Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 501: In which subcellular compartment does fatty acid synthesis occur in a hepatocyte?

A. Cytoplasm (Correct Answer)
B. Mitochondria
C. Nucleus
D. Endosomes

Explanation: **Explanation:** Fatty acid synthesis (Lipogenesis) primarily occurs in the **cytoplasm** of hepatocytes, as well as in mammary glands and adipose tissue. The process begins with the conversion of Acetyl-CoA to Malonyl-CoA by the enzyme Acetyl-CoA Carboxylase (the rate-limiting step). The subsequent reactions are catalyzed by the **Fatty Acid Synthase (FAS) multienzyme complex**, which is located exclusively in the cytosol. This spatial localization is essential because the required reducing equivalent, **NADPH**, is generated in the cytoplasm via the Pentose Phosphate Pathway (PPP). **Analysis of Incorrect Options:** * **B. Mitochondria:** While the precursor Acetyl-CoA is produced here, fatty acid *synthesis* does not occur in the mitochondria. Instead, the mitochondria are the primary site for **$\beta$-oxidation** (breakdown of fatty acids) and the Kreb’s cycle. * **C. Nucleus:** The nucleus houses genetic material and is responsible for replication and transcription; it does not contain the enzymatic machinery for lipid synthesis. * **D. Endosomes:** These are involved in the sorting and trafficking of proteins and lipids (endocytosis) rather than the de novo synthesis of fatty acids. **High-Yield Clinical Pearls for NEET-PG:** * **The Citrate Shuttle:** Since Acetyl-CoA cannot cross the mitochondrial membrane, it condenses with oxaloacetate to form **Citrate**, which is transported to the cytoplasm and then cleaved back into Acetyl-CoA for synthesis. * **Key Enzyme:** Acetyl-CoA Carboxylase (ACC) is inhibited by Palmitoyl-CoA and activated by Citrate. * **Cofactor:** NADPH is the essential electron donor for lipogenesis. * **Location Mnemonic:** "Synthesis in the Sol (Cytosol), Breakdown in the Box (Mitochondria)."

Question 502: What is the principal apolipoprotein in chylomicrons?

A. Apo A-I
B. Apo A-II
C. Apo B-100
D. Apo B-48 (Correct Answer)

Explanation: **Explanation:** The correct answer is **Apo B-48**. **1. Why Apo B-48 is correct:** Apolipoprotein B-48 is the structural hallmark of **chylomicrons**. It is synthesized exclusively in the **enterocytes** of the small intestine. It is derived from the same gene as Apo B-100 (APOB gene), but through a process called **RNA editing**. In the intestine, the enzyme *cytidine deaminase* introduces a premature stop codon (UAA), resulting in a protein that is only 48% of the length of Apo B-100. Because it lacks the LDL receptor-binding domain found in the C-terminal end of B-100, it serves primarily as a structural scaffold for transporting dietary (exogenous) lipids. **2. Why the other options are incorrect:** * **Apo B-100:** This is the principal apoprotein of **VLDL, IDL, and LDL**. It is synthesized in the **liver** and contains the ligand required for binding to the LDL receptor. * **Apo A-I:** This is the major apoprotein found in **HDL**. It is a potent activator of the enzyme **LCAT** (Lecithin-Cholesterol Acyltransferase), essential for reverse cholesterol transport. * **Apo A-II:** Also found primarily in **HDL**, its exact physiological role is less clear, though it may inhibit hepatic lipase. **3. High-Yield Clinical Pearls for NEET-PG:** * **Abetalipoproteinemia:** A deficiency of Microsomal Triglyceride Transfer Protein (MTP) leads to an inability to load Apo B-48 and B-100 with lipids, resulting in the absence of chylomicrons, VLDL, and LDL. * **Apo C-II:** Acts as a cofactor for **Lipoprotein Lipase (LPL)**; deficiency leads to Type I Hyperlipoproteinemia. * **Apo E:** Essential for the hepatic uptake of **chylomicron remnants** and IDL via the LRP and LDL receptors.

