Lipid Metabolism Practice Questions

Q: Tendon xanthomas are characteristic findings in which of the following conditions?

Familial hypercholesterolemia. **Explanation:** **Familial Hypercholesterolemia (Type IIa Hyperlipoproteinemia)** is the correct answer. This condition is primarily caused by a genetic mutation in the **LDL receptor gene**, leading to defective clearance of LDL from the plasma. The resulting severe hypercholesterolemia leads to the deposition of cholesterol in macrophages within tendons, manifesting clinically as **Tendon Xanthomas** (most commonly involving the Achilles tendon and extensor tendons of the hands). **Analysis of Incorrect Options:** * **Familial Hyperlipidemia (Type IIb):** While this involves elevated LDL and VLDL, it is more commonly associated with xanthelasma and tuberous xanthomas rather than the classic tendon xanthomas seen in Type IIa. * **Familial Beta-lipoproteinemia (Type III / Dysbetalipoproteinemia):** This is characterized by a deficiency in Apo-E, leading to the accumulation of IDL and chylomicron remnants. Its pathognomonic clinical sign is **Palmar Xanthomas** (orange-yellow discoloration of palmar creases). * **Familial Lipoprotein Lipase (LPL) Deficiency (Type I):** This results in severe hypertriglyceridemia (elevated chylomicrons). Clinical hallmarks include **Eruptive Xanthomas** (small yellow papules on an erythematous base), hepatosplenomegaly, and acute pancreatitis, but not tendon xanthomas. **High-Yield Clinical Pearls for NEET-PG:** * **Tendon Xanthoma:** Pathognomonic for Familial Hypercholesterolemia (Type IIa). * **Palmar Xanthoma:** Pathognomonic for Type III Hyperlipoproteinemia. * **Eruptive Xanthoma:** Associated with high Triglycerides (Type I, IV, V). * **Corneal Arcus:** Early onset (Arcus senilis) is a sign of hypercholesterolemia in young patients. * **Statins** are the first-line treatment for Type IIa to upregulate remaining LDL receptors.

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: Cholesterol is a type of which molecule?

Sterol. **Explanation:** **Why Sterol is Correct:** Cholesterol is structurally classified as a **sterol** (a contraction of steroid and alcohol). It consists of a characteristic **cyclopentanoperhydrophenanthrene (CPPP) nucleus**, also known as the steroid nucleus, which contains four fused rings (A, B, C, and D). It is categorized as a sterol because it possesses a hydroxyl (-OH) group at the C3 position, making it a solid alcohol. In humans, it serves as an essential structural component of cell membranes (regulating fluidity) and as a precursor for bile acids, steroid hormones, and Vitamin D. **Why Other Options are Incorrect:** * **Ester:** While cholesterol can form **cholesteryl esters** when a fatty acid is attached to its C3 hydroxyl group (the storage form), cholesterol itself is a free alcohol, not an ester. * **Phospholipid:** These are compound lipids containing a phosphate group (e.g., Lecithin). Cholesterol lacks a phosphate group and a glycerol/sphingosine backbone. * **Lipoprotein:** These are complex molecular aggregates (like LDL or HDL) that **transport** cholesterol through the blood. Cholesterol is a component of a lipoprotein, not a lipoprotein itself. **High-Yield NEET-PG Clinical Pearls:** * **Precursor:** All 27 carbon atoms of cholesterol are derived from **Acetyl-CoA**. * **Rate-limiting enzyme:** **HMG-CoA reductase** (inhibited by Statins). * **Identification:** The **Libermann-Burchard reaction** is the chemical test used to detect cholesterol (turns emerald green). * **Excretion:** Humans cannot metabolize the steroid ring to $CO_2$ and $H_2O$; it must be excreted in the bile as cholesterol or bile salts.

Q: Which of the following is the receptor for Low-Density Lipoprotein (LDL)?

