Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 391: What is the primary starting material for ketogenesis?

A. Propionyl-CoA
B. Acetyl-CoA
C. Acetoacetyl-CoA (Correct Answer)
D. Acyl-CoA

Explanation: **Explanation:** Ketogenesis occurs primarily in the mitochondria of hepatocytes during periods of starvation or uncontrolled diabetes. While the overall process begins with the condensation of two Acetyl-CoA molecules, the **immediate precursor** or the "primary starting material" that enters the specific ketogenic pathway is **Acetoacetyl-CoA**. 1. **Why Acetoacetyl-CoA is correct:** The rate-limiting step of ketogenesis is catalyzed by **HMG-CoA synthase**. This enzyme requires **Acetoacetyl-CoA** and a third molecule of Acetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). Without the formation of Acetoacetyl-CoA (via the enzyme Thiolase), the ketogenic cascade cannot proceed. 2. **Analysis of Incorrect Options:** * **Acetyl-CoA:** While it is the ultimate building block, two molecules must first condense into Acetoacetyl-CoA before entering the HMG-CoA cycle. * **Propionyl-CoA:** This is a product of odd-chain fatty acid oxidation and enters the TCA cycle via Succinyl-CoA; it is glucogenic, not ketogenic. * **Acyl-CoA:** This refers to an activated fatty acid intended for Beta-oxidation, not a direct substrate for ketone body synthesis. **NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme:** Mitochondrial HMG-CoA Synthase. * **Site:** Liver mitochondria (the liver produces ketones but cannot use them because it lacks the enzyme **Thiophorase/β-ketoacyl-CoA transferase**). * **Ketone Bodies:** Acetoacetate, 3-hydroxybutyrate (most abundant), and Acetone (non-metabolizable, causes "fruity breath"). * **Stimulus:** High Glucagon/Insulin ratio and high levels of Acetyl-CoA from Beta-oxidation.

Question 392: Cholesterol structurally belongs to which class of molecules?

A. Carbohydrate
B. Steroid (Correct Answer)
C. Urea
D. Peptide

Explanation: **Explanation:** Cholesterol is the most abundant animal sterol and serves as a vital structural component of mammalian cell membranes. Structurally, it belongs to the **Steroid** class because it contains the characteristic **cyclopentanoperhydrophenanthrene (CPPP) nucleus**, also known as the steroid nucleus. This nucleus consists of four fused rings (three six-membered cyclohexane rings and one five-membered cyclopentane ring). Cholesterol is specifically a "sterol" because it possesses an alcohol (-OH) group at the C3 position. **Analysis of Options:** * **Carbohydrate (A):** These are polyhydroxy aldehydes or ketones (e.g., glucose, glycogen). Cholesterol is a lipid, not a sugar, and lacks the characteristic (CH₂O)n formula. * **Urea (C):** This is a simple nitrogenous diamide ($NH_2-CO-NH_2$) which is the end product of protein metabolism. It has no ring structure or lipid properties. * **Peptide (D):** These are chains of amino acids linked by peptide bonds (e.g., insulin). Cholesterol is synthesized from Acetyl-CoA units, not amino acids. **High-Yield Clinical Pearls for NEET-PG:** * **Precursor Function:** Cholesterol is the parent compound for the synthesis of **bile acids, vitamin D, and steroid hormones** (glucocorticoids, mineralocorticoids, and sex hormones). * **Rate-Limiting Step:** The conversion of HMG-CoA to Mevalonate by the enzyme **HMG-CoA Reductase** is the key regulatory step in cholesterol biosynthesis (target of Statin drugs). * **Amphipathic Nature:** Due to the -OH group at C3 and the hydrocarbon chain at C17, cholesterol is amphipathic, allowing it to regulate membrane fluidity. * **Identification:** The **Libermann-Burchard reaction** is the classic colorimetric test used to detect cholesterol (turns emerald green).

Question 393: Adrenoleukodystrophy is associated with which of the following?