Question 503: Gangliosides consist of which of the following components?

A. Nitrogenous base
B. Glycerol
C. Phosphate
D. Sialic acid (Correct Answer)

Explanation: **Explanation:** **Gangliosides** are a subclass of **sphingolipids**, specifically categorized as acidic glycosphingolipids. The defining feature of a ganglioside is the presence of one or more residues of **N-acetylneuraminic acid (NANA)**, commonly known as **Sialic acid**. 1. **Why Sialic Acid is Correct:** The basic structure of a ganglioside consists of a **Ceramide** backbone (Sphingosine + Fatty acid) attached to an oligosaccharide chain. What distinguishes gangliosides from neutral glycosphingolipids (like cerebrosides) is the attachment of sialic acid to this sugar chain, which imparts a negative charge to the molecule at physiological pH. 2. **Why Other Options are Incorrect:** * **Glycerol:** Gangliosides are sphingolipids, not phospholipids. They use **sphingosine** as the alcohol backbone instead of glycerol. * **Phosphate:** Gangliosides are non-phosphorylated lipids. Phosphate is a characteristic component of phospholipids (like Lecithin) and Sphingomyelins, but it is absent in glycosphingolipids. * **Nitrogenous base:** While sphingosine contains nitrogen, the term "nitrogenous base" in biochemistry typically refers to purines/pyrimidines (DNA/RNA) or specific bases like choline/ethanolamine found in phospholipids. It is not a defining component of gangliosides. **High-Yield Clinical Pearls for NEET-PG:** * **GM1 Ganglioside:** Acts as the intestinal mucosal receptor for the **Cholera toxin**. * **Tay-Sachs Disease:** Caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of **GM2 ganglioside**. Key findings: Cherry-red spot on macula, no hepatosplenomegaly. * **Guillain-Barré Syndrome (GBS):** Often involves the formation of antibodies against gangliosides (e.g., anti-GM1 antibodies). * **Location:** Gangliosides are most abundant in the **gray matter** of the brain.

Question 504: Dietary triglycerides are transported by which lipoprotein?

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

Explanation: **Explanation:** The correct answer is **Chylomicrons**. **1. Why Chylomicrons are correct:** Dietary (exogenous) lipids, primarily triglycerides, are absorbed by the intestinal mucosal cells. Because triglycerides are hydrophobic, they are packaged into large, protein-coated droplets called **Chylomicrons**. These enter the lymphatic system via lacteals and eventually reach the systemic circulation through the thoracic duct. Their primary role is to deliver dietary triglycerides to peripheral tissues (adipose and muscle). **2. Why other options are incorrect:** * **VLDL (Very Low-Density Lipoprotein):** These transport **endogenous** triglycerides synthesized in the liver. They do not carry dietary lipids directly from the gut. * **LDL (Low-Density Lipoprotein):** Formed from VLDL/IDL, LDL is the primary carrier of **cholesterol** to peripheral tissues. It contains very little triglyceride. * **HDL (High-Density Lipoprotein):** Known for **"Reverse Cholesterol Transport,"** it picks up excess cholesterol from peripheral tissues and returns it to the liver. **3. NEET-PG High-Yield Pearls:** * **Apolipoprotein Marker:** **Apo B-48** is the unique structural protein for Chylomicrons (synthesized in the intestine), while **Apo B-100** is found in VLDL and LDL (synthesized in the liver). * **Milky Plasma:** After a fatty meal, plasma appears milky due to the presence of Chylomicrons. * **Enzyme Action:** **Lipoprotein Lipase (LPL)**, activated by **Apo C-II**, hydrolyzes the triglycerides within Chylomicrons at the capillary endothelium. * **Type I Hyperlipoproteinemia:** Characterized by a deficiency in LPL or Apo C-II, leading to massive accumulation of Chylomicrons in the blood.

