Lipid Metabolism — MCQs

A young man participates in a nutritional research study. After ingesting a very fatty meal, serum samples are taken for research studies at 1 hour and 3 hours. The studies measure the average diameter of chylomicrons, showing an average diameter of 500 nm at 1 hour, which drops to an average diameter of 150 nm at 3 hours. Where is the enzyme responsible for this change primarily located?

Q512Medium

Which of the following is produced by the complete oxidation of one molecule of palmitic acid in mitochondrial beta-oxidation?

Q513Easy

Which of the following is a scavenger of cholesterol from tissues and prevents atherosclerosis?

Q514Easy

Respiratory distress syndrome in premature infants is due to inadequate secretion of which one of the following lipids?

Q515Easy

What is the most important predictor of coronary artery disease?

Q516Easy

Apolipoprotein A is a major component of which lipoprotein particle?

Q517Easy

Which of the following is an omega-3 fatty acid?

Q518Easy

Which is the best marker of dyslipidemia?

Q519Easy

Which lipoprotein is the predominant carrier of cholesterol in the blood?

Q520Easy

The PAN-SH site of the fatty acid synthase complex accepts which of the following?

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 511: A young man participates in a nutritional research study. After ingesting a very fatty meal, serum samples are taken for research studies at 1 hour and 3 hours. The studies measure the average diameter of chylomicrons, showing an average diameter of 500 nm at 1 hour, which drops to an average diameter of 150 nm at 3 hours. Where is the enzyme responsible for this change primarily located?

A. Adipocytes
B. Endothelial cells (Correct Answer)
C. Enterocytes
D. Hepatocytes

Explanation: ### Explanation The reduction in chylomicron diameter from 500 nm to 150 nm represents the process of **lipolysis**, where triglycerides are hydrolyzed into free fatty acids and glycerol. This conversion transforms large, triglyceride-rich chylomicrons into smaller, denser **chylomicron remnants**. The enzyme responsible for this process is **Lipoprotein Lipase (LPL)**. LPL is synthesized by parenchymal cells (like adipocytes and myocytes) but is subsequently secreted and anchored to the **luminal surface of endothelial cells** in capillary walls via heparan sulfate proteoglycans. It is most abundant in the capillaries of adipose tissue, cardiac muscle, and skeletal muscle. When chylomicrons circulate, their surface **Apo C-II** activates LPL, leading to the rapid shrinkage of the lipoprotein particle. #### Why other options are incorrect: * **Adipocytes:** While adipocytes *synthesize* LPL, the enzyme must be translocated to the **endothelium** to interact with circulating lipoproteins. Adipocytes also contain Hormone-Sensitive Lipase (HSL), which acts on intracellular lipids, not circulating chylomicrons. * **Enterocytes:** These cells are responsible for the *assembly* and secretion of nascent chylomicrons into the lacteals, not their degradation. * **Hepatocytes:** The liver is the site for the clearance of chylomicron remnants (via the LDL receptor-related protein/LRP) and the synthesis of VLDL, but it is not the primary site for the initial lipolysis of dietary chylomicrons. #### High-Yield Clinical Pearls for NEET-PG: * **Cofactor:** Apo C-II is the essential activator of LPL. * **Inhibitor:** Apo C-III inhibits LPL. * **Heparin Effect:** Injecting heparin releases LPL from the endothelial surface into the plasma (increasing "post-heparin lipolytic activity"). * **Deficiency:** Type I Hyperlipoproteinemia (Familial Chylomicronemia Syndrome) is caused by a deficiency in either LPL or Apo C-II, leading to eruptive xanthomas and pancreatitis.

Question 512: Which of the following is produced by the complete oxidation of one molecule of palmitic acid in mitochondrial beta-oxidation?

A. 8 FADH2, 8 NADH, and 8 acetyl-CoA molecules
B. 7 FADH2, 7 NADH, and 7 acetyl-CoA molecules
C. 8 FADH2, 8 NADH, and 7 acetyl-CoA molecules
D. 7 FADH2, 7 NADH, and 8 acetyl-CoA molecules (Correct Answer)

