Lipid Metabolism Practice Questions

Q: Which of the following is NOT involved in imparting rancidity to fats?

Reduction. **Explanation:** Rancidity refers to the chemical decomposition of fats and oils, resulting in an unpleasant odor and taste. It occurs primarily through the degradation of fatty acid chains. **Why Reduction is the Correct Answer:** Reduction is the process of adding hydrogen atoms to unsaturated bonds (hydrogenation). In the food industry, partial reduction is actually used to **prevent** rancidity by converting unstable unsaturated fatty acids into more stable saturated ones (e.g., making margarine). Therefore, reduction inhibits rather than imparts rancidity. **Analysis of Other Options:** * **Oxidation (Oxidative Rancidity):** This is the most common cause. Free radicals attack the double bonds of unsaturated fatty acids, forming peroxides and aldehydes (like malondialdehyde), which produce the characteristic foul smell. * **Hydrolysis (Hydrolytic Rancidity):** This occurs when lipase enzymes (from bacteria) or moisture break the ester bonds of triglycerides, releasing free fatty acids. Short-chain fatty acids like butyric acid are particularly pungent. * **Cyclization of Hydrocarbons:** During high-heat processing (like deep frying), fatty acid chains can undergo cyclization and polymerization. These cyclic compounds contribute to the chemical spoilage and potential toxicity of the fat. **High-Yield NEET-PG Pearls:** * **Antioxidants:** Vitamin E (Tocopherol), Vitamin C, BHA, and BHT are added to fats to prevent oxidative rancidity by scavenging free radicals. * **Marker of Lipid Peroxidation:** Malondialdehyde (MDA) is a key biochemical marker used to measure the degree of lipid peroxidation/oxidative stress. * **Vegetable Oils:** These are more prone to oxidative rancidity than animal fats because they contain higher levels of polyunsaturated fatty acids (PUFAs).

Q: A newborn infant presents with severe respiratory problems, muscle problems, minimal development, and neurological deficits. A liver biopsy shows very low acetyl CoA carboxylase activity, with normal activity of enzymes involved in glycolysis, gluconeogenesis, the citric acid cycle, and the pentose phosphate pathway. What is the most likely cause of the infant's respiratory problems?

Low levels of phosphatidylcholine. **Explanation:** The core of this question lies in understanding the role of **Acetyl CoA Carboxylase (ACC)** in fatty acid synthesis and its downstream effects on lung physiology. **1. Why Option A is Correct:** Acetyl CoA Carboxylase is the **rate-limiting enzyme** for the synthesis of long-chain fatty acids (converting Acetyl CoA to Malonyl CoA). Fatty acids are essential building blocks for **Dipalmitoylphosphatidylcholine (DPPC)**, also known as **Lecithin**. DPPC is the primary phospholipid component of **pulmonary surfactant**. A deficiency in ACC leads to decreased fatty acid production, resulting in surfactant deficiency. This causes high alveolar surface tension, leading to atelectasis and severe respiratory distress syndrome (RDS). **2. Why Incorrect Options are Wrong:** * **B. Biotin deficiency:** While ACC is a biotin-dependent enzyme, a primary biotin deficiency would also affect other carboxylases (like Pyruvate Carboxylase), disrupting gluconeogenesis. The question states gluconeogenesis is normal. * **C. Ketoacidosis:** Low ACC activity actually *decreases* Malonyl CoA levels. Since Malonyl CoA normally inhibits CPT-1, its absence would accelerate fatty acid oxidation and potentially increase ketone bodies, but this does not explain the primary respiratory pathology. * **D. High levels of citrate:** Citrate is an allosteric activator of ACC. If ACC activity is low due to a primary enzyme defect, citrate levels might rise as it isn't being utilized, but this is a metabolic consequence, not the *cause* of respiratory failure. **High-Yield Clinical Pearls for NEET-PG:** * **Surfactant Composition:** 90% lipids (mainly DPPC/Lecithin) and 10% proteins (SP-A, B, C, D). * **L/S Ratio:** A Lecithin/Sphingomyelin ratio > 2.0 in amniotic fluid indicates fetal lung maturity. * **ACC Regulation:** Stimulated by Citrate and Insulin; inhibited by Palmitoyl-CoA and Glucagon (via phosphorylation).

