Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 681: What primarily forms the core of chylomicrons?

A. Triglycerides and Cholesterol together
B. Triglycerides (Correct Answer)
C. Free fatty acids
D. Triglyceride, Cholesterol and Phospholipids

Explanation: ***Triglycerides*** - Chylomicrons are primarily responsible for transporting **dietary triglycerides** from the intestines to other tissues. - Their large core, composed mainly of **triglycerides**, allows efficient transport of these hydrophobic molecules. *Triglycerides and Cholesterol together* - While **cholesterol** is present in chylomicrons, it is less abundant than **triglycerides** and primarily exists as **cholesterol esters** in the core. - The core is not an equal mixture; **triglycerides** overwhelmingly dominate the volume. *Free fatty acids* - **Free fatty acids** are transported in the blood primarily bound to **albumin**, not within the core of chylomicrons. - Chylomicrons typically carry **esterified fatty acids** as part of triglycerides. *Triglyceride, Cholesterol and Phospholipids* - **Phospholipids** form the outer monolayer of the chylomicron, along with apoproteins, making them **amphipathic**. - They do not constitute a core component but rather the **surface interface** with the aqueous environment.

Question 682: Which protein does the domain of plasminogen resemble?

A. Fibrinogen (a clotting protein)
B. LDL receptor (a lipid metabolism protein)
C. Apolipoprotein (a) (a lipoprotein) (Correct Answer)
D. Prothrombin (a coagulation protein)

Explanation: ***Apolipoprotein (a) (a lipoprotein)*** - **Plasminogen** and **apolipoprotein (a)** share structural homology, specifically due to the presence of **kringle domains**. - This structural similarity suggests a potential for apolipoprotein (a) to **interfere with plasminogen’s fibrinolytic activity**, contributing to **atherosclerosis**. *Fibrinogen (a clotting protein)* - While plasmin acts on fibrinogen (and its derivative fibrin), its domain structure does not **resemble fibrinogen**. - **Fibrinogen** is a large, multi-domain glycoprotein crucial for **clot formation**, distinct from plasminogen's primarily **kringle-rich structure**. *LDL receptor (a lipid metabolism protein)* - The **LDL receptor** is involved in **cholesterol uptake** by cells and has structural features like ligand-binding repeats and epidermal growth factor (EGF) repeats. - Its domain structure is **not similar to plasminogen**, which is characterized by **kringle domains** and a protease domain. *Prothrombin (a coagulation protein)* - **Prothrombin** is a precursor to thrombin, featuring **gla domains**, kringle-like domains (though structurally distinct from plasminogen's), and a serine protease domain. - While both are involved in coagulation/fibrinolysis, their **overall domain arrangements and specific kringle structures differ** significantly.

Question 683: Transport of lipids from the intestine to other tissues is by -

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

Explanation: ***Chylomicrons*** - **Chylomicrons** are the **largest lipoprotein particles** that transport **dietary (exogenous) lipids** from the **intestine** to peripheral tissues - They are synthesized in **intestinal enterocytes** after fat absorption and enter the bloodstream via the **lymphatic system (thoracic duct)** - They carry **triglycerides (85-95%), cholesterol, phospholipids, and fat-soluble vitamins** (A, D, E, K) - **Apolipoprotein B-48** is the characteristic structural protein of chylomicrons - After delivering triglycerides to tissues (via lipoprotein lipase), chylomicron remnants are taken up by the **liver** *LDL (Low-Density Lipoprotein)* - LDL transports **cholesterol from the liver to peripheral tissues** (not from intestine) - It carries **endogenous cholesterol**, not dietary lipids from the intestine - Often called "**bad cholesterol**" due to its role in atherosclerosis - Contains **Apolipoprotein B-100** *HDL (High-Density Lipoprotein)* - HDL performs **reverse cholesterol transport** - moving excess cholesterol from peripheral tissues **back to the liver** - It does **not transport lipids from the intestine** to tissues - Called "**good cholesterol**" for its protective cardiovascular role - Contains **Apolipoprotein A-I and A-II** *VLDL (Very-Low-Density Lipoprotein)* - VLDL is synthesized in the **liver** (not intestine) and transports **endogenous triglycerides** to peripheral tissues - It carries lipids **from the liver**, not from the intestine - VLDL is converted to IDL and then LDL after losing triglycerides - Contains **Apolipoprotein B-100**

Question 684: Bile acids are synthesized from ?

