Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 381: Apolipoprotein A-1 is associated with which of the following lipoprotein classes?

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

Explanation: ### Explanation **Correct Answer: C. High-density lipoprotein (HDL)** **Why it is correct:** Apolipoprotein A-1 (Apo A-1) is the major structural protein constituent of **HDL**. It is synthesized in the liver and small intestine. Its primary physiological role is to act as a cofactor for the enzyme **Lecithin-Cholesterol Acyltransferase (LCAT)**, which esterifies free cholesterol. This process is critical for **Reverse Cholesterol Transport**, where HDL removes excess cholesterol from peripheral tissues and transports it back to the liver for excretion. **Why the other options are incorrect:** * **A. LDL:** The primary apolipoprotein associated with LDL is **Apo B-100**. LDL is the main carrier of cholesterol to peripheral tissues. * **B. VLDL:** VLDL is characterized by **Apo B-100**, along with Apo C-II and Apo E. It transports endogenous triglycerides from the liver. * **D. Chylomicrons:** The hallmark apolipoprotein for chylomicrons is **Apo B-48**. While nascent chylomicrons do contain some Apo A-1, it is quickly transferred to HDL in the plasma; therefore, Apo B-48 remains the definitive marker. **High-Yield NEET-PG Pearls:** * **Apo A-1:** Activates LCAT; marker for "Good Cholesterol" (HDL). * **Apo B-48:** Unique to Chylomicrons (formed via mRNA editing in the intestine). * **Apo B-100:** Found in VLDL, IDL, and LDL; ligand for the LDL receptor. * **Apo C-II:** Activates **Lipoprotein Lipase (LPL)**; deficiency leads to Type I Hyperlipoproteinemia. * **Apo E:** Mediates remnant uptake by the liver; deficiency leads to Type III Hyperlipoproteinemia (Dysbetalipoproteinemia).

Question 382: Which of the following is not an essential unsaturated fatty acid?

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

Explanation: ### Explanation **Correct Answer: D. Palmitoleic acid** **1. Why Palmitoleic acid is the correct answer:** Essential fatty acids (EFAs) are those that the human body cannot synthesize de novo because humans lack the enzymes (**$\Delta^{12}$ and $\Delta^{15}$ desaturases**) required to introduce double bonds beyond the $\Delta^9$ position. **Palmitoleic acid** is an omega-7 monounsaturated fatty acid (16:1; $\Delta^9$). Since the double bond is at the 9th carbon, the human body can synthesize it from palmitic acid using the $\Delta^9$ desaturase enzyme. Therefore, it is **non-essential**. **2. Analysis of Incorrect Options:** * **Linoleic acid (18:2; $\Delta^{9,12}$):** An Omega-6 fatty acid. It is strictly essential because the double bond at $\Delta^{12}$ cannot be created by human enzymes. It serves as the precursor for Arachidonic acid. * **Linolenic acid (18:3; $\Delta^{9,12,15}$):** An Omega-3 fatty acid. It is strictly essential due to the double bonds at $\Delta^{12}$ and $\Delta^{15}$. * **Arachidonic acid (20:4; $\Delta^{5,8,11,14}$):** An Omega-6 fatty acid. It is considered **conditionally essential**. It can be synthesized from Linoleic acid; however, if Linoleic acid is deficient in the diet, Arachidonic acid becomes essential. In the context of NEET-PG, it is traditionally grouped with essential fatty acids. **3. High-Yield Clinical Pearls for NEET-PG:** * **True Essential Fatty Acids:** Only Linoleic and $\alpha$-Linolenic acid are "true" EFAs. * **EFA Deficiency:** Characterized by **Phrynoderma** (follicular hyperkeratosis/toad skin), scaly dermatitis, and poor wound healing. * **Precursor Role:** Arachidonic acid is the immediate precursor for **prostaglandins, leukotrienes, and thromboxanes** (Eicosanoids). * **Omega Nomenclature:** Count from the methyl ($\omega$) end; $\Delta$ nomenclature counts from the carboxyl (COOH) end.

Question 383: Lithogenic bile has which of the following properties?

