Lipid Metabolism Practice Questions

Q: Leukotrienes are produced from which precursor molecule?

Arachidonic acid. **Explanation:** **Leukotrienes** are a family of inflammatory mediators belonging to a class of compounds called **Eicosanoids** (20-carbon fatty acid derivatives). 1. **Why Arachidonic Acid is Correct:** Arachidonic acid is a 20-carbon polyunsaturated fatty acid (PUFA) found in the phospholipids of cell membranes. Upon stimulation (e.g., inflammation), it is released by the enzyme **Phospholipase A2**. It then enters one of two major pathways: * **Lipoxygenase (LOX) pathway:** Specifically, 5-LOX converts arachidonic acid into Leukotrienes (LTA4, LTB4, LTC4, LTD4, and LTE4). * **Cyclooxygenase (COX) pathway:** Converts it into Prostaglandins and Thromboxanes. 2. **Why Other Options are Incorrect:** * **Cholesterol (A):** This is a steroid precursor used for synthesizing steroid hormones (cortisol, estrogen), bile acids, and Vitamin D, but not eicosanoids. * **Stearic acid (C):** A 18-carbon saturated fatty acid. While it can be desaturated and elongated, it is not the direct precursor for leukotrienes. * **Palmitic acid (D):** A 16-carbon saturated fatty acid; it is the first fatty acid produced during *de novo* fatty acid synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **LTA4** is the precursor for all other leukotrienes. * **LTB4** is a potent **chemotactic agent** for neutrophils ("B" for "Bacteria/Binding" neutrophils). * **LTC4, LTD4, and LTE4** (Cysteinyl leukotrienes) are known as the **Slow-Reacting Substance of Anaphylaxis (SRS-A)**, causing potent bronchoconstriction and mucus secretion in asthma. * **Zileuton** inhibits 5-LOX, while **Montelukast/Zafirlukast** are leukotriene receptor antagonists (LTRAs) used in asthma management.

Q: Fabry’s disease is caused due to deficiency of which enzyme?

Alpha Galactosidase. **Explanation:** **Fabry’s disease** is a lysosomal storage disorder (Sphingolipidosis) characterized by the deficiency of the enzyme **Alpha-galactosidase A**. This deficiency leads to the systemic accumulation of **Globotriaosylceramide (Gb3)**, also known as ceramide trihexoside, within the lysosomes of vascular endothelial cells and other tissues. * **Why Option A is correct:** Alpha-galactosidase A is responsible for cleaving the terminal galactose from globotriaosylceramide. Its absence causes progressive cellular damage, primarily affecting the kidneys, heart, and nervous system. * **Why Option B is incorrect:** Deficiency of **Ceramidase** leads to **Farber disease**, characterized by painful joint swelling, hoarseness (laryngeal involvement), and subcutaneous nodules. * **Why Option C is incorrect:** Deficiency of the **Debranching enzyme** (Amylo-1,6-glucosidase) causes **Cori’s disease (GSD Type III)**, a glycogen storage disorder presenting with hepatomegaly and hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** 1. **Inheritance:** Fabry’s is unique among sphingolipidoses as it is **X-linked Recessive** (most others are Autosomal Recessive). 2. **Early Signs:** Characterized by **Angiokeratomas** (dark red skin spots), **Hypohidrosis** (decreased sweating), and **Acroparesthesia** (burning pain in extremities). 3. **Late Complications:** Progressive **Renal failure** (most common cause of death), hypertrophic cardiomyopathy, and early-onset stroke. 4. **Ocular Finding:** **Cornea verticillata** (whorl-like corneal opacities) is a classic diagnostic sign. 5. **Treatment:** Enzyme Replacement Therapy (ERT) with Agalsidase beta.

Q: A child presents with hepatosplenomegaly and pancytopenia. Bone marrow shows "crumbled tissue paper appearance". It is due to accumulation of which substance?

