Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 71: What is the mechanism for the transport of fatty acids across the mitochondrial membrane?

A. Active transport
B. Facilitated transport (Correct Answer)
C. Diffusion
D. Lipases

Explanation: **Explanation:** The transport of long-chain fatty acids (LCFA) into the mitochondria is the rate-limiting step of **$\beta$-oxidation**. While short and medium-chain fatty acids can cross the mitochondrial membranes directly, LCFAs require a specialized shuttle system known as the **Carnitine Shuttle**. **Why Facilitated Transport is Correct:** The Carnitine Shuttle utilizes specific carrier proteins to move fatty acyl-CoA molecules across the inner mitochondrial membrane. This process involves **Carnitine Palmitoyltransferase I (CPT-I)** on the outer membrane and **CPT-II** on the inner membrane, along with a **Carnitine-acylcarnitine translocase**. Because this movement depends on specific membrane-bound transporters to move molecules across a biological membrane, it is classified as **facilitated transport**. **Analysis of Incorrect Options:** * **Active Transport:** This requires direct ATP hydrolysis to move solutes against a concentration gradient. The carnitine shuttle is a facilitated exchange mechanism, not a primary active pump. * **Diffusion:** LCFAs are too large and polar (when activated to Acyl-CoA) to freely diffuse through the impermeable inner mitochondrial membrane. * **Lipases:** These are enzymes (like Hormone-Sensitive Lipase) that hydrolyze triglycerides into free fatty acids and glycerol; they are not transport mechanisms. **High-Yield Clinical Pearls for NEET-PG:** * **Inhibitor:** **Malonyl-CoA** (the first intermediate of fatty acid synthesis) potently inhibits **CPT-I**, preventing a futile cycle where synthesis and oxidation occur simultaneously. * **Carnitine Deficiency:** Presents with non-ketotic hypoglycemia, muscle weakness, and cardiomyopathy during fasting. * **Location:** CPT-I is located in the outer mitochondrial membrane, while CPT-II is in the inner mitochondrial membrane.

Question 72: How do long-chain fatty acids enter the mitochondria?

A. Through pores in the inner mitochondrial membrane
B. Through pores in the outer mitochondrial membrane
C. By combining with carnitine (Correct Answer)
D. By combining with ornithine

Explanation: **Explanation:** The entry of long-chain fatty acids (LCFAs) into the mitochondria is the rate-limiting step of **beta-oxidation**. While short-chain and medium-chain fatty acids can diffuse freely, LCFAs cannot cross the impermeable inner mitochondrial membrane (IMM) on their own. **Why Option C is correct:** To cross the IMM, LCFAs must undergo the **Carnitine Shuttle**. First, they are activated to Fatty Acyl-CoA in the cytosol. The enzyme **Carnitine Palmitoyltransferase-I (CPT-I)** then replaces the CoA group with **carnitine**, forming Acyl-carnitine. This complex is transported across the IMM by a translocase. Once inside the matrix, **CPT-II** converts it back into Fatty Acyl-CoA and free carnitine. **Why other options are incorrect:** * **Options A & B:** The outer mitochondrial membrane contains porins (VDAC) that allow many molecules to pass, but the **inner membrane** is highly selective and lacks pores for fatty acids. Transport requires specific protein carriers. * **Option D:** **Ornithine** is an amino acid involved in the **Urea Cycle**, not lipid metabolism. It helps transport nitrogenous waste, not fatty acids. **High-Yield Clinical Pearls for NEET-PG:** * **Inhibitor:** **Malonyl-CoA** (the first intermediate of fatty acid synthesis) potently inhibits CPT-I. This prevents a "futile cycle" by ensuring synthesis and breakdown do not occur simultaneously. * **Systemic Carnitine Deficiency:** Presents with non-ketotic hypoglycemia, muscle weakness, and liver dysfunction because the body cannot utilize LCFAs for energy during fasting. * **Location:** CPT-I is located on the outer mitochondrial membrane; CPT-II is on the inner mitochondrial membrane.

