Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 271: The activity of which plasma enzyme is responsible virtually for all plasma cholesterol esters in humans?

A. Lecithin cholesterol acyl transferase (LCAT) (Correct Answer)
B. Pyrophospho mevalonate decarboxylase
C. HMG CoA reductase
D. Phospho mevalonate kinase

Explanation: **Explanation:** The correct answer is **Lecithin cholesterol acyl transferase (LCAT)**. **1. Why LCAT is correct:** In humans, cholesterol exists in two forms: free cholesterol and cholesterol esters. While the liver can esterify cholesterol intracellularly (via ACAT), **virtually all plasma cholesterol esters** are generated by the enzyme **LCAT**. * **Mechanism:** LCAT is synthesized by the liver and secreted into the blood, where it is activated by **Apo A-I** (found on HDL). It transfers a fatty acid from the C2 position of Lecithin (Phosphatidylcholine) to free cholesterol, forming a cholesterol ester and Lysolecithin. * **Function:** This process allows HDL to sequester cholesterol within its core, facilitating **Reverse Cholesterol Transport** (carrying cholesterol from peripheral tissues back to the liver). **2. Why other options are incorrect:** * **HMG CoA Reductase (Option C):** This is the **rate-limiting enzyme** for *de novo* cholesterol synthesis within cells (converting HMG-CoA to Mevalonate). It does not esterify cholesterol in the plasma. * **Phospho mevalonate kinase (Option D) and Pyrophospho mevalonate decarboxylase (Option B):** These are intermediate enzymes in the **Mevalonate pathway** of cholesterol biosynthesis. They function intracellularly and are not involved in plasma lipid modification. **High-Yield Clinical Pearls for NEET-PG:** * **Activator:** LCAT is activated by **Apo A-I**. * **Fish-Eye Disease:** A partial LCAT deficiency characterized by corneal opacities but no significant renal disease. * **Classic LCAT Deficiency:** Leads to the "Triad" of corneal opacities, hemolytic anemia, and **proteinuria/renal failure** due to abnormal lipid deposits. * **ACAT vs. LCAT:** Remember, **A**CAT is **A**ntracellular (Intracellular), while **L**CAT is for **L**ipoproteins (Plasma).

Question 272: Which cellular compartment is the primary site of fatty acid synthesis?

A. Endoplasmic Reticulum
B. Mitochondria
C. Cytosol (Correct Answer)
D. Microsomes

Explanation: ### Explanation **Correct Answer: C. Cytosol** Fatty acid synthesis (De novo lipogenesis) occurs primarily in the **cytosol**. This process involves the conversion of Acetyl-CoA into Palmitate. The key enzyme complex, **Fatty Acid Synthase (FAS)**, is located in the cytosol. Since Acetyl-CoA is produced in the mitochondria but cannot cross the inner mitochondrial membrane, it is transported to the cytosol via the **Citrate-Malate Shuttle** (Citrate is the carrier). **Analysis of Incorrect Options:** * **A. Endoplasmic Reticulum:** While the ER is not the site of *de novo* synthesis, it is the primary site for **fatty acid elongation** (beyond 16 carbons) and **desaturation** (adding double bonds). * **B. Mitochondria:** This is the primary site for **$\beta$-oxidation** (breakdown of fatty acids) and ketogenesis. While a minor pathway for elongation exists here, it is not the site of synthesis. * **D. Microsomes:** Microsomes are vesicles derived from the ER. They are involved in elongation and desaturation but not the primary synthesis of the palmitate chain. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Enzyme:** Acetyl-CoA Carboxylase (ACC), which requires **Biotin** (Vitamin B7) as a cofactor. * **Reductant Requirement:** **NADPH** is the essential electron donor for fatty acid synthesis, primarily supplied by the **HMP Shunt** (Pentose Phosphate Pathway). * **Inhibitor:** Glucagon and Epinephrine inhibit synthesis, while **Insulin** promotes it by activating ACC. * **Mnemonic:** "Sy-to-sol" for **Sy**nthesis; "Mito-Chondria" for **C**atabolism ($\beta$-oxidation).

