Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 411: What is the primary fatty acid found in Spermaceti wax?

A. Myristic acid
B. Lauric acid (Correct Answer)
C. Palmitic acid
D. Caproic acid

Explanation: **Explanation:** **Spermaceti wax** is a waxy substance found in the head cavities of the sperm whale (*Physeter macrocephalus*). Chemically, it is a **wax ester**, which is formed by the esterification of a long-chain fatty acid with a long-chain monohydric alcohol. 1. **Why Lauric acid is correct:** The primary constituent of spermaceti wax is **cetyl laurate**, which is an ester of **Lauric acid (C12)** and cetyl alcohol (C16). While older textbooks occasionally mentioned palmitic acid, modern biochemical analysis and standard NEET-PG references identify Lauric acid as the predominant fatty acid component in this specific wax. 2. **Analysis of Incorrect Options:** * **Myristic acid (C14):** Found in nutmeg, palm oil, and butter fat, but it is not the primary component of spermaceti. * **Palmitic acid (C16):** This is the most common saturated fatty acid in the human body and the end product of the Fatty Acid Synthase (FAS) complex. While present in many waxes, it is not the primary acid in spermaceti. * **Caproic acid (C6):** A short-chain fatty acid found in animal fats and oils (like coconut oil); it is too short to be the major component of a structural wax like spermaceti. **High-Yield Clinical Pearls for NEET-PG:** * **Definition of Wax:** Esters of long-chain fatty acids (C14–C36) with long-chain monohydric alcohols (C16–C30). * **Biological Function:** Waxes are highly insoluble in water and serve as protective coatings on fruits, leaves, and skin. * **Spermaceti Use:** Historically used in ointments and cosmetics due to its excellent emollient properties. * **Key Fatty Acid Lengths:** Lauric (12C), Myristic (14C), Palmitic (16C), Stearic (18C). Memorizing these even-numbered chains is essential for lipid metabolism questions.

Question 412: Which enzyme regulates the synthesis of ketone bodies?

A. Beta-hydroxy-beta-methylglutaryl CoA synthase (Correct Answer)
B. Beta-hydroxybutyrate
C. Acetyl CoA
D. None of the above

Explanation: **Explanation:** **1. Why Option A is Correct:** The synthesis of ketone bodies (ketogenesis) occurs primarily in the mitochondria of liver cells. The **rate-limiting enzyme** for this pathway is **HMG-CoA synthase (mitochondrial isoform)**. This enzyme catalyzes the condensation of Acetoacetyl-CoA with a third molecule of Acetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). Because it is the slowest step in the pathway, its activity determines the overall rate of ketone body production. **2. Why Other Options are Incorrect:** * **Option B (Beta-hydroxybutyrate):** This is not an enzyme; it is one of the three primary ketone bodies (along with acetoacetate and acetone). It serves as a major energy source for peripheral tissues during starvation. * **Option C (Acetyl CoA):** This is the **substrate** (starting material) for ketogenesis, not the regulatory enzyme. While high levels of Acetyl-CoA (derived from fatty acid oxidation) drive the pathway, the regulation is governed by the enzyme HMG-CoA synthase. **3. NEET-PG High-Yield Clinical Pearls:** * **Location Specificity:** HMG-CoA synthase has two isoforms. The **mitochondrial** enzyme is for **ketogenesis**, while the **cytosolic** enzyme is for **cholesterol synthesis**. * **HMG-CoA Reductase:** Do not confuse the two; HMG-CoA *Reductase* is the rate-limiting enzyme for cholesterol synthesis and is the target of Statin drugs. * **Ketolysis:** The liver produces ketone bodies but **cannot utilize them** because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). * **Stimulus:** Ketogenesis is stimulated by a high Glucagon:Insulin ratio, typically seen in prolonged fasting or uncontrolled Diabetes Mellitus (leading to DKA).

Question 413: Which is the most essential fatty acid?

