Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 201: What is the action of lipoprotein lipase?

A. Activation of IP3-DAG
B. Hydrolysis of triglycerides in chylomicrons
C. Coupling of DAG and MAG to form triglycerides
D. Hydrolysis of dietary and endogenous triglycerides (Correct Answer)

Explanation: **Explanation:** **Lipoprotein Lipase (LPL)** is a key enzyme in lipid metabolism, primarily located on the luminal surface of capillary endothelial cells in extrahepatic tissues (especially adipose tissue, cardiac, and skeletal muscle). 1. **Why Option D is Correct:** LPL is responsible for the hydrolysis of triglycerides (TAGs) found within circulating lipoproteins. It acts on **chylomicrons** (which carry dietary/exogenous lipids) and **VLDL** (which carry endogenous lipids synthesized by the liver). By breaking down these TAGs into free fatty acids and glycerol, LPL allows tissues to utilize or store fat. 2. **Analysis of Incorrect Options:** * **Option A:** IP3-DAG are secondary messengers involved in G-protein coupled receptor signaling (e.g., via Phospholipase C), not lipid transport. * **Option B:** While LPL does hydrolyze chylomicrons, this option is incomplete because it ignores VLDL (endogenous triglycerides). * **Option C:** This describes the re-esterification process (TAG synthesis) occurring within the intestinal mucosa or adipocytes, which is the opposite of LPL's hydrolytic function. **High-Yield Clinical Pearls for NEET-PG:** * **Activator:** **Apo C-II** is the essential co-factor for LPL activation. * **Inhibitor:** **Apo C-III** and **Apo A-IV** inhibit LPL. * **Hormonal Regulation:** Insulin stimulates LPL synthesis and release in adipose tissue (promoting fat storage). * **Clinical Correlation:** A deficiency of LPL or Apo C-II leads to **Type I Hyperlipoproteinemia** (Familial Chylomicronemia Syndrome), characterized by eruptive xanthomas and pancreatitis. * **Heparin Connection:** Injecting heparin releases LPL from the endothelium into the blood, increasing "post-heparin lipolytic activity."

Question 202: A scientist has developed an adipocyte cell line that, at 42°C, cannot degrade triglycerides to glycerol and free fatty acids. At 25°C, the triglyceride degradation is normal. Which one of the following best reflects enzymes in which a temperature-sensitive mutation may lead to this phenotype?

A. Protein Kinase A - yes; Adenylate Kinase - no; Insulin Receptor - no; Glucagon Receptor - yes; Glycerol Kinase - yes
B. Protein Kinase A - no; Adenylate Kinase - no; Insulin Receptor - yes; Glucagon Receptor - no; Glycerol Kinase - yes
C. Protein Kinase A - yes; Adenylate Kinase - no; Insulin Receptor - no; Glucagon Receptor - yes; Glycerol Kinase - no (Correct Answer)
D. Protein Kinase A - no; Adenylate Kinase - yes; Insulin Receptor - yes; Glucagon Receptor - no; Glycerol Kinase - no