A. Accumulation of very long chain fatty acids (Correct Answer)
B. Accumulation of medium chain fatty acid
C. Increased plasmalogen
D. Decreased pipecolic acid

Explanation: **Explanation:** **Adrenoleukodystrophy (ALD)** is an X-linked recessive peroxisomal disorder caused by a mutation in the **ABCD1 gene**. This gene encodes a membrane transporter protein responsible for importing **Very Long Chain Fatty Acids (VLCFAs)**—fatty acids with more than 22 carbons—into the peroxisome for degradation via **beta-oxidation**. 1. **Why Option A is Correct:** In ALD, the defective transporter prevents VLCFAs from entering the peroxisome. Consequently, these fatty acids accumulate in the blood and tissues, particularly in the **adrenal cortex** (causing adrenal insufficiency/Addison’s disease) and the **white matter of the brain** (causing progressive demyelination). 2. **Why Other Options are Incorrect:** * **Option B:** Accumulation of medium-chain fatty acids is seen in **MCAD deficiency**, a mitochondrial disorder, not a peroxisomal one. * **Option C:** **Plasmalogens** (essential myelin lipids) are actually **decreased** in peroxisomal biogenesis disorders like Zellweger Syndrome, as their synthesis begins in the peroxisome. * **Option D:** **Pipecolic acid** levels are typically **increased** in generalized peroxisomal disorders (like Zellweger Syndrome) due to impaired lysine catabolism, not decreased. **NEET-PG High-Yield Pearls:** * **Zellweger Syndrome:** The most severe peroxisomal disorder ("Empty Peroxisome" syndrome) involving a total failure of peroxisome biogenesis (PEX gene mutations). * **Refsum Disease:** Characterized by the inability to alpha-oxidize **Phytanic acid** due to Phytanoyl-CoA hydroxylase deficiency. * **Clinical Presentation of ALD:** Look for a young boy with behavioral changes, vision/hearing loss, and signs of adrenal failure (hyperpigmentation).

Question 394: Insulin increases the activity of which of the following enzymes?

A. HMG-CoA reductase (Correct Answer)
B. HMG-CoA lyase
C. HMG-CoA synthase
D. Thiolase

Explanation: **Explanation:** **HMG-CoA reductase** is the rate-limiting and key regulatory enzyme of cholesterol biosynthesis. Insulin, a hormone secreted in the well-fed state, promotes anabolic processes, including cholesterol synthesis. It increases the activity of HMG-CoA reductase through **dephosphorylation** (via protein phosphatase-1) and by inducing gene expression. This ensures that when energy and substrates (Acetyl-CoA) are abundant, the body synthesizes cholesterol. **Analysis of Incorrect Options:** * **HMG-CoA lyase:** This enzyme is involved in **ketogenesis** (the breakdown of fats into ketone bodies) and leucine catabolism. Ketogenesis occurs during fasting or starvation when insulin levels are low and glucagon is high. * **HMG-CoA synthase:** While this enzyme participates in both cholesterol synthesis (cytosolic isoform) and ketogenesis (mitochondrial isoform), it is not the primary rate-limiting step regulated by insulin in the same manner as the reductase. * **Thiolase:** This enzyme catalyzes the initial condensation of two Acetyl-CoA molecules to Acetoacetyl-CoA. It is a reversible enzyme involved in both the synthesis and breakdown pathways and is not the primary target for insulin-mediated hormonal regulation. **High-Yield Clinical Pearls for NEET-PG:** * **Statins:** These are competitive inhibitors of HMG-CoA reductase, used to treat hypercholesterolemia. * **Hormonal Regulation:** HMG-CoA reductase is **activated by Insulin** and Thyroxine, but **inhibited by Glucagon** and Epinephrine (via phosphorylation). * **Subcellular Location:** Cholesterol synthesis occurs in the **cytosol and ER**, whereas ketogenesis occurs in the **mitochondria**. * **AMPK:** High AMP levels (low energy) inhibit HMG-CoA reductase to conserve energy.

Question 395: What is the most important role of cholesterol?