Glucocerebroside. ### Explanation The clinical presentation of **hepatosplenomegaly**, **pancytopenia** (due to hypersplenism), and the pathognomonic **"crumbled tissue paper"** appearance of macrophages in the bone marrow is diagnostic of **Gaucher disease**, the most common lysosomal storage disorder. **1. Why Glucocerebroside is correct:** Gaucher disease is caused by a deficiency of the enzyme **β-Glucosidase** (also known as glucocerebrosidase). This deficiency leads to the accumulation of **glucocerebroside** within the lysosomes of macrophages. These overloaded macrophages are called "Gaucher cells," and their cytoplasm appears fibrillary or like "crumbled tissue paper" under the microscope. **2. Why the other options are incorrect:** * **Sphingomyelin:** Accumulates in **Niemann-Pick disease** (deficiency of sphingomyelinase). It presents with a "foam cell" appearance (vacuolated) and a cherry-red spot on the macula, but not the crumbled tissue paper look. * **Ganglioside (GM2):** Accumulates in **Tay-Sachs disease**. It is characterized by neurodegeneration and a cherry-red spot, but notably lacks hepatosplenomegaly. * **Galactocerebroside:** Accumulates in **Krabbe disease** (deficiency of galactocerebrosidase). It presents with "globoid cells" and severe demyelination, not hepatosplenomegaly. **Clinical Pearls for NEET-PG:** * **Enzyme Deficient:** Acid β-glucosidase / Glucocerebrosidase. * **Gaucher Cells:** PAS-positive macrophages with wrinkled paper cytoplasm. * **Skeletal Involvement:** Look for "Erlenmeyer flask deformity" of the distal femur and avascular necrosis of the femoral head. * **Biochemical Marker:** Elevated levels of **Chitotriosidase** are used for diagnosis and monitoring treatment. * **Treatment:** Enzyme Replacement Therapy (ERT) with Alglucerase or Imiglucerase.

Q: Which of the following steroid hormones is a C-21 steroid?

Progesterone. **Explanation:** Steroid hormones are derivatives of cholesterol and are classified based on the number of carbon atoms in their structure. Understanding these carbon counts is a high-yield topic for NEET-PG. **1. Why Progesterone is Correct:** Progesterone belongs to the **Pregnane** group. All hormones in this group, including **Progesterone**, **Aldosterone**, and **Cortisol**, contain **21 carbon atoms (C-21)**. These are synthesized from pregnenolone, which is the immediate 21-carbon derivative of cholesterol (C-27). **2. Analysis of Incorrect Options:** * **A & C. Testosterone and Androgens:** These belong to the **Androstane** group. Androgens (like Testosterone and Dehydroepiandrosterone/DHEA) contain **19 carbon atoms (C-19)**. They are formed by the cleavage of the side chain of C-21 steroids. * **B. Estrogen:** Estrogens (like Estradiol, Estrone, and Estriol) belong to the **Estrane** group. They contain **18 carbon atoms (C-18)**. They are formed via the aromatization of C-19 androgens, a process that involves the loss of the C-19 methyl group. **High-Yield NEET-PG Pearls:** * **Carbon Count Summary:** * **C-27:** Cholesterol (Precursor) * **C-21:** Progestogens, Glucocorticoids, Mineralocorticoids (Pregnane series) * **C-19:** Androgens (Androstane series) * **C-18:** Estrogens (Estrane series) * **Rate-limiting step:** The conversion of Cholesterol (C-27) to Pregnenolone (C-21) by the enzyme **Desmolase** (CYP11A1) in the mitochondria. * **Aromatase:** The enzyme responsible for converting C-19 androgens to C-18 estrogens.

Q: Where are glycolipids synthesized?