Explanation: ### Explanation **Concept: Hormonal Regulation of Lipolysis** The degradation of triglycerides (lipolysis) in adipocytes is primarily regulated by **Hormone-Sensitive Lipase (HSL)**. This process is stimulated by catabolic hormones (Glucagon, Epinephrine) and inhibited by anabolic hormones (Insulin). 1. **Glucagon/Epinephrine** bind to G-protein coupled receptors, activating Adenylate Cyclase to produce cAMP. 2. **cAMP** activates **Protein Kinase A (PKA)**. 3. **PKA** phosphorylates and activates HSL, leading to the breakdown of triglycerides into free fatty acids and glycerol. **Why Option C is Correct:** * **Glucagon Receptor & PKA:** Both are essential components of the stimulatory pathway for lipolysis. A mutation in either would prevent the activation of HSL, halting triglyceride degradation. * **Insulin Receptor:** Insulin *inhibits* lipolysis. A mutation here would lead to *uncontrolled* lipolysis, not a failure to degrade fats. * **Adenylate Kinase:** This enzyme interconverts adenine nucleotides (2 ADP ↔ ATP + AMP); it is not a primary regulator of the lipolytic signaling cascade. * **Glycerol Kinase:** Adipocytes lack Glycerol Kinase. Glycerol released during lipolysis must be transported to the liver for metabolism. Therefore, its mutation would not affect the initial breakdown of triglycerides within the adipocyte. **Analysis of Incorrect Options:** * **Options A & B:** Incorrect because they suggest Glycerol Kinase is involved. Adipocytes cannot reuse glycerol due to the absence of this enzyme. * **Option D:** Incorrect because it includes the Insulin Receptor (which inhibits degradation) and Adenylate Kinase, while excluding PKA and Glucagon receptors. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme of lipolysis:** Hormone-Sensitive Lipase (HSL). * **Key Inhibitor:** Insulin is the most potent antilipolytic hormone (via phosphodiesterase activation which lowers cAMP). * **Glycerol Metabolism:** Adipose tissue lacks **Glycerol Kinase**; this is why glycerol is a marker of lipolysis in clinical studies—it must exit the cell to be metabolized by the liver.

Question 203: Which metabolic pathway provides the reducing agent primarily used in lipogenesis?

A. Pentose phosphate pathway (Correct Answer)
B. Glycolysis
C. TCA cycle
D. Gluconeogenesis

Explanation: **Explanation:** The correct answer is **A. Pentose Phosphate Pathway (PPP)**. **1. Why it is correct:** Lipogenesis (fatty acid synthesis) occurs in the cytosol and requires **NADPH** as a reducing agent to reduce keto groups to hydroxyl groups and double bonds to single bonds. The primary source of this NADPH is the **Pentose Phosphate Pathway** (specifically the oxidative phase via Glucose-6-phosphate dehydrogenase). Another secondary source is the activity of the **Malic enzyme**, which converts malate to pyruvate. **2. Why other options are incorrect:** * **B. Glycolysis:** This pathway occurs in the cytosol but produces **NADH**, not NADPH. NADH is primarily used for ATP production in the electron transport chain, not for reductive biosynthesis. * **C. TCA Cycle:** This mitochondrial pathway produces **NADH and FADH₂**, which are used for oxidative phosphorylation. While it provides the substrate (Citrate) for lipogenesis, it does not provide the reducing power. * **D. Gluconeogenesis:** This is an anabolic pathway that consumes energy (ATP/GTP) and reducing equivalents (NADH) to synthesize glucose; it does not generate NADPH. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of PPP:** Glucose-6-phosphate dehydrogenase (G6PD). * **Tissues with high PPP activity:** Adrenal cortex, liver, lactating mammary glands, and RBCs (all require NADPH for either steroid/fatty acid synthesis or maintaining reduced glutathione). * **The "Citrate Shuttle":** Since Acetyl-CoA cannot cross the mitochondrial membrane, it exits as **Citrate**. In the cytosol, Citrate is cleaved by *ATP Citrate Lyase* to provide Acetyl-CoA for lipogenesis. * **Key Regulatory Enzyme:** Acetyl-CoA Carboxylase (inhibited by Glucagon/Palmitoyl-CoA; activated by Insulin/Citrate).

Question 204: Which of the following would lead to an increase in hepatocyte synthesis of primary bile acids?

A. An injection of cholecystokinin
B. An ileal resection (Correct Answer)
C. Stimulation of the vagus nerve
D. Lovastatin