Question 73: Which apolipoprotein is associated with Alzheimer's disease?

A. APOE1
B. APOE2
C. APOE3
D. APOE4 (Correct Answer)

Explanation: **Explanation:** The correct answer is **APOE4**. Apolipoprotein E (ApoE) is a critical glycoprotein involved in the transport of lipids and cholesterol within the central nervous system. In humans, the APOE gene exists in three common alleles: ε2, ε3, and ε4. **Why APOE4 is correct:** The **APOE-ε4** allele is the strongest genetic risk factor for late-onset **Alzheimer’s Disease (AD)**. Mechanistically, ApoE4 is less efficient than other isoforms at clearing amyloid-beta (Aβ) peptides from the brain. This leads to increased Aβ aggregation and the formation of senile plaques, a hallmark of AD pathology. Carrying one copy of ε4 increases risk 3-fold, while two copies increase risk up to 12-fold. **Analysis of Incorrect Options:** * **APOE1:** This is not a common functional isoform in the human population and is not clinically relevant to Alzheimer’s risk. * **APOE2:** This isoform is actually considered **protective** against Alzheimer’s disease. However, it is associated with an increased risk of Type III Hyperlipoproteinemia (Dysbetalipoproteinemia). * **APOE3:** This is the most common (wild-type) isoform in the general population. It is considered "neutral" regarding Alzheimer’s risk. **High-Yield Clinical Pearls for NEET-PG:** * **ApoE Function:** It serves as a ligand for the LDL receptor, facilitating the uptake of chylomicron remnants and VLDL. * **Type III Hyperlipoproteinemia:** Associated with **ApoE2** homozygosity, resulting in elevated cholesterol and triglycerides due to poor clearance of IDL and remnants. * **Alzheimer’s Pathology:** Remember the "Amyloid Cascade Hypothesis"—ApoE4 promotes amyloid deposition and neurofibrillary tangles (Tau protein). * **Protective Factor:** APOE2 is the "good" allele for the brain; APOE4 is the "bad" allele.

Question 74: What is the primary enzymatic conversion performed by aromatase?

A. Estrogen to androgen
B. Androgen to estrogen (Correct Answer)
C. Progesterone to estrogen
D. Androgen to progesterone

Explanation: **Explanation:** **Aromatase** (also known as estrogen synthase or CYP19A1) is a member of the Cytochrome P450 superfamily. Its primary function is the conversion of **androgens to estrogens**. Specifically, it catalyzes the aromatization of the 'A' ring of the steroid nucleus, converting **androstenedione to estrone** and **testosterone to estradiol**. This process involves three successive hydroxylation steps requiring NADPH and molecular oxygen. **Analysis of Options:** * **Option B (Correct):** Aromatase is the rate-limiting enzyme that transforms C19 steroids (androgens) into C18 steroids (estrogens) by removing a methyl group and creating an aromatic ring. * **Option A:** This is the reverse process. Androgens are precursors to estrogens; there is no direct enzymatic pathway for aromatase to convert estrogens back into androgens. * **Option C:** Progesterone is a C21 steroid. It must first be converted to androgens (via 17α-hydroxylase and 17,20-lyase) before it can be converted to estrogens. * **Option D:** Androgens are downstream of progesterone in the steroidogenic pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Localization:** Aromatase is highly expressed in the granulosa cells of the ovaries, adipose tissue, placenta, and the brain. * **Clinical Application:** **Aromatase Inhibitors** (e.g., Letrozole, Anastrozole, Exemestane) are first-line treatments for hormone-receptor-positive breast cancer in postmenopausal women. * **Aromatase Deficiency:** A rare condition leading to virilization of the female fetus and maternal virilization during pregnancy (due to the inability of the placenta to convert fetal androgens to estrogens). * **PCOS Connection:** In Polycystic Ovary Syndrome, there is often an altered LH/FSH ratio leading to relative aromatase inactivity in granulosa cells, contributing to hyperandrogenism.