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

Explanation: **Explanation:** The concept of "essentiality" in fatty acids refers to the body's inability to synthesize them due to the lack of enzymes (**desaturases**) capable of introducing double bonds beyond the $\Delta^9$ position. **1. Why Linoleic Acid is the Correct Answer:** Linoleic acid (18:2; $\omega$-6) is considered the **most essential** fatty acid because it cannot be synthesized by the human body and must be obtained from the diet. It serves as the primary precursor for the synthesis of Arachidonic acid. If Linoleic acid is present in sufficient quantities, the body can produce other $\omega$-6 derivatives. **2. Analysis of Incorrect Options:** * **Linolenic acid (18:3; $\omega$-3):** While also an essential fatty acid, it is generally considered secondary to Linoleic acid in the context of standard medical examinations unless specifically discussing $\omega$-3 pathways. * **Arachidonic acid (20:4; $\omega$-6):** It is a **semi-essential** fatty acid. It becomes essential only if its precursor, Linoleic acid, is deficient in the diet. * **Palmitic acid (16:0):** This is a saturated fatty acid and is the first fatty acid produced by the fatty acid synthase complex in the body; hence, it is non-essential. **High-Yield Clinical Pearls for NEET-PG:** * **True Essential Fatty Acids:** Linoleic acid ($\omega$-6) and $\alpha$-Linolenic acid ($\omega$-3). * **Deficiency Manifestations:** Phrynoderma (Toad skin) characterized by follicular hyperkeratosis, poor wound healing, and alopecia. * **Eicosanoid Precursor:** Arachidonic acid is the immediate precursor for Prostaglandins, Thromboxanes, and Leukotrienes. * **Key Enzyme:** Humans lack $\Delta^{12}$ and $\Delta^{15}$ desaturases.

Question 414: In liver cells, triglycerides are primarily formed in which organelle?

A. Rough endoplasmic reticulum
B. Smooth endoplasmic reticulum (Correct Answer)
C. Golgi apparatus
D. Ribosomes

Explanation: ### Explanation **Correct Option: B. Smooth Endoplasmic Reticulum (SER)** The **Smooth Endoplasmic Reticulum (SER)** is the primary site for the synthesis of lipids, including triglycerides, phospholipids, and cholesterol. In liver cells (hepatocytes), the enzymes required for triglyceride synthesis—such as *glycerol-3-phosphate acyltransferase*—are membrane-bound proteins located specifically in the SER. Once synthesized, these triglycerides are either stored as lipid droplets or packaged with apolipoproteins to form Very Low-Density Lipoproteins (VLDL) for secretion into the blood. **Analysis of Incorrect Options:** * **A. Rough Endoplasmic Reticulum (RER):** The RER is studded with ribosomes and is primarily responsible for **protein synthesis** (specifically secretory, lysosomal, and membrane proteins). While it provides the apolipoprotein component (like Apo B-100) for lipoproteins, it does not synthesize the lipid core. * **C. Golgi Apparatus:** The Golgi serves as the "post office" of the cell. Its role in lipid metabolism is limited to the **modification, packaging, and sorting** of lipoproteins into secretory vesicles, rather than the primary synthesis of triglycerides. * **D. Ribosomes:** These are the cellular machinery for **translation (protein synthesis)**. They have no enzymatic capacity for lipid or triglyceride assembly. **High-Yield Clinical Pearls for NEET-PG:** * **VLDL Assembly:** Triglycerides synthesized in the SER are loaded onto Apo B-100 via the **Microsomal Triglyceride Transfer Protein (MTP)**. A deficiency in MTP leads to **Abetalipoproteinemia**. * **Fatty Liver (Steatosis):** An imbalance between triglyceride synthesis in the SER and its export as VLDL leads to the accumulation of lipid droplets in hepatocytes. * **Detoxification:** Besides lipid synthesis, the hepatic SER is also the site for **Cytochrome P450** mediated drug metabolism and **gluconeogenesis** (via Glucose-6-phosphatase).