Explanation: **Explanation:** The synthesis of primary bile acids (Cholic acid and Chenodeoxycholic acid) from cholesterol is regulated by the rate-limiting enzyme **7-alpha-hydroxylase**. This enzyme is controlled via a **negative feedback mechanism** by bile salts returning to the liver through the enterohepatic circulation. **Why Option B is correct:** Approximately 95% of bile salts are reabsorbed in the **terminal ileum**. An **ileal resection** disrupts this enterohepatic circulation, preventing bile salts from returning to the liver. The loss of negative feedback (due to low bile salt levels in the portal blood) leads to the up-regulation of 7-alpha-hydroxylase, significantly increasing the hepatic synthesis of new bile acids to compensate for the fecal loss. **Why other options are incorrect:** * **A & C (CCK and Vagus Nerve):** Cholecystokinin and vagal stimulation cause gallbladder contraction and the release of pre-formed bile into the duodenum. They affect the *secretion* and *storage* of bile, not its *de novo* synthesis in the hepatocyte. * **D (Lovastatin):** This is an HMG-CoA reductase inhibitor. By inhibiting cholesterol synthesis, it reduces the availability of the precursor required for bile acid production, potentially decreasing synthesis rather than increasing it. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** 7-alpha-hydroxylase (inhibited by bile acids, stimulated by cholesterol). * **Primary Bile Acids:** Cholic acid and Chenodeoxycholic acid. * **Secondary Bile Acids:** Deoxycholic acid and Lithocholic acid (formed by bacterial action in the colon). * **Clinical Correlation:** Ileal resection > 100 cm often leads to "bile acid diarrhea" and steatorrhea because the liver's compensatory synthesis cannot keep up with the massive fecal loss.

Question 205: Which of the following steroids have 8-10 carbon side chains at C-17 and an hydroxyl group at C3?

A. Androgens
B. Adrenal cortical steroids
C. Progestins
D. Sterols (Correct Answer)

Explanation: **Explanation:** The core structure of all steroids is the **cyclopentanoperhydrophenanthrene (CPPP)** nucleus. The classification of steroid molecules depends primarily on the length of the side chain attached to **Carbon-17 (C-17)** and the functional groups present. **1. Why Sterols are correct:** Sterols (like **Cholesterol**) are a specific subgroup of steroids characterized by: * An **8 to 10 carbon aliphatic side chain** at the C-17 position (Cholesterol has 8 carbons). * A **hydroxyl (-OH) group** at the C-3 position, making them alcohols (hence the suffix "-ol"). * Cholesterol is the most abundant sterol in humans and serves as the precursor for all other steroid hormones. **2. Why other options are incorrect:** * **Androgens (e.g., Testosterone):** These are C-19 steroids. They have **no side chain** at C-17; instead, they typically have a hydroxyl or ketone group directly at that position. * **Adrenal Cortical Steroids (e.g., Cortisol, Aldosterone):** These are C-21 steroids. They possess a short **2-carbon side chain** at C-17. * **Progestins (e.g., Progesterone):** Like corticosteroids, these are C-21 steroids with a **2-carbon side chain** at C-17. **High-Yield Clinical Pearls for NEET-PG:** * **Carbon Counts:** Sterols (C-27), Progestins/Adrenocorticals (C-21), Androgens (C-19), and Estrogens (C-18). * **Estrogens** are unique because they have an **aromatic A-ring** and lack a methyl group at C-10. * **Rate-limiting step:** The conversion of Cholesterol (C-27) to Pregnenolone (C-21) by the enzyme **Desmolase** (CYP11A1) in the mitochondria.

Question 206: What is generated during beta-oxidation in peroxisomes?