Question 273: The broad beta disease is due to a defect in which apolipoprotein?

A. Apolipoprotein A
B. Apolipoprotein B
C. Apolipoprotein C
D. Apolipoprotein E (Correct Answer)

Explanation: **Explanation:** **Broad Beta Disease**, also known as **Type III Hyperlipoproteinemia** or Dysbetalipoproteinemia, is caused by a genetic deficiency or defect in **Apolipoprotein E (Apo E)**. Apo E is essential for the hepatic recognition and uptake of **Chylomicron remnants** and **IDL (Intermediate-Density Lipoproteins)** via the LDL receptor-related protein (LRP). In this condition, patients typically possess the **E2/E2 isoform** (homozygosity), which has a low affinity for the receptor. This leads to the accumulation of cholesterol-rich "remnant" particles in the plasma. On electrophoresis, these remnants migrate in the beta region but appear broader than usual, giving the disease its name. **Why other options are incorrect:** * **Apolipoprotein A:** Primarily found in HDL; it is involved in reverse cholesterol transport and activates LCAT. Defects lead to Tangier disease. * **Apolipoprotein B:** Apo B-100 is the structural protein for VLDL/LDL, and Apo B-48 is for chylomicrons. Defects lead to Abetalipoproteinemia or Familial Hypobetalipoproteinemia. * **Apolipoprotein C:** Apo C-II is a co-factor for Lipoprotein Lipase (LPL). Deficiency causes Type I Hyperlipoproteinemia (Chylomicronemia syndrome). **High-Yield Clinical Pearls for NEET-PG:** * **Pathognomonic Sign:** **Palmar Xanthomas** (yellowish deposits in the creases of the palms) and Tuberoeruptive xanthomas. * **Lipid Profile:** Elevated both Cholesterol and Triglycerides (often in a 1:1 ratio). * **Inheritance:** Autosomal Recessive (usually requiring a secondary factor like obesity or hypothyroidism to manifest). * **Risk:** Significant increase in premature Atherosclerosis and Peripheral Vascular Disease.

Question 274: Which of the following is an allosteric activator of Acetyl CoA carboxylase?

A. Malonyl CoA
B. Acetyl CoA
C. Citrate (Correct Answer)
D. Biotin

Explanation: **Explanation:** **Acetyl CoA Carboxylase (ACC)** is the rate-limiting enzyme in fatty acid synthesis (lipogenesis). It catalyzes the conversion of Acetyl CoA to Malonyl CoA, a process that requires ATP, Biotin, and $\text{CO}_2$. **Why Citrate is the Correct Answer:** Citrate acts as a high-energy signal. When the TCA cycle is saturated due to high energy levels, citrate is transported from the mitochondria into the cytosol. Here, it acts as a potent **allosteric activator** of ACC. It promotes the polymerization of inactive ACC dimers into active long filaments, thereby triggering fatty acid synthesis. **Analysis of Incorrect Options:** * **A. Malonyl CoA:** This is the immediate product of the ACC reaction. It acts as a **feedback inhibitor** of the enzyme, not an activator. * **B. Acetyl CoA:** This is the substrate for the reaction. While substrate availability affects the rate, it is not classified as an allosteric activator. * **D. Biotin:** Biotin is an essential **co-enzyme** (prosthetic group) for ACC, required for the carboxylation step. It is a structural requirement for the reaction but does not function as an allosteric regulator. **High-Yield Clinical Pearls for NEET-PG:** * **Hormonal Regulation:** ACC is activated by **Insulin** (via dephosphorylation) and inhibited by **Glucagon/Epinephrine** (via phosphorylation by AMPK). * **Malonyl CoA’s Dual Role:** Besides being an intermediate, Malonyl CoA inhibits **Carnitine Palmitoyltransferase-I (CPT-1)**, preventing the newly synthesized fatty acids from entering the mitochondria for $\beta$-oxidation (preventing a futile cycle). * **Mnemonic:** "Citrate **S**timulates **S**ynthesis; **M**alonyl **M**utes it."