Endoplasmic reticulum. **Explanation:** The synthesis of glycolipids (specifically glycosphingolipids) is a multi-step process that primarily occurs within the membrane systems of the **Endoplasmic Reticulum (ER)** and the Golgi apparatus. 1. **Why the Endoplasmic Reticulum is correct:** The lipid backbone of most glycolipids, known as **ceramide**, is synthesized on the cytosolic face of the smooth ER. While the subsequent addition of sugar moieties (glycosylation) primarily occurs in the Golgi apparatus, the foundational assembly and the initial steps of sphingolipid synthesis are anchored in the ER. In the context of medical exams, the ER is recognized as the primary site for the synthesis of membrane lipids, including phospholipids and the ceramide base for glycolipids. 2. **Why other options are incorrect:** * **Mitochondria:** Primarily involved in ATP production, the Citric Acid Cycle, and beta-oxidation of fatty acids; they do not synthesize complex glycolipids. * **Cytosol:** While some initial precursors (like malonyl-CoA) are formed here, the complex assembly of hydrophobic glycolipids requires membrane-bound enzymes found in organelles. * **Peroxisomes:** These are responsible for the initial steps of **plasmalogen** synthesis and the oxidation of very-long-chain fatty acids (VLCFA), but not glycolipid assembly. **High-Yield Clinical Pearls for NEET-PG:** * **Ceramide** is the fundamental structural unit of all sphingolipids (Ceramide = Sphingosine + Fatty Acid). * **Sphingolipidoses:** Genetic deficiencies in the lysosomal enzymes that *degrade* glycolipids lead to storage diseases like **Gaucher’s** (glucocerebrosidase deficiency) and **Tay-Sachs** (hexosaminidase A deficiency). * **Site Summary:** Lipid synthesis = Smooth ER; Protein synthesis = Rough ER; Lipid/Protein modification = Golgi.

Q: A 10-year-old boy presents with a white ring around the cornea. Physical examination reveals corneal arcus and xanthoma on the Achilles tendon. His fasting blood cholesterol is >300 mg/dl, and triglycerides are within the normal limit. His father had a history of coronary heart disease. What is the most likely diagnosis?

Type II A Hyperlipoproteinemia. ### **Explanation** **Correct Answer: B. Type II A Hyperlipoproteinemia** The clinical presentation of **coronary heart disease (CHD)** at a young age, **corneal arcus**, and **tendon xanthomas** (specifically on the Achilles tendon) is classic for **Familial Hypercholesterolemia (Type IIa)**. * **Underlying Concept:** Type IIa is characterized by a deficiency or defect in **LDL receptors**, leading to decreased clearance of LDL from the plasma. This results in isolated elevation of **LDL cholesterol** (Total Cholesterol >300 mg/dl) while **Triglycerides (TG) remain normal**, as seen in this patient. --- ### **Why Incorrect Options are Wrong:** * **Type I (Familial Chylomicronemia):** Caused by Lipoprotein Lipase (LPL) or Apo C-II deficiency. It presents with extremely high **Triglycerides**, eruptive xanthomas, and pancreatitis, but *not* premature CHD or tendon xanthomas. * **Type II B (Combined Hyperlipidemia):** Characterized by elevations in both **LDL and VLDL**. Therefore, both cholesterol and **triglycerides** would be elevated, which contradicts the normal TG levels in this case. * **Type III (Dysbetalipoproteinemia):** Caused by Apo E deficiency, leading to the accumulation of IDL (remnants). It typically presents with **palmar xanthomas** and elevation of both cholesterol and triglycerides. --- ### **High-Yield Clinical Pearls for NEET-PG:** * **Tendon Xanthoma:** Pathognomonic for Type IIa Hyperlipoproteinemia. * **Palmar Xanthoma (Xanthoma Striatum Palmare):** Pathognomonic for Type III Hyperlipoproteinemia. * **Eruptive Xanthoma:** Associated with severe Hypertriglyceridemia (Type I, IV, V). * **Fredrickson Classification:** Remember that Type IIa is "Pure Hypercholesterolemia" (↑LDL), while Type IIb is "Mixed Hyperlipidemia" (↑LDL + ↑VLDL).

Q: Under metabolic conditions associated with a high rate of fatty acid oxidation, what does the liver produce?

