Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 191: Most of the reducing equivalents utilized for the synthesis of fatty acids can be generated from which of the following pathways?

A. The pentose phosphate pathway (Correct Answer)
B. Glycolysis
C. The citric acid cycle
D. Mitochondrial malate dehydrogenase

Explanation: **Explanation:** Fatty acid synthesis (lipogenesis) is a reductive process that occurs in the cytosol and requires a significant amount of **NADPH** as a reducing equivalent. For every molecule of Palmitate (C16) synthesized, 14 molecules of NADPH are required. **1. Why Option A is Correct:** The **Pentose Phosphate Pathway (PPP)**, specifically the oxidative phase catalyzed by *Glucose-6-Phosphate Dehydrogenase (G6PD)* and *6-Phosphogluconate Dehydrogenase*, is the primary source of NADPH in the body. It provides approximately **60%** of the total NADPH required for reductive biosynthesis (fatty acids, cholesterol, and steroids). **2. Why Other Options are Incorrect:** * **Glycolysis (B):** This pathway generates **NADH**, not NADPH. NADH is primarily used for ATP production via the electron transport chain, not for biosynthetic reductions. * **The Citric Acid Cycle (C):** This mitochondrial pathway generates **NADH and FADH₂**. While it provides the Citrate necessary to transport Acetyl-CoA into the cytosol, it does not directly provide the reducing power for fatty acid synthesis. * **Mitochondrial Malate Dehydrogenase (D):** This enzyme converts Oxaloacetate to Malate using NADH. However, it is the **cytosolic Malic Enzyme** (which converts Malate to Pyruvate) that generates NADPH. Mitochondrial MDH is part of the malate-aspartate shuttle and does not contribute to the NADPH pool. **High-Yield Clinical Pearls for NEET-PG:** * **Two Main Sources of NADPH:** 1. Pentose Phosphate Pathway (Major) and 2. Malic Enzyme (Minor). * **Tissues with active PPP:** Liver, lactating mammary glands, adipose tissue, and adrenal cortex (all sites of active lipid/steroid synthesis). * **Rate-limiting step of Lipogenesis:** Acetyl-CoA Carboxylase (requires Biotin). * **G6PD Deficiency:** Leads to hemolytic anemia because RBCs lack mitochondria and depend solely on the PPP for NADPH to maintain reduced glutathione.

Question 192: Endogenous triglycerides in plasma are maximally carried in which lipoprotein?

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

Explanation: **Explanation:** The transport of triglycerides (TGs) in the plasma is divided into two pathways: exogenous (dietary) and endogenous (synthesized by the liver). **1. Why VLDL is correct:** **Very Low-Density Lipoprotein (VLDL)** is synthesized in the liver. Its primary physiological role is to transport **endogenously** synthesized triglycerides from the liver to peripheral tissues (muscle and adipose tissue). It contains approximately 50-60% triglycerides by weight and carries the **Apo B-100** marker. **2. Why other options are incorrect:** * **Chylomicrons:** These carry the highest percentage of triglycerides (up to 90%), but they transport **exogenous (dietary)** lipids from the intestines. They are characterized by the **Apo B-48** marker. * **LDL (Low-Density Lipoprotein):** This is the end product of VLDL metabolism. It is the primary carrier of **cholesterol** in the plasma, not triglycerides. * **HDL (High-Density Lipoprotein):** Known for "reverse cholesterol transport," it carries cholesterol from peripheral tissues back to the liver. It has the highest protein content and the lowest lipid content. **High-Yield Clinical Pearls for NEET-PG:** * **Electrophoretic Mobility:** On electrophoresis, the order of migration (from origin to anode) is: Chylomicrons (stay at origin) < LDL (Beta) < VLDL (Pre-Beta) < HDL (Alpha). * **Apo B-100 vs. B-48:** Both are products of the same gene; B-48 is formed via **RNA editing** (C to U conversion) in the intestine. * **Friedewald Formula:** Used to calculate LDL. [LDL = Total Cholesterol – HDL – (TG/5)]. This formula is invalid if TG levels are >400 mg/dL. * **LPL (Lipoprotein Lipase):** The enzyme responsible for clearing TGs from both Chylomicrons and VLDL, activated by **Apo C-II**.

Question 193: Which of the following structures is common to all sphingolipids?