Q: HMG-CoA can be directly converted to all except?

Acetoacetyl CoA. ### Explanation The question tests your knowledge of the metabolic fates of **3-hydroxy-3-methylglutaryl-CoA (HMG-CoA)**, a central intermediate in both ketogenesis and cholesterol synthesis. #### Why Acetoacetyl CoA is the Correct Answer HMG-CoA is **synthesized from** Acetoacetyl CoA and Acetyl-CoA by the enzyme *HMG-CoA synthase*. The reaction is: **Acetoacetyl CoA + Acetyl-CoA → HMG-CoA** Metabolic pathways do not "directly" convert HMG-CoA back into Acetoacetyl CoA in a single step; rather, it is broken down into Acetoacetate and Acetyl-CoA. Therefore, Acetoacetyl CoA is a precursor, not a direct product. #### Analysis of Incorrect Options * **A & B (Acetoacetate and Acetyl-CoA):** In the mitochondria (Ketogenesis), the enzyme **HMG-CoA Lyase** cleaves HMG-CoA directly into Acetoacetate (a ketone body) and Acetyl-CoA. * **C (Mevalonate):** In the cytosol (Cholesterol synthesis), the rate-limiting enzyme **HMG-CoA Reductase** reduces HMG-CoA directly into Mevalonate using NADPH. #### NEET-PG High-Yield Pearls * **Compartmentalization:** HMG-CoA for **ketogenesis** occurs in the **mitochondria** (Liver), while HMG-CoA for **cholesterol synthesis** occurs in the **cytosol/ER**. * **Rate-Limiting Step:** HMG-CoA Reductase is the target of **Statins**, which are competitive inhibitors used to treat hypercholesterolemia. * **Ketogenic Enzyme:** HMG-CoA Lyase deficiency is a rare organic aciduria that presents with non-ketotic hypoglycemia. * **Mnemonic:** "Lyase for Lipids (Ketones), Reductase for Ring (Cholesterol)."

Q: Which of the following has the maximum medium-chain fatty acid content?

Coconut oil. **Explanation:** The correct answer is **Coconut oil**. **1. Why Coconut Oil is Correct:** Medium-chain fatty acids (MCFAs) are fatty acids with a carbon chain length of 6 to 12 carbons (e.g., Caprylic and Lauric acid). **Coconut oil** is the richest natural source of MCFAs, containing approximately **60-65%** medium-chain triglycerides (MCTs). Unlike long-chain fatty acids, MCTs are unique because they are absorbed directly into the portal circulation without requiring bile salts or chylomicron formation, making them a vital energy source for patients with malabsorption syndromes. **2. Why the Other Options are Incorrect:** * **Sunflower oil:** Primarily composed of Long-Chain Fatty Acids (LCFAs), specifically Omega-6 polyunsaturated fatty acids (PUFA) like Linoleic acid. * **Flaxseed oil:** A major source of Alpha-linolenic acid (ALA), which is an essential Omega-3 polyunsaturated **long-chain** fatty acid. * **Fish oil:** Renowned for its high content of Eicosapentaenoic acid (EPA) and Docosahexaenoic acid (DHA). These are **very-long-chain** Omega-3 fatty acids (20-22 carbons). **3. High-Yield Clinical Pearls for NEET-PG:** * **Therapeutic Use:** MCTs are used in the dietary management of **Chyluria**, **Short Bowel Syndrome**, and **Pancreatic insufficiency** because they bypass the lymphatic system. * **Essential Fatty Acids:** Remember that Linoleic (ω-6) and Linolenic (ω-3) acids are "essential" because humans lack the enzymes (Δ12 and Δ15 desaturases) to introduce double bonds beyond carbon 9. * **Rancidity:** Oils with high PUFA content (like Sunflower) are more prone to oxidative rancidity compared to saturated-rich Coconut oil.