Explanation: **Explanation:** The complete oxidation of a fatty acid occurs via the **$\beta$-oxidation spiral** in the mitochondria. To determine the yield of Palmitic acid (a 16-carbon saturated fatty acid), we use two primary rules: 1. **Number of Acetyl-CoA molecules:** Since each Acetyl-CoA has 2 carbons, the yield is $C/2$. For Palmitate (C16), this results in **8 Acetyl-CoA** molecules ($16/2 = 8$). 2. **Number of Cycles:** The number of rounds of $\beta$-oxidation required is $(C/2) - 1$. For Palmitate, this is **7 cycles** ($8 - 1 = 7$). In each cycle of $\beta$-oxidation, one molecule of **FADH$_2$** (via acyl-CoA dehydrogenase) and one molecule of **NADH** (via 3-hydroxyacyl-CoA dehydrogenase) are produced. Therefore, 7 cycles yield **7 FADH$_2$ and 7 NADH**. The final cycle splits a 4-carbon unit (Butyryl-CoA) into two 2-carbon Acetyl-CoA molecules, which is why there is one less cycle than the total number of Acetyl-CoA produced. **Analysis of Incorrect Options:** * **Option A & C:** These suggest 8 cycles of oxidation. This is incorrect because the 8th Acetyl-CoA is produced automatically at the end of the 7th cycle without further oxidation. * **Option B:** This suggests only 7 Acetyl-CoA molecules are produced, which would only account for 14 carbons, leaving the 16-carbon Palmitate incomplete. **High-Yield NEET-PG Pearls:** * **Net ATP Yield:** The total ATP yield for Palmitate is **106 ATP** (108 generated minus 2 used for initial activation). * **Rate-Limiting Step:** The transport of long-chain fatty acids into the mitochondria via the **Carnitine Shuttle** (inhibited by Malonyl-CoA). * **Clinical Correlation:** Deficiency of **MCAD** (Medium-Chain Acyl-CoA Dehydrogenase) is the most common inborn error of $\beta$-oxidation, presenting as non-ketotic hypoglycemia during fasting.

Question 513: Which of the following is a scavenger of cholesterol from tissues and prevents atherosclerosis?

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

Explanation: ### Explanation **Correct Option: D (HDL)** High-Density Lipoprotein (HDL) is known as the **"Good Cholesterol"** because of its role in **Reverse Cholesterol Transport**. It acts as a scavenger by picking up excess cholesterol from peripheral tissues and vascular walls and transporting it back to the liver for excretion in bile. This process is mediated by the enzyme **LCAT (Lecithin-Cholesterol Acyltransferase)**, which esterifies free cholesterol. By removing cholesterol from the arterial endothelium, HDL prevents the formation of foam cells and inhibits the progression of atherosclerosis. **Incorrect Options:** * **A (VLDL):** Produced by the liver, its primary role is to transport endogenous triglycerides to peripheral tissues. High levels are associated with increased cardiovascular risk, not protection. * **B (LDL):** Known as **"Bad Cholesterol,"** LDL transports cholesterol *from* the liver *to* the peripheral tissues. High levels lead to cholesterol deposition in arterial walls, leading to atherosclerosis. * **C (Chylomicrons):** These are responsible for transporting exogenous (dietary) lipids from the intestines to the liver and peripheral tissues. They are primarily composed of triglycerides. **NEET-PG High-Yield Pearls:** * **Apo A-I:** The major apoprotein associated with HDL; it activates LCAT. * **CETP (Cholesteryl Ester Transfer Protein):** Facilitates the exchange of cholesteryl esters from HDL for triglycerides from VLDL/LDL. * **Tangier Disease:** A rare genetic disorder characterized by a deficiency in the **ABCA1 transporter**, leading to extremely low HDL levels and orange-colored tonsils. * **Protective Level:** An HDL level >60 mg/dL is considered a "negative" risk factor for coronary heart disease.

Question 514: Respiratory distress syndrome in premature infants is due to inadequate secretion of which one of the following lipids?

