Lipid Metabolism Practice Questions

Q: Which cellular compartment is the primary site of fatty acid synthesis?

Cytosol. ### Explanation **Correct Answer: C. Cytosol** Fatty acid synthesis (De novo lipogenesis) occurs primarily in the **cytosol**. This process involves the conversion of Acetyl-CoA into Palmitate. The key enzyme complex, **Fatty Acid Synthase (FAS)**, is located in the cytosol. Since Acetyl-CoA is produced in the mitochondria but cannot cross the inner mitochondrial membrane, it is transported to the cytosol via the **Citrate-Malate Shuttle** (Citrate is the carrier). **Analysis of Incorrect Options:** * **A. Endoplasmic Reticulum:** While the ER is not the site of *de novo* synthesis, it is the primary site for **fatty acid elongation** (beyond 16 carbons) and **desaturation** (adding double bonds). * **B. Mitochondria:** This is the primary site for **$\beta$-oxidation** (breakdown of fatty acids) and ketogenesis. While a minor pathway for elongation exists here, it is not the site of synthesis. * **D. Microsomes:** Microsomes are vesicles derived from the ER. They are involved in elongation and desaturation but not the primary synthesis of the palmitate chain. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Enzyme:** Acetyl-CoA Carboxylase (ACC), which requires **Biotin** (Vitamin B7) as a cofactor. * **Reductant Requirement:** **NADPH** is the essential electron donor for fatty acid synthesis, primarily supplied by the **HMP Shunt** (Pentose Phosphate Pathway). * **Inhibitor:** Glucagon and Epinephrine inhibit synthesis, while **Insulin** promotes it by activating ACC. * **Mnemonic:** "Sy-to-sol" for **Sy**nthesis; "Mito-Chondria" for **C**atabolism ($\beta$-oxidation).

Q: The broad beta disease is due to a defect in which apolipoprotein?

Apolipoprotein E. **Explanation:** **Broad Beta Disease**, also known as **Type III Hyperlipoproteinemia** or Dysbetalipoproteinemia, is caused by a genetic deficiency or defect in **Apolipoprotein E (Apo E)**. Apo E is essential for the hepatic recognition and uptake of **Chylomicron remnants** and **IDL (Intermediate-Density Lipoproteins)** via the LDL receptor-related protein (LRP). In this condition, patients typically possess the **E2/E2 isoform** (homozygosity), which has a low affinity for the receptor. This leads to the accumulation of cholesterol-rich "remnant" particles in the plasma. On electrophoresis, these remnants migrate in the beta region but appear broader than usual, giving the disease its name. **Why other options are incorrect:** * **Apolipoprotein A:** Primarily found in HDL; it is involved in reverse cholesterol transport and activates LCAT. Defects lead to Tangier disease. * **Apolipoprotein B:** Apo B-100 is the structural protein for VLDL/LDL, and Apo B-48 is for chylomicrons. Defects lead to Abetalipoproteinemia or Familial Hypobetalipoproteinemia. * **Apolipoprotein C:** Apo C-II is a co-factor for Lipoprotein Lipase (LPL). Deficiency causes Type I Hyperlipoproteinemia (Chylomicronemia syndrome). **High-Yield Clinical Pearls for NEET-PG:** * **Pathognomonic Sign:** **Palmar Xanthomas** (yellowish deposits in the creases of the palms) and Tuberoeruptive xanthomas. * **Lipid Profile:** Elevated both Cholesterol and Triglycerides (often in a 1:1 ratio). * **Inheritance:** Autosomal Recessive (usually requiring a secondary factor like obesity or hypothyroidism to manifest). * **Risk:** Significant increase in premature Atherosclerosis and Peripheral Vascular Disease.

Q: Where are unsaturated fatty acids synthesized?