A. Heme
B. Ribulose
C. Arachidonic acid
D. Cholesterol (Correct Answer)

Explanation: ***Cholesterol*** - **Bile acids** are derivatives of **cholesterol**, synthesized in the liver through a multi-step enzymatic pathway. - The conversion of cholesterol to bile acids is a primary mechanism for the excretion and transport of cholesterol from the body. *Heme* - **Heme** is a component of hemoglobin and myoglobin, primarily involved in oxygen transport and storage. - Its degradation product is **bilirubin**, which forms part of bile but is distinct from bile acids. *Ribulose* - **Ribulose** is a 5-carbon sugar, playing a key role in the **pentose phosphate pathway** and the **Calvin cycle** in photosynthesis. - It is not a precursor for bile acid synthesis. *Arachidonic acid* - **Arachidonic acid** is a polyunsaturated fatty acid that serves as a precursor for **eicosanoids** (prostaglandins, thromboxanes, and leukotrienes). - These molecules are involved in inflammation and immune responses but are unrelated to bile acid synthesis.

Question 685: Apo B48 is synthesized in -

A. Liver
B. Kidney
C. Intestine (Correct Answer)
D. RBCs

Explanation: ***Intestine*** - **Apo B48** is a truncated form of apolipoprotein B-100, uniquely synthesized in the **intestine** through RNA editing. - It is a crucial structural component of **chylomicrons**, which are lipoprotein particles responsible for transporting exogenous dietary lipids from the intestine to other tissues. *Liver* - The liver primarily synthesizes **Apo B100**, which is a full-length apolipoprotein B and a major component of VLDL, IDL, and LDL. - It does not produce Apo B48. *Kidney* - The kidneys are involved in filtering waste products and regulating fluid balance, but they do not play a role in the synthesis of apolipoproteins like Apo B48. - Kidney cells are not equipped with the specific machinery for Apo B mRNA editing. *RBCs* - Red blood cells (RBCs) are primarily responsible for oxygen transport and lack a nucleus and most organelles, including those required for protein synthesis. - Therefore, RBCs cannot synthesize proteins such as Apo B48.

Question 686: What is the effect of moderate alcohol consumption on lipid profiles in dyslipidemia?

A. Decreased HDL levels
B. Increased HDL levels (Correct Answer)
C. Increased triglyceride levels
D. Decreased LDL levels

Explanation: ***Increased HDL levels*** - Moderate alcohol consumption is known to **increase high-density lipoprotein (HDL) cholesterol levels**, which is often considered beneficial for cardiovascular health. - This effect is thought to be mediated by alcohol's influence on **hepatic lipoprotein metabolism**, leading to enhanced HDL production and reduced catabolism. *Decreased HDL levels* - This is incorrect, as multiple studies have consistently shown that **moderate alcohol consumption** tends to elevate, rather than decrease, HDL cholesterol. - Low HDL levels are associated with increased cardiovascular risk, making this effect an undesirable outcome that is not typical of moderate drinking. *Increased triglyceride levels* - While heavy or chronic alcohol consumption can lead to **increased triglyceride levels**, moderate intake typically has a neutral or only slightly elevated effect, if any, often overshadowed by the HDL increase. - Significant hypertriglyceridemia is a concern with **excessive alcohol use**, not usually with moderate consumption in healthy individuals. *Decreased LDL levels* - Moderate alcohol consumption generally has **little to no significant effect** on **low-density lipoprotein (LDL) cholesterol levels**, often referred to as "bad" cholesterol. - While HDL increases are observed, alcohol does not effectively lower LDL, which is a primary target in the management of dyslipidemia.

Question 687: What is the primary effect of moderate alcohol consumption on cholesterol levels?

A. Total cholesterol
B. Low-Density Lipoprotein (LDL)
C. Very Low-Density Lipoprotein (VLDL)
D. High-Density Lipoprotein (HDL) (Correct Answer)

Explanation: ***High-Density Lipoprotein (HDL)*** - Moderate alcohol consumption is known to **increase HDL cholesterol** levels. - HDL cholesterol helps in the **reverse cholesterol transport**, removing excess cholesterol from tissues and transporting it back to the liver for excretion. *Total cholesterol* - The effect of moderate alcohol on **total cholesterol** is less consistent and may vary, as it is a sum of HDL, LDL, and 20% of VLDL. - While HDL increases, other components might remain unchanged or show minimal variation, thus not making it the primary and direct effect. *Low-Density Lipoprotein (LDL)* - Moderate alcohol consumption generally has **little to no significant effect** on **LDL cholesterol** levels. - Some studies suggest a slight decrease or no change, but it is not the primary lipid affected. *Very Low-Density Lipoprotein (VLDL)* - There is generally **no significant direct effect** of moderate alcohol consumption on **VLDL cholesterol** levels. - Excessive alcohol intake, however, can elevate triglycerides, which are the main component of VLDL particles.