A. H2O2 (Correct Answer)
B. NADPH
C. Long chain fatty acid
D. FADH2

Explanation: **Explanation:** The correct answer is **A. H2O2**. **Underlying Concept:** Beta-oxidation in peroxisomes is specialized for the initial breakdown of **Very Long Chain Fatty Acids (VLCFAs)** (C22 or longer). While the process is similar to mitochondrial beta-oxidation, the first step is catalyzed by **Acyl-CoA oxidase** rather than Acyl-CoA dehydrogenase. In peroxisomes, the electrons from FADH₂ are transferred directly to molecular oxygen ($O_2$), reducing it to **Hydrogen Peroxide ($H_2O_2$)**. This $H_2O_2$ is subsequently neutralized by the enzyme **catalase**. Unlike mitochondrial oxidation, this step does not generate ATP. **Analysis of Incorrect Options:** * **B. NADPH:** NADPH is primarily generated in the Hexose Monophosphate (HMP) shunt and is used for fatty acid *synthesis* (reductive biosynthesis), not oxidation. * **C. Long chain fatty acid:** Peroxisomes do not generate long-chain fatty acids; they *shorten* VLCFAs into medium or short-chain fatty acids (like octanoyl-CoA), which are then exported to mitochondria for complete oxidation. * **D. FADH2:** While FADH₂ is formed as an intermediate, it is immediately re-oxidized by oxygen to produce $H_2O_2$. In mitochondria, FADH₂ enters the Electron Transport Chain to produce ATP; in peroxisomes, its energy is dissipated as heat. **Clinical Pearls for NEET-PG:** * **Zellweger Syndrome:** An autosomal recessive disorder caused by the absence of functional peroxisomes, leading to the accumulation of VLCFAs (especially in the liver and brain). * **X-linked Adrenoleukodystrophy (X-ALD):** A defect in the transport of VLCFAs into peroxisomes (ABCD1 gene mutation), causing myelin breakdown and adrenal insufficiency. * **Key Enzyme:** **Catalase** is the marker enzyme for peroxisomes.

Question 207: Which of the following is NOT derived from cholesterol?

A. Bile acids
B. Steroid hormones
C. Vitamin D
D. Energy (Correct Answer)

Explanation: ### Explanation **Why "Energy" is the correct answer:** Unlike carbohydrates and fatty acids, **cholesterol cannot be oxidized to CO₂ and water** in the human body. Humans lack the enzymes necessary to break down the four-ring cyclopentanoperhydrophenanthrene (sterane) nucleus. Consequently, cholesterol provides **zero calories** (0 kcal/g) of energy. It is eliminated from the body primarily by conversion into bile acids or by excretion as neutral sterols in feces. **Why the other options are incorrect:** Cholesterol serves as the essential precursor for several vital biomolecules: * **A. Bile Acids:** Synthesized in the liver (e.g., cholic acid), these are the primary metabolic products of cholesterol and are essential for lipid digestion and absorption. * **B. Steroid Hormones:** Cholesterol is the parent compound for all steroid hormones, including glucocorticoids (cortisol), mineralocorticoids (aldosterone), and sex hormones (estrogen, progesterone, testosterone). * **C. Vitamin D:** 7-Dehydrocholesterol (an intermediate in cholesterol synthesis) is converted to Cholecalciferol (Vitamin D3) in the skin upon exposure to UV light. **NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme:** HMG-CoA reductase is the key enzyme in cholesterol synthesis (target of Statins). * **Excretion:** The only significant route for cholesterol excretion is through **bile**. * **Membrane Fluidity:** Cholesterol is a critical component of the plasma membrane, where it regulates fluidity and stability. * **Precursor:** Pregnenolone is the first steroid formed from cholesterol in the pathway to hormone synthesis.

Question 208: All of the following are polyunsaturated fatty acids (PUFAs) except:

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

Explanation: **Explanation:** The classification of fatty acids is based on the presence and number of double bonds in their hydrocarbon chain. **Palmitic acid** is a **saturated fatty acid (SFA)** because it contains no double bonds. It is a 16-carbon chain (16:0) and is the most common saturated fatty acid found in animals, plants, and microorganisms. **Analysis of Options:** * **Linoleic Acid (18:2, ω-6):** An essential PUFA with two double bonds. It is the precursor for arachidonic acid. * **Linolenic Acid (18:3, ω-3):** An essential PUFA with three double bonds. Specifically, Alpha-linolenic acid (ALA) is a vital plant-based omega-3 fatty acid. * **Arachidonic Acid (20:4, ω-6):** A PUFA with four double bonds. It is a key component of cell membrane phospholipids and serves as the precursor for eicosanoids (prostaglandins, leukotrienes, and thromboxanes). **High-Yield NEET-PG Pearls:** 1. **Essential Fatty Acids:** Humans lack the enzymes (**$\Delta^{12}$ and $\Delta^{15}$ desaturases**) to introduce double bonds beyond the $\Delta^9$ position; therefore, Linoleic and Linolenic acids must be obtained from the diet. 2. **Mnemonic for PUFAs:** "L-L-A" (Linoleic, Linolenic, Arachidonic). 3. **Palmitic Acid Synthesis:** It is the primary end-product of the **Fatty Acid Synthase (FAS)** multienzyme complex in the cytosol. 4. **Clinical Correlation:** Deficiency of essential PUFAs leads to **Phrynoderma** (follicular hyperkeratosis or "toad skin").