Question 275: Where are unsaturated fatty acids synthesized?

A. Plasma membrane
B. Endoplasmic reticulum (Correct Answer)
C. Cytosol
D. Golgi complex

Explanation: **Explanation:** The synthesis of unsaturated fatty acids involves the introduction of double bonds into a saturated fatty acid chain, a process known as **desaturation**. This reaction is catalyzed by **fatty acid desaturases** (e.g., $\Delta^9$ desaturase). These enzymes are located on the **smooth endoplasmic reticulum (ER)** membrane. The process requires molecular oxygen, NADH, and cytochrome $b_5$, forming an electron transport chain within the ER membrane to facilitate the redox reaction. **Analysis of Options:** * **Endoplasmic Reticulum (Correct):** It is the primary site for both fatty acid **elongation** (adding carbons to palmitate) and **desaturation** (introducing double bonds). * **Cytosol (Incorrect):** This is the site for *de novo* synthesis of palmitate (saturated fatty acid) via the Fatty Acid Synthase (FAS) complex. It does not contain the desaturase enzyme system. * **Plasma Membrane (Incorrect):** While it contains phospholipids with unsaturated fatty acids, it lacks the enzymatic machinery for their synthesis. * **Golgi Complex (Incorrect):** The Golgi is primarily involved in the modification, sorting, and packaging of proteins and lipids, not the primary synthesis of unsaturated bonds. **High-Yield NEET-PG Pearls:** 1. **Essential Fatty Acids:** Humans lack $\Delta^{12}$ and $\Delta^{15}$ desaturases; therefore, we cannot synthesize **Linoleic** and **Linolenic acid**. These must be obtained from the diet. 2. **Mitochondria:** While primarily the site of $\beta$-oxidation, mitochondria also possess a minor pathway for fatty acid elongation, distinct from the ER system. 3. **Rate-limiting step of Synthesis:** The conversion of Acetyl-CoA to Malonyl-CoA by **Acetyl-CoA Carboxylase** (in the cytosol).

Question 276: What is the defect in Type II hyperlipidemia?

A. Apolipoprotein E
B. Lipoprotein lipase
C. LDL receptor (Correct Answer)
D. None of the above

Explanation: **Explanation:** **Type II Hyperlipidemia (Familial Hypercholesterolemia)** is primarily characterized by a defect in the **LDL receptor (LDLR)** or its ligand, **Apolipoprotein B-100**. 1. **Why Option C is Correct:** In Type IIa hyperlipidemia, a deficiency or dysfunction of the LDL receptor prevents the liver and peripheral tissues from clearing LDL-cholesterol from the blood. This leads to a massive elevation of plasma LDL and total cholesterol levels. In Type IIb, VLDL is also elevated alongside LDL. 2. **Why Options A and B are Incorrect:** * **Option A (Apolipoprotein E):** A defect in Apo E leads to **Type III Hyperlipidemia** (Dysbetalipoproteinemia), resulting in the accumulation of chylomicron remnants and IDL. * **Option B (Lipoprotein lipase):** A deficiency in LPL (or its cofactor Apo C-II) causes **Type I Hyperlipidemia** (Familial Chylomicronemia), characterized by extreme elevations in triglycerides and chylomicrons. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Dominant. * **Clinical Features:** Look for **Tendon Xanthomas** (especially the Achilles tendon) and **Xanthelasma** (yellowish deposits around eyelids). * **Coronary Artery Disease (CAD):** Patients are at extremely high risk for premature atherosclerosis and myocardial infarction, often occurring before age 20 in homozygous cases. * **Diagnosis:** Characterized by "Clear Serum" (unlike Type I or IV) because LDL does not scatter light like VLDL or chylomicrons.

