Lipoprotein Metabolism and Transport — MCQs
Type-I hyperlipoproteinemia is caused by deficiency of:-
Apolipoprotein B-48 is made by which process?
Apo B48 is synthesized in -
Lipoprotein involved in reverse cholesterol transport is:
Which of the following statements about LDL is false?
Which lipoprotein is involved in reverse cholesterol transport?
A patient has multiple tendon xanthomas. Serum cholesterol ( $398 \mathrm{mg} / \mathrm{dL}$ ) and LDL ( 220 $\mathrm{mg} / \mathrm{dL}$ ) were found to be elevated. What is the most likely defect?
In a patient with lipoprotein lipase deficiency, which of the following is increased following a fatty meal?
What is the primary receptor for High-Density Lipoprotein (HDL) in cholesterol metabolism?
Which gene defect causes familial hypercholesterolemia?
Lipoprotein Metabolism and Transport Indian Medical PG Practice Questions and MCQs
Question 1: Type-I hyperlipoproteinemia is caused by deficiency of:-
- A. Lipoprotein lipase (Correct Answer)
- B. Elevated triglycerides in plasma
- C. Elevated LDL
- D. Elevated cholesterol
Explanation: ***Lipoprotein lipase*** - **Type I hyperlipoproteinemia**, also known as **familial lipoprotein lipase deficiency**, is caused by a genetic defect leading to **deficiency or defect in lipoprotein lipase (LPL)** or its cofactor **apolipoprotein C-II**. - LPL is crucial for the **hydrolysis of triglycerides** in chylomicrons and VLDL at the capillary endothelium. - This enzymatic deficiency leads to **massive accumulation of chylomicrons** and severe hypertriglyceridemia (often >1000 mg/dL). - Clinical features include **eruptive xanthomas, lipemia retinalis, hepatosplenomegaly**, and **recurrent pancreatitis**. *Elevated triglycerides in plasma* - This is indeed the **most prominent laboratory finding** in Type I hyperlipoproteinemia, with triglyceride levels often exceeding 1000-2000 mg/dL. - However, this is the **consequence/manifestation** of the LPL deficiency, not the underlying cause. - The question asks what causes Type I hyperlipoproteinemia, which is the enzyme deficiency itself. *Elevated LDL* - Type I hyperlipoproteinemia typically has **normal or even reduced LDL levels**. - **Elevated LDL** is characteristic of **Type IIa hyperlipoproteinemia (familial hypercholesterolemia)**, which involves defects in LDL receptor or ApoB-100. - Type I primarily affects **chylomicron metabolism**, not LDL. *Elevated cholesterol* - Cholesterol levels are typically **normal or only mildly elevated** in Type I hyperlipoproteinemia. - The triglyceride elevation is disproportionately massive compared to any cholesterol elevation. - Significant isolated cholesterol elevation points to Type IIa or IIb dyslipidemias.
Question 2: Apolipoprotein B-48 is made by which process?
- A. DNA editing
- B. RNA editing (Correct Answer)
- C. RNA alternate splicing
- D. RNA interference
Explanation: ***RNA editing*** - Apolipoprotein B-48 is synthesized from ApoB-100 mRNA through a process called **RNA editing** (specifically ApoB mRNA editing) - This involves a **cytidine deaminase enzyme (APOBEC-1)** that converts cytidine to uridine at position 6666, changing a glutamine codon (CAA) to a premature stop codon (UAA) in the small intestine - This results in a truncated protein that is 48% the length of ApoB-100 - ApoB-48 is produced in the **intestine**, while ApoB-100 (unedited) is produced in the **liver** *DNA editing* - DNA editing refers to permanent modifications in the DNA sequence itself - The ApoB gene remains unchanged; only the mRNA transcript is edited in intestinal cells - This is not the mechanism for producing ApoB-48 *RNA alternate splicing* - Alternative splicing involves selecting different combinations of exons from pre-mRNA to produce multiple mRNA isoforms - This process creates different protein variants through exon inclusion/exclusion - ApoB-48 production does not involve alternative splicing but rather direct nucleotide modification (C to U) within the coding sequence *RNA interference* - RNA interference (RNAi) is a biological process involving small RNA molecules (siRNA, miRNA) that silence gene expression - RNAi typically degrades mRNA or blocks translation - This process is not involved in generating a truncated protein like ApoB-48 from the same mRNA transcript
Question 3: 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 4: Lipoprotein involved in reverse cholesterol transport is:
- A. LDL
- B. VLDL
- C. CETP
- D. HDL (Correct Answer)
Explanation: ***HDL*** - **High-density lipoprotein (HDL)** is responsible for **reverse cholesterol transport**, which removes excess cholesterol from peripheral cells and transports it back to the liver for excretion or recycling. - HDL particles are formed in the liver and intestine and acquire cholesterol from peripheral tissues via the **ABCA1 transporter** and **LCAT enzyme**, which esterifies cholesterol. *LDL* - **Low-density lipoprotein (LDL)** is primarily responsible for transporting **cholesterol from the liver to peripheral tissues**. - High levels of LDL are associated with an increased risk of **atherosclerosis** and cardiovascular disease. *VLDL* - **Very low-density lipoprotein (VLDL)** primarily transports **triglycerides** synthesized in the liver to peripheral tissues. - VLDL is converted to **LDL** after releasing triglycerides. *CETP* - **Cholesteryl ester transfer protein (CETP)** is a plasma protein that facilitates the transfer of **cholesteryl esters** and triglycerides between lipoproteins, primarily from HDL to VLDL and LDL. - While CETP plays a role in lipoprotein metabolism, it is a **protein that mediates lipid exchange**, rather than a lipoprotein directly involved in reverse cholesterol transport itself.