A. Carnitine
B. Ceramide (Correct Answer)
C. Diacylglycerol
D. Sphingomyelin

Explanation: **Explanation:** **Why Ceramide is the correct answer:** Sphingolipids are a major class of membrane lipids derived from the 18-carbon amino alcohol **sphingosine**. When a long-chain fatty acid is attached to the amino group of sphingosine via an **amide linkage**, the resulting molecule is called **Ceramide**. Ceramide serves as the fundamental structural core and common precursor for all complex sphingolipids. Depending on the head group attached to the terminal hydroxyl group of ceramide, different sphingolipids are formed (e.g., phosphorylcholine forms Sphingomyelin; sugars form Glycosphingolipids). **Analysis of Incorrect Options:** * **A. Carnitine:** This is a quaternary ammonium compound involved in the transport of long-chain fatty acids across the inner mitochondrial membrane for beta-oxidation (the "Carnitine Shuttle"). It is not a component of sphingolipids. * **C. Diacylglycerol (DAG):** This is the structural backbone of **Glycerophospholipids** (like lecithin), where two fatty acids are esterified to a glycerol backbone. Sphingolipids do not contain glycerol. * **D. Sphingomyelin:** While this is a sphingolipid, it is a *specific type* (a sphingophospholipid) containing a phosphorylcholine head group. It is not the "common structure" for others like cerebrosides or gangliosides. **High-Yield Clinical Pearls for NEET-PG:** 1. **Sphingolipidoses:** Deficiencies in lysosomal enzymes that break down sphingolipids lead to storage diseases. * *Niemann-Pick Disease:* Deficiency of Sphingomyelinase (accumulation of Sphingomyelin). * *Gaucher Disease:* Deficiency of β-Glucosidase (accumulation of Glucosylceramide). 2. **Key Enzyme:** Serine palmitoyltransferase is the rate-limiting enzyme in sphingosine synthesis (requires Vitamin B6). 3. **Location:** Sphingomyelin is the only phospholipid in membranes that is **not** derived from glycerol.

Question 194: Which of the following is not typically found in nephrotic syndrome?

A. Increased LDL-Cholesterol (Correct Answer)
B. Increased Triglycerides (TG)
C. Decreased HDL-Cholesterol
D. Increased VLDL-Cholesterol

Explanation: In Nephrotic Syndrome, the hallmark lipid profile is **Hyperlipidemia**, characterized by an increase in almost all lipid fractions. ### **Explanation of the Correct Answer** The question asks which finding is **not typically found** (or is the least characteristic) in the context of the options provided. However, there is a technical nuance in medical exams: Nephrotic syndrome classically presents with **increased** LDL, VLDL, and Triglycerides. **Wait, why is A the answer?** In many standard NEET-PG patterns, this question highlights that while LDL and VLDL rise significantly, **HDL levels are often normal or decreased** due to urinary loss of alpha-lipoproteins (HDL). If the question implies which parameter is *consistently* elevated versus variable, **HDL-Cholesterol (Option C)** is usually the one that decreases or stays normal. *Note: If the key marks A as correct, it may be a "distractor" or error in the provided key, as LDL is characteristically **increased** in nephrotic syndrome due to hepatic overproduction. In standard clinical teaching, **Decreased HDL** is the most common "exception" to the generalized hyperlipidemia.* ### **Analysis of Options** * **Increased LDL & VLDL (A & D):** The liver compensates for low oncotic pressure (hypoalbuminemia) by increasing the synthesis of proteins, including **Apolipoprotein B**. This leads to a massive increase in LDL and VLDL production. * **Increased Triglycerides (B):** Hypertriglyceridemia occurs because of both increased hepatic synthesis and **decreased clearance** (reduced activity of Lipoprotein Lipase). * **Decreased HDL (C):** Unlike other lipids, HDL can be lost in the urine due to its small molecular size, often leading to low or normal levels. ### **High-Yield Clinical Pearls for NEET-PG** 1. **Mechanism:** Hypoalbuminemia → ↓ Plasma Oncotic Pressure → ↑ Hepatic Lipogenesis (VLDL/LDL). 2. **Lipiduria:** "Oval fat bodies" and "Maltese cross" appearance under polarized microscopy are classic findings in nephrotic sediment. 3. **Key Enzyme:** There is a deficiency/inhibition of **Lipoprotein Lipase (LPL)**, which impairs the conversion of VLDL to IDL, further raising TG levels.