Question 209: Which of the following is a component of glycosphingolipids?

A. Glucose (Correct Answer)
B. Glycerol
C. Sphingosine
D. Fatty acids

Explanation: **Explanation:** **Glycosphingolipids** are a subtype of glycolipids found primarily in cell membranes, particularly in nervous tissue. They are composed of a **ceramide** backbone (sphingosine + fatty acid) attached to one or more **sugar units** via a glycosidic bond. 1. **Why Glucose is correct:** In many glycosphingolipids, such as **Glucosylceramide** (the simplest neutral glycosphingolipid), a molecule of **glucose** serves as the polar head group. In more complex structures like gangliosides, glucose forms the base of the oligosaccharide chain. 2. **Why other options are incorrect:** * **Glycerol:** This is the backbone for *phosphoglycerides* (e.g., Lecithin). Sphingolipids specifically lack a glycerol backbone, using sphingosine instead. * **Sphingosine & Fatty acids:** While these are indeed components of glycosphingolipids, they combine to form **Ceramide**. In the context of multiple-choice questions, when asked for the defining "glyco" component that distinguishes them from other sphingolipids (like sphingomyelin), the carbohydrate (Glucose/Galactose) is the specific answer sought. **High-Yield Clinical Pearls for NEET-PG:** * **Ceramide** is the fundamental structural unit of all sphingolipids (Sphingosine + Fatty Acid). * **Cerebrosides:** Contain a single sugar. Galactocerebroside is the major lipid of myelin. * **Gangliosides:** Complex glycosphingolipids containing **N-acetylneuraminic acid (NANA/Sialic acid)**. * **Sphingolipidoses:** Genetic deficiencies in lysosomal enzymes that degrade these lipids lead to storage diseases. * *Gaucher Disease:* Deficiency of **Glucocerebrosidase** (accumulation of Glucosylceramide). * *Tay-Sachs Disease:* Deficiency of **Hexosaminidase A** (accumulation of GM2 ganglioside).

Question 210: Which organ is not directly involved in cholesterol transport?

A. Liver
B. Kidney (Correct Answer)
C. Intestine
D. Adipose tissue

Explanation: **Explanation:** The transport of cholesterol is primarily mediated by the **Exogenous** and **Endogenous** lipoprotein pathways. The **Kidney** is the correct answer because it does not play a direct role in the synthesis, packaging, or systemic transport of cholesterol. Its primary metabolic functions relate to filtration, acid-base balance, and vitamin D activation, rather than lipoprotein metabolism. **Why the other options are involved:** * **Liver (Option A):** The central hub of lipid metabolism. It synthesizes endogenous cholesterol, packages it into **VLDL** for systemic delivery, and expresses **LDL receptors** to clear cholesterol from circulation. It also initiates Reverse Cholesterol Transport (RCT). * **Intestine (Option B):** The site of the exogenous pathway. It absorbs dietary cholesterol and packages it into **Chylomicrons** for transport into the lymphatic system and blood. * **Adipose Tissue (Option D):** While primarily a storage site for triglycerides, adipose tissue is a major reservoir for free cholesterol. It interacts with **HDL** for cholesterol efflux and expresses **Lipoprotein Lipase (LPL)**, which is crucial for the remodeling of transport vehicles (Chylomicrons and VLDL). **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** HMG-CoA Reductase (inhibited by Statins). * **Key Transporter:** **ABCA1** is essential for effluxing cholesterol to nascent HDL; its deficiency causes **Tangier Disease**. * **Apolipoprotein B-48** is unique to the intestine (Chylomicrons), while **Apo B-100** is unique to the liver (VLDL/LDL). * **CETP (Cholesteryl Ester Transfer Protein)** facilitates the exchange of cholesteryl esters from HDL to 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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