A. It is a component of the cell membrane. (Correct Answer)
B. It is a precursor of polyunsaturated fatty acids.
C. It stores energy.
D. None of the above.

Explanation: **Explanation:** **1. Why Option A is Correct:** Cholesterol is an essential structural component of all mammalian **cell membranes**. It intercalates between phospholipids, playing a critical role in modulating **membrane fluidity** and stability. At high temperatures, it stabilizes the membrane and raises its melting point; at low temperatures, it prevents phospholipids from packing too tightly, maintaining fluidity. Furthermore, it is vital for the formation of **lipid rafts**, which are specialized membrane microdomains involved in cell signaling and protein trafficking. **2. Why Other Options are Incorrect:** * **Option B:** Cholesterol is **not** a precursor for polyunsaturated fatty acids (PUFAs). In fact, humans cannot synthesize certain PUFAs (like linoleic and linolenic acid) and must obtain them from the diet. However, cholesterol *is* a precursor for steroid hormones, bile acids, and Vitamin D. * **Option C:** Cholesterol is not used for energy storage. Unlike Triacylglycerols (TAGs), which are stored in adipose tissue and oxidized for ATP, the steroid ring of cholesterol cannot be degraded to $CO_2$ and $H_2O$ in humans. It is excreted primarily via bile. **3. NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme:** HMG-CoA Reductase (target of Statins). * **Transport:** Transported in the blood via lipoproteins; LDL is the primary carrier to tissues ("Bad cholesterol"), while HDL mediates reverse cholesterol transport ("Good cholesterol"). * **Amphipathic nature:** It has a polar hydroxyl group and a non-polar steroid nucleus, allowing it to sit perfectly within the lipid bilayer. * **Clinical Correlation:** High levels of LDL-cholesterol are strongly associated with atherosclerosis and coronary artery disease.

Question 396: A broad beta band in electrophoretic pattern of lipoproteins is characteristic of which type of hyperlipoproteinemia?

A. Type I hyperlipoproteinemia
B. Type IIa hyperlipoproteinemia
C. Type III hyperlipoproteinemia (Correct Answer)
D. Type IV hyperlipoproteinemia

Explanation: ### Explanation **Type III Hyperlipoproteinemia**, also known as **Dysbetalipoproteinemia** or Remnant Removal Disease, is characterized by the accumulation of **IDL (Intermediate-Density Lipoprotein)** and **Chylomicron remnants**. 1. **Why Type III is correct:** In normal electrophoresis, VLDL (pre-beta) and LDL (beta) migrate separately. In Type III, the accumulating IDL (which has a density between VLDL and LDL) creates a single, continuous, wide band that spans the pre-beta and beta regions. This is classically referred to as the **"Broad Beta Band."** It is caused by a genetic deficiency in **Apolipoprotein E (Apo E)**, specifically the E2/E2 phenotype, which prevents the liver from recognizing and clearing remnants. 2. **Why other options are incorrect:** * **Type I:** Characterized by high Chylomicrons due to Lipoprotein Lipase (LPL) or Apo C-II deficiency. Electrophoresis shows a heavy band at the origin. * **Type IIa:** Characterized by high LDL. Electrophoresis shows an intense, sharp **Beta band**. * **Type IV:** Characterized by high VLDL. Electrophoresis shows an intense **Pre-beta band**. ### High-Yield Clinical Pearls for NEET-PG: * **Clinical Hallmark:** Pathognomonic **Palmar Xanthomas** (orange-yellow discoloration of palmar creases) and Tuberoeruptive xanthomas. * **Genetic Defect:** Homozygosity for **Apo E2** isoform (which has low affinity for the LDL receptor). * **Lipid Profile:** Simultaneous elevation of both Cholesterol and Triglycerides (often in a 1:1 ratio). * **Treatment:** Fibrates are highly effective as they increase fatty acid oxidation and remnant clearance.

Question 397: What is the main lipid component of LDL?