Q: All of the following inhibit hormone-dependent lipase, EXCEPT:

Adrenaline. **Explanation:** The enzyme **Hormone-Sensitive Lipase (HSL)** is the rate-limiting enzyme for lipolysis in adipose tissue. It catalyzes the breakdown of stored triacylglycerols into free fatty acids and glycerol. Its activity is regulated via **cAMP-dependent phosphorylation**. **1. Why Adrenaline is the Correct Answer:** Adrenaline (and Noradrenaline) acts via **$\beta_3$-adrenergic receptors** to activate Adenylate Cyclase. This increases intracellular cAMP levels, which activates Protein Kinase A (PKA). PKA then phosphorylates and **activates** HSL. Therefore, Adrenaline is a potent **stimulator**, not an inhibitor, of HSL. **2. Why the other options are incorrect (Inhibitors of HSL):** * **Insulin:** The most significant inhibitor of HSL. It activates a phosphatase that dephosphorylates (inactivates) HSL and promotes the degradation of cAMP via phosphodiesterase. * **Prostaglandin E1 (PGE1):** Inhibits Adenylate Cyclase, leading to decreased cAMP levels and reduced HSL activity. * **Nicotinic Acid (Niacin):** Potently inhibits lipolysis in adipose tissue by inhibiting Adenylate Cyclase, which is why it is used clinically to lower plasma free fatty acids. **Clinical Pearls for NEET-PG:** * **HSL vs. LPL:** Do not confuse Hormone-Sensitive Lipase (adipose tissue; mobilizes fat) with **Lipoprotein Lipase** (capillary endothelium; clears chylomicrons/VLDL). * **Glucagon & ACTH:** Like Adrenaline, these hormones also stimulate HSL. * **Caffeine/Theophylline:** These inhibit phosphodiesterase, maintaining high cAMP levels and thus **stimulating** HSL activity (promoting lipolysis).

Q: Conversion of stearic acid to oleic acid is catalyzed by which enzyme?

D9 desaturase. **Explanation:** The conversion of **stearic acid** (a 18-carbon saturated fatty acid, 18:0) to **oleic acid** (a 18-carbon monounsaturated fatty acid, 18:1; Δ9) is a process of desaturation. **1. Why Option C is Correct:** In humans, the synthesis of monounsaturated fatty acids is catalyzed by the **Fatty Acid Desaturase system** (specifically **Δ9-desaturase**), located in the endoplasmic reticulum. This enzyme introduces a double bond between **carbons 9 and 10** of the fatty acid chain. Stearic acid is the primary substrate for this enzyme, resulting in the formation of oleic acid. This reaction requires molecular oxygen, NADH, and cytochrome b5. **2. Why Other Options are Incorrect:** * **Δ3 and Δ6 desaturases (Options A & B):** These enzymes are involved in the further desaturation of polyunsaturated fatty acids (PUFAs) like linoleic and α-linolenic acid to form longer-chain PUFAs (e.g., arachidonic acid). They do not act directly on saturated stearic acid to form oleic acid. * **Δ12 desaturase (Option D):** This enzyme introduces a double bond at the 12th position. **Humans lack Δ12 and Δ15 desaturases**, which is why linoleic (18:2; Δ9,12) and linolenic (18:3; Δ9,12,15) acids cannot be synthesized endogenously and are classified as **Essential Fatty Acids (EFA)**. **Clinical Pearls & High-Yield Facts:** * **Essential Fatty Acids:** Humans cannot introduce double bonds beyond the Δ9 position. Therefore, any fatty acid with a double bond at Δ12 or Δ15 must be obtained from the diet. * **Non-essentiality of Oleic Acid:** Since we possess Δ9-desaturase, oleic acid (Omega-9) is considered a non-essential fatty acid. * **Enzyme Components:** The desaturase system is a multienzyme complex consisting of **Desaturase, Cytochrome b5, and Cytochrome b5 reductase.**

Q: Delayed clotting in abetalipoproteinemia is due to:

Reduced clotting factor synthesis. **Explanation:** **Abetalipoproteinemia** is an autosomal recessive disorder caused by a mutation in the **Microsomal Triglyceride Transfer Protein (MTP)**. This defect prevents the assembly and secretion of ApoB-containing lipoproteins (Chylomicrons, VLDL, and LDL). **Why the correct answer is right:** The hallmark of abetalipoproteinemia is severe **fat malabsorption**. Because dietary fats cannot be packaged into chylomicrons, fat-soluble vitamins (A, D, E, and K) are not absorbed from the intestine. **Vitamin K** is a vital cofactor for the γ-carboxylation of clotting factors **II, VII, IX, and X**. A deficiency in Vitamin K leads to the production of non-functional clotting factors, resulting in a bleeding diathesis and delayed clotting. **Why incorrect options are wrong:** * **A & C (Decreased VLDL/Chylomicrons):** While these are characteristic biochemical findings in abetalipoproteinemia, they are the *cause* of malabsorption, not the direct mechanism for delayed clotting. Clotting is a protein-mediated process, not a lipid-mediated one. * **D (Decreased fatty acid synthesis):** De novo fatty acid synthesis occurs primarily in the liver and is not the primary defect in this condition. **Clinical Pearls for NEET-PG:** * **Blood Smear:** Look for **Acanthocytes** (spur cells) due to altered membrane lipid composition. * **Neurological symptoms:** Ataxia and retinitis pigmentosa occur due to severe **Vitamin E** deficiency. * **Biopsy:** Intestinal biopsy shows **lipid-laden enterocytes** (vacuoles) because triglycerides are trapped and cannot be exported. * **Labs:** Extremely low levels of Cholesterol and Triglycerides; absent ApoB-48 and ApoB-100.

Question 1

Which of the following is NOT involved in imparting rancidity to fats?

Accepted Answer

Reduction

Answer

Oxidation

Answer

Hydrolysis

Answer

Cyclization of hydrocarbons

Question 2

A newborn infant presents with severe respiratory problems, muscle problems, minimal development, and neurological deficits. A liver biopsy shows very low acetyl CoA carboxylase activity, with normal activity of enzymes involved in glycolysis, gluconeogenesis, the citric acid cycle, and the pentose phosphate pathway. What is the most likely cause of the infant's respiratory problems?

Accepted Answer

Low levels of phosphatidylcholine

Answer

Biotin deficiency

Answer

Ketoacidosis

Answer

High levels of citrate

Question 3

HMG-CoA can be directly converted to all except?

Accepted Answer

Acetoacetyl CoA

Answer

Acetoacetate

Answer

Acetyl-CoA

Answer

Mevalonate

Question 4

PUFA consumption is associated with which of the following changes in serum lipid profile?

Accepted Answer

Lowering of serum cholesterol and lowering of LDL cholesterol

Answer

Lowering of serum cholesterol and a rise in LDL cholesterol

Answer

A rise in serum cholesterol and a rise in LDL cholesterol

Answer

A rise in serum cholesterol and lowering of LDL cholesterol

Question 5

Which gene present in theca cells and absent in granulosa cells enables theca cells to produce androgens?

Accepted Answer

CYP17 gene

Answer

CYP12 gene

Answer

p53

Answer

KRAS

Question 6

Which of the following has the maximum medium-chain fatty acid content?

Accepted Answer

Coconut oil

Answer

Sunflower oil

Answer

Flaxseed oil

Answer

Fish oil

Question 7

All of the following inhibit hormone-dependent lipase, EXCEPT:

Accepted Answer

Adrenaline

Answer

Insulin

Answer

Prostaglandin E1

Answer

Nicotinic acid

Question 8

In which of the following enzyme deficiencies is steatorrhea associated with increased triglycerides?

Accepted Answer

Lipoprotein lipase

Answer

Pancreatic lipase

Answer

Serum lipase

Answer

Acetyl CoA carboxylase

Question 9

Conversion of stearic acid to oleic acid is catalyzed by which enzyme?

Accepted Answer

D9 desaturase

Answer

D3 desaturase

Answer

D6 desaturase

Answer

D12 desaturase

Question 10

Delayed clotting in abetalipoproteinemia is due to:

Accepted Answer

Reduced clotting factor synthesis

Answer

A decreased VLDL

Answer

Decreased chylomicrons

Answer

Decreased fatty acid synthesis

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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