Endoplasmic reticulum. **Explanation:** The synthesis of unsaturated fatty acids involves the introduction of double bonds into a saturated fatty acid chain, a process known as **desaturation**. This reaction is catalyzed by **fatty acid desaturases** (e.g., $\Delta^9$ desaturase). These enzymes are located on the **smooth endoplasmic reticulum (ER)** membrane. The process requires molecular oxygen, NADH, and cytochrome $b_5$, forming an electron transport chain within the ER membrane to facilitate the redox reaction. **Analysis of Options:** * **Endoplasmic Reticulum (Correct):** It is the primary site for both fatty acid **elongation** (adding carbons to palmitate) and **desaturation** (introducing double bonds). * **Cytosol (Incorrect):** This is the site for *de novo* synthesis of palmitate (saturated fatty acid) via the Fatty Acid Synthase (FAS) complex. It does not contain the desaturase enzyme system. * **Plasma Membrane (Incorrect):** While it contains phospholipids with unsaturated fatty acids, it lacks the enzymatic machinery for their synthesis. * **Golgi Complex (Incorrect):** The Golgi is primarily involved in the modification, sorting, and packaging of proteins and lipids, not the primary synthesis of unsaturated bonds. **High-Yield NEET-PG Pearls:** 1. **Essential Fatty Acids:** Humans lack $\Delta^{12}$ and $\Delta^{15}$ desaturases; therefore, we cannot synthesize **Linoleic** and **Linolenic acid**. These must be obtained from the diet. 2. **Mitochondria:** While primarily the site of $\beta$-oxidation, mitochondria also possess a minor pathway for fatty acid elongation, distinct from the ER system. 3. **Rate-limiting step of Synthesis:** The conversion of Acetyl-CoA to Malonyl-CoA by **Acetyl-CoA Carboxylase** (in the cytosol).

Q: What is the defect in Type II hyperlipidemia?

LDL receptor. **Explanation:** **Type II Hyperlipidemia (Familial Hypercholesterolemia)** is primarily characterized by a defect in the **LDL receptor (LDLR)** or its ligand, **Apolipoprotein B-100**. 1. **Why Option C is Correct:** In Type IIa hyperlipidemia, a deficiency or dysfunction of the LDL receptor prevents the liver and peripheral tissues from clearing LDL-cholesterol from the blood. This leads to a massive elevation of plasma LDL and total cholesterol levels. In Type IIb, VLDL is also elevated alongside LDL. 2. **Why Options A and B are Incorrect:** * **Option A (Apolipoprotein E):** A defect in Apo E leads to **Type III Hyperlipidemia** (Dysbetalipoproteinemia), resulting in the accumulation of chylomicron remnants and IDL. * **Option B (Lipoprotein lipase):** A deficiency in LPL (or its cofactor Apo C-II) causes **Type I Hyperlipidemia** (Familial Chylomicronemia), characterized by extreme elevations in triglycerides and chylomicrons. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Dominant. * **Clinical Features:** Look for **Tendon Xanthomas** (especially the Achilles tendon) and **Xanthelasma** (yellowish deposits around eyelids). * **Coronary Artery Disease (CAD):** Patients are at extremely high risk for premature atherosclerosis and myocardial infarction, often occurring before age 20 in homozygous cases. * **Diagnosis:** Characterized by "Clear Serum" (unlike Type I or IV) because LDL does not scatter light like VLDL or chylomicrons.

Q: ApoB is related to all EXCEPT:

HDL. **Explanation:** The question tests the knowledge of **Apolipoprotein B (ApoB)** distribution across different lipoprotein classes. ApoB is the primary structural protein for all non-HDL lipoproteins. **1. Why HDL is the correct answer:** **HDL (High-Density Lipoprotein)** does not contain ApoB. Instead, its primary structural protein is **ApoA-I**. HDL is involved in reverse cholesterol transport, and the presence of ApoB is a marker for "atherogenic" particles, which HDL is not. **2. Analysis of incorrect options:** * **Chylomicrons:** These contain **ApoB-48**, which is synthesized in the intestine. It represents the N-terminal 48% of the ApoB gene and is essential for the secretion of chylomicrons into the lymph. * **VLDL:** These contain **ApoB-100**, synthesized in the liver. ApoB-100 acts as the structural scaffold for VLDL assembly. * **LDL:** Since LDL is a metabolic product of VLDL (via IDL), it retains the **ApoB-100** molecule. ApoB-100 on LDL serves as the ligand for the **LDL receptor**, facilitating cellular uptake. **3. High-Yield Clinical Pearls for NEET-PG:** * **ApoB-100 vs. ApoB-48:** Both are derived from the same gene. The difference arises due to **RNA editing** (C to U conversion by the enzyme *cytidine deaminase*), which creates a premature stop codon in the intestine. * **Atherogenic Index:** Total ApoB count is often considered a better predictor of cardiovascular risk than LDL-C because each atherogenic particle (VLDL, IDL, LDL) contains exactly one molecule of ApoB. * **Abetalipoproteinemia:** A deficiency in **Microsomal Triglyceride Transfer Protein (MTP)** leads to an inability to load ApoB with lipids, resulting in the near-absence of Chylomicrons, VLDL, and LDL in plasma.