Question 75: Which of the following is a primary ketone body that is formed from leucine, lysine, phenylalanine, and tyrosine?

A. Acetoacetate (Correct Answer)
B. Acetone
C. Beta-hydroxybutyrate
D. All of the above

Explanation: **Explanation:** The synthesis of ketone bodies (ketogenesis) occurs primarily in the liver mitochondria. The process begins with the condensation of acetyl-CoA molecules to form **Acetoacetate**, which is the **first and primary ketone body** produced. **Why Acetoacetate is correct:** Ketogenic amino acids (Leucine and Lysine) and partially ketogenic amino acids (Phenylalanine and Tyrosine) are catabolized into either Acetyl-CoA or Acetoacetyl-CoA. These precursors enter the HMG-CoA pathway to form Acetoacetate. Because Acetoacetate is the parent molecule from which other ketone bodies are derived, it is considered the primary ketone body formed during the breakdown of these amino acids. **Why other options are incorrect:** * **B. Acetone:** This is a secondary ketone body formed by the non-enzymatic, spontaneous decarboxylation of acetoacetate. It is a waste product excreted via the lungs (causing "fruity breath"). * **C. Beta-hydroxybutyrate:** This is formed from the reduction of acetoacetate by the enzyme *β-hydroxybutyrate dehydrogenase*. While it is the predominant ketone body in the blood during ketosis, it is a secondary derivative, not the initial product. * **D. All of the above:** While all three are ketone bodies, only acetoacetate is the direct primary product formed from the ketogenic skeletons of the listed amino acids. **High-Yield NEET-PG Pearls:** * **Purely Ketogenic Amino Acids:** Leucine and Lysine (The "L"s). * **Rate-limiting enzyme of Ketogenesis:** HMG-CoA Synthase (Mitochondrial). * **Site of Utilization:** Ketone bodies are used by extrahepatic tissues (brain, heart, muscle) but **not by the liver**, because the liver lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). * **Detection:** The Rothera’s test detects Acetoacetate and Acetone, but **not** Beta-hydroxybutyrate.

Question 76: Which of the following is a glycosphingolipid?

A. Lecithin
B. Cardiolipin
C. Plasmalogens
D. Sphingomyelin (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Sphingomyelin** **Concept:** Lipids are classified based on their backbone. **Sphingolipids** contain a complex amino alcohol called **sphingosine** instead of glycerol. Sphingomyelin is a unique phospholipid because it is the only sphingolipid that contains **phosphate** (as phosphorylcholine) but no sugar. While the question asks for a glycosphingolipid, in many standard medical classifications and exam contexts, Sphingomyelin is grouped under the broader category of sphingolipids due to its sphingosine backbone, distinguishing it from glycerol-based lipids. *Note: Strictly speaking, glycosphingolipids (like Cerebrosides and Gangliosides) contain carbohydrates. However, among the given options, Sphingomyelin is the only one belonging to the Sphingolipid family.* **Analysis of Incorrect Options:** * **A. Lecithin (Phosphatidylcholine):** This is a **glycerophospholipid**. It consists of a glycerol backbone, two fatty acids, and a choline group attached to a phosphate. * **B. Cardiolipin (Diphosphatidylglycerol):** Found exclusively in the inner mitochondrial membrane. It consists of two molecules of phosphatidic acid connected by a glycerol bridge. * **C. Plasmalogens:** These are specialized ether lipids where the fatty acid at the C1 position of glycerol is attached via an **ether linkage** (alkenyl group) rather than an ester bond. **High-Yield NEET-PG Pearls:** * **Niemann-Pick Disease:** Caused by a deficiency of **Sphingomyelinase**, leading to the accumulation of sphingomyelin in the liver, spleen, and brain (look for "Cherry Red Spot" on the macula). * **Surfactant:** Lecithin (Dipalmitoylphosphatidylcholine) is the major component of lung surfactant; an L/S ratio > 2 indicates fetal lung maturity. * **Barth Syndrome:** A rare X-linked disorder associated with abnormal cardiolipin metabolism, leading to cardiomyopathy.

