Lipid Metabolism Practice Questions

Q: Eicosanoids are derived from which of the following fatty acids?

Arachidonic acid. **Explanation:** **1. Why Arachidonic Acid is Correct:** Eicosanoids (prostaglandins, thromboxanes, and leukotrienes) are a family of potent signaling molecules containing 20 carbon atoms. The term "eicosanoid" is derived from the Greek word *eikosi*, meaning twenty. **Arachidonic acid** is a 20-carbon polyunsaturated fatty acid (PUFA) with four double bonds (C20:4, ω-6). It is the primary precursor for eicosanoid synthesis in humans. It is typically esterified in membrane phospholipids and is released by the enzyme **Phospholipase A2** in response to physiological or pathological stimuli. **2. Why Other Options are Incorrect:** * **Oleic acid (A):** A 18-carbon monounsaturated fatty acid (C18:1, ω-9) found in olive oil; it does not serve as a precursor for eicosanoids. * **Linoleic acid (B):** An essential 18-carbon ω-6 fatty acid (C18:2). While it is the metabolic precursor to arachidonic acid, it must first be elongated and desaturated before it can form eicosanoids. * **Linolenic acid (C):** Specifically α-linolenic acid (C18:3, ω-3), it is an essential fatty acid that serves as a precursor for EPA and DHA, not the primary arachidonic acid-derived eicosanoids. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The release of arachidonic acid from the cell membrane by **Phospholipase A2** is the rate-limiting step in eicosanoid synthesis. This enzyme is inhibited by **Corticosteroids**. * **Pathways:** Arachidonic acid follows two main pathways: 1. **Cyclooxygenase (COX) pathway:** Leads to Prostaglandins and Thromboxanes (Inhibited by NSAIDs/Aspirin). 2. **Lipoxygenase (LOX) pathway:** Leads to Leukotrienes (involved in asthma and inflammation). * **Prostacyclin (PGI2) vs. Thromboxane (TXA2):** PGI2 (from endothelium) causes vasodilation and inhibits platelet aggregation, while TXA2 (from platelets) causes vasoconstriction and promotes aggregation.

Q: Free fatty acids are transported into the mitochondria for oxidation by which mechanism?

Albumin. **Explanation:** The transport of free fatty acids (FFAs) in the blood is a critical step in lipid metabolism. Although the question mentions "into the mitochondria," it refers to the systemic transport of non-esterified fatty acids (NEFAs) from adipose tissue to target organs (like the liver or muscle) where mitochondrial oxidation occurs. **Why Albumin is Correct:** Free fatty acids are hydrophobic and cannot dissolve in the aqueous environment of the plasma. Upon release from adipocytes via lipolysis, FFAs bind non-covalently to **Albumin**. Albumin has multiple high-affinity binding sites for long-chain fatty acids, acting as the primary carrier protein. This ensures that FFAs remain soluble and are prevented from forming detergent-like aggregates that could damage cell membranes. **Why Other Options are Incorrect:** * **Chylomicrons:** These transport **exogenous (dietary) triglycerides** from the intestines to peripheral tissues, not free fatty acids. * **Globulins:** These proteins primarily transport hormones (e.g., TBG, SHBG), vitamins, and ions, or function in the immune system (Immunoglobulins). They do not serve as the primary carriers for FFAs. * **Note on Mitochondrial Entry:** Once inside the cell, the transport *across* the mitochondrial membrane is mediated by the **Carnitine Shuttle**, but the systemic transport in the blood is the role of Albumin. **High-Yield Clinical Pearls for NEET-PG:** * **Carnitine Palmitoyltransferase-I (CPT-I):** The rate-limiting enzyme of $\beta$-oxidation, located on the outer mitochondrial membrane. * **Inhibitor:** CPT-I is inhibited by **Malonyl-CoA**, preventing a futile cycle during fatty acid synthesis. * **Short/Medium Chain Fatty Acids:** Unlike long-chain FAs, these do not require the carnitine shuttle and can enter the mitochondria directly.

Q: Premature atherosclerosis causing death is seen in which of the following conditions?