Question 415: What is the end product of cytosolic fatty acid synthetase in humans?

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

Explanation: **Explanation:** The correct answer is **Palmitic acid (D)**. In humans, de novo fatty acid synthesis occurs primarily in the **cytosol** of liver and mammary gland cells. The process is catalyzed by the **Fatty Acid Synthase (FAS) complex**, a multi-enzyme system. The synthesis begins with Acetyl-CoA and Malonyl-CoA, undergoing a repeating four-step cycle (condensation, reduction, dehydration, and reduction). This cycle repeats until a **16-carbon saturated fatty acid chain** is formed. At this point, the enzyme **thioesterase** releases the final product, which is **Palmitic acid (C16:0)**. **Analysis of Incorrect Options:** * **Oleic acid (A):** This is a C18 monounsaturated fatty acid. It is produced by the elongation and desaturation of palmitic acid in the endoplasmic reticulum, not directly by the cytosolic FAS complex. * **Arachidonic acid (B):** This is a C20 polyunsaturated fatty acid (omega-6). It is synthesized from linoleic acid and is a precursor for eicosanoids (prostaglandins/leukotrienes). * **Linoleic acid (C):** This is an **essential fatty acid** (C18:2). Humans lack the enzymes (desaturases) to introduce double bonds beyond carbon 9; therefore, it cannot be synthesized by the body and must be obtained from the diet. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Acetyl-CoA Carboxylase (ACC), which requires **Biotin** as a cofactor. * **Reductant:** **NADPH** is the essential electron donor, primarily supplied by the Hexose Monophosphate (HMP) Shunt. * **Transport:** Acetyl-CoA enters the cytosol from the mitochondria via the **Citrate Shuttle** ("Citrate is for Synthesis"). * **Inhibitor:** Palmitoyl-CoA (the end product) provides feedback inhibition to ACC.

Question 416: Which lipoprotein carries the maximum amount of cholesterol?

A. Very Low-Density Lipoprotein (VLDL)
B. Low-Density Lipoprotein (LDL) (Correct Answer)
C. High-Density Lipoprotein (HDL)
D. Chylomicrons

Explanation: ### Explanation The correct answer is **Low-Density Lipoprotein (LDL)**. **1. Why LDL is the Correct Answer:** Lipoproteins are classified based on their density and composition. LDL is the primary carrier of cholesterol in the blood, containing approximately **50% cholesterol** (mostly as cholesterol esters). It is formed from the metabolism of VLDL via IDL. Its physiological role is to transport cholesterol from the liver to peripheral tissues. Because it has the highest percentage of cholesterol among all lipoproteins, it is often referred to as "Bad Cholesterol." **2. Why the Other Options are Incorrect:** * **VLDL (Very Low-Density Lipoprotein):** Its primary cargo is **endogenous triglycerides** (approx. 55-65%). It contains only about 10-15% cholesterol. * **HDL (High-Density Lipoprotein):** While it is rich in protein (approx. 50%), its cholesterol content is roughly 20-30%. Its role is "Reverse Cholesterol Transport" (carrying cholesterol from tissues back to the liver). * **Chylomicrons:** These are the largest and least dense lipoproteins. They are primarily composed of **exogenous (dietary) triglycerides** (85-90%) and contain very little cholesterol (approx. 3-5%). **3. High-Yield Clinical Pearls for NEET-PG:** * **Apolipoprotein Marker:** LDL is characterized by **Apo B-100**, which acts as a ligand for the LDL receptor. * **Friedewald Equation:** LDL-C = [Total Cholesterol] – [HDL-C] – [TG/5]. (Note: This is invalid if TG > 400 mg/dL). * **Rate-Limiting Enzyme:** HMG-CoA Reductase is the key enzyme in cholesterol synthesis, targeted by Statins to lower LDL levels. * **Density vs. Size:** As density increases (HDL > LDL > VLDL > Chylomicrons), the size of the particle decreases.

