Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 121: Which of the following is an essential fatty acid?

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

Explanation: **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 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. Linoleic acid serves as a precursor for Arachidonic acid, which is vital for prostaglandin and leukotriene synthesis. **2. Analysis of Incorrect Options:** * **A. Oleic acid (18:1; $\omega$-9):** This is a monounsaturated fatty acid. Humans can synthesize it from stearic acid using the $\Delta^9$-desaturase enzyme; hence, it is non-essential. * **B. Stearic acid (18:0):** This is a long-chain saturated fatty acid found in animal fats. It is synthesized in the body via the fatty acid synthase complex. * **C. Cervonic acid (22:6; $\omega$-3):** Also known as **Docosahexaenoic acid (DHA)**. While it is physiologically critical (especially for the retina and brain), it is technically considered "semi-essential" because it can be synthesized from $\alpha$-Linolenic acid, though the conversion efficiency is low. **3. High-Yield Clinical Pearls for NEET-PG:** * **Phrynoderma (Toad Skin):** A clinical deficiency of EFAs characterized by follicular hyperkeratosis on the extensor surfaces of extremities. * **Arachidonic Acid:** Becomes "essential" only if Linoleic acid is deficient in the diet. * **Eicosanoids:** EFAs are the parent compounds for all eicosanoids (prostaglandins, thromboxanes, and leukotrienes). * **Mnemonic:** Remember **"LL"** for Essential—**L**inoleic and **L**inolenic.

Question 122: The solubility of cholesterol is primarily dependent on which of the following?

A. Water
B. Phosphatidylcholine
C. Bile salts (Correct Answer)
D. Cholesterol itself

Explanation: **Explanation:** Cholesterol is a highly hydrophobic (water-insoluble) lipid. In the gallbladder and bile, its solubility is maintained through the formation of **mixed micelles**. These micelles are composed of **bile salts** and phospholipids (primarily phosphatidylcholine). **Why Bile Salts are the correct answer:** Bile salts are the primary detergent-like molecules that solubilize cholesterol. They are amphipathic, meaning they have both polar and non-polar faces. In the bile, they surround cholesterol molecules, orienting their hydrophobic sides toward the lipid and their hydrophilic sides toward the aqueous environment. While phosphatidylcholine (lecithin) aids this process, the concentration and presence of bile salts are the most critical factors in preventing cholesterol from crystallizing. **Analysis of Incorrect Options:** * **A. Water:** Cholesterol is virtually insoluble in water due to its hydrocarbon structure. * **B. Phosphatidylcholine:** While it increases the capacity of bile salts to solubilize cholesterol, it is a secondary component. Without bile salts, phosphatidylcholine alone cannot effectively maintain cholesterol in a soluble state. * **D. Cholesterol itself:** Increasing the concentration of cholesterol actually decreases its own solubility, leading to supersaturation and precipitation. **Clinical Pearls for NEET-PG:** * **Lithogenic Index:** This refers to the ratio of cholesterol to bile salts and phospholipids. If the concentration of bile salts decreases (e.g., due to ileal resection/malabsorption) or cholesterol increases, the bile becomes "lithogenic." * **Cholelythiasis:** When the solubilizing capacity of bile salts is exceeded, cholesterol precipitates to form **cholesterol gallstones**. * **Rate-limiting step:** The conversion of cholesterol to bile acids is catalyzed by **7-alpha-hydroxylase**, which is inhibited by bile acids (feedback inhibition).

Question 123: In the synthesis of fatty acids, energy is supplied by which coenzyme?

A. NAD
B. NADPH (Correct Answer)
C. FAD
D. GTP

Explanation: **Explanation:** Fatty acid synthesis (Lipogenesis) is a **reductive anabolic process** that occurs in the cytosol. To build a long-chain fatty acid from acetyl-CoA units, high-energy electrons are required for the reduction steps catalyzed by the **Fatty Acid Synthase (FAS)** complex. **1. Why NADPH is correct:** NADPH (Nicotinamide Adenine Dinucleotide Phosphate) serves as the essential electron donor (reducing agent) in two specific steps of each elongation cycle: the reduction of the keto group to an alcohol and the reduction of the double bond to a single bond. In humans, the primary sources of NADPH for lipogenesis are the **Hexose Monophosphate (HMP) Shunt** (via G6PD) and the **Malic Enzyme** reaction. **2. Why other options are incorrect:** * **NAD+ / NADH:** These are primarily involved in **catabolic** pathways (like Glycolysis and the TCA cycle) to transfer electrons to the electron transport chain for ATP production. * **FAD / FADH2:** These act as electron carriers in redox reactions (like Beta-oxidation or the TCA cycle) but do not provide the reducing power for fatty acid assembly. * **GTP:** While GTP provides energy for protein synthesis (translation) and gluconeogenesis (PEPCK reaction), it is not used in the enzymatic steps of fatty acid synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Acetyl-CoA Carboxylase (requires **Biotin**). * **Key Activator:** Citrate (shuttles acetyl-CoA from mitochondria to cytosol). * **Key Inhibitor:** Palmitoyl-CoA (feedback inhibition). * **Location:** Primarily in the liver, lactating mammary glands, and adipose tissue.