A. Increased bile and cholesterol ratio
B. Decreased bile and cholesterol ratio (Correct Answer)
C. Equal bile and cholesterol ratio
D. Decreased cholesterol only

Explanation: **Explanation:** The formation of gallstones (cholelithiasis) is primarily driven by the production of **lithogenic bile**. Bile is a complex fluid containing bile salts, phospholipids (lecithin), and cholesterol. Under normal physiological conditions, cholesterol—which is water-insoluble—is kept in a soluble state by being incorporated into mixed micelles formed by bile salts and phospholipids. **1. Why Option B is Correct:** Bile becomes lithogenic when it is **supersaturated with cholesterol**. This occurs due to either an absolute increase in cholesterol secretion or a relative decrease in bile salts and phospholipids. Therefore, a **decreased bile salt-to-cholesterol ratio** means there is insufficient "detergent" (bile salts) to keep the cholesterol in solution. This leads to the precipitation of cholesterol crystals, which eventually aggregate to form stones. **2. Why Incorrect Options are Wrong:** * **Option A:** An increased bile-to-cholesterol ratio would mean more bile salts are available to solubilize cholesterol, making the bile *less* likely to form stones (non-lithogenic). * **Option C:** An equal ratio does not account for the specific saturation index required to maintain solubility; supersaturation is the key driver. * **Option D:** Decreased cholesterol would actually decrease the risk of stone formation, as the bile would be undersaturated. **NEET-PG High-Yield Pearls:** * **The Solubility Triangle:** The stability of bile depends on the molar ratios of bile salts, lecithin, and cholesterol (Admirand-Small Triangle). * **Rate-Limiting Enzyme:** **7-alpha-hydroxylase** is the rate-limiting enzyme for bile acid synthesis. Its deficiency or inhibition leads to lithogenic bile. * **Risk Factors (The 4 F's):** Fat, Female, Fertile, and Forty. * **Statin Mechanism:** HMG-CoA reductase inhibitors (Statins) can reduce the lithogenicity of bile by decreasing biliary cholesterol secretion.

Question 384: Which protein, having structural homology with plasminogen, is responsible for myocardial infarction and stroke?

A. HDL
B. LP(a) (Correct Answer)
C. LDL
D. Homocysteine

Explanation: **Explanation:** **Lipoprotein(a) [Lp(a)]** is the correct answer because it consists of an LDL-like particle containing **Apolipoprotein B-100** linked to a unique glycoprotein called **Apolipoprotein(a)**. 1. **Mechanism of Action:** Apo(a) shares significant **structural homology with Plasminogen**, the precursor to plasmin. Due to this similarity, Lp(a) competitively inhibits the binding of plasminogen to fibrin and endothelial cell receptors. This impairs fibrinolysis (clot breakdown) and promotes thrombosis. Furthermore, Lp(a) is highly pro-atherogenic as it deposits cholesterol into the arterial wall. This dual **pro-thrombotic and pro-atherogenic** nature significantly increases the risk of Myocardial Infarction (MI) and Stroke. **Analysis of Incorrect Options:** * **A. HDL:** Known as "good cholesterol," it mediates reverse cholesterol transport. High levels are cardioprotective, not causative of MI. * **C. LDL:** While the primary carrier of cholesterol and a major risk factor for atherosclerosis, it lacks the plasminogen-like Apo(a) component. * **D. Homocysteine:** High levels (Hyperhomocysteinemia) are a risk factor for vascular disease due to endothelial damage, but it is an amino acid derivative, not a protein with structural homology to plasminogen. **High-Yield Facts for NEET-PG:** * **Kringles:** The structural homology between Apo(a) and plasminogen lies in the **"Kringle IV"** repeats. * **Genetic Determinant:** Lp(a) levels are primarily genetically determined and are not significantly altered by diet or statins. * **Niacin:** Historically, Niacin was used to lower Lp(a), though its clinical benefit on outcomes remains debated. * **Clinical Marker:** Lp(a) is considered an independent risk factor for premature coronary artery disease (CAD).

Question 385: Which of the following is a member of the ω9 family of fatty acids?