Question 195: What is the rate-limiting enzyme in fatty acid synthesis?

A. Acetoacetate synthesis
B. Acyl transferase
C. Acetyl CoA carboxylase (Correct Answer)
D. Biotin

Explanation: **Explanation:** **Acetyl CoA carboxylase (ACC)** is the correct answer because it catalyzes the first committed and rate-limiting step of de novo fatty acid synthesis (lipogenesis). This enzyme converts Acetyl CoA into Malonyl CoA via a carboxylation reaction. This step is highly regulated: it is **activated by Citrate** (signaling high energy) and **inhibited by Palmitoyl-CoA** (feedback inhibition) and Glucagon/Epinephrine (via phosphorylation). **Analysis of Incorrect Options:** * **A. Acetoacetate synthesis:** This refers to ketogenesis, which occurs in the mitochondria during fasting or starvation, rather than fatty acid synthesis which occurs in the cytosol. * **B. Acyl transferase:** Specifically, Carnitine Acyl Transferase-I (CAT-I) is the rate-limiting enzyme for **fatty acid oxidation** (beta-oxidation), not synthesis. It is actually inhibited by Malonyl CoA to prevent a futile cycle. * **D. Biotin:** While Biotin is a vital **co-factor** for Acetyl CoA carboxylase, it is not an enzyme itself. **High-Yield Clinical Pearls for NEET-PG:** * **Co-factors required:** ACC requires **ABCs**: **A**TP, **B**iotin, and **C**O₂. * **Hormonal Control:** Insulin stimulates ACC by promoting its dephosphorylation, thereby increasing fatty acid synthesis. * **Location:** Fatty acid synthesis occurs in the **cytosol**, primarily in the liver, lactating mammary glands, and adipose tissue. * **The "Citrate Shuttle":** Since Acetyl CoA cannot cross the mitochondrial membrane, it moves to the cytosol in the form of Citrate.

Question 196: Hormone-sensitive lipase is not activated by which of the following?

A. Insulin (Correct Answer)
B. Glucagon
C. Catecholamines
D. Thyroxine (T4)

Explanation: **Explanation:** **Hormone-sensitive lipase (HSL)** is the rate-limiting enzyme of lipolysis, responsible for mobilizing stored triglycerides from adipose tissue into free fatty acids and glycerol. Its activity is strictly regulated by covalent modification (phosphorylation). **1. Why Insulin is the correct answer:** Insulin is the primary **anabolic** hormone. It promotes fat storage and inhibits fat mobilization. Mechanistically, insulin activates **Protein Phosphatase 2**, which dephosphorylates HSL, rendering it **inactive**. Additionally, insulin activates phosphodiesterase, which lowers cAMP levels, further preventing the activation of the Protein Kinase A (PKA) pathway required for HSL activation. **2. Why the other options are incorrect:** These are **catabolic** (lipolytic) hormones that activate HSL via the cAMP-mediated phosphorylation pathway: * **Glucagon & Catecholamines (Epinephrine/Norepinephrine):** These bind to G-protein coupled receptors, increasing cAMP levels, which activates PKA. PKA then phosphorylates and **activates** HSL. * **Thyroxine (T4):** Thyroid hormones increase the sensitivity of adipocytes to catecholamines (upregulation of $\beta$-receptors), thereby indirectly promoting HSL activation and lipolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** HSL acts intracellularly within adipocytes. Do not confuse it with **Lipoprotein Lipase (LPL)**, which acts on the vascular endothelium to clear chylomicrons/VLDL and is *activated* by insulin. * **Perilipin:** When HSL is phosphorylated, perilipin (a protein coating the lipid droplet) is also phosphorylated, allowing HSL access to the triglyceride core. * **Glucocorticoids & ACTH:** These also stimulate lipolysis and activate HSL.

Question 197: How many ATP molecules are formed from one complete cycle of beta-oxidation?