Apolipoprotein B-100. ### Explanation The correct answer is **Apolipoprotein B-100**. **1. Why Apolipoprotein B-100 is Correct:** Low-Density Lipoprotein (LDL) is the primary carrier of cholesterol in the blood. The LDL receptor (LDLR), located on the surface of hepatocytes and peripheral tissues, recognizes and binds specifically to **Apo B-100**. This interaction triggers receptor-mediated endocytosis, allowing the cell to internalize the LDL particle. Apo B-100 is a large, structural protein synthesized in the liver and is the hallmark apoprotein of VLDL, IDL, and LDL. **2. Analysis of Incorrect Options:** * **Apolipoprotein B-48 (Option B):** This is a truncated version of Apo B-100 synthesized in the intestine. It is found on **Chylomicrons** and their remnants. It lacks the LDL receptor-binding domain found in the C-terminal portion of Apo B-100. * **Apolipoprotein A-I (Option C):** This is the major structural protein of **HDL**. Its primary role is to activate the enzyme Lecithin-Cholesterol Acyltransferase (LCAT) for reverse cholesterol transport. * **Apolipoprotein A-II (Option D):** Also found primarily in HDL, its exact physiological function is less clear, but it is not involved in LDL receptor binding. **3. NEET-PG High-Yield Pearls:** * **Apo B-48 vs. B-100:** Both are products of the same gene. Apo B-48 is formed via **RNA editing** (C to U conversion by cytidine deaminase), which creates a premature stop codon. * **Clinical Correlation:** Mutations in the LDL receptor or the Apo B-100 ligand lead to **Familial Hypercholesterolemia (Type IIa)**, characterized by elevated LDL levels, xanthomas, and early-onset atherosclerosis. * **Apo E:** While Apo B-100 is the primary ligand for LDL, **Apo E** is the ligand for the LDL-Receptor-Related Protein (LRP), which clears chylomicron remnants and IDL.

Q: Unsaturated fatty acids are converted to:

Prostaglandins. **Explanation:** The correct answer is **Prostaglandins**. **1. Why Prostaglandins is correct:** Unsaturated fatty acids, specifically **Polyunsaturated Fatty Acids (PUFAs)** like Arachidonic acid (an omega-6 fatty acid), serve as the primary precursors for the synthesis of **Eicosanoids**. Through the **Cyclooxygenase (COX) pathway**, arachidonic acid is converted into prostaglandins, prostacyclins, and thromboxanes. These molecules act as potent local hormones mediating inflammation, pain, and fever. **2. Why other options are incorrect:** * **Cholesterol:** Cholesterol is synthesized from **Acetyl-CoA** via the HMG-CoA reductase pathway, not directly from the conversion of unsaturated fatty acids. * **Cell membrane lipid:** While unsaturated fatty acids are *components* of phospholipids in the cell membrane (providing fluidity), they are incorporated into the membrane structure rather than being "converted to" the lipid itself in a metabolic transformation sense. * **Saturated fatty acid:** While hydrogenation can convert unsaturated fats to saturated fats, this is primarily an industrial process (producing trans-fats). In human metabolism, the body typically performs desaturation (adding double bonds) rather than the reverse. **3. High-Yield Clinical Pearls for NEET-PG:** * **Essential Fatty Acids:** Linoleic acid and α-Linolenic acid are essential because humans lack the enzymes (**$\Delta^{12}$ and $\Delta^{15}$ desaturases**) to introduce double bonds beyond carbon 9. * **Rate-limiting step:** The release of arachidonic acid from membrane phospholipids by **Phospholipase A2** is the rate-limiting step for prostaglandin synthesis. This enzyme is inhibited by **Glucocorticoids**. * **NSAIDs Mechanism:** Aspirin and other NSAIDs work by irreversibly or reversibly inhibiting the COX enzymes, preventing the conversion of arachidonic acid to prostaglandins.

Q: Which of the following statements about High Density Lipoprotein (HDL) is false?

HDL increases oxidation of LDL. ### Explanation **1. Why Option A is the Correct Answer (False Statement):** HDL is known as the "Good Cholesterol" primarily because of its **antioxidant and anti-inflammatory properties**. It contains enzymes like **Paraoxonase (PON1)**, which actively **inhibits** the oxidation of LDL. Since oxidized LDL (ox-LDL) is the primary driver of atherosclerosis, HDL’s ability to prevent this oxidation is a key cardioprotective mechanism. Therefore, the statement that HDL *increases* oxidation is factually incorrect. **2. Analysis of Other Options:** * **Option B (Reduces foam cell production):** Foam cells are formed when macrophages ingest ox-LDL. By preventing LDL oxidation and promoting cholesterol efflux from macrophages, HDL significantly reduces foam cell formation. * **Option C (Predictor of CAD):** In clinical practice, the **LDL/HDL ratio** or low levels of HDL are considered among the strongest independent predictors of Coronary Artery Disease (CAD) risk, often more sensitive than total cholesterol alone. * **Option D (Clears lipids from atheromas):** This refers to **Reverse Cholesterol Transport (RCT)**. HDL picks up excess cholesterol from peripheral tissues and atherosclerotic plaques (via ABCA1 transporters) and transports it back to the liver for excretion. **3. NEET-PG High-Yield Pearls:** * **Apo-A1:** The primary apolipoprotein associated with HDL. * **LCAT (Lecithin-Cholesterol Acyltransferase):** Activated by Apo-A1; it converts free cholesterol into cholesterol esters inside HDL (maturation of discoid HDL to spherical HDL). * **CETP (Cholesterol Ester Transfer Protein):** Mediates the exchange of cholesterol esters from HDL for triglycerides from VLDL/LDL. * **Tangier Disease:** A rare genetic disorder characterized by a deficiency in ABCA1 transporters, leading to extremely low HDL levels and orange-colored tonsils.