Fabry's disease. **Explanation:** **Fabry’s Disease (Correct Answer):** Fabry’s disease is an **X-linked recessive** lysosomal storage disorder caused by a deficiency of the enzyme **$\alpha$-galactosidase A**. This leads to the systemic accumulation of **globotriaosylceramide ($Gb_3$)** within the vascular endothelium. The progressive deposition of these glycosphingolipids in the endothelial lining of blood vessels causes narrowing and ischemia, leading to **premature atherosclerosis**, myocardial infarction, and stroke (often at a young age). It is the only sphingolipidosis that is X-linked. **Analysis of Incorrect Options:** * **Vitamin E deficiency:** Vitamin E is a potent antioxidant that protects against lipid peroxidation. Its deficiency typically presents with hemolytic anemia, posterior column signs, and ataxia, but it is not a primary cause of premature atherosclerosis. * **Abetalipoproteinemia:** This is caused by a mutation in the **Microsomal Triglyceride Transfer Protein (MTP)**, leading to a near-absence of Apo-B48 and Apo-B100. Patients have extremely low cholesterol and LDL levels; therefore, they are actually protected against atherosclerosis. * **Spinocerebellar ataxia:** This is a group of genetic neurodegenerative disorders characterized by progressive incoordination. It does not involve lipid metabolism or vascular endothelial damage. **High-Yield Clinical Pearls for NEET-PG:** * **Fabry’s Triad:** Episodic peripheral neuropathy (burning pain in hands/feet), angiokeratomas (red-purple skin rashes), and hypohidrosis (decreased sweating). * **Late Complications:** Renal failure (most common cause of death) and hypertrophic cardiomyopathy. * **Diagnosis:** "Maltese cross" appearance in urine sediment and "Zebra bodies" on electron microscopy of renal biopsy.

Q: Cerebrosides are primarily composed of which monosaccharide?

Galactose. **Explanation:** **Cerebrosides** are the simplest form of **neutral glycosphingolipids**. They consist of a ceramide unit (sphingosine + fatty acid) linked to a single monosaccharide unit via a covalent glycosidic bond. 1. **Why Galactose is correct:** The most common monosaccharide found in cerebrosides, particularly those located in the myelin sheath of the brain and nervous tissue, is **Galactose**. These are specifically termed **Galactocerebrosides** (Galactosylceramides). While glucocerebrosides do exist in non-neural tissues, the term "cerebroside" classically refers to the galactosyl variety prevalent in the central nervous system. 2. **Why other options are incorrect:** * **Glucose:** While **Glucocerebrosides** are intermediates in the synthesis of complex glycosphingolipids (like gangliosides), they are not the primary component of the structural cerebrosides found in the brain. * **Fructose:** This is a ketohexose primarily involved in energy metabolism (glycolysis) and is not a structural component of sphingolipids. * **Arabinose:** This is a five-carbon sugar (pentose) found mainly in plant polysaccharides and is not part of human sphingolipid structure. **High-Yield Clinical Pearls for NEET-PG:** * **Krabbe’s Disease:** Caused by a deficiency of the enzyme **Galactocerebrosidase**, leading to the accumulation of galactocerebrosides and destruction of myelin (leukodystrophy). * **Gaucher’s Disease:** The most common lysosomal storage disorder, caused by a deficiency of **Glucocerebrosidase**, leading to the accumulation of glucocerebrosides in the liver, spleen, and bone marrow (Gaucher cells). * **Ceramide** is the fundamental structural unit of all sphingolipids. * **Sulfatides** are galactocerebrosides that contain a sulfate group attached to the galactose moiety.

Q: Which apolipoprotein is the main ligand for the LDL receptor?

Apolipoprotein B-100. **Explanation:** The **LDL receptor (LDLR)**, also known as the ApoB/E receptor, is responsible for the cellular uptake of cholesterol-rich lipoproteins via receptor-mediated endocytosis. **Why Apolipoprotein B-100 is correct:** Apolipoprotein B-100 (Apo B-100) is the primary structural protein of VLDL, IDL, and LDL. It serves as the **obligatory ligand** for the LDL receptor. When LDL particles circulate in the plasma, the LDLR recognizes the specific binding domain on Apo B-100, allowing the liver and peripheral tissues to internalize the cholesterol. **Analysis of Incorrect Options:** * **Apolipoprotein A:** Primarily found on HDL (Apo A-I). It acts as an activator of Lecithin-Cholesterol Acyltransferase (LCAT) and is involved in reverse cholesterol transport, not LDL receptor binding. * **Apolipoprotein C-100:** This is a **distractor**. There is no "C-100." Apo C-II is a cofactor for Lipoprotein Lipase (LPL), and Apo C-III inhibits it. * **Apolipoprotein E:** While Apo E is a ligand for the LDL receptor (and the LRP receptor), it is the primary ligand for **Chylomicron remnants** and **IDL**. While it has a higher affinity for the receptor than Apo B-100, Apo B-100 remains the "main" ligand specifically associated with the clearance of **LDL** particles. **High-Yield Clinical Pearls for NEET-PG:** * **Familial Hypercholesterolemia (Type IIa):** Caused by mutations in the LDL receptor or the binding domain of **Apo B-100**, leading to severely elevated LDL levels and early atherosclerosis. * **Apo B-48:** Produced in the intestine (via RNA editing of the Apo B gene); it lacks the LDL receptor-binding domain, which is why chylomicrons are not cleared by the LDLR. * **PCSK9:** A protein that degrades LDL receptors; PCSK9 inhibitors are a modern class of cholesterol-lowering drugs.

Q: The chyle from the intestine is rich with chylomicrons. Which of the following forms the protein core of chylomicrons?