Acetoacetate. **Explanation:** The correct answer is **Acetoacetate**. **Why it is correct:** Under conditions of high fatty acid oxidation (such as starvation, prolonged fasting, or uncontrolled diabetes mellitus), there is an overproduction of **Acetyl-CoA** via $\beta$-oxidation. This excess Acetyl-CoA exceeds the capacity of the TCA cycle (partly because oxaloacetate is being diverted for gluconeogenesis). The liver diverts this surplus Acetyl-CoA into the **Ketogenesis** pathway. Acetoacetate is the "primary" ketone body formed, which can then be reduced to $\beta$-hydroxybutyrate or spontaneously decarboxylated to acetone. **Why the other options are incorrect:** * **Glutamate:** This is an amino acid involved in transamination and nitrogen metabolism, not a direct product of fatty acid oxidation. * **Cholesterol:** While cholesterol is synthesized from Acetyl-CoA, its synthesis is inhibited during high fatty acid oxidation (low energy states/fasting) because the rate-limiting enzyme, HMG-CoA reductase, is inactivated. * **Glycine:** This is the simplest non-essential amino acid and is not a product of lipid catabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Ketogenesis:** Occurs exclusively in the **mitochondria of hepatocytes**. * **Rate-limiting enzyme:** **HMG-CoA Synthase** (Mitochondrial). Note that the cytosolic version of this enzyme is used for cholesterol synthesis. * **Utilization:** The liver **cannot** use ketone bodies for energy because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). * **Ketone Bodies:** Acetoacetate and $\beta$-hydroxybutyrate are acidic; their accumulation leads to metabolic acidosis (Ketoacidosis).

Q: A 10-year-old boy admitted for cleft lip surgery suddenly developed acute abdominal pain and was found to have xanthomas and milky plasma on investigation. Which lipoprotein is increased?

Chylomicron. **Explanation:** The clinical presentation of **milky plasma** (lipemic serum) and **acute abdominal pain** (suggestive of acute pancreatitis) in a young child is a classic hallmark of **Type I Hyperlipoproteinemia** (Familial Chylomicronemia Syndrome). 1. **Why Chylomicron is correct:** Chylomicrons are the largest, least dense lipoproteins, primarily composed of dietary triglycerides. When there is a deficiency in **Lipoprotein Lipase (LPL)** or its cofactor **Apo C-II**, chylomicrons cannot be cleared from the blood. This massive accumulation causes the plasma to appear "milky" or "creamy." The high levels of chylomicrons can lead to capillary plugging in the pancreas, causing acute pancreatitis (abdominal pain) and the formation of eruptive xanthomas. 2. **Why other options are incorrect:** * **VLDL Remnant:** These are increased in Type III Hyperlipoproteinemia (Dysbetalipoproteinemia). While they cause xanthomas (specifically palmar xanthomas), they do not typically cause the classic "milky plasma" seen in Type I. * **Triglycerides:** While triglycerides are indeed elevated, the question asks which **lipoprotein** is increased. Triglycerides are a lipid component, not a lipoprotein particle. * **Cholesterol:** Cholesterol is primarily carried by LDL. Elevated LDL (Type IIa) leads to xanthelasmas and tendon xanthomas but does not cause milky plasma or acute pancreatitis. **High-Yield Clinical Pearls for NEET-PG:** * **Refrigeration Test:** In Type I, a creamy layer forms on top of the plasma when left overnight, while the layer below remains clear. * **Defect:** Mutation in *LPL* gene (most common) or *APOC2* gene. * **Risk:** The primary life-threatening complication is **Acute Pancreatitis**, not atherosclerosis. * **Treatment:** Strict fat-restricted diet (medium-chain triglycerides are preferred as they bypass chylomicron formation).

Q: HDL3 is converted to HDL2 by which enzyme?