Question 77: What is the best predictor for coronary artery disease?

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

Explanation: **Explanation:** **Why LDL is the correct answer:** Low-Density Lipoprotein (LDL) is the primary carrier of cholesterol from the liver to peripheral tissues. It is considered the most significant independent risk factor and the **best predictor** for Coronary Artery Disease (CAD) because it is highly atherogenic. When LDL levels are elevated, they infiltrate the arterial intima, undergo oxidation, and are engulfed by macrophages to form "foam cells," the hallmark of atherosclerotic plaques. Clinical management of CAD primarily targets the lowering of LDL-C levels. **Analysis of Incorrect Options:** * **HDL (High-Density Lipoprotein):** Known as "good cholesterol," HDL facilitates reverse cholesterol transport (carrying cholesterol back to the liver). While low HDL is a risk factor, high levels are actually **cardioprotective**. * **VLDL (Very Low-Density Lipoprotein):** VLDL primarily transports endogenous triglycerides. While elevated VLDL contributes to metabolic syndrome, it is a precursor to LDL and not as direct a predictor of CAD as LDL itself. * **Chylomicrons:** These transport dietary (exogenous) triglycerides from the intestines. They are the least dense lipoproteins and are not directly implicated in the pathogenesis of atherosclerosis. **NEET-PG High-Yield Pearls:** * **Friedewald Formula:** LDL = Total Cholesterol – [HDL + (Triglycerides/5)]. (Note: This is invalid if TG >400 mg/dL). * **Apo-B100:** The characteristic apoprotein found in VLDL, IDL, and LDL. It is often cited as an even more precise marker of atherogenic particles than LDL-C. * **Lipoprotein (a):** An independent genetic risk factor for CAD; it is essentially an LDL particle with an attached Apo(a). * **Oxidized LDL:** The specific form of LDL that is most toxic to the endothelium and leads to plaque formation.

Question 78: Which of the following lipids is associated with respiratory distress syndrome?

A. Phosphatidylethanolamine (cephalin)
B. Phosphatidylserine
C. Dipalmitoyl lecithin (Correct Answer)
D. Phosphatidylinositol

Explanation: ### Explanation **Correct Option: C. Dipalmitoyl lecithin** **Mechanism and Clinical Correlation:** Respiratory Distress Syndrome (RDS), specifically Neonatal RDS (Hyaline Membrane Disease), is caused by a deficiency of **pulmonary surfactant**. Surfactant is a lipoprotein complex synthesized by **Type II pneumocytes**. Its primary function is to reduce surface tension at the alveolar air-liquid interface, preventing alveolar collapse (atelectasis) during expiration. The major lipid component of surfactant (comprising about 80%) is phospholipids, and the most abundant and active constituent is **Dipalmitoylphosphatidylcholine (DPPC)**, also known as **Dipalmitoyl lecithin**. It acts as a detergent; its amphipathic nature allows it to spread across the alveoli, lowering the pressure required to keep them inflated. **Analysis of Incorrect Options:** * **A. Phosphatidylethanolamine (Cephalin):** Found primarily in nervous tissue and cell membranes; it is not a major component of lung surfactant. * **B. Phosphatidylserine:** Important for cell signaling and apoptosis (it flips to the outer leaflet to signal "eat me" to macrophages), but plays no role in surface tension reduction. * **D. Phosphatidylinositol:** A precursor for second messengers like $IP_3$ and $DAG$; while present in small amounts in surfactant, it is not the primary functional lipid associated with RDS. **High-Yield NEET-PG Pearls:** * **L/S Ratio:** Fetal lung maturity is assessed via the **Lecithin/Sphingomyelin ratio** in amniotic fluid. A ratio **> 2.0** indicates mature lungs. * **Glucocorticoids:** Given to mothers in preterm labor (e.g., Betamethasone) to stimulate surfactant production by inducing fibroblast pneumocyte factor. * **Surfactant Proteins:** SP-A and SP-D are involved in innate immunity, while **SP-B and SP-C** are crucial for the mechanical spreading of the surfactant film. Deficiency of SP-B is a rare genetic cause of RDS.