Question 124: Which lipoprotein has a scavenging action?

A. HDL (Correct Answer)
B. LDL
C. VLDL
D. None of the above

Explanation: **Explanation:** **HDL (High-Density Lipoprotein)** is known as the "Good Cholesterol" because of its unique **scavenging action**. This process, termed **Reverse Cholesterol Transport (RCT)**, involves HDL picking up excess cholesterol from peripheral tissues and the arterial walls and transporting it back to the liver for excretion in bile. This action prevents lipid accumulation in the sub-endothelial space, making HDL highly anti-atherogenic. **Why other options are incorrect:** * **LDL (Low-Density Lipoprotein):** Often called "Bad Cholesterol," its primary role is to transport cholesterol from the liver to peripheral tissues. High levels lead to cholesterol deposition in arteries, promoting atherosclerosis (the opposite of scavenging). * **VLDL (Very Low-Density Lipoprotein):** Produced by the liver, its main function is to transport endogenous triglycerides to peripheral tissues. It is a precursor to LDL and does not possess scavenging properties. **High-Yield Clinical Pearls for NEET-PG:** * **Apo A-I:** The major apoprotein associated with HDL; it activates the enzyme **LCAT** (Lecithin-Cholesterol Acyltransferase), which is essential for esterifying cholesterol during the scavenging process. * **ABCA1 Transporter:** This protein helps move cholesterol from cells onto HDL; a deficiency leads to **Tangier Disease** (characterized by very low HDL and orange tonsils). * **CETP (Cholesterol Ester Transfer Protein):** Facilitates the exchange of cholesterol esters from HDL for triglycerides from VLDL/LDL. * **Protective Effect:** For every 1 mg/dL increase in HDL, the risk of coronary artery disease decreases by 2–3%.

Question 125: Progesterone is synthesized from which precursor molecule?

A. Pregnenolone (Correct Answer)
B. 17-Hydroxypregnenolone
C. Pregnanediol
D. Pregnanetriol

Explanation: **Explanation:** The synthesis of steroid hormones (steroidogenesis) begins with **Cholesterol**. The rate-limiting step is the conversion of cholesterol to **Pregnenolone** by the enzyme *Desmolase* (CYP11A1) in the mitochondria. **Why Pregnenolone is correct:** Pregnenolone is known as the "mother of all steroid hormones." It is directly converted into **Progesterone** by the enzyme **3β-hydroxysteroid dehydrogenase (3β-HSD)**. This reaction involves the oxidation of the 3-hydroxyl group and the isomerization of the double bond from the B ring to the A ring. **Analysis of Incorrect Options:** * **17-Hydroxypregnenolone:** This is a metabolite of pregnenolone (via 17α-hydroxylase) used in the "delta-5 pathway" to produce dehydroepiandrosterone (DHEA) and cortisol precursors, not progesterone. * **Pregnanediol:** This is the **inactive urinary metabolite** of progesterone. It is used clinically to monitor progesterone levels but is a breakdown product, not a precursor. * **Pregnanetriol:** This is the primary urinary metabolite of **17-hydroxyprogesterone**. Elevated levels are a diagnostic marker for 21-hydroxylase deficiency (Congenital Adrenal Hyperplasia). **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Cholesterol side-chain cleavage enzyme (Desmolase/CYP11A1). * **StAR Protein:** The Steroidogenic Acute Regulatory (StAR) protein is essential for transporting cholesterol into the mitochondria. * **Site of Synthesis:** Progesterone is synthesized in the Corpus Luteum (ovary), Adrenal Cortex, and Placenta. * **Key Enzyme Deficiency:** A deficiency in 3β-HSD leads to a decrease in progesterone, mineralocorticoids, and glucocorticoids, while increasing DHEA.

Question 126: Which one of the following acts as lung surfactant?