Question 505: A patient presents with a history of recurring attacks of pancreatitis, eruptive xanthomas, and markedly increased plasma triglyceride levels (2,000 mg/dL). Which of the following deficiencies is most likely present?

A. Lipoprotein Lipase (Correct Answer)
B. LDL receptors
C. HMG-CoA reductase
D. ABCA1 receptor

Explanation: **Explanation:** The clinical presentation of **recurrent pancreatitis**, **eruptive xanthomas**, and severe hypertriglyceridemia (often >1,000 mg/dL) is characteristic of **Type I Hyperlipoproteinemia** (Familial Chylomicronemia Syndrome). 1. **Why Lipoprotein Lipase (LPL) is correct:** LPL is the key enzyme responsible for hydrolyzing triglycerides in chylomicrons and VLDL into free fatty acids. A deficiency in LPL (or its cofactor, Apo C-II) leads to a massive accumulation of chylomicrons in the plasma. These large particles obstruct pancreatic capillaries, causing ischemia and **pancreatitis**. Eruptive xanthomas (small yellow papules) occur due to lipid uptake by macrophages in the skin. 2. **Why other options are incorrect:** * **LDL Receptors:** Deficiency leads to **Type IIa Hypercholesterolemia**. It presents with high LDL and cholesterol, causing xanthelasmas and tendon xanthomas, but not severe hypertriglyceridemia or pancreatitis. * **HMG-CoA Reductase:** This is the rate-limiting enzyme for cholesterol synthesis. It is the target of statins; its deficiency is not a recognized cause of hyperlipidemia. * **ABCA1 Receptor:** Deficiency causes **Tangier Disease**, characterized by extremely low HDL levels and orange tonsils, not high triglycerides. **NEET-PG High-Yield Pearls:** * **Appearance of Plasma:** In LPL deficiency, if plasma is left standing, a **creamy layer** forms on top (chylomicrons). * **Apo C-II:** Acts as an obligate co-factor for LPL; its deficiency mimics LPL deficiency. * **Treatment:** Primarily a **very low-fat diet** (medium-chain triglycerides are preferred as they bypass chylomicron formation). * **Type IV vs. Type I:** Both involve high triglycerides, but Type IV (VLDL excess) rarely presents with the extreme levels (>2,000 mg/dL) seen in Type I.

Question 506: Which of the following conditions is characterized by the accumulation of sphingomyelin in phagocytic cells?

A. Gaucher disease
B. Niemann-Pick disease (Correct Answer)
C. Tay-Sachs disease
D. Down syndrome

Explanation: **Explanation:** **Niemann-Pick Disease (Option B)** is the correct answer. It is an autosomal recessive lysosomal storage disorder caused by a deficiency of the enzyme **sphingomyelinase**. This deficiency leads to the pathological accumulation of **sphingomyelin** within the lysosomes of phagocytic cells (macrophages). Under microscopy, these lipid-laden macrophages appear as characteristic **"Foam cells"** (vacuolated cytoplasm). Clinical hallmarks include hepatosplenomegaly, progressive neurodegeneration, and a cherry-red spot on the macula. **Analysis of Incorrect Options:** * **Gaucher Disease (Option A):** The most common lysosomal storage disease, caused by **glucocerebrosidase** deficiency. It results in the accumulation of **glucocerebroside**. Histology shows "Gaucher cells" (macrophages with a "wrinkled tissue paper" appearance). * **Tay-Sachs Disease (Option C):** Caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of **GM2 gangliosides**. While it features a cherry-red spot, there is **no hepatosplenomegaly**, which distinguishes it from Niemann-Pick. * **Down Syndrome (Option D):** A chromosomal anomaly (Trisomy 21) unrelated to sphingolipid metabolism or lysosomal storage. **High-Yield NEET-PG Pearls:** * **Mnemonic for Niemann-Pick:** "No-man picks (Niemann-Pick) his nose with his **foam**y finger" (Foam cells). * **Cherry-red spot:** Seen in Niemann-Pick, Tay-Sachs, and Central Retinal Artery Occlusion (CRAO). * **Sphingomyelinase** is a type of phospholipase C. * **Type A** Niemann-Pick is the severe infantile form with CNS involvement; **Type B** is the visceral form (no CNS involvement).

