Lipid Metabolism Practice Questions

Q: In primary familial hypercholesterolemia, what is the defect?

LDL receptors. **Explanation:** **Primary Familial Hypercholesterolemia (Type IIa Hyperlipoproteinemia)** is an autosomal dominant disorder characterized by a significant elevation in serum LDL cholesterol. **Why LDL Receptors are the correct answer:** The primary defect lies in the **LDL receptor (LDLR) gene**. These receptors are responsible for the hepatic uptake of LDL particles from the circulation via receptor-mediated endocytosis. A deficiency or dysfunction of these receptors leads to decreased clearance of LDL, resulting in markedly elevated plasma cholesterol levels and premature atherosclerosis. **Analysis of Incorrect Options:** * **Apolipoprotein E:** Defects in Apo E (specifically the E2 isoform) lead to **Type III Hyperlipoproteinemia** (Dysbetalipoproteinemia), characterized by the accumulation of chylomicron remnants and IDL. * **Apolipoprotein B:** While a mutation in Apo B-100 (the ligand for the LDL receptor) can cause "Familial Defective Apo B-100," it is a distinct clinical entity. In the classic "Primary Familial Hypercholesterolemia," the receptor itself is the primary site of defect. * **VLDL:** VLDL is a transport lipoprotein. Elevated VLDL is seen in Type IV Hypertriglyceridemia, not primarily in Type IIa. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Look for **Tendon Xanthomas** (especially the Achilles tendon), **Xanthelasma** (eyelids), and **Corneal Arcus** in a young patient with high cholesterol. * **Genetics:** Homozygous individuals present in childhood with myocardial infarction before age 20; Heterozygotes present in their 30s-40s. * **Classification:** It is classified as **Type IIa** under the Fredrickson classification. * **Treatment:** Statins are the first-line treatment as they upregulate the expression of remaining functional LDL receptors.

Q: Sandhoff's disease is due to the absence of which enzyme?

Beta-hexosaminidase. **Explanation:** **Sandhoff’s Disease** is a lysosomal storage disorder (specifically a GM2 gangliosidosis) caused by a deficiency in **Beta-hexosaminidase**. The enzyme Beta-hexosaminidase exists in two major isoforms: * **Hexosaminidase A:** Composed of $\alpha$ and $\beta$ subunits. * **Hexosaminidase B:** Composed of two $\beta$ subunits. In Sandhoff’s disease, there is a mutation in the **HEXB gene**, which encodes the **$\beta$-subunit**. Consequently, **both Hexosaminidase A and B are deficient**, leading to the accumulation of GM2 gangliosides and globosides in the brain and other visceral organs. This distinguishes it from Tay-Sachs disease, where only Hexosaminidase A is deficient due to an $\alpha$-subunit mutation. **Analysis of Incorrect Options:** * **B. Beta-glucuronidase:** Deficiency leads to **Sly Syndrome** (Mucopolysaccharidosis VII), characterized by hepatosplenomegaly and skeletal deformities. * **C. Aryl sulphatase:** Deficiency of Aryl sulphatase A causes **Metachromatic Leukodystrophy**, leading to the accumulation of cerebroside sulfate. * **D. Alpha-galactosidase:** Deficiency leads to **Fabry Disease**, an X-linked disorder characterized by angiokeratomas, peripheral neuropathy, and renal failure. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Similar to Tay-Sachs (progressive neurodegeneration, **cherry-red spot** on macula, hyperacusis), but Sandhoff’s often presents with **hepatosplenomegaly** (visceral involvement), which is absent in Tay-Sachs. * **Inheritance:** Autosomal Recessive. * **Histology:** "Onion-skin" appearance of lysosomes due to whorled lamellar structures.

Q: Fish eye disease is a genetic disorder characterized by which of the following?