Question 277: During prolonged starvation, adipose tissue is utilized as a primary energy source. Which of the following statements regarding this process is NOT true?

A. Free fatty acids (FFAs) are released by the action of hormone-sensitive lipase on stored triacylglycerols.
B. FFAs are released into the plasma and transported as chylomicrons to adipose tissue. (Correct Answer)
C. Free glycerol is released from adipose cells.
D. Hormone-sensitive lipase is activated, leading to the hydrolysis of stored triacylglycerols.

Explanation: ### Explanation The correct answer is **B**, as it contains a fundamental physiological error regarding lipid transport. **1. Why Option B is NOT true:** During starvation (fasting state), adipose tissue undergoes **lipolysis**. The resulting Free Fatty Acids (FFAs) are released into the plasma, where they bind to **Albumin** for transport to peripheral tissues (like the liver and muscle). **Chylomicrons**, conversely, are lipoproteins synthesized by the intestinal mucosa to transport *exogenous* (dietary) lipids following a meal. They are not involved in the transport of endogenous fatty acids during starvation. **2. Analysis of Incorrect Options:** * **Option A & D:** These are correct statements. In starvation, low insulin and high glucagon/epinephrine levels trigger a cAMP-mediated cascade that phosphorylates and activates **Hormone-Sensitive Lipase (HSL)**. HSL is the rate-limiting enzyme that hydrolyzes stored triacylglycerols (TAGs) into FFAs and glycerol. * **Option C:** This is a correct statement. Adipose tissue lacks the enzyme **glycerokinase**. Therefore, the glycerol released during lipolysis cannot be reused by the adipocyte and must be released into the blood to be taken up by the liver for gluconeogenesis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of Lipolysis:** Hormone-Sensitive Lipase (HSL). * **Inhibitor of HSL:** Insulin (via dephosphorylation). * **Activators of HSL:** Glucagon, Epinephrine, ACTH, and Glucocorticoids. * **Fate of Glycerol:** Transported to the liver to enter **Gluconeogenesis** or Glycolysis via Glycerol-3-Phosphate. * **Albumin:** The primary carrier for non-esterified fatty acids (NEFA/FFA) in the blood.

Question 278: Prostaglandins were initially discovered in which bodily fluid?

A. Tear
B. Saliva
C. Seminal fluid (Correct Answer)
D. Blood

Explanation: **Explanation:** **Correct Answer: C. Seminal fluid** Prostaglandins were first discovered in the 1930s by Ulf von Euler and Maurice Goldblatt. They observed that human **seminal fluid** and extracts from the prostate gland contained substances capable of stimulating uterine smooth muscle contraction and lowering blood pressure. Because they initially believed these compounds were produced exclusively by the **prostate gland**, they named them "Prostaglandins." We now know they are produced by almost all nucleated cells in the body. **Why other options are incorrect:** * **A & B (Tear and Saliva):** While prostaglandins can be found in various secretions as inflammatory mediators, they were not the source of their initial discovery. * **D (Blood):** Prostaglandins act as **autocoids** (local hormones). They have an extremely short half-life in the systemic circulation and are rapidly inactivated (primarily in the lungs), making blood an unlikely source for their initial isolation and identification. **High-Yield Clinical Pearls for NEET-PG:** * **Precursor:** Most prostaglandins in humans are synthesized from **Arachidonic acid** (a 20-carbon polyunsaturated fatty acid) via the **Cyclooxygenase (COX) pathway**. * **Rate-limiting step:** The release of arachidonic acid from membrane phospholipids by **Phospholipase A2**. * **Inhibition:** **NSAIDs** (like Aspirin) inhibit COX enzymes, while **Glucocorticoids** inhibit Phospholipase A2. * **Key Function:** PGE2 is a potent vasodilator and is responsible for maintaining the patency of the **Ductus Arteriosus** in utero.