A. 3
B. 5 (Correct Answer)
C. 7
D. 12

Explanation: ### Explanation **Underlying Concept:** Beta-oxidation is the catabolic process by which fatty acid molecules are broken down in the mitochondria to generate energy. Each **single cycle** of beta-oxidation involves four sequential steps: oxidation, hydration, oxidation, and thiolysis. During these steps, two specific reduced coenzymes are generated: 1. **FADH₂:** Produced during the first oxidation step (catalyzed by Acyl-CoA dehydrogenase). 2. **NADH:** Produced during the second oxidation step (catalyzed by 3-hydroxyacyl-CoA dehydrogenase). According to the Electron Transport Chain (ETC) yields: * 1 FADH₂ yields **1.5 ATP** (traditionally 2 ATP) * 1 NADH yields **2.5 ATP** (traditionally 3 ATP) * **Total per cycle: 1.5 + 2.5 = 4 ATP** (using modern yields) or **2 + 3 = 5 ATP** (using traditional yields). In the context of NEET-PG and standard medical textbooks (like Harper’s or Vasudevan), the **traditional yield of 5 ATP** is the standard answer for a single cycle. **Analysis of Options:** * **Option A (3):** This is incorrect as it does not account for the full yield of both coenzymes. * **Option B (5):** **Correct.** This represents the sum of ATP from 1 FADH₂ (2 ATP) and 1 NADH (3 ATP) generated in one spiral. * **Option C (7):** This value is incorrect for a single cycle of beta-oxidation. * **Option D (12):** This is the ATP yield for **one molecule of Acetyl-CoA** entering the TCA cycle, not the yield of the beta-oxidation cycle itself. **High-Yield Clinical Pearls for NEET-PG:** * **Total ATP Calculation:** For a Palmitic acid (16C) molecule, there are **7 cycles**, producing 35 ATP. Adding the 8 Acetyl-CoA (8 × 12 = 96 ATP) and subtracting 2 ATP for activation, the net yield is **129 ATP**. * **Rate-limiting enzyme:** Carnitine Palmitoyltransferase I (CPT-I). * **Inhibitor:** Malonyl-CoA (an intermediate of fatty acid synthesis) inhibits CPT-I, preventing a futile cycle. * **Clinical Correlation:** Sudden Infant Death Syndrome (SIDS) is sometimes associated with **MCAD deficiency** (Medium-chain acyl-CoA dehydrogenase deficiency), leading to hypoketotic hypoglycemia.

Question 198: Which of the following fatty acids does not belong to the omega-6 (W6) series?

A. Linoleic acid
B. Arachidonic acid
C. Gamma linolenic acid
D. Alpha linolenic acid (Correct Answer)

Explanation: ### Explanation The classification of unsaturated fatty acids into **Omega (ω) series** is determined by the position of the first double bond, counting from the methyl (omega) end of the carbon chain. **Why Alpha-Linolenic Acid (ALA) is the correct answer:** Alpha-linolenic acid is an **Omega-3 (ω-3)** fatty acid. It has 18 carbons and 3 double bonds (18:3; Δ9, 12, 15). The first double bond from the methyl end is at the 3rd carbon. It is an essential fatty acid and serves as the precursor for Eicosapentaenoic acid (EPA) and Docosahexaenoic acid (DHA). **Analysis of Incorrect Options (Omega-6 Series):** * **Linoleic Acid (LA):** The parent fatty acid of the ω-6 series (18:2; Δ9, 12). The first double bond from the methyl end is at carbon 6. It is an essential fatty acid found in vegetable oils. * **Gamma-Linolenic Acid (GLA):** An intermediate in the ω-6 pathway (18:3; Δ6, 9, 12). It is formed from Linoleic acid by the enzyme Δ6-desaturase. * **Arachidonic Acid:** A 20-carbon ω-6 fatty acid (20:4; Δ5, 8, 11, 14). It is synthesized from Linoleic acid and serves as the primary precursor for pro-inflammatory prostaglandins and leukotrienes. **High-Yield Clinical Pearls for NEET-PG:** 1. **Essential Fatty Acids (EFA):** Humans lack the enzymes (**Δ12 and Δ15 desaturases**) to introduce double bonds beyond carbon 9; hence, Linoleic and Alpha-linolenic acids must be obtained from the diet. 2. **EFA Deficiency:** Characterized by **Phrynoderma** (follicular hyperkeratosis), scaly skin, and poor wound healing. 3. **Mnemonic:** "A**L**A is **3** letters" (Omega-3), whereas "Linoleic is **6**" (Omega-6). 4. **Arachidonic Acid:** Becomes "conditionally essential" only if its precursor, Linoleic acid, is deficient in the diet.