A. Sphingomyelin
B. Lysolecithin
C. Dipalmitoyl lecithin (Correct Answer)
D. Cephalin

Explanation: **Explanation:** **Dipalmitoyl lecithin** (also known as Dipalmitoylphosphatidylcholine or DPPC) is the correct answer because it is the major phospholipid component of **lung surfactant** (comprising about 80%). Lung surfactant is a surface-active lipoprotein complex secreted by **Type II alveolar cells**. Its primary physiological role is to reduce surface tension at the air-liquid interface of the alveoli, preventing them from collapsing during expiration (atelectasis) and reducing the work of breathing. **Analysis of Options:** * **Sphingomyelin (A):** This is a membrane phospholipid found in the myelin sheath of nerve fibers. While it is found in amniotic fluid, its levels remain relatively constant, unlike lecithin which rises with lung maturity. * **Lysolecithin (B):** This is a derivative of lecithin formed by the action of Phospholipase A2. It acts as a potent hemolytic agent and is not a component of surfactant. * **Cephalin (D):** Also known as Phosphatidylethanolamine, this is a major structural phospholipid in biological membranes (especially nervous tissue) but does not possess the surface-tension-reducing properties required for lung function. **Clinical Pearls for NEET-PG:** * **L/S Ratio:** The Lecithin-Sphingomyelin ratio in amniotic fluid is a classic marker for fetal lung maturity. A ratio **> 2.0** indicates mature lungs. * **RDS:** Deficiency of surfactant in premature infants leads to **Respiratory Distress Syndrome (RDS)** or Hyaline Membrane Disease. * **Glucocorticoids:** These are administered to mothers in preterm labor to stimulate surfactant production by inducing enzymes in Type II pneumocytes. * **Composition:** DPPC contains two **palmitic acid** residues at the 1st and 2nd positions of glycerol.

Question 127: Which of the following features is ABSENT in the regulation of lipid metabolism by insulin?

A. Reduced activity of HMG CoA synthetase
B. Increased release of fatty acids from stored fat in adipose tissue (Correct Answer)
C. Increased availability of glycerol-3-phosphate
D. Increased Acetyl CoA carboxylase activity

Explanation: **Explanation** Insulin is an **anabolic hormone** that promotes energy storage and inhibits the mobilization of fuels. Its primary role in lipid metabolism is to stimulate lipogenesis and inhibit lipolysis. **Why Option B is the correct answer:** Insulin **inhibits** the enzyme **Hormone-Sensitive Lipase (HSL)** in adipose tissue via dephosphorylation. HSL is responsible for breaking down stored triglycerides into free fatty acids and glycerol. Therefore, insulin leads to a **decreased** release of fatty acids into the circulation. An "increased release" occurs only in insulin-deficient states (like Diabetes Mellitus) or under the influence of counter-regulatory hormones like glucagon and epinephrine. **Analysis of Incorrect Options:** * **Option A:** Insulin promotes cholesterol synthesis by activating HMG CoA Reductase. However, in the context of mitochondrial ketone body synthesis, insulin suppresses **HMG CoA Synthetase**, thereby reducing ketogenesis. * **Option C:** Insulin increases glucose uptake in adipocytes via **GLUT-4**. This glucose undergoes glycolysis to produce **Glycerol-3-Phosphate**, which serves as the backbone for triglyceride synthesis (esterification). * **Option D:** Insulin activates **Acetyl CoA Carboxylase (ACC)**, the rate-limiting enzyme of fatty acid synthesis, by promoting its dephosphorylation and polymerization. **High-Yield Clinical Pearls for NEET-PG:** * **Key Enzyme Inhibition:** Insulin inhibits HSL (lipolysis) but activates **Lipoprotein Lipase (LPL)** in capillary walls to enhance fatty acid uptake from chylomicrons/VLDLs. * **Malonyl CoA:** By activating ACC, insulin increases Malonyl CoA levels, which inhibits **Carnitine Palmitoyltransferase-I (CPT-1)**, effectively shutting down beta-oxidation (fatty acid breakdown). * **Net Effect:** Insulin = ↑ Lipogenesis, ↑ Cholesterol synthesis, ↓ Ketogenesis, ↓ Lipolysis.

Question 128: Hypertriglyceridemia is seen in which of the following conditions?