Question 688: What is the primary reason for the detergent action of bile salts?

A. Hydrophobic properties
B. Acts as a zwitterion
C. Amphipathic nature (Correct Answer)
D. None of the options

Explanation: ***Amphipathic nature*** - Bile salts are **amphipathic molecules**, meaning they have both **hydrophilic (water-loving)** and **hydrophobic (water-fearing)** regions. - This dual nature allows them to emulsify fats by surrounding lipid droplets with their hydrophobic ends dissolving in the fat and their hydrophilic ends facing the aqueous environment, stabilizing the emulsion. *Hydrophobic properties* - While bile salts do possess **hydrophobic regions**, these alone are not sufficient for detergent action. - The ability to interact with both oil and water phases simultaneously is crucial for their role in **emulsification**. *Acts as a zwitterion* - A zwitterion is a molecule with both a **positive and negative charge**, but an overall neutral charge. - This property is not the primary mechanism behind the **detergent action** of bile salts, which relies more on their ability to solubilize fats. *None of the options* - The **amphipathic nature** is indeed the primary reason for the detergent action; therefore, this option is incorrect.

Question 689: Albumin is the primary transport protein in blood for which of the following substances?

A. Free fatty acids (FFA) (Correct Answer)
B. Thyroxine
C. Steroid
D. Calcium

Explanation: ***Free fatty acids (FFA)*** - **Albumin is the PRIMARY and MAJOR transport protein for free fatty acids** in the bloodstream, with each albumin molecule having **6-7 high-affinity binding sites** for FFAs. - This binding is essential for transporting water-insoluble fatty acids from **adipose tissue** (during lipolysis) to peripheral tissues for **β-oxidation and energy production**. - In the context of lipid metabolism, albumin-FFA transport is the **most quantitatively significant** and physiologically important binding function. - **Clinical relevance:** Impaired albumin levels directly affect FFA transport capacity. *Thyroxine* - While albumin does bind thyroid hormones, **thyroxine-binding globulin (TBG)** is the **primary carrier** (~70% of T4), followed by transthyretin (~15%). - Albumin binds only ~10-15% of circulating T4 with **low affinity**, serving as a secondary reserve. - TBG has **much higher affinity** for thyroid hormones than albumin. *Steroid* - Steroids are primarily transported by **specific binding globulins**: **corticosteroid-binding globulin (CBG)** for cortisol and **sex hormone-binding globulin (SHBG)** for testosterone/estrogen. - While albumin binds ~10-20% of steroids, it is a **secondary carrier** with lower affinity than the specific globulins. *Calcium* - Although ~40-45% of plasma calcium is albumin-bound (important for calcium homeostasis), this is a **passive binding function** rather than active transport. - Albumin's role with calcium is primarily **buffering** rather than the dedicated transport function it provides for FFAs. - In the context of **lipid metabolism** and **transport proteins**, FFA binding is the hallmark function of albumin.

Question 690: Apolipoprotein E is rich in

A. Methionine
B. Lysine
C. Histidine
D. Arginine (Correct Answer)

Explanation: ***Arginine*** - **Apolipoprotein E (apoE)** is notably rich in **basic amino acids**, with **arginine** being particularly abundant. - The high content of **positively charged arginine residues** is critical for apoE's ability to bind to negatively charged lipid surfaces and interact with receptors such as the **LDL receptor** and **LDL receptor-related protein (LRP)**. - This arginine-rich composition is a defining characteristic of apoE and is essential for its role in **lipid metabolism** and **receptor-mediated lipoprotein uptake**. *Lysine* - While apoE does contain **lysine** (another basic amino acid), it is **arginine** that is particularly abundant and functionally emphasized. - Both lysine and arginine contribute positive charges, but **arginine residues** are specifically highlighted in apoE's **receptor binding domains** and are more characteristic of this apolipoprotein. *Histidine* - **Histidine** is also a **basic amino acid**, but it is not present in the same high proportions as **arginine** in apoE. - Its pKa (~6.0) is closer to physiological pH, meaning its charge state can vary, making it less consistently positive than arginine or lysine in biological contexts. - Histidine is not a defining feature of apoE's amino acid composition. *Methionine* - **Methionine** is a **sulfur-containing, nonpolar amino acid**, not a basic amino acid. - It does not contribute to the positive charge characteristic of apoE. - Its role in proteins is typically structural or as the initiator of protein synthesis (as the first amino acid), but it is not relevant to apoE's receptor-binding properties.

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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