Partial LCAT deficiency. **Explanation:** **Fish Eye Disease (FED)** is a rare genetic disorder caused by a **partial deficiency of the enzyme Lecithin-Cholesterol Acyltransferase (LCAT)**. 1. **Why the correct answer is right:** LCAT is responsible for esterifying free cholesterol into cholesterol esters. In FED, there is a selective loss of **alpha-LCAT activity** (which acts on HDL), while **beta-LCAT activity** (acting on VLDL/LDL) is preserved. This leads to a significant reduction in HDL-cholesterol levels and the deposition of unesterified cholesterol in the corneal stroma, giving the eyes a characteristic "boiled fish" appearance. 2. **Why incorrect options are wrong:** * **Classic LCAT Deficiency:** This involves a **complete** (both alpha and beta) deficiency of the enzyme. Unlike FED, it presents with more severe systemic features, including hemolytic anemia and progressive renal failure, alongside corneal opacities. * **Tangier Disease:** This is caused by a mutation in the **ABCA1 transporter** gene. While it also features extremely low HDL, its hallmark clinical sign is **enlarged, orange-colored tonsils** and hepatosplenomegaly, not the specific corneal pattern of FED. * **Familial Hyperalphalipoproteinemia:** This is a condition characterized by **elevated** HDL levels (often due to CETP deficiency), which is cardioprotective, the opposite of the low-HDL state seen in FED. **High-Yield Clinical Pearls for NEET-PG:** * **LCAT Activator:** Apo A-I is the primary activator of LCAT. * **Key Difference:** FED = Partial deficiency (Corneal opacities only); Classic LCAT deficiency = Complete deficiency (Cornea + Anemia + Renal failure). * **HDL Metabolism:** LCAT is essential for the maturation of discoidal nascent HDL into spherical mature HDL (Reverse Cholesterol Transport).

Q: Which of the following fatty acids contains the maximum amount of polyunsaturated fatty acids (PUFA)?

Linoleic acid. **Explanation:** The classification of fatty acids is based on the number of double bonds present in their hydrocarbon chain. **Linoleic acid** is the correct answer because it is a **Polyunsaturated Fatty Acid (PUFA)** containing two double bonds (18:2, ω-6). It is an essential fatty acid, meaning the human body cannot synthesize it and must obtain it from dietary sources like sunflower, safflower, and corn oils. **Analysis of Options:** * **A. Palmitic acid (16:0):** This is a **Saturated Fatty Acid (SFA)** with no double bonds. It is the most common SFA found in palm oil and animal fats. * **B. Stearic acid (18:0):** This is also a **Saturated Fatty Acid (SFA)** with no double bonds. It is commonly found in animal fats and cocoa butter. * **C. Oleic acid (18:1, ω-9):** This is a **Monounsaturated Fatty Acid (MUFA)** containing only one double bond. It is the primary constituent of olive oil. **High-Yield NEET-PG Pearls:** 1. **Essential Fatty Acids (EFA):** There are two primary EFAs: **Linoleic acid** (Omega-6) and **Alpha-linolenic acid** (Omega-3). Arachidonic acid becomes essential only if Linoleic acid is deficient. 2. **P/S Ratio:** The ratio of Polyunsaturated to Saturated fatty acids. A ratio of **0.8 to 1.0** is considered ideal for cardiovascular health. 3. **Rich Sources:** Safflower oil has the highest PUFA content (~75%), followed by Sunflower oil (~65%). 4. **Clinical Significance:** High PUFA intake helps lower LDL cholesterol, but excessive intake may lower HDL (the "good" cholesterol). MUFAs (like Oleic acid) are preferred as they lower LDL without significantly affecting HDL.

Q: Which lipoprotein is primarily associated with carrying cholesterol from peripheral tissues back to the liver?

HDL. ### Explanation **Correct Answer: A. HDL (High-Density Lipoprotein)** The correct answer is **HDL** because it is the primary mediator of **Reverse Cholesterol Transport (RCT)**. HDL picks up excess cholesterol from peripheral tissues and vascular endothelium (using the ABCA1 transporter) and transports it back to the liver for excretion in bile or conversion into bile acids. This process is cardioprotective, which is why HDL is colloquially known as "Good Cholesterol." **Analysis of Incorrect Options:** * **B. LDL (Low-Density Lipoprotein):** Known as "Bad Cholesterol," its primary role is the opposite of HDL. It transports cholesterol **from the liver to peripheral tissues**. High levels are strongly associated with atherosclerosis. * **C. VLDL (Very-Low-Density Lipoprotein):** Produced by the liver, its main function is to transport **endogenous triglycerides** to peripheral tissues (muscles and adipose tissue). * **D. IDL (Intermediate-Density Lipoprotein):** Formed during the degradation of VLDL. It serves as a precursor to LDL and is not involved in reverse transport. **NEET-PG High-Yield Pearls:** * **LCAT (Lecithin-Cholesterol Acyltransferase):** An enzyme activated by **Apo A-I** (found on HDL) that esterifies cholesterol within HDL, allowing it to be packed into the core of the lipoprotein. * **CETP (Cholesteryl Ester Transfer Protein):** Facilitates the exchange of cholesteryl esters from HDL for triglycerides from VLDL/LDL. * **Tangier Disease:** A rare genetic disorder caused by a mutation in the **ABCA1 transporter**, leading to extremely low HDL levels and orange-colored tonsils. * **Apo B-100** is the characteristic apolipoprotein for VLDL, IDL, and LDL, while **Apo A-I** is the hallmark of HDL.