A. Dipalmitoyl phosphatidycholine (Correct Answer)
B. Sphingomyelin
C. Cholesterol
D. Phosphatidylinositol

Explanation: ### Explanation **1. Why Dipalmitoyl Phosphatidylcholine (DPPC) is Correct:** Respiratory Distress Syndrome (RDS), also known as Hyaline Membrane Disease, occurs in premature infants due to a deficiency of **pulmonary surfactant**. Surfactant is a lipoprotein complex produced by **Type II pneumocytes**. Its primary function is to reduce surface tension at the air-liquid interface of the alveoli, preventing alveolar collapse during expiration (atelectasis). The major lipid component (approx. 80%) of surfactant is phospholipids, and the most critical functional constituent is **Dipalmitoyl Phosphatidylcholine (DPPC)**, also known as **Lecithin**. It acts as a detergent to lower surface tension, ensuring lung compliance. **2. Why the Other Options are Incorrect:** * **Sphingomyelin:** While found in the amniotic fluid, it is a structural lipid of cell membranes and myelin sheaths. It does not possess surfactant properties. Its concentration remains relatively constant during pregnancy, making it the "denominator" in the L/S ratio. * **Cholesterol:** Although present in small amounts in surfactant to help with the fluidity of the lipid layer, it is not the primary functional component responsible for preventing alveolar collapse. * **Phosphatidylinositol:** This is a precursor for secondary messengers (like IP3/DAG) and a minor component of surfactant, but it does not play the primary role in reducing surface tension compared to DPPC. **3. NEET-PG High-Yield Clinical Pearls:** * **L/S Ratio:** Fetal lung maturity is assessed by the Lecithin/Sphingomyelin ratio in amniotic fluid. A **ratio > 2.0** indicates mature lungs. * **Surfactant Markers:** Phosphatidylglycerol (PG) is another marker; its presence in amniotic fluid indicates advanced lung maturity. * **Glucocorticoids:** Antenatal administration of steroids (e.g., Betamethasone) to the mother accelerates surfactant production by stimulating Type II pneumocytes. * **Surfactant Proteins:** SP-A and SP-D are involved in innate immunity, while **SP-B and SP-C** are essential for the mechanical spreading of the surfactant film.

Question 515: What is the most important predictor of coronary artery disease?

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

Explanation: **Explanation:** The correct answer is **HDL (High-Density Lipoprotein)**. In clinical practice and biochemistry, the level of HDL is considered the most significant independent predictor of Coronary Artery Disease (CAD). **1. Why HDL is the correct answer:** HDL is known as "Good Cholesterol" because it facilitates **Reverse Cholesterol Transport**. It picks up excess cholesterol from peripheral tissues and vascular endothelium and transports it back to the liver for excretion in bile. High levels of HDL are **cardioprotective** because they prevent the formation of foam cells and atherosclerotic plaques. Conversely, low HDL levels are the strongest lipid predictor of increased cardiovascular risk. **2. Why the other options are incorrect:** * **VLDL (Very Low-Density Lipoprotein):** Primarily transports endogenous triglycerides. While elevated VLDL contributes to metabolic syndrome, it is not as strong a predictor as HDL or LDL. * **LDL (Low-Density Lipoprotein):** Known as "Bad Cholesterol," it transports cholesterol to peripheral tissues. While high LDL is a major risk factor and the primary target for statin therapy, statistically, low HDL is often a more sensitive predictor of future cardiac events. * **Chylomicron:** These transport dietary (exogenous) triglycerides. They are not directly atherogenic; their primary clinical significance relates to pancreatitis when levels are extremely high. **Clinical Pearls for NEET-PG:** * **Apo A-I** is the major apoprotein associated with HDL (anti-atherogenic). * **Apo B-100** is the major apoprotein associated with VLDL and LDL (atherogenic). * **Friedewald Formula:** LDL = Total Cholesterol – (HDL + TG/5). (Note: This is invalid if TG >400 mg/dL). * **The Best Ratio:** The Total Cholesterol/HDL ratio is often used in risk scoring; a higher ratio indicates a higher risk of CAD.

Question 516: Apolipoprotein A is a major component of which lipoprotein particle?

A. Chylomicrons
B. Very-low-density lipoprotein (VLDL)
C. High-density lipoprotein (HDL) (Correct Answer)
D. Low-density lipoprotein (LDL)

Explanation: **Explanation:** The correct answer is **High-density lipoprotein (HDL)**. Apolipoprotein A (specifically **Apo A-I**) is the primary structural protein of HDL, accounting for approximately 70% of its protein content. It plays a critical role in **Reverse Cholesterol Transport** by acting as a cofactor for the enzyme **LCAT (Lecithin-Cholesterol Acyltransferase)**, which esterifies free cholesterol, allowing it to be packed into the core of the HDL particle for transport to the liver. **Analysis of Incorrect Options:** * **Chylomicrons:** Their characteristic structural protein is **Apo B-48**. While they do contain some Apo A-I (acquired from the intestine), it is rapidly transferred to HDL in the circulation. * **VLDL:** The primary structural protein for VLDL is **Apo B-100**. It also contains Apo C-II (activates Lipoprotein Lipase) and Apo E. * **LDL:** Formed from the metabolism of VLDL, LDL contains only **Apo B-100**, which serves as the ligand for the LDL receptor. **High-Yield Clinical Pearls for NEET-PG:** * **Apo A-I:** Activates **LCAT** (Reverse Cholesterol Transport). * **Apo B-48:** Required for chylomicron assembly and secretion from the intestine. * **Apo B-100:** Required for VLDL assembly and acts as a ligand for the **LDL receptor**. * **Apo C-II:** A potent activator of **Lipoprotein Lipase (LPL)**. * **Apo E:** Mediates the uptake of chylomicron remnants and IDL by the liver. * **Tangier Disease:** A rare genetic disorder characterized by a deficiency of the ABCA1 transporter, leading to extremely low levels of HDL and Apo A-I.