Question 279: What are the building blocks for most lipids?

A. Carbohydrates
B. Proteins
C. Fatty acids (Correct Answer)
D. None of the above

Explanation: ### Explanation **1. Why Fatty Acids are the Correct Answer:** Lipids are a heterogeneous group of organic compounds defined by their solubility in non-polar solvents. **Fatty acids** serve as the fundamental structural building blocks for the majority of complex lipids. In **Triacylglycerols (TAGs)**—the primary storage form of energy—three fatty acids are esterified to a glycerol backbone. Similarly, in **Phospholipids** (the primary components of cell membranes) and **Sphingolipids**, fatty acids provide the hydrophobic "tails" essential for forming lipid bilayers. While some lipids like cholesterol do not contain fatty acids, the bulk of lipid mass in the human body is derived from fatty acid chains. **2. Why Other Options are Incorrect:** * **Carbohydrates (A):** These are polyhydroxy aldehydes or ketones (sugars). While excess carbohydrates can be converted into fatty acids via *de novo* lipogenesis (Acetyl-CoA pathway), they are distinct macromolecular building blocks themselves. * **Proteins (B):** These are polymers of amino acids. While certain amino acids are ketogenic and can provide carbon skeletons for lipid synthesis, they are not structural components of lipids. **3. NEET-PG High-Yield Clinical Pearls:** * **Essential Fatty Acids:** Humans lack the enzymes (**$\Delta^{12}$ and $\Delta^{15}$ desaturases**) to introduce double bonds beyond carbon 9; therefore, **Linoleic acid ($\omega$-6)** and **Linolenic acid ($\omega$-3)** must be obtained from the diet. * **Storage:** Fatty acids are stored as TAGs in adipose tissue. This is the most concentrated form of energy (9 kcal/g) because fatty acids are highly reduced and anhydrous. * **Amphipathic Nature:** Fatty acids have a polar carboxyl group and a non-polar hydrocarbon chain, a property crucial for the formation of micelles and lung surfactant (Dipalmitoylphosphatidylcholine).

Question 280: ApoB is related to all EXCEPT:

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

Explanation: **Explanation:** The question tests the knowledge of **Apolipoprotein B (ApoB)** distribution across different lipoprotein classes. ApoB is the primary structural protein for all non-HDL lipoproteins. **1. Why HDL is the correct answer:** **HDL (High-Density Lipoprotein)** does not contain ApoB. Instead, its primary structural protein is **ApoA-I**. HDL is involved in reverse cholesterol transport, and the presence of ApoB is a marker for "atherogenic" particles, which HDL is not. **2. Analysis of incorrect options:** * **Chylomicrons:** These contain **ApoB-48**, which is synthesized in the intestine. It represents the N-terminal 48% of the ApoB gene and is essential for the secretion of chylomicrons into the lymph. * **VLDL:** These contain **ApoB-100**, synthesized in the liver. ApoB-100 acts as the structural scaffold for VLDL assembly. * **LDL:** Since LDL is a metabolic product of VLDL (via IDL), it retains the **ApoB-100** molecule. ApoB-100 on LDL serves as the ligand for the **LDL receptor**, facilitating cellular uptake. **3. High-Yield Clinical Pearls for NEET-PG:** * **ApoB-100 vs. ApoB-48:** Both are derived from the same gene. The difference arises due to **RNA editing** (C to U conversion by the enzyme *cytidine deaminase*), which creates a premature stop codon in the intestine. * **Atherogenic Index:** Total ApoB count is often considered a better predictor of cardiovascular risk than LDL-C because each atherogenic particle (VLDL, IDL, LDL) contains exactly one molecule of ApoB. * **Abetalipoproteinemia:** A deficiency in **Microsomal Triglyceride Transfer Protein (MTP)** leads to an inability to load ApoB with lipids, resulting in the near-absence of Chylomicrons, VLDL, and LDL in plasma.

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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