Q: If a person has Total Cholesterol = 300 mg/dL, HDL = 25 mg/dL, and Triglycerides = 150 mg/dL, calculate the value of LDL.

245 mg/dL. ### Explanation The calculation of LDL cholesterol is a high-yield topic for NEET-PG, primarily utilizing the **Friedewald Formula**. #### 1. Why Option A is Correct To find the LDL (Low-Density Lipoprotein) value, we use the Friedewald Equation: **LDL = Total Cholesterol – HDL – (Triglycerides / 5)** *Note: (Triglycerides / 5) is used to estimate VLDL cholesterol.* **Calculation:** * Total Cholesterol = 300 mg/dL * HDL = 25 mg/dL * VLDL = TG / 5 = 150 / 5 = 30 mg/dL * **LDL = 300 – 25 – 30 = 245 mg/dL** #### 2. Why Other Options are Incorrect * **Option B (125 mg/dL):** This result occurs if one incorrectly subtracts both HDL and TG directly from Total Cholesterol (300 - 25 - 150) without dividing TG by 5. * **Option C (95 mg/dL):** This is a distractor value that does not correlate with standard calculation errors in this formula. * **Option D (55 mg/dL):** This represents the sum of HDL and VLDL (25 + 30), rather than the LDL value itself. #### 3. Clinical Pearls & High-Yield Facts * **Limitation of Friedewald Formula:** It becomes **inaccurate** if Triglycerides are **>400 mg/dL**. In such cases, LDL must be measured directly. * **Non-HDL Cholesterol:** Calculated as (Total Cholesterol – HDL). It is increasingly used as a better predictor of cardiovascular risk than LDL alone. * **Target Levels:** For a healthy individual, LDL should ideally be <100 mg/dL. A value of 245 mg/dL indicates severe hypercholesterolemia, often seen in Familial Hypercholesterolemia (Type IIa). * **Sample Requirement:** For accurate TG and LDL calculation, a **12-14 hour fasting** sample is mandatory to clear chylomicrons from the blood.

Q: Fatty acid elongation occurs in which part of the cell?

Mitochondria. **Explanation:** The synthesis of fatty acids occurs via two distinct processes: *de novo* synthesis and elongation. While *de novo* synthesis (up to Palmitate, C16) occurs in the cytosol, the further lengthening of these chains (elongation) occurs primarily in the **Mitochondria** and the **Endoplasmic Reticulum (Microsomal system)**. 1. **Why Mitochondria is Correct:** The mitochondrial system for fatty acid elongation involves the addition of acetyl-CoA units to existing fatty acid chains. It essentially operates as a reversal of $\beta$-oxidation, with the exception that NADPH is used as the reductant instead of FADH₂ in the final step. This system is highly active in the heart and brain. 2. **Why other options are incorrect:** * **Cytosol:** This is the site for *de novo* fatty acid synthesis (catalyzed by the Fatty Acid Synthase complex). It produces Palmitate (C16) but is not the site for subsequent elongation. * **Lysosome:** These are "suicide bags" involved in the degradation of macromolecules via acid hydrolases; they play no role in fatty acid synthesis or elongation. * **Nucleus:** This organelle houses genetic material and is involved in transcription/replication, not lipid metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Microsomal System:** The Endoplasmic Reticulum is the *other* major site of elongation, using Malonyl-CoA and NADPH. * **Rate-limiting step of Synthesis:** Acetyl-CoA Carboxylase (converts Acetyl-CoA to Malonyl-CoA). * **Essential Fatty Acids:** Humans lack the enzymes (desaturases) to introduce double bonds beyond carbon 9, which is why Linoleic and Linolenic acids must be obtained from the diet. * **Reducing Equivalent:** NADPH is the universal electron donor for fatty acid synthesis/elongation, primarily sourced from the HMP Shunt.