Question 517: Which of the following is an omega-3 fatty acid?

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

Explanation: **Explanation:** Fatty acids are classified based on the position of the first double bond from the methyl ($\omega$) end. **Omega-3 ($\omega$-3) fatty acids** have their first double bond at the third carbon atom from the methyl terminal. **Why Cervonic Acid is Correct:** **Cervonic acid** is the systematic name for **Docosahexaenoic acid (DHA)** ($22:6, \omega\text{-}3$). It is a long-chain polyunsaturated fatty acid (PUFA) essential for retinal and brain development. Since the first double bond occurs at the third carbon from the $\omega$-end, it is a classic $\omega$-3 fatty acid. **Analysis of Incorrect Options:** * **Linoleic acid ($18:2, \omega\text{-}6$):** This is an essential fatty acid, but it belongs to the **$\omega$-6 family**. It is the precursor for arachidonic acid. * **Arachidonic acid ($20:4, \omega\text{-}6$):** Derived from linoleic acid, this is an **$\omega$-6 fatty acid**. It serves as a precursor for pro-inflammatory eicosanoids (prostaglandins and leukotrienes). * **Oleic acid ($18:1, \omega\text{-}9$):** This is a monounsaturated fatty acid (MUFA) commonly found in olive oil. It belongs to the **$\omega$-9 family**. **High-Yield NEET-PG Pearls:** 1. **Essential Fatty Acids (EFA):** Humans lack enzymes ($\Delta^{12}$ and $\Delta^{15}$ desaturases) to introduce double bonds beyond carbon 9. Thus, Linoleic ($\omega$-6) and $\alpha$-Linolenic acid ($\omega$-3) must be obtained from the diet. 2. **$\omega$-3 Family Members:** Includes $\alpha$-Linolenic acid (ALA), Timnodonic acid (EPA), and Cervonic acid (DHA). 3. **Clinical Significance:** $\omega$-3 fatty acids are cardioprotective as they decrease serum triglycerides and inhibit VLDL synthesis. 4. **Nomenclature Tip:** **Cervonic** acid (DHA) is often confused with **Clupanodonic** acid (DPA - also an $\omega$-3). Both are high-yield terms in lipid biochemistry.

Question 518: Which is the best marker of dyslipidemia?

A. Triglycerides and cholesterol
B. LDL/HDL ratio (Correct Answer)
C. Cholesterol
D. Apolipoprotein AI

Explanation: **Explanation:** The **LDL/HDL ratio** is considered the best marker of dyslipidemia because it reflects the balance between "bad" cholesterol (pro-atherogenic) and "good" cholesterol (anti-atherogenic). While individual lipid levels provide data, the ratio offers a superior predictive value for cardiovascular risk. A high ratio indicates that the rate of cholesterol deposition in peripheral tissues (mediated by LDL) significantly outweighs the rate of reverse cholesterol transport (mediated by HDL). **Analysis of Options:** * **Option A & C (Triglycerides and Cholesterol):** These are absolute values. While elevated levels are risk factors, they do not account for the protective effect of HDL. For example, a patient with high total cholesterol but very high HDL may have a lower cardiovascular risk than someone with "normal" cholesterol but very low HDL. * **Option D (Apolipoprotein AI):** Apo-AI is the primary protein component of HDL. While it is a good marker for the number of HDL particles, it is only one side of the lipid equation and is less comprehensive than the LDL/HDL ratio in clinical practice. **Clinical Pearls for NEET-PG:** * **Atherogenic Index of Plasma (AIP):** Calculated as $log(TG/HDL-C)$. It is an emerging potent marker for atherosclerosis. * **Friedewald Formula:** Used to calculate LDL ($LDL = Total\ Cholesterol – HDL – TG/5$). Note: This formula is inaccurate if $TG > 400\ mg/dL$. * **Apo B:** Often cited as a better predictor of cardiovascular events than LDL-C alone because it measures the total number of atherogenic particles (LDL, VLDL, IDL). * **Target Ratio:** For primary prevention, an LDL/HDL ratio **< 3.0** is generally considered desirable.

