Lipid Metabolism Practice Questions

Q: Pancreatic secretions are a rich source of which enzyme?

Phospholipase A2. **Explanation:** **Why Phospholipase A2 (PLA2) is the correct answer:** The pancreas is the primary source of digestive enzymes required for lipid breakdown in the small intestine. **Phospholipase A2** is secreted by the exocrine pancreas as a proenzyme (pro-PLA2), which is subsequently activated by trypsin in the duodenal lumen. Its primary function is to hydrolyze the ester bond at the **second carbon (C2)** of phospholipids (like lecithin), yielding a free fatty acid and a **lysophospholipid**. This process is essential for the micellar solubilization of dietary lipids. **Analysis of Incorrect Options:** * **Phospholipase A1:** This enzyme cleaves the ester bond at the first carbon (C1). While present in various mammalian tissues and some venoms, it is not a major constituent of pancreatic secretions. * **Phospholipase C:** This enzyme cleaves the bond before the phosphate group, releasing diacylglycerol (DAG) and an inositol triphosphate (IP3). It is primarily involved in **intracellular secondary messenger signaling** (G-protein coupled receptor pathways) rather than intestinal digestion. * **Phospholipase D:** This enzyme cleaves after the phosphate group, releasing phosphatidic acid. It is primarily found in plant tissues and is not a significant human digestive enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Acute Pancreatitis:** Serum levels of Phospholipase A2 are often elevated in acute pancreatitis. Excessive activation of PLA2 within the pancreas can lead to the destruction of cell membranes and parenchymal necrosis. * **Snake Venom:** The venom of many snakes (e.g., Cobras and Vipers) is rich in Phospholipase A2, which causes extensive tissue necrosis and hemolysis by damaging red blood cell membranes. * **Requirement:** Pancreatic PLA2 requires **bile salts** for optimal activity and calcium ions as a cofactor.

Q: Which of the following is an essential fatty acid?

Linoleic acid. **Explanation:** **1. Why Linoleic Acid is Correct:** Essential fatty acids (EFAs) are those that the human body cannot synthesize de novo because humans lack the desaturase enzymes ($\Delta^{12}$ and $\Delta^{15}$ desaturases) required to introduce double bonds beyond the $\Delta^9$ position. **Linoleic acid (18:2; $\omega$-6)** and **$\alpha$-Linolenic acid (18:3; $\omega$-3)** are the two primary EFAs. They must be obtained through the diet (e.g., vegetable oils, nuts) to support cell membrane integrity and serve as precursors for bioactive molecules. **2. Analysis of Incorrect Options:** * **Palmitoleic acid (16:1; $\omega$-7):** This is a monounsaturated fatty acid that the body can synthesize from palmitic acid via $\Delta^9$-desaturase. * **Oleic acid (18:1; $\omega$-9):** The most common dietary fatty acid (found in olive oil), it is non-essential as it is synthesized in the body from stearic acid. * **Arachidonic acid (20:4; $\omega$-6):** While often grouped with EFAs, it is technically **"conditionally essential."** It can be synthesized in the body from linoleic acid. It only becomes essential if there is a dietary deficiency of linoleic acid. **3. High-Yield Clinical Pearls for NEET-PG:** * **EFA Deficiency:** Characterized by scaly dermatitis (phrynoderma or "toad skin"), alopecia, and poor wound healing. * **Precursor Role:** Linoleic acid is the precursor for Arachidonic acid, which is the starting point for the synthesis of **prostaglandins, thromboxanes, and leukotrienes** (Eicosanoids). * **Omega Nomenclature:** Remember that $\omega$-3 ($\alpha$-Linolenic) is cardioprotective and anti-inflammatory, whereas an imbalance favoring $\omega$-6 can be pro-inflammatory.

Q: In fatty acid synthesis, CO2 loss occurs in which step?

Condensation reaction. ### Explanation In fatty acid synthesis (Lipogenesis), the process occurs in the cytosol via the **Fatty Acid Synthase (FAS) multienzyme complex**. **1. Why "Condensation reaction" is correct:** The synthesis begins with the condensation of an **Acetyl group** (2C) and a **Malonyl group** (3C). This reaction is catalyzed by the enzyme *3-ketoacyl synthase*. During this step, the malonyl group undergoes **decarboxylation** (loss of $CO_2$). The energy released from this decarboxylation drives the endergonic reaction forward, allowing the two carbons of the malonyl group to attach to the growing acyl chain. Thus, while $CO_2$ is added to Acetyl-CoA to form Malonyl-CoA (via Acetyl-CoA Carboxylase), it is **lost** during the condensation step. **2. Why other options are incorrect:** * **Reduction:** There are two reduction steps in each cycle of fatty acid synthesis. These use **NADPH** as a reducing agent but do not involve $CO_2$ release. * **Dehydration:** This involves the removal of a water molecule ($H_2O$) to create a double bond (forming trans-2-enoyl-ACP); it does not involve $CO_2$. * **Hydration:** This is the reverse process (adding water), which occurs in $\beta$-oxidation (fatty acid breakdown), not synthesis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Acetyl-CoA Carboxylase (requires **Biotin**). * **Reductant source:** **NADPH** is essential, primarily supplied by the **HMP Shunt** (Pentose Phosphate Pathway). * **The "Primer":** Acetyl-CoA acts as the initial primer, but all subsequent 2-carbon units are added via **Malonyl-CoA**. * **End product:** The primary end product of the FAS complex is **Palmitate (16C)**. * **Inhibitor:** Long-chain Acyl-CoA (feedback inhibition); **Activator:** Citrate (shuttles Acetyl-CoA from mitochondria to cytosol).