Question 5: Which of the following statements about LDL is false?
- A. More dense than chylomicron
- B. Transports maximum amount of lipid (Correct Answer)
- C. Contains maximum cholesterol
- D. Smaller than VLDL
Explanation: ***Transports maximum amount of lipid*** - This statement is false because **chylomicrons**, not LDL, are primarily responsible for transporting the **maximum amount of dietary lipids** (triglycerides) from the intestines to various tissues. - While LDL does transport lipids, its primary role is to deliver **cholesterol** to cells, and it contains a lower proportion of triglyceride compared to chylomicrons and VLDL. *More dense than chylomicron* - This statement is true; **LDL is denser than chylomicrons** because it has a higher protein-to-lipid ratio. - **Chylomicrons** are the least dense lipoproteins due to their very high triglyceride content. *Smaller than VLDL* - This statement is true; **LDL is smaller than VLDL** (Very Low-Density Lipoprotein). - VLDL particles are larger and contain more triglycerides, which are gradually removed, leading to the formation of smaller LDL particles. *Contains maximum cholesterol* - This statement is true; **LDL contains the highest proportion of cholesterol** (specifically, **cholesterol esters**) among the lipoproteins. - This characteristic makes LDL the primary carrier for delivering cholesterol to peripheral tissues.
Question 6: Which lipoprotein is involved in reverse cholesterol transport?
- A. LDL
- B. HDL (Correct Answer)
- C. VLDL
- D. Chylomicrons
Explanation: ***HDL*** - **High-density lipoprotein (HDL)** is responsible for **reverse cholesterol transport**, which involves picking up excess cholesterol from peripheral tissues. - This cholesterol is then transported back to the liver for excretion or recycling, hence HDL is often referred to as "good cholesterol." *LDL* - **Low-density lipoprotein (LDL)** primarily transports **cholesterol from the liver to peripheral tissues**, contributing to plaque formation in arteries. - It is often called "bad cholesterol" because high levels are associated with an increased risk of **atherosclerosis** and cardiovascular disease. *VLDL* - **Very-low-density lipoprotein (VLDL)** is synthesized in the liver and primarily transports **endogenous triglycerides** to peripheral tissues. - As triglycerides are removed, VLDL is converted into **intermediate-density lipoprotein (IDL)** and eventually to LDL. *Chylomicrons* - **Chylomicrons** are formed in the small intestine and are responsible for transporting **exogenous (dietary) triglycerides** and cholesterol absorbed from the gut to various tissues. - They are the largest and least dense lipoproteins, appearing after a fatty meal.
Question 7: A patient has multiple tendon xanthomas. Serum cholesterol ( $398 \mathrm{mg} / \mathrm{dL}$ ) and LDL ( 220 $\mathrm{mg} / \mathrm{dL}$ ) were found to be elevated. What is the most likely defect?
- A. Lipoprotein lipase deficiency
- B. LDL receptor defect (Correct Answer)
- C. Apo E defect
- D. LCAT deficiency
- E. Apo B-100 defect
Explanation: ***LDL receptor defect*** - **Tendon xanthomas** are a classic sign of **familial hypercholesterolemia**, which is most commonly caused by a genetic defect in the **LDL receptor**. - **Elevated LDL cholesterol** levels are a hallmark of this condition, as dysfunctional LDL receptors lead to impaired clearance of LDL particles from the blood. *Lipoprotein lipase deficiency* - This condition primarily causes severe **hypertriglyceridemia** and can lead to **eruptive xanthomas**, but not typically tendon xanthomas. - While cholesterol levels might be elevated, the defining feature would be very high triglyceride levels, often exceeding 1000 mg/dL. *Apo E defect* - A defect in **ApoE** (specifically the **ApoE2/E2 genotype**) is associated with **familial dysbetalipoproteinemia** (Type III hyperlipoproteinemia). - This condition causes elevated remnants of chylomicrons and VLDL, leading to **palmar xanthomas**, but less commonly tendon xanthomas, and often presents with high triglyceride levels in addition to cholesterol. *Apo B-100 defect* - **Familial defective apoB-100** can present similarly to familial hypercholesterolemia with elevated LDL cholesterol. - However, this is much **rarer** than LDL receptor defects (affecting ~1:700 vs 1:250-500 for LDL receptor mutations). - The clinical presentation and lipid profile overlap significantly, but LDL receptor defects remain the most common cause of this clinical picture. *LCAT deficiency* - **Lecithin-cholesterol acyltransferase (LCAT)** deficiency leads to an accumulation of **unesterified cholesterol** in plasma and tissues. - This typically presents with **corneal opacities**, **hemolytic anemia**, and proteinuria, rather than predominantly tendon xanthomas and isolated severe LDL elevation.
