it contains the amino alcohol sphingosine); it is essential for the myelin sheath of nerve fibers.

Question 160: Where is apolipoprotein A primarily found?

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

Explanation: ### Explanation **Apolipoprotein B-48 (Apo B-48)** is the structural hallmark of **Chylomicrons**. It is synthesized exclusively in the enterocytes of the small intestine. #### Why Chylomicrons is the Correct Answer: Apo B-48 is a truncated version of Apo B-100 (containing only the N-terminal 48% of the protein). This truncation occurs due to **post-transcriptional RNA editing** by the enzyme *cytidine deaminase*, which introduces a premature stop codon in the mRNA. Because this process is specific to the intestinal mucosa, Apo B-48 is found only in chylomicrons and their remnants, serving as a marker for exogenous (dietary) lipid transport. #### Why Other Options are Incorrect: * **VLDL (Option B):** These are synthesized in the liver and contain **Apo B-100**, not Apo B-48. Apo B-100 is required for VLDL assembly and acts as a ligand for the LDL receptor. * **LDL (Option C):** LDL is the end product of VLDL metabolism. It retains the **Apo B-100** from its parent VLDL molecule. * **HDL (Option D):** HDL does not contain Apo B. Its primary structural protein is **Apo A-I**, which is involved in reverse cholesterol transport. #### High-Yield NEET-PG Pearls: * **RNA Editing:** The conversion of Apo B-100 mRNA to Apo B-48 mRNA is a classic example of tissue-specific RNA editing (C to U transition). * **Abetalipoproteinemia:** A deficiency in **Microsomal Triglyceride Transfer Protein (MTP)** leads to an inability to load Apo B with lipids, resulting in the absence of Chylomicrons, VLDL, and LDL. * **Apo B-48 vs. B-100:** Remember: **B-48** is for the **B**owel (Chylomicrons); **B-100** is for the **L**iver (VLDL/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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