Q: All of the following statements about Lipoprotein Lipase are true, except:

Does not require apoC-II as cofactor. **Explanation:** **Lipoprotein Lipase (LPL)** is a key enzyme in lipid metabolism responsible for the hydrolysis of triglycerides (TAGs) found in Chylomicrons and Very Low-Density Lipoproteins (VLDL) into free fatty acids and glycerol. **Why Option D is the correct answer:** LPL is an **apoC-II dependent enzyme**. ApoC-II, which is donated by HDL to chylomicrons and VLDL, acts as an obligatory cofactor that activates LPL. Without apoC-II, LPL remains inactive, preventing the clearance of triglycerides from the blood. Therefore, the statement that it "does not require apoC-II" is false. **Analysis of other options:** * **Options A & B:** LPL is synthesized and secreted by **adipocytes** (adipose tissue) and **myocytes** (cardiac and skeletal muscle). It is then anchored to the luminal surface of capillary endothelial cells by heparan sulfate proteoglycans to interact with circulating lipoproteins. * **Option C:** Since LPL is the primary enzyme for clearing triglyceride-rich lipoproteins, its deficiency (or deficiency of its cofactor apoC-II) leads to **Type I Hyperlipoproteinemia (Familial Chylomicronemia Syndrome)**, characterized by severe hypertriglyceridemia and eruptive xanthomas. **High-Yield Clinical Pearls for NEET-PG:** * **Insulin Effect:** Insulin stimulates the synthesis and secretion of LPL in adipose tissue (promoting fat storage) but inhibits it in muscle. * **Heparin Release:** Intravenous heparin releases LPL from the endothelial wall into the plasma, a property used to measure "post-heparin lipolytic activity." * **Comparison:** Do not confuse LPL with **Hormone Sensitive Lipase (HSL)**; HSL acts *inside* adipocytes to mobilize stored fat during fasting and is inhibited by insulin.

Question 1

In primary familial hypercholesterolemia, what is the defect?

Accepted Answer

LDL receptors

Answer

Apolipoprotein

Answer

Apolipoprotein B

Answer

VLDL

Question 2

Sandhoff's disease is due to the absence of which enzyme?

Accepted Answer

Beta-hexosaminidase

Answer

Beta-glucuronidase

Answer

Aryl sulphatase

Answer

Alpha galactosidase

Question 3

Fish eye disease is a genetic disorder characterized by which of the following?

Accepted Answer

Partial LCAT deficiency

Answer

Classic form of LCAT deficiency

Answer

Tangier disease

Answer

Familial hyperalphalipoproteinemia

Question 4

Which of the following fatty acids contains the maximum amount of polyunsaturated fatty acids (PUFA)?

Accepted Answer

Linoleic acid

Answer

Palmitic acid

Answer

Stearic acid

Answer

Oleic acid

Question 5

Increased formation of ketone bodies during fasting is a result of which of the following?

Accepted Answer

Increased oxidation of fatty acids as a source of fuel

Answer

Decreased formation of acetyl CoA in the liver

Answer

Decreased levels of glucagon

Answer

Increased glycogenesis in muscle

Question 6

A 4-month-old child presented with extreme tiredness, irritable mood, poor appetite, and fasting hypoglycemia associated with vomiting and muscle weakness. Blood tests revealed elevated levels of free fatty acids but low levels of acylcarnitine. A muscle biopsy demonstrated significant fatty acid infiltration in the cytoplasm. What is the most likely molecular defect in this child?

Accepted Answer

Carnitine transporter deficiency

Answer

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency

Answer

Acetyl-CoA carboxylase deficiency

Answer

Carnitine palmitoyltransferase II (CPT II) deficiency

Question 7

Which lipoprotein is primarily associated with carrying cholesterol from peripheral tissues back to the liver?

Accepted Answer

HDL

Answer

LDL

Answer

VLDL

Answer

IDL

Question 8

If a person has Total Cholesterol = 300 mg/dL, HDL = 25 mg/dL, and Triglycerides = 150 mg/dL, calculate the value of LDL.

Accepted Answer

245 mg/dL

Answer

125 mg/dL

Answer

95 mg/dL

Answer

55 mg/dL

Question 9

Fatty acid elongation occurs in which part of the cell?

Accepted Answer

Mitochondria

Answer

Cytosol

Answer

Lysosome

Answer

Nucleus

Question 10

All of the following statements about Lipoprotein Lipase are true, except:

Accepted Answer

Does not require apoC-II as cofactor

Answer

Found in adipose tissue

Answer

Found in myocytes

Answer

Deficiency leads to hypertriglyceridemia

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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