Question 199: All of the following statements are true about the regulation of cholesterol synthesis, EXCEPT?

A. HMG-CoA reductase is inhibited by mevalonate
B. HMG-CoA reductase is inhibited by cholesterol
C. SREBP increases the transcription of HMG-CoA reductase
D. Thyroid hormone decreases the HMG-CoA reductase activity (Correct Answer)

Explanation: ### Explanation The rate-limiting step of cholesterol synthesis is the conversion of HMG-CoA to mevalonate, catalyzed by the enzyme **HMG-CoA reductase**. **Why Option D is the Correct Answer (The False Statement):** Thyroid hormone (T3/T4) actually **increases** the activity and expression of HMG-CoA reductase. However, it simultaneously increases the expression of **LDL receptors** on the liver, leading to a net effect of increased cholesterol clearance from the blood. This is why patients with hypothyroidism often present with hypercholesterolemia. **Analysis of Incorrect Options (True Statements):** * **A & B (Feedback Inhibition):** HMG-CoA reductase is regulated by end-product inhibition. Both **cholesterol** and its immediate precursor, **mevalonate**, act as feedback inhibitors to prevent the overproduction of lipids. * **C (Transcriptional Regulation):** When cellular cholesterol levels are low, **SREBP** (Sterol Regulatory Element-Binding Protein) is activated. It translocates to the nucleus and binds to the Sterol Regulatory Element (SRE), increasing the transcription of the HMG-CoA reductase gene. **High-Yield Clinical Pearls for NEET-PG:** * **Statins:** These are competitive inhibitors of HMG-CoA reductase (structural analogs of HMG-CoA). * **Hormonal Control:** Insulin and Thyroid hormones **stimulate** HMG-CoA reductase (dephosphorylated state = active), while Glucagon and Epinephrine **inhibit** it (phosphorylated state = inactive). * **AMPK:** High AMP levels (low energy) inhibit the enzyme via phosphorylation to conserve energy. * **Diurnal Rhythm:** Cholesterol synthesis is maximal at night; hence, short-acting statins are traditionally administered at bedtime.

Question 200: Fatty acids of cholesterol are mainly:

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

Explanation: ### Explanation **Correct Answer: D. Palmitic acid** **1. Why Palmitic Acid is Correct:** Cholesterol exists in the body in two forms: free cholesterol and esterified cholesterol (cholesterol esters). In the **liver** and most peripheral tissues, the enzyme **ACAT (Acyl-CoA:cholesterol acyltransferase)** is responsible for esterifying cholesterol. ACAT has a high substrate specificity for **Palmitoyl-CoA**. Consequently, in intracellular storage and within the structure of many lipoproteins, the fatty acid most commonly attached to cholesterol is **Palmitic acid** (a 16-carbon saturated fatty acid). **2. Analysis of Incorrect Options:** * **A. Oleic acid:** While Oleic acid is a common monounsaturated fatty acid found in the body, it is not the primary fatty acid found in the majority of cholesterol esters synthesized intracellularly. * **B. Linoleic acid:** This is the predominant fatty acid found in cholesterol esters in **plasma/HDL**. This occurs via the enzyme **LCAT (Lecithin-cholesterol acyltransferase)**, which transfers a fatty acid from the second position of Lecithin (usually Linoleic acid) to cholesterol. However, globally and structurally, Palmitic acid remains the standard answer for general cholesterol composition. * **C. Linolenic acid:** This is an essential omega-3 fatty acid. While present in the diet, it is not a major component of the cholesterol ester pool. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **LCAT vs. ACAT:** Remember that **LCAT** (activated by Apo A-I) works in the plasma (HDL), while **ACAT** works intracellularly. * **Rate-limiting step:** The rate-limiting enzyme of cholesterol synthesis is **HMG-CoA Reductase**. * **Storage:** Cholesterol is stored in the cytoplasm as cholesterol esters in "lipid droplets." * **Palmitic Acid Synthesis:** It is the primary product of the **Fatty Acid Synthase (FAS)** multienzyme complex in the cytosol.

Practice by Chapter

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