Q: A patient presents with high LDL and increased total cholesterol, but normal levels of LDL receptors. What is the most probable cause?

Apo B100 mutation. **Explanation:** The correct answer is **Apo B100 mutation**. This condition is clinically known as **Familial Defective Apolipoprotein B100**. **1. Why Apo B100 mutation is correct:** Low-Density Lipoprotein (LDL) particles are cleared from the blood when the **LDL receptor (LDLR)** on the liver recognizes and binds to **Apolipoprotein B100** on the LDL surface. In this scenario, the LDL receptors are normal in number and function, but the "ligand" (Apo B100) is mutated. This structural defect prevents LDL from binding to its receptor, leading to decreased clearance and elevated plasma LDL and total cholesterol levels. This mimics the clinical presentation of Type IIa Hyperlipoproteinemia (Familial Hypercholesterolemia) but with normal receptors. **2. Why other options are incorrect:** * **Apo E defect:** Apo E is essential for the clearance of chylomicron remnants and VLDL remnants (IDL). A defect leads to **Type III Hyperlipoproteinemia** (Dysbetalipoproteinemia), characterized by elevated IDL and cholesterol/triglycerides, not isolated high LDL. * **LCAT Deficiency:** Lecithin-cholesterol acyltransferase (LCAT) is required for cholesterol esterification in HDL. Deficiency leads to **Fish-eye disease**, characterized by low HDL and corneal opacities, not high LDL. * **Lipoprotein Lipase (LPL) Deficiency:** LPL breaks down triglycerides in chylomicrons and VLDL. Deficiency causes **Type I Hyperlipoproteinemia**, leading to severe hypertriglyceridemia and eruptive xanthomas, rather than isolated hypercholesterolemia. **High-Yield NEET-PG Pearls:** * **Apo B100** is the primary structural protein of VLDL, IDL, and LDL. * **Apo B48** (found in chylomicrons) lacks the LDL-receptor binding domain found in Apo B100. * **PCSK9 inhibitors** are a modern drug class that increases LDL receptor density by preventing their degradation, used when LDL remains high despite statin therapy.

Q: Which of the following is not a steroid?

Leukotrienes. ### Explanation **Why Leukotrienes are the correct answer:** Steroids are derivatives of **Cyclopentanoperhydrophenanthrene (CPPP)**, also known as the sterane nucleus. **Leukotrienes**, however, are **Eicosanoids**. They are derived from Arachidonic acid (a 20-carbon polyunsaturated fatty acid) via the Lipoxygenase (LOX) pathway. Unlike steroids, which have a four-ring fused structure, leukotrienes are linear (aliphatic) molecules and do not contain the steroid nucleus. **Analysis of Incorrect Options:** * **Estrogen:** This is a steroid hormone synthesized from cholesterol. It contains the characteristic tetracyclic steroid nucleus. * **Cholic acid:** This is a primary **bile acid**. Bile acids are end-products of cholesterol metabolism in the liver and retain the steroid nucleus (specifically the cholane structure). * **Vitamin D:** Often called a "secosteroid," Vitamin D is derived from 7-dehydrocholesterol. Although one of its rings is broken (B-ring cleavage by UV light), it is chemically classified as a steroid derivative. **High-Yield NEET-PG Pearls:** * **The Parent Molecule:** All human steroids are synthesized from **Cholesterol**. * **Leukotriene Clinical Link:** Leukotriene B4 (LTB4) is a potent chemotactic agent for neutrophils ("**B**4 attracts **B**acteria-fighters"). * **Pharmacology Tip:** Corticosteroids inhibit Phospholipase A2, thereby blocking the production of *both* Prostaglandins and Leukotrienes. In contrast, NSAIDs only block the Cyclooxygenase (COX) pathway. * **Steroid Nucleus:** Remember the name **CPPP** (Cyclopentanoperhydrophenanthrene); it is a frequent examiner favorite.