A. LDL receptor defect
B. Dysbetalipoproteinemia (Correct Answer)
C. Abetalipoproteinemia
D. All of the above

Explanation: **Explanation:** **Dysbetalipoproteinemia (Type III Hyperlipoproteinemia)** is the correct answer because it is characterized by a deficiency in **Apolipoprotein E (Apo E)**. Apo E is essential for the hepatic recognition and clearance of chylomicron remnants and VLDL remnants (IDL). When Apo E is defective, these "remnant" particles accumulate in the plasma. Since these remnants are rich in triglycerides (though they also contain cholesterol), their accumulation leads to significant **hypertriglyceridemia** and hypercholesterolemia. **Analysis of Incorrect Options:** * **LDL Receptor Defect (Option A):** This defines **Familial Hypercholesterolemia (Type IIa)**. The primary defect is the inability to clear LDL from the blood. Since LDL is almost exclusively composed of cholesterol, this condition presents with isolated hypercholesterolemia; triglyceride levels are typically normal. * **Abetalipoproteinemia (Option C):** This is a rare genetic disorder caused by a defect in the **Microsomal Triglyceride Transfer Protein (MTP)**, leading to an inability to synthesize Apolipoprotein B-48 and B-100. Consequently, chylomicrons and VLDL cannot be formed. This results in **hypolipidemia** (extremely low levels of triglycerides and cholesterol), not hypertriglyceridemia. **High-Yield Clinical Pearls for NEET-PG:** * **Dysbetalipoproteinemia (Type III):** Look for the presence of **Palmar Xanthomas** (pathognomonic) and a "broad beta band" on electrophoresis. * **Abetalipoproteinemia:** Look for clinical features like malabsorption, steatorrhea, **Acanthocytosis** (thorny red blood cells), and retinitis pigmentosa. * **Friedewald Equation:** Remember that LDL = Total Cholesterol – HDL – (TG/5). This formula is invalid if TG >400 mg/dL.

Question 129: Reducing equivalents in fatty acid synthesis are supplied by?

A. NADH + H+
B. NADPH + H+ (Correct Answer)
C. FADH2
D. QH2

Explanation: **Explanation:** Fatty acid synthesis (Lipogenesis) is a **reductive anabolic process** that occurs in the cytosol. To build a long-chain fatty acid like palmitate, the enzyme Fatty Acid Synthase (FAS) requires a source of electrons to reduce keto groups to hydroxy groups and double bonds to single bonds. **1. Why NADPH + H⁺ is correct:** NADPH is the essential reducing equivalent for biosynthetic (anabolic) pathways. In fatty acid synthesis, **two molecules of NADPH** are consumed for every two-carbon unit added to the chain. The primary sources of this NADPH are: * **Hexose Monophosphate (HMP) Shunt:** The oxidative phase (via G6PD) is the major contributor. * **Malic Enzyme:** Converts malate to pyruvate, generating NADPH. * **Cytosolic Isocitrate Dehydrogenase.** **2. Why other options are incorrect:** * **NADH + H⁺:** Primarily used in **catabolic** pathways (like Glycolysis or TCA cycle) to carry electrons to the Electron Transport Chain for ATP production. * **FADH₂:** Used as a prosthetic group in redox reactions (e.g., Succinate dehydrogenase) and is a product of **Beta-oxidation** (the breakdown of fats), not synthesis. * **QH₂ (Ubiquinol):** A component of the mitochondrial respiratory chain involved in electron transfer between Complex I/II and Complex III. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Acetyl-CoA Carboxylase (requires Biotin). * **Location:** Occurs in the **Cytosol** ("Synthesis in the Cytosol"). * **The "Citrate Shuttle":** Acetyl-CoA leaves the mitochondria in the form of Citrate to participate in lipogenesis. * **Key Inhibitor:** Glucagon and Epinephrine (via phosphorylation); **Activator:** Insulin and Citrate.

Question 130: Fatty acids are the main source of energy for which of the following organs?

A. Muscles
B. Heart (Correct Answer)
C. Liver
D. Red Blood Cells

Explanation: **Explanation:** The **heart** is a metabolic omnivore but relies predominantly on **fatty acid oxidation (FAO)** for its continuous energy needs. Approximately **60-80%** of the ATP required for cardiac contraction is derived from the oxidation of long-chain fatty acids. This is because fatty acids yield the highest amount of ATP per gram, providing a steady, high-density energy source necessary for an organ that never rests. **Analysis of Options:** * **Muscles (Option A):** While skeletal muscles use fatty acids during rest and low-intensity exercise, they switch to glucose (glycolysis) during high-intensity anaerobic activity. Unlike the heart, their fuel source is variable. * **Liver (Option B):** The liver is the primary site for fatty acid synthesis and ketogenesis. While it oxidizes fatty acids for its own energy, its metabolic profile is diverse, involving significant carbohydrate and amino acid metabolism. * **Red Blood Cells (Option D):** RBCs lack **mitochondria**. Since fatty acid oxidation (beta-oxidation) occurs exclusively in the mitochondria, RBCs are entirely dependent on **anaerobic glycolysis** for energy. **High-Yield NEET-PG Pearls:** 1. **The "Glucose-Sparing Effect":** In the fasting state, the heart and muscles increase fatty acid use to spare glucose for the brain and RBCs. 2. **Brain Metabolism:** The brain cannot use fatty acids (they cannot cross the blood-brain barrier) but can adapt to use **ketone bodies** during prolonged starvation. 3. **Carnitine Shuttle:** This is the rate-limiting step of fatty acid oxidation. A deficiency leads to cardiomyopathy and muscle weakness because the heart cannot transport long-chain fatty acids into the mitochondria.

Practice by Chapter

Lipid Classification and Chemistry

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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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