A. Timnodonic Acid
B. Cervonic Acid
C. Oleic Acid (Correct Answer)
D. Arachidonic Acid

Explanation: **Explanation:** Fatty acids are classified into families (ω3, ω6, ω7, or ω9) based on the position of the first double bond relative to the methyl (omega) end of the carbon chain. **Why Oleic Acid is Correct:** **Oleic Acid (18:1; Δ9)** is the most common monounsaturated fatty acid (MUFA) in the human body. Counting from the methyl end (omega carbon), the first (and only) double bond occurs at the 9th carbon. Therefore, it belongs to the **ω9 family**. It is synthesized in the body from Stearic acid by the enzyme Δ9-desaturase. **Analysis of Incorrect Options:** * **Timnodonic Acid (20:5; ω3):** Also known as Eicosapentaenoic acid (EPA). It is a polyunsaturated fatty acid (PUFA) found in fish oil and belongs to the **ω3 family**. * **Cervonic Acid (22:6; ω3):** Also known as Docosahexaenoic acid (DHA). It is critical for retinal and brain development and belongs to the **ω3 family**. * **Arachidonic Acid (20:4; ω6):** A precursor for prostaglandins and leukotrienes. Since the first double bond from the methyl end is at carbon 6, it belongs to the **ω6 family**. **High-Yield Clinical Pearls for NEET-PG:** * **Essential Fatty Acids (EFA):** Humans lack Δ12 and Δ15 desaturases; thus, **Linoleic acid (ω6)** and **Linolenic acid (ω3)** must be obtained from the diet. * **Arachidonic acid** becomes "conditionally essential" only if its precursor, Linoleic acid, is deficient in the diet. * **EFA Deficiency:** Characterized by scaly dermatitis (phrynoderma or toad skin), poor wound healing, and fatty liver.

Question 386: Alpha oxidation of dietary phytanic acid takes place in which of the following cell structures?

A. ER
B. Golgi apparatus
C. Mitochondria
D. Peroxisomes (Correct Answer)

Explanation: **Explanation:** **Correct Option: D. Peroxisomes** Alpha (α) oxidation is a specialized pathway for the catabolism of branched-chain fatty acids, most notably **phytanic acid** (derived from chlorophyll in the diet). Unlike most fatty acids, phytanic acid has a methyl group at the beta-carbon, which blocks standard beta-oxidation. To bypass this, the enzyme **Phytanoyl-CoA hydroxylase** removes one carbon atom from the carboxyl end as $CO_2$. This process occurs exclusively within the **peroxisomes**. Once the methyl group is shifted to the alpha position, the resulting pristanic acid can undergo standard beta-oxidation. **Incorrect Options:** * **A. ER:** The endoplasmic reticulum is the site for **omega (ω) oxidation**, a minor pathway for fatty acid metabolism that becomes active when beta-oxidation is defective. * **B. Golgi apparatus:** This organelle is involved in protein modification, sorting, and packaging, but not in the oxidative metabolism of fatty acids. * **C. Mitochondria:** While mitochondria are the primary site for **beta (β) oxidation** of short, medium, and long-chain fatty acids, they lack the specific enzymes required for alpha-oxidation. **Clinical Pearls & High-Yield Facts:** * **Refsum Disease:** A rare autosomal recessive disorder caused by a deficiency of the peroxisomal enzyme **Phytanoyl-CoA hydroxylase**. * **Clinical Presentation:** Characterized by the accumulation of phytanic acid in tissues, leading to **retinitis pigmentosa**, peripheral neuropathy, cerebellar ataxia, and nerve deafness. * **Management:** Treatment involves a diet restricted in phytanic acid (avoiding dairy and ruminant fats). * **Zellweger Syndrome:** A generalized defect in peroxisome biogenesis that also affects alpha and beta-oxidation of very-long-chain fatty acids (VLCFA).

Question 387: Which of the following is a ketone body?