Question 507: Which apolipoprotein is present in LDL?

A. Apolipoprotein B-48
B. Apolipoprotein B-100 (Correct Answer)
C. Apolipoprotein C-I
D. Apolipoprotein C-III

Explanation: ### Explanation **Correct Answer: B. Apolipoprotein B-100** **Underlying Concept:** Low-Density Lipoprotein (LDL) is derived from the metabolism of VLDL (Very Low-Density Lipoprotein). **Apolipoprotein B-100 (Apo B-100)** is the primary structural protein found in VLDL, IDL, and LDL. It serves as the essential ligand for the **LDL receptor (ApoB/E receptor)**, facilitating the endocytosis of LDL into peripheral tissues and the liver. Since LDL is the final product of the endogenous lipoprotein pathway, it retains Apo B-100 as its sole apolipoprotein. **Analysis of Incorrect Options:** * **A. Apolipoprotein B-48:** This is the hallmark protein of **Chylomicrons**. It is synthesized in the intestine (via mRNA editing of the ApoB gene). It lacks the LDL receptor-binding domain found in B-100. * **C. Apolipoprotein C-I:** This is primarily found in VLDL and HDL. It plays a role in activating LCAT (Lecithin-Cholesterol Acyltransferase) but is not the defining protein of LDL. * **D. Apolipoprotein C-III:** Found in VLDL, Chylomicrons, and HDL. Its primary function is to **inhibit Lipoprotein Lipase (LPL)** and hepatic lipase; it is lost as VLDL matures into LDL. **High-Yield Clinical Pearls for NEET-PG:** * **"Bad Cholesterol":** LDL is the primary carrier of cholesterol to peripheral tissues. High levels are strongly associated with atherosclerosis. * **Apo B-100 vs. B-48:** Remember: **B-48** is for the **B**owel (Chylomicrons); **B-100** is for the **B**lood/Liver (VLDL/LDL). * **Type IIa Hyperlipoproteinemia:** Caused by a deficiency or defect in LDL receptors, leading to elevated LDL and Apo B-100 levels. * **Wolman Disease:** A lysosomal storage disease where a deficiency in cholesteryl ester hydrolase prevents the breakdown of LDL-derived esters.

Question 508: Cholesterol is a(an):

A. Ester
B. Phospholipid
C. Sterol (Correct Answer)
D. Lipoprotein

Explanation: **Explanation:** **Why the correct answer is right:** Cholesterol is classified as a **Sterol** (a steroid alcohol) because it contains a characteristic **cyclopentanoperhydrophenanthrene (CPPP) ring** (sterane nucleus) and a hydroxyl (-OH) group at the C3 position. It is the most abundant sterol in animal tissues and serves as a vital structural component of cell membranes, regulating fluidity. **Why the incorrect options are wrong:** * **A. Ester:** While cholesterol can react with fatty acids to form *cholesteryl esters* (the storage form), cholesterol itself is a free alcohol, not an ester. * **B. Phospholipid:** Phospholipids (like lecithin) contain a phosphate group and a glycerol or sphingosine backbone. Cholesterol lacks phosphate and has a completely different ring structure. * **C. Lipoprotein:** Lipoproteins (like LDL, HDL) are complex molecular aggregates composed of lipids and proteins used for transport. Cholesterol is a *cargo* carried within these lipoproteins, not a lipoprotein itself. **NEET-PG High-Yield Pearls:** 1. **Precursor Molecule:** Cholesterol is the parent compound for the synthesis of **Bile acids, Vitamin D, and Steroid hormones** (Glucocorticoids, Mineralocorticoids, and Sex hormones). 2. **Rate-Limiting Enzyme:** The synthesis of cholesterol occurs in the cytosol, and the rate-limiting step is catalyzed by **HMG-CoA Reductase** (inhibited by Statins). 3. **Identification:** It can be identified by the **Libermann-Burchard reaction**, which produces a characteristic green color. 4. **Excretion:** Humans cannot metabolize the sterol ring to $CO_2$ and $H_2O$; it is excreted primarily via bile as neutral sterols or bile acids.