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

Explanation: **Explanation:** The correct answer is **Cholesterol**. Lipoproteins are classified based on their density and the specific ratio of lipids (triacylglycerols, cholesterol, and phospholipids) to proteins (apolipoproteins) they contain. **Why Cholesterol is correct:** Low-Density Lipoprotein (LDL) is the primary carrier of cholesterol in the blood. It is formed from the metabolism of VLDL (via IDL). As VLDL loses triacylglycerols through the action of lipoprotein lipase, the relative concentration of cholesterol increases. Approximately **50% of the mass of an LDL particle is cholesterol** (primarily cholesterol esters), making it the dominant lipid component. Its physiological role is to transport cholesterol from the liver to peripheral tissues. **Why the other options are incorrect:** * **A. Triacylglycerol:** This is the main lipid component of **Chylomicrons** (dietary) and **VLDL** (endogenous). By the time these particles transition to LDL, most triacylglycerols have been hydrolyzed. * **C. Phospholipids:** While present in the outer shell of all lipoproteins to maintain solubility, they are never the "main" lipid component by weight compared to core lipids like TAGs or cholesterol. * **D. Free fatty acids:** These are not transported within lipoproteins; instead, they circulate in the blood bound to **Albumin**. **High-Yield Clinical Pearls for NEET-PG:** * **"Bad Cholesterol":** LDL is termed "bad" because high levels are strongly associated with atherosclerosis and Coronary Artery Disease (CAD). * **Apolipoprotein:** The characteristic apoprotein of LDL is **Apo B-100**. * **LDL Uptake:** LDL is taken up by cells via LDL receptors (Apo B-100 receptors). A defect in these receptors leads to **Type IIa Familial Hypercholesterolemia**. * **Friedewald Formula:** LDL = [Total Cholesterol] – [HDL] – [TG/5] (Note: This formula is invalid if TG > 400 mg/dL).

Question 398: CoA and Acyl carrier protein (ACP) are similar in that they

A. are both present in mitochondria
B. both contain pantothenic acid (Correct Answer)
C. are both used in fatty acid degradation
D. are both required for activation of fatty acids

Explanation: **Explanation:** The correct answer is **B: both contain pantothenic acid.** **1. Why the correct answer is right:** Both Coenzyme A (CoA) and Acyl Carrier Protein (ACP) serve as carriers of acyl groups during lipid metabolism. The functional core of both molecules is **4'-phosphopantetheine**, which is derived from **Pantothenic acid (Vitamin B5)**. This moiety contains a terminal sulfhydryl (-SH) group that forms a high-energy thioester bond with fatty acid chains, allowing them to be activated and processed by enzymes. **2. Why the incorrect options are wrong:** * **Option A:** While CoA is found in both the mitochondria (for Beta-oxidation and TCA cycle) and the cytosol, **ACP is primarily located in the cytosol** as part of the Fatty Acid Synthase (FAS) multienzyme complex. * **Option C:** CoA is used in fatty acid **degradation** (Beta-oxidation), but ACP is exclusively used in fatty acid **synthesis** (lipogenesis). * **Option D:** Activation of fatty acids (conversion to Acyl-CoA) specifically requires **CoA** and the enzyme Thiokinase. ACP does not activate free fatty acids; it holds the growing chain during the elongation steps of synthesis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Vitamin B5 Deficiency:** Rare, but can lead to "Burning Foot Syndrome." * **Fatty Acid Synthase (FAS) Complex:** In eukaryotes, this is a dimer of two identical polypeptides, each containing seven enzyme activities and one ACP domain. * **The "Shuttle" Concept:** Remember that CoA is the carrier for **catabolism** (breakdown), while ACP is the carrier for **anabolism** (synthesis). * **Key Difference:** In CoA, the phosphopantetheine is linked to adenosine diphosphate; in ACP, it is linked to a serine residue of the protein.

Question 399: Beta-oxidation found in peroxisomes leads to the formation of which products?