Q: Abetalipoproteinemia is due to deficiency of which protein?

Mitochondrial Triglyceride Transfer Protein. **Explanation:** **Abetalipoproteinemia** (Bassen-Kornzweig syndrome) is an autosomal recessive disorder caused by a mutation in the gene encoding **Mitochondrial Triglyceride Transfer Protein (MTP)**. MTP is essential for the assembly and secretion of ApoB-containing lipoproteins. It functions by loading lipids (triglycerides) onto **ApoB-48** in the enterocytes and **ApoB-100** in the hepatocytes. In its absence, Chylomicrons, VLDL, and LDL cannot be formed or secreted into the plasma, leading to near-zero levels of these lipoproteins. **Analysis of Incorrect Options:** * **Option A (LCAT):** Deficiency leads to Fish Eye Disease or Norum disease. LCAT is responsible for esterifying cholesterol within HDL. * **Option B (ABC-1):** Deficiency causes **Tangier Disease**, characterized by the inability to clear cholesterol from peripheral cells, leading to extremely low HDL levels and orange tonsils. * **Option D (ApoCII):** Deficiency causes **Type I Hyperlipoproteinemia** (Familial Chylomicronemia Syndrome) because ApoCII is a required cofactor for Lipoprotein Lipase (LPL). **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Malabsorption of fats, steatorrhea, and failure to thrive in infancy. * **Vitamin Deficiencies:** Severe deficiency of fat-soluble vitamins (A, D, E, K), particularly **Vitamin E**, leading to spinocerebellar degeneration and retinitis pigmentosa. * **Hematology:** Presence of **Acanthocytes** (spur cells) on peripheral blood smear due to altered RBC membrane lipid composition. * **Biopsy:** Intestinal biopsy shows lipid-laden enterocytes (vacuoles) after a fatty meal.

Q: Leukotrienes are secreted by all of the following cells except?

Platelets. **Explanation** Leukotrienes (LTs) are potent inflammatory mediators derived from arachidonic acid via the **5-lipoxygenase (5-LOX) pathway**. The synthesis of leukotrienes is primarily restricted to specific inflammatory cells that express the 5-LOX enzyme. **Why Platelets is the Correct Answer:** Platelets lack the enzyme **5-lipoxygenase** and therefore cannot synthesize leukotrienes independently. Instead, platelets possess the enzyme **12-lipoxygenase**, which produces 12-HETE. While platelets can participate in "transcellular biosynthesis" (taking up LTA4 from neutrophils to produce Lipoxins), they do not secrete leukotrienes themselves. **Analysis of Other Options:** * **Macrophages:** These are major sources of LTB4 and LTC4, which play critical roles in chemotaxis and vascular permeability during chronic inflammation. * **T4 (Helper) and T8 (Cytotoxic) Cells:** Lymphocytes, including both CD4+ and CD8+ subsets, express the 5-LOX pathway and secrete leukotrienes to modulate immune responses and leukocyte trafficking. **High-Yield Clinical Pearls for NEET-PG:** * **LTA4:** The precursor for all other leukotrienes. * **LTB4:** A potent **chemotactic agent** for neutrophils ("B for Bacteria/Binding"). * **LTC4, LTD4, LTE4:** Known as the **Slow-Reacting Substance of Anaphylaxis (SRS-A)**; they cause intense bronchoconstriction and are central to the pathogenesis of asthma. * **Zileuton:** A 5-LOX inhibitor used in asthma. * **Montelukast/Zafirlukast:** Cysteinyl leukotriene receptor (CysLT1) antagonists.

Q: Esters of fat-soluble vitamins are digested by:

Cholesterol esterase. ### Explanation **Correct Option: B. Cholesterol esterase** **Concept:** Fat-soluble vitamins (A, D, E, and K) are often consumed as esters (e.g., retinyl esters). To be absorbed by the intestinal mucosa, these must be hydrolyzed into free vitamins and fatty acids. **Cholesterol esterase** (also known as **Nonspecific Lipid Esterase** or Bile Salt-Activated Lipase) is the primary enzyme responsible for this process. It has broad specificity and hydrolyzes not only cholesterol esters but also esters of fat-soluble vitamins, monoglycerides, and phospholipids. **Analysis of Incorrect Options:** * **A. Pancreatic lipase:** This is the primary enzyme for the digestion of dietary **triacylglycerols (TAGs)**. It specifically targets the 1 and 3 positions of the glycerol backbone but does not efficiently hydrolyze vitamin esters. * **C. Colipase:** This is a protein co-factor secreted by the pancreas. It does not have enzymatic activity itself; rather, it displaces bile salts from the oil-water interface to allow pancreatic lipase to bind to its substrate. * **D. Carboxypeptidase:** This is a **proteolytic enzyme** (exopeptidase) involved in protein digestion, specifically cleaving amino acids from the C-terminal end of peptides. It has no role in lipid or vitamin metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Absorption Requirement:** Fat-soluble vitamin absorption is entirely dependent on **micelle formation** and adequate bile salt concentration. * **Steatorrhea:** Conditions causing fat malabsorption (e.g., Chronic Pancreatitis, Celiac disease) lead to secondary deficiencies of fat-soluble vitamins. * **Vitamin A Storage:** Once absorbed, Vitamin A is re-esterified and transported in chylomicrons to the liver, where it is stored in **Ito cells** (Stellate cells) as retinyl palmitate.