Question 519: Which lipoprotein is the predominant carrier of cholesterol in the blood?

A. LDL and HDL (Correct Answer)
B. HDL and VLDL
C. Chylomicron and VLDL
D. IDL and HDL

Explanation: **Explanation:** Lipoproteins are classified based on their density and the specific lipids they transport. To identify the predominant carriers of cholesterol, one must distinguish between exogenous/endogenous triglycerides and cholesterol esters. **1. Why LDL and HDL are correct:** * **LDL (Low-Density Lipoprotein):** Known as "bad cholesterol," LDL is the primary carrier of cholesterol from the liver to peripheral tissues. It contains the highest percentage of cholesterol (approx. 50%) among all lipoproteins. * **HDL (High-Density Lipoprotein):** Known as "good cholesterol," it is responsible for **Reverse Cholesterol Transport**, carrying excess cholesterol from peripheral tissues back to the liver. Together, these two fractions account for the vast majority of cholesterol circulating in the plasma. **2. Why other options are incorrect:** * **Chylomicrons:** These primarily transport **exogenous (dietary) triglycerides** from the intestines to the tissues. * **VLDL (Very Low-Density Lipoprotein):** These primarily transport **endogenous triglycerides** synthesized in the liver. * **IDL (Intermediate-Density Lipoprotein):** A transient state in the conversion of VLDL to LDL; while it contains cholesterol, its concentration in the blood is normally very low. **Clinical Pearls for NEET-PG:** * **Apo-B100** is the characteristic apolipoprotein for VLDL, IDL, and LDL. * **Apo-A1** is the primary apolipoprotein for HDL. * **Friedewald Equation:** LDL Cholesterol = Total Cholesterol – [HDL + (Triglycerides/5)]. (Note: This is invalid if TG >400 mg/dL). * **Rate-limiting enzyme** for cholesterol synthesis: HMG-CoA Reductase (target of Statins).

Question 520: The PAN-SH site of the fatty acid synthase complex accepts which of the following?

A. Acetyl CoA
B. Malonyl CoA (Correct Answer)
C. Propionyl CoA
D. All of the above

Explanation: **Explanation:** The **Fatty Acid Synthase (FAS) complex** is a multi-enzyme system that functions as a dimer. Each monomer contains two essential thiol (-SH) groups: 1. **Cys-SH:** Located on the 3-ketoacyl synthase (condensing enzyme) subunit. 2. **Pan-SH:** Located on the **Acyl Carrier Protein (ACP)** subunit, derived from the prosthetic group 4'-phosphopantetheine. **Why Malonyl CoA is correct:** During the elongation cycle of fatty acid synthesis, the **Pan-SH** site is the primary "loading dock" for the 3-carbon donor, **Malonyl CoA**. The enzyme *Malonyl acetyl transacylase* catalyzes the transfer of the malonyl group to the Pan-SH group of ACP. While the very first Acetyl CoA molecule initially binds to Pan-SH, it is immediately transferred to the Cys-SH site to vacate Pan-SH for the incoming Malonyl CoA. Therefore, in the repetitive cycles of synthesis, Pan-SH specifically accepts Malonyl CoA. **Analysis of Incorrect Options:** * **Acetyl CoA:** This acts as the "primer" or "starter" molecule. While it briefly touches Pan-SH, its functional residence during the condensation reaction is the **Cys-SH** site. * **Propionyl CoA:** This is used as a primer only for the synthesis of odd-chain fatty acids (rare in humans). It does not represent the standard substrate for the FAS complex Pan-SH site in general metabolism. **High-Yield Facts for NEET-PG:** * **Rate-limiting step:** The conversion of Acetyl CoA to Malonyl CoA by *Acetyl CoA Carboxylase (ACC)*, which requires **Biotin**. * **End product:** The FAS complex primarily produces **Palmitate** (a 16-carbon saturated fatty acid). * **Reductant:** **NADPH** is the essential electron donor, primarily sourced from the Hexose Monophosphate (HMP) shunt. * **Location:** Fatty acid synthesis occurs in the **Cytosol** (the "Citrate-Malate Shuttle" transports Acetyl CoA out of the mitochondria).

Practice by Chapter

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Fatty Acid Oxidation

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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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