Q: All of the following are ketone bodies EXCEPT?

Alpha ketoglutarate. **Explanation:** Ketone bodies are water-soluble molecules produced by the liver from fatty acids during periods of low glucose availability (starvation, prolonged exercise, or untreated Type 1 Diabetes). **Why Alpha-ketoglutarate is the correct answer:** Alpha-ketoglutarate is **not** a ketone body. It is a key intermediate in the **Tricarboxylic Acid (TCA) Cycle** and plays a vital role in amino acid metabolism (transamination). Despite its name containing "keto," it belongs to the category of dicarboxylic acids and functions primarily in energy production and nitrogen transport, not as a fuel source derived from ketogenesis. **Analysis of incorrect options:** * **Acetoacetate:** This is the primary ketone body formed in the liver mitochondria. It is the precursor to the other two ketone bodies. * **Beta-hydroxybutyrate:** Formed by the reduction of acetoacetate. Quantitatively, it is the **most abundant** ketone body in the blood during ketosis. Technically, it is a hydroxy acid, not a ketone, but it is clinically classified as a ketone body. * **Acetone:** Produced by the spontaneous non-enzymatic decarboxylation of acetoacetate. It is volatile and excreted via the lungs, giving the characteristic "fruity odor" to the breath in ketoacidosis. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Synthesis:** Liver mitochondria (but the liver **cannot** utilize them because it lacks the enzyme **Thiophorase** / Succinyl-CoA:3-ketoacid CoA transferase). * **Rate-limiting enzyme:** HMG-CoA Synthase. * **Detection:** The **Rothera’s test** detects Acetoacetate and Acetone, but **not** Beta-hydroxybutyrate. * **Fuel Source:** Ketone bodies are the preferred fuel for the heart and renal cortex; the brain uses them only during prolonged starvation.

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.

Question 1

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 2

Tendon xanthomas are characteristic findings in which of the following conditions?

Accepted Answer

Familial hypercholesterolemia

Answer

Familial hyperlipidemia

Answer

Familial beta-lipoproteinemia

Answer

Familial lipoprotein lipase deficiency

Question 3

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 4

Cholesterol is a type of which molecule?

Accepted Answer

Sterol

Answer

Ester

Answer

Phospholipid

Answer

Lipoprotein

Question 5

Which of the following is the receptor for Low-Density Lipoprotein (LDL)?

Accepted Answer

Apolipoprotein B-100

Answer

Apolipoprotein B-48

Answer

Apolipoprotein A-I

Answer

Apolipoprotein A-II

Question 6

Unsaturated fatty acids are converted to:

Accepted Answer

Prostaglandins

Answer

Cholesterol

Answer

Cell membrane lipid

Answer

Saturated fatty acid

Question 7

Atherosclerosis is due to

Accepted Answer

Apolipoprotein E deficiency

Answer

HDL receptor defect

Answer

Decreased LDL activity

Answer

Decreased lipoprotein lipase

Question 8

Which of the following statements about High Density Lipoprotein (HDL) is false?

Accepted Answer

HDL increases oxidation of LDL

Answer

HDL reduces foam cell production by LDL

Answer

HDL is the best predictor of CAD

Answer

HDL helps to clear lipids from atheromas

Question 9

All of the following are ketone bodies EXCEPT?

Accepted Answer

Alpha ketoglutarate

Answer

Acetone

Answer

Acetoacetate

Answer

Beta-hydroxy butyrate

Question 10

Cerebrosides are primarily composed of which monosaccharide?

Accepted Answer

Galactose

Answer

Glucose

Answer

Fructose

Answer

Arabinose

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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