Q: Which of the following is not a component of Sphingomyelin?

Glycerol. **Explanation:** The correct answer is **Glycerol**. Sphingomyelin is a unique membrane phospholipid because it is the only significant phospholipid that does **not** contain a glycerol backbone. Instead, it belongs to the class of **Sphingophospholipids**, where the structural foundation is an amino alcohol called **Sphingosine**. 1. **Why Glycerol is the correct answer:** In Glycerophospholipids (like Lecithin), fatty acids are esterified to glycerol. However, in Sphingomyelin, the backbone is Sphingosine. Therefore, glycerol is absent in its structure. 2. **Why other options are incorrect:** * **Sphingosine:** This is the 18-carbon amino alcohol backbone. When a fatty acid is attached to its amino group via an amide bond, it forms **Ceramide**, the precursor of sphingomyelin. * **Phosphoric acid:** Sphingomyelin is a phospholipid; the primary hydroxyl group of sphingosine is esterified to phosphoric acid. * **Choline:** The phosphate group is further linked to a nitrogenous base, usually **Phosphocholine** (or occasionally phosphoethanolamine). **High-Yield Clinical Pearls for NEET-PG:** * **Niemann-Pick Disease:** A lysosomal storage disorder caused by a deficiency of the enzyme **Sphingomyelinase**, leading to the accumulation of sphingomyelin in the liver, spleen, and brain (characterized by "Foam cells" and a cherry-red spot on the macula). * **Myelin Sheath:** Sphingomyelin is a major structural component of the myelin sheath insulating nerve fibers. * **L/S Ratio:** The Lecithin/Sphingomyelin ratio in amniotic fluid is used to assess fetal lung maturity (a ratio >2 indicates maturity).

Q: Which of the following processes occurs during a low insulin to glucagon ratio?

Ketogenesis. **Explanation:** A **low insulin-to-glucagon ratio** signifies a fasting or "starvation" state. In this hormonal milieu, the body shifts from energy storage to energy mobilization. **Why Ketogenesis is Correct:** When insulin is low and glucagon is high, **Hormone-Sensitive Lipase (HSL)** in adipose tissue is activated, leading to massive lipolysis. This floods the liver with free fatty acids (FFAs). Inside the liver, glucagon stimulates **Carnitine Palmitoyltransferase-1 (CPT-1)** by decreasing Malonyl-CoA levels. This allows FFAs to enter the mitochondria for $\beta$-oxidation, resulting in an excess of Acetyl-CoA. Since the TCA cycle is overwhelmed (and oxaloacetate is diverted toward gluconeogenesis), the liver converts this excess Acetyl-CoA into **ketone bodies** (Acetoacetate, $\beta$-hydroxybutyrate) to provide fuel for extrahepatic tissues like the brain. **Why the other options are incorrect:** * **Cholesterol Synthesis (A), Glycogen Synthesis (B), and Fatty Acid Synthesis (D)** are all **anabolic processes** stimulated by **Insulin** (high insulin-to-glucagon ratio). Insulin activates key rate-limiting enzymes: HMG-CoA reductase (cholesterol), Glycogen synthase (glycogen), and Acetyl-CoA Carboxylase (fatty acids) via dephosphorylation. **NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme of Ketogenesis:** HMG-CoA Synthase (Mitochondrial). * **Ketone bodies** are utilized by the brain, heart, and skeletal muscle, but **never by the liver** (due to the absence of the enzyme Thiophorase/$\beta$-ketoacyl-CoA transferase). * **Hormone-Sensitive Lipase (HSL)** is the "starvation enzyme," while **Lipoprotein Lipase (LPL)** is the "feeding enzyme."