A. Oxalocetate
B. Pyruvic acid
C. Acetyl CoA
D. Acetoacetate (Correct Answer)

Explanation: **Explanation:** Ketone bodies are water-soluble molecules produced by the liver from fatty acids during periods of low glucose availability (e.g., fasting, starvation, or untreated diabetes). The three primary ketone bodies are **Acetoacetate**, **3-beta-hydroxybutyrate**, and **Acetone**. 1. **Why Acetoacetate is correct:** Acetoacetate is the "primary" ketone body formed in the mitochondrial matrix of hepatocytes via the HMG-CoA lyase reaction. It can either be reduced to 3-beta-hydroxybutyrate or spontaneously decarboxylated to acetone. 2. **Why other options are incorrect:** * **Oxaloacetate:** A key intermediate in the TCA cycle and gluconeogenesis. In ketogenesis, a deficiency of oxaloacetate (due to its diversion to gluconeogenesis) actually triggers the diversion of Acetyl CoA toward ketone body synthesis. * **Pyruvic acid:** The end-product of glycolysis; it is a three-carbon alpha-keto acid, not a ketone body. * **Acetyl CoA:** While it is the *precursor* for ketone body synthesis, it is not a ketone body itself. It must undergo condensation via the HMG-CoA pathway to form acetoacetate. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Synthesis:** Liver (mitochondria). * **Site of Utilization:** Extrahepatic tissues (Brain, Heart, Skeletal Muscle). The liver **cannot** use ketone bodies because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). * **Rate-limiting Enzyme:** HMG-CoA Synthase (mitochondrial isoform). * **Detection:** Rothera’s test detects Acetoacetate and Acetone, but **not** 3-beta-hydroxybutyrate.

Question 388: Which of the following is an Omega-3 fatty acid?

A. Linoleic acid
B. a-Linolenic acid (Correct Answer)
C. Oleic acid
D. Arachidonic acid

Explanation: **Explanation:** Fatty acids are classified based on the position of the first double bond from the methyl (omega, ω) end of the carbon chain. **$\alpha$-Linolenic acid (ALA)** is an essential **Omega-3 (n-3)** fatty acid because its first double bond is located at the third carbon atom from the methyl end. It serves as the precursor for other vital omega-3 fatty acids like Eicosapentaenoic acid (EPA) and Docosahexaenoic acid (DHA). **Analysis of Options:** * **Linoleic acid (Option A):** This is an **Omega-6** fatty acid (18:2; Δ9,12). It is an essential fatty acid but belongs to the n-6 family. * **Oleic acid (Option C):** This is an **Omega-9** fatty acid (18:1; Δ9). It is a monounsaturated fatty acid (MUFA) commonly found in olive oil and is non-essential as the body can synthesize it. * **Arachidonic acid (Option D):** This is an **Omega-6** fatty acid (20:4; Δ5,8,11,14). It is synthesized from linoleic acid and serves as a precursor for pro-inflammatory eicosanoids (prostaglandins and leukotrienes). **High-Yield NEET-PG Pearls:** 1. **Essential Fatty Acids (EFA):** Humans lack the enzymes (**$\Delta$12 and $\Delta$15 desaturases**) to introduce double bonds beyond carbon 9; hence, Linoleic and $\alpha$-Linolenic acid must be obtained from the diet. 2. **DHA (Docosahexaenoic acid):** An omega-3 derivative critical for **retinal phospholipids** and **brain development** in infants. 3. **Clinical Significance:** Omega-3 fatty acids are cardioprotective as they lower serum triglycerides and reduce platelet aggregation. 4. **Mnemonic:** "Lino**len**ic has three 'n's (nearly)" $\rightarrow$ Omega-**3**. "Linoleic" has no extra 'n' $\rightarrow$ Omega-**6**.

Question 389: What is the immediate precursor of mevalonic acid?