Question 509: Which apoprotein is primarily present in LDL?

A. ApoB-48
B. ApoB-100 (Correct Answer)
C. ApoC-I
D. ApoC-III

Explanation: **Explanation:** Low-Density Lipoprotein (LDL) is the primary carrier of cholesterol in the blood. It is derived from the metabolism of VLDL (Very Low-Density Lipoprotein) and IDL (Intermediate-Density Lipoprotein). **Why ApoB-100 is correct:** ApoB-100 is the structural apoprotein for VLDL, IDL, and LDL. As VLDL is depleted of triglycerides by lipoprotein lipase, it eventually becomes LDL. During this transition, other apoproteins (like ApoC and ApoE) are lost, leaving **ApoB-100** as the sole and defining apoprotein of LDL. It serves as the ligand for the **LDL receptor**, facilitating cholesterol uptake into peripheral tissues. **Analysis of Incorrect Options:** * **ApoB-48:** This is the truncated version of ApoB synthesized in the intestine. It is the characteristic marker for **Chylomicrons** and their remnants. * **ApoC-I & ApoC-III:** These are "exchangeable" apoproteins found primarily on VLDL and HDL. ApoC-III is notable for inhibiting lipoprotein lipase and hepatic lipase, but neither is the primary structural protein of LDL. **High-Yield Clinical Pearls for NEET-PG:** * **ApoB-100 vs. ApoB-48:** Both are products of the same gene. ApoB-48 is produced via **RNA editing** (C to U conversion creating a stop codon) specifically in the intestine. * **Friedewald Equation:** LDL Cholesterol = Total Cholesterol – [HDL + (Triglycerides/5)]. (Note: Not valid if TG >400 mg/dL). * **Type IIa Hyperlipoproteinemia:** Characterized by a deficiency in LDL receptors, leading to elevated LDL and ApoB-100 levels.

Question 510: What is the most essential fatty acid?

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

Explanation: **Explanation:** 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 create double bonds beyond the $\Delta^9$ position. **Why Linoleic Acid is the Correct Answer:** **Linoleic acid (18:2; $\omega$-6)** is considered the "most essential" because it serves as the primary precursor for the synthesis of other $\omega$-6 fatty acids, including Arachidonic acid. While both Linoleic and $\alpha$-Linolenic acid are essential, Linoleic acid is often prioritized in medical literature as the fundamental EFA because its deficiency leads to clinical symptoms more rapidly and it cannot be synthesized from any other precursor. **Analysis of Incorrect Options:** * **B. Linolenic acid (18:3; $\omega$-3):** This is also an essential fatty acid. However, in the hierarchy of "most essential," Linoleic acid is the primary dietary requirement from which others are derived. * **C. Arachidonic acid (20:4; $\omega$-6):** This is a **semi-essential** fatty acid. It can be synthesized in the body from Linoleic acid. It only becomes essential if there is a dietary deficiency of Linoleic acid. * **D. Palmitic acid (16:0):** This is a saturated fatty acid and is the first fatty acid produced by the **Fatty Acid Synthase (FAS)** complex in the body. It is non-essential. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Deficiency:** EFA deficiency presents as **Phrynoderma** (follicular hyperkeratosis/toad skin), hair loss, and poor wound healing. * **Precursor Role:** Arachidonic acid is the vital precursor for **Prostaglandins, Leukotrienes, and Thromboxanes** (Eicosanoids). * **Ratio:** The ideal dietary ratio of $\omega$-6 to $\omega$-3 is approximately 4:1 to 10:1.

Practice by Chapter

Lipid Classification and Chemistry

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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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