Question 417: Bile salts act as which of the following?

A. Emulsifying agents
B. Detergents
C. Surface tension lowering agents
D. All of the above (Correct Answer)

Explanation: **Explanation:** Bile salts (such as sodium glycocholate and sodium taurocholate) are polar derivatives of cholesterol synthesized in the liver. They are **amphipathic** molecules, meaning they possess both a hydrophilic (water-soluble) and a hydrophobic (lipid-soluble) face. This unique structure allows them to perform multiple physical functions essential for lipid digestion: 1. **Surface Tension Lowering Agents:** Bile salts concentrate at the oil-water interface. By reducing the surface tension of lipid droplets, they allow large fat globules to be broken down into smaller particles. 2. **Emulsifying Agents:** By lowering surface tension, they stabilize these smaller droplets, preventing them from coalescing. This process, called **emulsification**, increases the total surface area available for pancreatic lipase to act upon. 3. **Detergents:** Due to their amphipathic nature, they act as biological detergents. At a specific concentration (Critical Micellar Concentration), they form **micelles**. These polymolecular aggregates ferry insoluble lipids (monoglycerides, free fatty acids, and cholesterol) to the intestinal brush border for absorption. **Why "All of the Above" is correct:** Since bile salts simultaneously lower surface tension, facilitate emulsification, and act as detergents to form micelles, all three descriptors are functionally accurate. **Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Cholesterol 7-α-hydroxylase (inhibited by bile acids via feedback). * **Enterohepatic Circulation:** 95% of bile salts are reabsorbed in the **terminal ileum**. Resection of the ileum leads to steatorrhea and malabsorption of fat-soluble vitamins (A, D, E, K). * **Hay’s Test:** A diagnostic test for jaundice where sulfur powder sinks in urine if bile salts are present (due to their property of lowering surface tension).

Question 418: Which of the following is an omega-9 fatty acid?

A. Arachidonic acid
B. Oleic acid (Correct Answer)
C. Linolenic acid
D. Cervonic acid

Explanation: ### Explanation **1. Why Oleic Acid is Correct:** Fatty acids are classified as **Omega ($\omega$)** based on the position of the first double bond starting from the methyl (omega) end of the carbon chain. **Oleic acid** is a monounsaturated fatty acid (MUFA) with the chemical notation **18:1; $\omega$-9**. This means it has 18 carbons and one double bond located at the 9th carbon from the methyl end. It is the most common fatty acid in human adipose tissue and a primary component of olive oil. **2. Analysis of Incorrect Options:** * **Arachidonic acid (20:4; $\omega$-6):** An essential fatty acid precursor for prostaglandins and leukotrienes. Its first double bond is at the 6th carbon from the methyl end. * **Linolenic acid ($\alpha$-Linolenic acid, 18:3; $\omega$-3):** An essential fatty acid found in flaxseed and green leafy vegetables. Its first double bond is at the 3rd carbon. * **Cervonic acid (22:6; $\omega$-3):** Also known as **Docosahexaenoic acid (DHA)**. It is a long-chain polyunsaturated fatty acid (PUFA) vital for retinal and brain development. **3. High-Yield Clinical Pearls for NEET-PG:** * **Essential Fatty Acids (EFA):** Humans lack the enzymes ($\Delta^{12}$ and $\Delta^{15}$ desaturases) to introduce double bonds beyond the 9th carbon. Therefore, **Linoleic ($\omega$-6)** and **$\alpha$-Linolenic ($\omega$-3)** acids must be obtained from the diet. * **Non-Essential:** Oleic acid is **not** an essential fatty acid because the human body possesses $\Delta^9$ desaturase, allowing it to synthesize $\omega$-9 fatty acids from stearic acid. * **Clinical Deficiency:** EFA deficiency leads to **Phrynoderma** (follicular hyperkeratosis) and poor wound healing. * **Mnemonic:** "9-O, 6-L, 3-A" (Oleic-9, Linoleic-6, Alpha-linolenic-3).