Question 79: Which of the following is the richest source of Linoleic acid?

A. Safflower oil (Correct Answer)
B. Sunflower oil
C. Olive oil
D. Groundnut oil

Explanation: **Explanation:** **Linoleic acid (18:2; n-6)** is an essential polyunsaturated fatty acid (PUFA) that the human body cannot synthesize. It serves as the precursor for Arachidonic acid, which is vital for prostaglandin and leukotriene synthesis. **Why Safflower Oil is correct:** Safflower oil is clinically recognized as the richest dietary source of Linoleic acid, containing approximately **75-78%**. In medical nutrition, it is often the gold standard for treating or preventing Essential Fatty Acid Deficiency (EFAD). **Analysis of Incorrect Options:** * **Sunflower oil:** While a very good source of PUFAs, it typically contains about **60-65%** Linoleic acid, ranking lower than Safflower oil. * **Olive oil:** This is primarily a source of **Oleic acid** (a Monounsaturated Fatty Acid - MUFA), containing only about 10% Linoleic acid. It is the hallmark of the heart-healthy Mediterranean diet. * **Groundnut (Peanut) oil:** Contains a mix of MUFAs and PUFAs, with Linoleic acid content averaging around **20-30%**. **High-Yield Clinical Pearls for NEET-PG:** * **Essential Fatty Acids (EFAs):** Only two are strictly essential—Linoleic acid (ω-6) and α-Linolenic acid (ω-3). * **Deficiency Symptoms:** Clinical signs of EFAD include **Phrynoderma** (toad skin/follicular hyperkeratosis), poor wound healing, and alopecia. * **P/S Ratio:** Safflower oil has a high Polyunsaturated to Saturated fat ratio, making it effective in lowering serum cholesterol by increasing LDL receptor activity. * **Hierarchy of Linoleic Acid Content:** Safflower > Corn > Sunflower > Soya bean > Groundnut > Palm oil > Coconut oil.

Question 80: Ketone bodies are synthesized in which part of the body?

A. Brain
B. Liver (Correct Answer)
C. Heart
D. Intestine

Explanation: **Explanation:** **1. Why the Liver is Correct:** Ketone bodies (Acetoacetate, 3-hydroxybutyrate, and Acetone) are synthesized primarily in the **mitochondria of liver hepatocytes**. This process, known as ketogenesis, occurs when there is an excess of Acetyl-CoA (derived from fatty acid β-oxidation) that cannot enter the TCA cycle due to a depletion of oxaloacetate (e.g., during starvation or uncontrolled diabetes). The rate-limiting enzyme for this process is **HMG-CoA synthase**, which is highly expressed in the liver. **2. Why Other Options are Incorrect:** * **Brain & Heart (Options A & C):** These organs are major **consumers** of ketone bodies, not producers. While the brain normally relies on glucose, it adapts to use ketones during prolonged fasting. The heart prefers fatty acids but can readily oxidize ketones. They lack the enzymatic machinery for synthesis. * **Intestine (Option D):** While the intestinal mucosa can produce small amounts of ketones during development or specific dietary states, it is not a primary site of synthesis and is clinically insignificant compared to the liver. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Liver Paradox":** Although the liver synthesizes ketone bodies, it **cannot utilize them** because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). * **Ketone Body Ratio:** The ratio of 3-hydroxybutyrate to acetoacetate depends on the NADH/NAD+ ratio in the mitochondria. * **Detection:** The **Rothera’s Test** detects Acetoacetate and Acetone, but **not** 3-hydroxybutyrate. * **Key Enzyme:** HMG-CoA **Synthase** is for Ketogenesis; HMG-CoA **Reductase** is for Cholesterol synthesis.

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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