Triglyceride + cholesterol + phospholipid. ### Explanation **Concept Overview:** Chylomicrons are the largest and least dense lipoproteins, synthesized in the intestinal mucosal cells to transport dietary (exogenous) lipids into the circulation. A lipoprotein particle is structured with a **hydrophobic core** and a **hydrophilic shell**. **Why Option C is Correct:** The "protein core" or the internal lipid cargo of a chylomicron consists primarily of **Triglycerides** (about 85-90%) and **Cholesteryl esters**. While phospholipids and free cholesterol are typically found on the surface membrane to provide solubility, the question refers to the overall lipid composition that constitutes the bulk of the particle. In the context of this question, the chylomicron is a complex assembly of **Triglycerides + Cholesterol + Phospholipids** (along with specific apolipoproteins like Apo B-48). **Analysis of Incorrect Options:** * **Options A, B, and D:** These are incomplete. Chylomicrons are never composed of a single lipid type. While triglycerides are the predominant component, they must be packaged with cholesterol and phospholipids to form a stable lipoprotein particle capable of entering the lymphatic system (chyle). **NEET-PG High-Yield Pearls:** * **Apolipoprotein Marker:** **Apo B-48** is the unique structural protein for chylomicrons (synthesized via RNA editing of the Apo B gene in the intestine). * **Route of Transport:** Chylomicrons are too large to enter capillaries directly; they enter **lacteals** (lymphatics), forming **chyle**, and reach the blood via the **thoracic duct**. * **Activation:** They receive **Apo C-II** and **Apo E** from HDL in the plasma. Apo C-II is essential for activating **Lipoprotein Lipase (LPL)** to release free fatty acids. * **Appearance:** Post-prandial plasma appears milky due to high chylomicron content (Chylomicronemia).

Question 1

Eicosanoids are derived from which of the following fatty acids?

Accepted Answer

Arachidonic acid

Answer

Oleic acid

Answer

Linoleic acid

Answer

Linolenic acid

Question 2

Free fatty acids are transported into the mitochondria for oxidation by which mechanism?

Accepted Answer

Albumin

Answer

None of the above

Answer

Globulins

Answer

Chylomicrons

Question 3

All of the following statements regarding Cholesterol Ester Transfer Protein (CETP) are true, EXCEPT:

Accepted Answer

CETP facilitates the transfer of triacylglycerol from HDL to LDL.

Answer

CETP is associated with HDL.

Answer

CETP facilitates the transfer of cholesteryl ester from HDL to LDL.

Answer

CETP facilitates the transfer of triacylglycerol from LDL to HDL.

Question 4

Premature atherosclerosis causing death is seen in which of the following conditions?

Accepted Answer

Fabry's disease

Answer

Vitamin E deficiency

Answer

Abetalipoproteinemia

Answer

Spinocerebellar ataxia

Question 5

Atherosclerosis is due to

Accepted Answer

Apolipoprotein E deficiency

Answer

HDL receptor defect

Answer

Decreased LDL activity

Answer

Decreased lipoprotein lipase

Question 6

Cerebrosides are primarily composed of which monosaccharide?

Accepted Answer

Galactose

Answer

Glucose

Answer

Fructose

Answer

Arabinose

Question 7

An eight-month-old female infant presented with recurrent episodes of hypoglycemia, especially if the time interval between feedings is increased. Dicarboxylic acid is present in the urine, and urine ketone bodies are negative. The child responded well to IV glucose, a diet low in fat and high in carbohydrate, and frequent feedings. The child was diagnosed with MCAD deficiency. What is the primary reason for hypoglycemia in this condition?

Accepted Answer

Lack of sufficient acetyl-CoA to support gluconeogenesis.

Answer

Increased dicarboxylic acid inhibits glycogenolysis.

Answer

Impaired beta-oxidation leading to decreased acetyl-CoA availability for gluconeogenesis.

Answer

Inadequate glycogen stores due to a defect in glycogen synthesis.

Question 8

Which apolipoprotein is the main ligand for the LDL receptor?

Accepted Answer

Apolipoprotein B-100

Answer

Apolipoprotein A

Answer

Apolipoprotein C-100

Answer

Apolipoprotein E

Question 9

The chyle from the intestine is rich with chylomicrons. Which of the following forms the protein core of chylomicrons?

Accepted Answer

Triglyceride + cholesterol + phospholipid

Answer

Triglyceride only

Answer

Triglyceride + cholesterol

Answer

Only cholesterol

Question 10

Which of the following regulates lipolysis in adipocytes?

Accepted Answer

Activation of triglyceride lipase as a result of hormone-stimulated increases in cyclic AMP levels

Answer

Activation of fatty acid synthesis mediated by cyclic AMP

Answer

Glycerol phosphorylation to prevent futile esterification of fatty acids

Answer

Activation of cyclic AMP production by insulin

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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