Question 419: Which of the following is a primary bile acid synthesized in the liver?

A. Lithocholic acid
B. Cholic acid (Correct Answer)
C. Deoxycholic acid
D. All of the above

Explanation: ### Explanation **1. Why Cholic Acid is Correct:** Bile acids are synthesized in the liver from **cholesterol**. This process is categorized into two stages: * **Primary Bile Acids:** These are synthesized directly in the hepatocytes. The two primary bile acids are **Cholic acid** (a trihydroxy acid) and **Chenodeoxycholic acid** (a dihydroxy acid). * The rate-limiting step in this synthesis is catalyzed by the enzyme **7-alpha-hydroxylase**, which is inhibited by bile acids (feedback inhibition). **2. Why the Other Options are Incorrect:** * **Deoxycholic acid (Option C) and Lithocholic acid (Option A):** These are **Secondary Bile Acids**. They are not synthesized by the liver. Instead, they are formed in the intestine through the action of bacterial enzymes (specifically 7-alpha-dehydroxylase) on primary bile acids. * Cholic acid is converted to **Deoxycholic acid**. * Chenodeoxycholic acid is converted to **Lithocholic acid**. * **Option D:** Incorrect because only Cholic acid is primary; the others are secondary metabolites. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Conjugation:** Before secretion into bile, primary bile acids are conjugated with **Glycine** or **Taurine** (forming Glycocholic or Taurocholic acid) to increase their solubility at physiological pH. * **Enterohepatic Circulation:** About 95% of bile acids are reabsorbed in the **terminal ileum** and returned to the liver via the portal vein. * **Steatorrhea:** Malabsorption of bile acids (e.g., in Crohn’s disease affecting the terminal ileum) leads to fat malabsorption and foul-smelling, fatty stools. * **Cholestyramine:** A bile acid sequestrant used to treat hypercholesterolemia by preventing the reabsorption of bile acids, forcing the liver to use more cholesterol to synthesize new ones.

Question 420: Tangier disease is characterized by which of the following lipid abnormalities?

A. Extremely low circulating level of plasma cholesterol (Correct Answer)
B. Low level of LDL but high HDL level
C. Extremely low level of triglycerides
D. Very high level of triglycerides

Explanation: **Explanation:** **Tangier Disease** is an autosomal recessive disorder caused by a mutation in the **ABCA1 gene** (ATP-binding cassette transporter A1). This transporter is responsible for the efflux of cholesterol and phospholipids from peripheral cells to lipid-poor **Apolipoprotein A-I (Apo A-I)** to form nascent HDL. 1. **Why Option A is Correct:** In Tangier disease, the defect in ABCA1 prevents the formation of HDL. Consequently, Apo A-I is rapidly cleared by the kidneys. This leads to a **near-total absence of HDL** in the plasma. Since HDL is a major carrier of cholesterol, its absence results in **extremely low circulating levels of plasma cholesterol** (typically <30 mg/dL). 2. **Why Incorrect Options are Wrong:** * **Option B:** In Tangier disease, HDL is virtually absent, not high. LDL levels are also typically reduced (hypocholesterolemia), not just LDL alone. * **Options C & D:** Triglyceride levels in Tangier disease are usually **normal or mildly elevated** (hypertriglyceridemia), but the hallmark of the disease is the profound drop in cholesterol and HDL, not a specific extreme fluctuation in triglycerides. **High-Yield Clinical Pearls for NEET-PG:** * **Pathognomonic Sign:** Large, **orange-colored tonsils** (due to accumulation of cholesteryl esters in reticuloendothelial cells). * **Clinical Features:** Hepatosplenomegaly, lymphadenopathy, and peripheral neuropathy. * **Biochemical Hallmark:** Absence of HDL and very low Apo A-I levels. * **Histology:** Presence of "foam cells" in tissues like the bone marrow and rectal mucosa.

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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