Q: Which among the following is not a saturated fatty acid?

Linoleic acid. ### Explanation The correct answer is **Linoleic acid**. **1. Why Linoleic acid is the correct answer:** Fatty acids are classified based on the presence of double bonds. **Saturated fatty acids (SFAs)** have no double bonds, while **Unsaturated fatty acids** contain one or more double bonds. Linoleic acid is a **Polyunsaturated Fatty Acid (PUFA)** containing 18 carbon atoms and **two double bonds** (18:2; Δ9,12). It is an essential fatty acid belonging to the Omega-6 (ω-6) family, meaning the body cannot synthesize it and it must be obtained through the diet. **2. Why the other options are incorrect:** * **Myristic acid (A):** A saturated fatty acid with 14 carbon atoms (14:0). * **Palmitic acid (C):** The most common saturated fatty acid in the human body, containing 16 carbon atoms (16:0). It is the primary product of the Fatty Acid Synthase (FAS) complex. * **Stearic acid (B):** A saturated fatty acid with 18 carbon atoms (18:0). **3. High-Yield Clinical Pearls for NEET-PG:** * **Essential Fatty Acids:** There are two—Linoleic acid (ω-6) and Linolenic acid (ω-3). Arachidonic acid becomes essential only if Linoleic acid is deficient. * **Mnemonic for Saturated FAs:** "**L**ittle **M**ice **P**lay **S**tealthily" (**L**auric 12C, **M**yristic 14C, **P**almitic 16C, **S**tearic 18C). * **Prostaglandin Precursor:** Arachidonic acid (20:4; Δ5,8,11,14) is the direct precursor for the synthesis of eicosanoids (prostaglandins, thromboxanes, and leukotrienes). * **Refsum Disease:** A metabolic disorder caused by a deficiency in the α-oxidation of **Phytanic acid** (a branched-chain fatty acid).

Question 1

Pancreatic secretions are a rich source of which enzyme?

Accepted Answer

Phospholipase A2

Answer

Phospholipase A1

Answer

Phospholipase C

Answer

Phospholipase D

Question 2

Which of the following is an essential fatty acid?

Accepted Answer

Linoleic acid

Answer

Palmitoleic acid

Answer

Oleic acid

Answer

Arachidonic acid

Question 3

In fatty acid synthesis, CO2 loss occurs in which step?

Accepted Answer

Condensation reaction

Answer

Hydration

Answer

Dehydration

Answer

Reduction

Question 4

A patient presents with high LDL and increased total cholesterol, but normal levels of LDL receptors. What is the most probable cause?

Accepted Answer

Apo B100 mutation

Answer

Apo E defect

Answer

LCAT Deficiency

Answer

Lipoprotein lipase Deficiency

Question 5

Which of the following is not a steroid?

Accepted Answer

Leukotrienes

Answer

Estrogen

Answer

Cholic acid

Answer

Vitamin D

Question 6

Abetalipoproteinemia is due to deficiency of which protein?

Accepted Answer

Mitochondrial Triglyceride Transfer Protein

Answer

Lecithin Cholesterol Acyl Transferase

Answer

ATP Binding Cassette Transporter-1

Answer

ApoCII

Question 7

To be defined as a ganglioside, a lipid substance isolated from nervous tissue must contain which of the following in its structure?

Accepted Answer

N-Acetylneuraminic acid (NANA), hexoses, sphingosine, long chain fatty acid

Answer

NANA, hexoses, a fatty acid, sphingosine, phosphorylcholine

Answer

NANA, sphingosine, ethanolamine

Answer

NANA, hexoses, fatty acid, glycerol

Question 8

Leukotrienes are secreted by all of the following cells except?

Accepted Answer

Platelets

Answer

Macrophages

Answer

T4 cells

Answer

T8 cells

Question 9

Esters of fat-soluble vitamins are digested by:

Accepted Answer

Cholesterol esterase

Answer

Pancreatic lipase

Answer

Colipase

Answer

Carboxypeptidase

Question 10

Which among the following is not a saturated fatty acid?

Accepted Answer

Linoleic acid

Answer

Myristic acid

Answer

Stearic acid

Answer

Palmitic acid

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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