Question 8: In a patient with lipoprotein lipase deficiency, which of the following is increased following a fatty meal?
- A. Chylomicron (Correct Answer)
- B. LDL
- C. HDL
- D. Apo-A
Explanation: ***Chylomicron*** - Lipoprotein lipase (LPL) is essential for the breakdown of **chylomicrons** in the bloodstream. A deficiency in LPL means chylomicrons cannot be cleared effectively. - After a **fatty meal**, the body absorbs dietary fats as chylomicrons. Without functional LPL, these chylomicrons accumulate in the plasma, leading to **marked elevation** of chylomicron levels. - This results in **lipemic (milky) serum**, a characteristic finding in Type I hyperlipoproteinemia. *LDL* - **LDL (Low-Density Lipoprotein)** levels are primarily affected by the metabolism of VLDL (Very Low-Density Lipoprotein), which is a separate pathway from chylomicron metabolism. - LPL deficiency specifically impacts chylomicron clearance, not directly causing an increase in LDL. In fact, LDL may be normal or even low in severe hypertriglyceridemia. *HDL* - **HDL (High-Density Lipoprotein)** is involved in reverse cholesterol transport and is typically **decreased** (not increased) in LPL deficiency. - During normal lipolysis by LPL, surface components from chylomicrons are transferred to HDL. In LPL deficiency, this process is impaired, leading to **reduced HDL levels**. *Apo-A* - **Apolipoprotein A-I (Apo-AI)** is the primary apolipoprotein found on HDL particles and is crucial for HDL formation and function. - Since HDL levels are decreased in LPL deficiency, Apo-AI levels would also be decreased (not increased) following a fatty meal.
Question 9: What is the primary receptor for High-Density Lipoprotein (HDL) in cholesterol metabolism?
- A. SR-BI (Correct Answer)
- B. LDLR
- C. HDLR
- D. SR-82
Explanation: ***SR-BI*** - **Scavenger Receptor class B type 1 (SR-BI)** is the primary receptor responsible for selective uptake of **cholesteryl esters** from HDL into cells, particularly the liver and steroidogenic tissues. - Unlike other lipoprotein receptors, SR-BI mediates the **selective transfer** of cholesterol without internalizing the entire HDL particle. *LDLR* - The **Low-Density Lipoprotein Receptor (LDLR)** is the primary receptor for **LDL** and very low-density lipoprotein (VLDL) remnants, mediating their endocytosis and degradation. - While it plays a crucial role in cholesterol metabolism, its main function is related to the uptake of **LDL cholesterol**, not HDL. *HDLR* - **HDLR** is not a recognized receptor in cholesterol metabolism. - This term may be a distracter created by combining HDL with the common receptor nomenclature. *SR-82* - **SR-82** is not a recognized receptor involved in cholesterol metabolism. - Similar to HDLR, this is a distracter term.
Question 10: Which gene defect causes familial hypercholesterolemia?
- A. LDL Receptor (Correct Answer)
- B. Apo E
- C. Apo CII
- D. Apo B48
Explanation: ***LDL Receptor*** - Familial hypercholesterolemia (FH) is primarily caused by mutations in the **LDL receptor (LDLR) gene**, which leads to impaired clearance of **low-density lipoprotein (LDL)** from the blood. - This defect results in significantly elevated levels of **LDL cholesterol** and an increased risk of premature cardiovascular disease. *Apo E* - Mutations in the **Apo E gene** are associated with **Type III hyperlipoproteinemia (dysbetalipoproteinemia)**, characterized by elevated **chylomicron remnants** and **VLDL remnants**. - This condition presents with xanthomas and premature atherosclerosis, but is distinct from the primary defect in FH. *Apo CII* - **Apo CII** is a cofactor for **lipoprotein lipase (LPL)**, an enzyme essential for the breakdown of **triglycerides** in chylomicrons and VLDL. - Deficiency in Apo CII or LPL causes **Type I hyperlipoproteinemia (familial chylomicronemia syndrome)**, leading to marked **hypertriglyceridemia**, not hypercholesterolemia. *Apo B48* - **Apo B48** is a structural component of **chylomicrons**, which are responsible for transporting dietary fats from the intestines. - It is not directly involved in the primary defect of **LDL clearing** that characterizes familial hypercholesterolemia.
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