Q: In which of the following metabolic disorders is this skin finding characteristically seen?

Fabry disease. ***Fabry disease*** - The skin finding shown is **angiokeratoma corporis diffusum**, which is pathognomonic for Fabry disease and presents in a characteristic **bathing trunk distribution**. - Caused by **alpha-galactosidase A deficiency** leading to **Gb3 accumulation** in blood vessels, resulting in these distinctive dark red, punctate vascular lesions. *Farber disease* - Characterized by **subcutaneous nodules** around joints and **hoarse voice** due to laryngeal involvement, not angiokeratomas. - Results from **acid ceramidase deficiency** causing ceramide accumulation, presenting with joint contractures and granulomatous lesions. *Galactosemia* - Associated with **cataracts**, **hepatomegaly**, and **intellectual disability** rather than characteristic skin lesions. - Caused by **galactose-1-phosphate uridyltransferase deficiency**, leading to galactose accumulation affecting liver, brain, and eyes. *Albinism* - Presents with **hypopigmentation** of skin, hair, and eyes due to **melanin synthesis defects**. - Characterized by **photosensitivity** and **nystagmus**, with no vascular skin lesions like angiokeratomas.

Question 1

Which cellular compartment is the primary site of fatty acid synthesis?

Accepted Answer

Cytosol

Answer

Endoplasmic Reticulum

Answer

Mitochondria

Answer

Microsomes

Question 2

The broad beta disease is due to a defect in which apolipoprotein?

Accepted Answer

Apolipoprotein E

Answer

Apolipoprotein A

Answer

Apolipoprotein B

Answer

Apolipoprotein C

Question 3

Where are unsaturated fatty acids synthesized?

Accepted Answer

Endoplasmic reticulum

Answer

Plasma membrane

Answer

Cytosol

Answer

Golgi complex

Question 4

What is the defect in Type II hyperlipidemia?

Accepted Answer

LDL receptor

Answer

Apolipoprotein E

Answer

Lipoprotein lipase

Answer

None of the above

Question 5

During prolonged starvation, adipose tissue is utilized as a primary energy source. Which of the following statements regarding this process is NOT true?

Accepted Answer

FFAs are released into the plasma and transported as chylomicrons to adipose tissue.

Answer

Free fatty acids (FFAs) are released by the action of hormone-sensitive lipase on stored triacylglycerols.

Answer

Free glycerol is released from adipose cells.

Answer

Hormone-sensitive lipase is activated, leading to the hydrolysis of stored triacylglycerols.

Question 6

A 1-year-old boy presented with failure to thrive and symptoms of severe fat malabsorption. The child was born at 39 weeks gestation via spontaneous vaginal delivery. The mother reported that the child's stools were pale, foul-smelling, and bulky. On examination, the child had a distended abdomen, absent deep tendon reflexes (DTRs), and slow intellectual development. Laboratory findings revealed low levels of serum cholesterol and serum triglycerides, with non-detectable VLDL and chylomicron levels. What of the following fatty acids should be most likely avoided in the diet in the described condition?

Accepted Answer

Very Long Chain Fatty Acids (VLCFA)

Answer

Medium Chain Fatty Acids (MCFA)

Answer

Short Chain Fatty Acids (SCFA)

Answer

Long Chain Fatty Acids (LCFA)

Question 7

ApoB is related to all EXCEPT:

Accepted Answer

HDL

Answer

LDL

Answer

VLDL

Answer

Chylomicrons

Question 8

Which of the following is not a component of Sphingomyelin?

Accepted Answer

Glycerol

Answer

Choline

Answer

Sphingosine

Answer

Phosphoric acid

Question 9

Which of the following processes occurs during a low insulin to glucagon ratio?

Accepted Answer

Ketogenesis

Answer

Cholesterol synthesis

Answer

Glycogen synthesis

Answer

Fatty acid synthesis

Question 10

In which of the following metabolic disorders is this skin finding characteristically seen?

Accepted Answer

Fabry disease

Answer

Farber disease

Answer

Galactosemia

Answer

Albinism

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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