A. Mevalonyl CoA
B. Mevalonyl pyrophosphate
C. Acetoacetyl CoA
D. Beta-hydroxy-beta-methylglutaryl CoA (Correct Answer)

Explanation: **Explanation:** The synthesis of **mevalonic acid (mevalonate)** is the committed and rate-limiting step in the **cholesterol biosynthetic pathway**. **Why the correct answer is right:** The immediate precursor of mevalonic acid is **3-hydroxy-3-methylglutaryl CoA (HMG-CoA)**. This reaction is catalyzed by the enzyme **HMG-CoA Reductase**, which utilizes two molecules of NADPH to reduce the thioester group of HMG-CoA into the primary alcohol group of mevalonate. This occurs in the cytosol of the cell. **Analysis of incorrect options:** * **A & B (Mevalonyl CoA/Pyrophosphate):** These are not standard intermediates in the early stages of cholesterol synthesis. Mevalonate is phosphorylated *after* its formation to become mevalonate-5-phosphate, not the other way around. * **C (Acetoacetyl CoA):** This is the precursor to HMG-CoA, not mevalonate. Two molecules of Acetyl-CoA condense to form Acetoacetyl-CoA, which then reacts with a third Acetyl-CoA (via HMG-CoA synthase) to produce HMG-CoA. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Enzyme:** HMG-CoA Reductase is the most important regulatory enzyme in cholesterol synthesis. * **Pharmacology Link:** **Statins** (e.g., Atorvastatin) are competitive inhibitors of HMG-CoA Reductase; they structurally resemble HMG-CoA and block the production of mevalonate. * **Location:** While HMG-CoA is also an intermediate in **ketogenesis**, that process occurs in the **mitochondria**. Cholesterol synthesis (and thus mevalonate formation) occurs in the **cytosol/ER**. * **Hormonal Control:** HMG-CoA reductase is activated by **Insulin** (via dephosphorylation) and inhibited by **Glucagon** and **AMP** (via phosphorylation).

Question 390: Cholesteryl ester transfer protein transports cholesterol from HDL to which of the following?

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

Explanation: **Explanation:** **Cholesteryl Ester Transfer Protein (CETP)** plays a pivotal role in the "Reverse Cholesterol Transport" pathway. Its primary function is to facilitate the exchange of lipids between high-density lipoproteins (HDL) and triglyceride-rich lipoproteins. **Why VLDL is the Correct Answer:** CETP mediates a reciprocal exchange where it collects **cholesteryl esters** from HDL and transfers them to **VLDL** (and to a lesser extent, LDL). In exchange, VLDL provides **triglycerides** to HDL. This process allows VLDL to be enriched with cholesterol, eventually being metabolized into LDL, while HDL becomes enriched with triglycerides (which are later hydrolyzed by hepatic lipase). **Analysis of Incorrect Options:** * **B. IDL & C. LDL:** While CETP can transfer cholesteryl esters to these lipoproteins, **VLDL** is the primary and initial acceptor in the endogenous pathway. In the context of NEET-PG, VLDL is the standard textbook answer for the primary site of exchange. * **D. Chylomicrons:** While CETP can interact with chylomicrons (exogenous pathway), the major flux of cholesterol exchange in the fasting state occurs between HDL and VLDL. **High-Yield Clinical Pearls for NEET-PG:** * **Reverse Cholesterol Transport:** This is the process of moving cholesterol from peripheral tissues back to the liver via HDL. CETP is a key regulator of this. * **CETP Inhibition:** Drugs that inhibit CETP (e.g., Anacetrapib) significantly increase HDL-C levels and decrease LDL-C, though their clinical utility in reducing cardiovascular events is still a subject of research. * **Atherogenic Profile:** High CETP activity can be pro-atherogenic as it lowers "good" HDL-cholesterol and increases "bad" VLDL/LDL-cholesterol.

Practice by Chapter

Lipid Classification and Chemistry

Practice Questions

Fatty Acid Oxidation

Practice Questions

Ketone Body Metabolism

Practice Questions

Fatty Acid Synthesis

Practice Questions

Metabolism of Triacylglycerols

Practice Questions

Phospholipid Metabolism

Practice Questions

Cholesterol Metabolism and Biosynthesis

Practice Questions

Bile Acids and Bile Salts

Practice Questions

Lipoprotein Metabolism and Transport

Practice Questions

Dyslipidemias and Atherosclerosis

Practice Questions

Prostaglandins and Eicosanoids

Practice Questions

Fatty Liver and Lipotropic Factors

Practice Questions

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