A. Two molecules of Acetyl CoA
B. Propionyl CoA and H2O2
C. Acetyl-CoA and Propionyl-CoA
D. Acetyl-CoA and H2O2 (Correct Answer)

Explanation: **Explanation:** Peroxisomal beta-oxidation is a specialized pathway designed to handle **Very Long Chain Fatty Acids (VLCFAs)** (C22 or longer) and branched-chain fatty acids. While it shares similarities with mitochondrial beta-oxidation, there is a critical difference in the first step. In peroxisomes, the first enzyme is **Acyl-CoA oxidase**. Instead of transferring electrons to the electron transport chain (as FADH2 does in mitochondria), this enzyme transfers electrons directly to molecular oxygen ($O_2$), reducing it to **Hydrogen Peroxide ($H_2O_2$)**. The process continues until the fatty acid chain is shortened to octanoyl-CoA (C8), yielding **Acetyl-CoA** as the primary end product. **Analysis of Options:** * **Option D (Correct):** Peroxisomal oxidation produces Acetyl-CoA (which is then exported to mitochondria) and $H_2O_2$ (which is neutralized by catalase). * **Option A:** Two molecules of Acetyl-CoA are produced in mitochondrial oxidation of short chains, but this ignores the unique byproduct ($H_2O_2$) of peroxisomes. * **Option B:** While $H_2O_2$ is correct, Propionyl-CoA is specifically a product of odd-chain fatty acid oxidation or branched-chain oxidation (alpha-oxidation), not the standard beta-oxidation byproduct. * **Option C:** This describes the end products of odd-chain fatty acid oxidation in mitochondria. **High-Yield Clinical Pearls for NEET-PG:** * **Zellweger Syndrome:** An autosomal recessive disorder due to the absence of functional peroxisomes, leading to the accumulation of VLCFAs in the brain and liver. * **X-linked Adrenoleukodystrophy (X-ALD):** Defect in the transport of VLCFAs into peroxisomes (ABCD1 transporter mutation). * **Key Enzyme:** Catalase is the marker enzyme for peroxisomes, responsible for breaking down the $H_2O_2$ produced during this process.

Question 400: Fatty acids are oxidized by all the following tissues, except?

A. Liver
B. Adipose tissue
C. Brain (Correct Answer)
D. Skeletal muscle

Explanation: ### Explanation The correct answer is **Brain (Option C)**. **Why the Brain cannot oxidize Fatty Acids:** Although the brain has a high energy requirement, it cannot utilize long-chain fatty acids as a primary fuel source. This is due to two main reasons: 1. **Blood-Brain Barrier (BBB):** Large, albumin-bound fatty acids cannot effectively cross the BBB. 2. **Enzymatic Limitation:** Neurons have relatively low levels of the enzymes required for **$\beta$-oxidation**. Furthermore, $\beta$-oxidation is a slow process and generates reactive oxygen species (ROS), which can cause oxidative stress in the delicate neural environment. Instead, the brain relies on **glucose** (primary) and **ketone bodies** (during prolonged fasting). **Analysis of Incorrect Options:** * **Liver (A):** The liver is the primary site for fatty acid metabolism. It oxidizes fatty acids to generate ATP and provides the acetyl-CoA necessary for ketogenesis. * **Adipose Tissue (B):** While adipose tissue primarily stores triacylglycerols, it contains mitochondria and can oxidize fatty acids to meet its own basal energy needs. * **Skeletal Muscle (D):** At rest and during low-to-moderate intensity exercise, skeletal muscle prefers fatty acids as its major fuel source via aerobic $\beta$-oxidation. **High-Yield NEET-PG Pearls:** * **Erythrocytes (RBCs):** Like the brain, RBCs also **cannot** oxidize fatty acids because they lack mitochondria. * **Ketone Bodies:** While the brain cannot use fatty acids, it can use ketone bodies (acetoacetate and $\beta$-hydroxybutyrate) during starvation. However, the **liver** cannot use ketone bodies because it lacks the enzyme **thiophorase** (succinyl-CoA:3-ketoacid CoA transferase). * **Heart Muscle:** The myocardium is highly dependent on fatty acid oxidation for energy (approx. 60-80% of its requirement).

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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