LCAT. **Explanation:** The conversion of **HDL3 to HDL2** is a critical step in **Reverse Cholesterol Transport (RCT)**. **Why LCAT is the correct answer:** HDL3 is a small, dense, protein-rich particle. The enzyme **LCAT (Lecithin-Cholesterol Acyltransferase)**, which is activated by **Apo A-I**, catalyzes the transfer of a fatty acid from lecithin to free cholesterol on the surface of HDL3. This creates **cholesterol esters**, which are highly hydrophobic and move into the core of the particle. As the core expands with these esters, the HDL3 particle increases in size and decreases in density, transforming into the larger, spherical **HDL2**. **Analysis of Incorrect Options:** * **CETP (Cholesteryl Ester Transfer Protein):** This protein facilitates the exchange of cholesteryl esters from HDL2 to VLDL/LDL in exchange for triglycerides. This process actually helps convert HDL2 back into HDL3 (the reverse of the question). * **PLTP (Phospholipid Transfer Protein):** This protein transfers phospholipids from triglyceride-rich lipoproteins (like VLDL) to HDL, helping in HDL remodeling, but it is not the primary driver of the HDL3 to HDL2 maturation. **High-Yield Clinical Pearls for NEET-PG:** * **HDL2** is considered the most cardioprotective form of HDL. * **Hepatic Lipase (HL)** converts HDL2 back to HDL3 by hydrolyzing triglycerides and phospholipids. * **Fish-Eye Disease:** A partial LCAT deficiency where only alpha-LCAT is affected. * **Tangier Disease:** Caused by a mutation in the **ABCA1 transporter**, leading to a near-total absence of HDL.

Q: All of the following statements about LDL are true EXCEPT:

It contains Apo-B48. **Explanation:** The correct answer is **D (It contains Apo-B48)** because this statement is factually incorrect. **Apolipoprotein B-100 (Apo-B100)** is the primary structural protein found in LDL, VLDL, and IDL. In contrast, **Apo-B48** is unique to **chylomicrons** and their remnants, synthesized exclusively in the intestinal mucosal cells. **Analysis of Options:** * **Option A (Delivers cholesterol to cells):** This is true. LDL is the primary carrier of cholesterol in the blood. It transports cholesterol from the liver to peripheral tissues via receptor-mediated endocytosis (LDL receptors). * **Option B (Contains only one apolipoprotein):** This is true. Unlike other lipoproteins that carry multiple types of apolipoproteins (like HDL), mature LDL particles contain **only one** molecule of **Apo-B100**. * **Option C (Marker for cardiovascular disease):** This is true. LDL is often termed "bad cholesterol." High levels lead to cholesterol deposition in arterial walls, leading to atherosclerosis and increased risk of myocardial infarction. **High-Yield NEET-PG Pearls:** * **Apo-B100 vs. Apo-B48:** Both are products of the same gene. Apo-B48 is formed via **RNA editing** (C to U conversion by cytidine deaminase), which creates a premature stop codon, resulting in a protein that is 48% the length of Apo-B100. * **LDL Formation:** LDL is not secreted directly; it is the "end product" of VLDL metabolism (VLDL → IDL → LDL). * **Friedewald Equation:** LDL Cholesterol = Total Cholesterol – [HDL + (Triglycerides/5)]. (Note: This is invalid if TG >400 mg/dL).

Question 1

Leukotrienes are produced from which precursor molecule?

Accepted Answer

Arachidonic acid

Answer

Cholesterol

Answer

Stearic acid

Answer

Palmitic acid

Question 2

Fabry’s disease is caused due to deficiency of which enzyme?

Accepted Answer

Alpha Galactosidase

Answer

Ceramidase

Answer

Debranching enzyme

Answer

None of the above

Question 3

A child presents with hepatosplenomegaly and pancytopenia. Bone marrow shows "crumbled tissue paper appearance". It is due to accumulation of which substance?

Accepted Answer

Glucocerebroside

Answer

Sphingomyelin

Answer

Ganglioside

Answer

Galactocerebroside

Question 4

Which of the following steroid hormones is a C-21 steroid?

Accepted Answer

Progesterone

Answer

Testosterone

Answer

Estrogen

Answer

Androgen

Question 5

Where are glycolipids synthesized?

Accepted Answer

Endoplasmic reticulum

Answer

Mitochondria

Answer

Cytosol

Answer

Peroxisomes

Question 6

A 10-year-old boy presents with a white ring around the cornea. Physical examination reveals corneal arcus and xanthoma on the Achilles tendon. His fasting blood cholesterol is >300 mg/dl, and triglycerides are within the normal limit. His father had a history of coronary heart disease. What is the most likely diagnosis?

Accepted Answer

Type II A Hyperlipoproteinemia

Answer

Type I Hyperlipoproteinemia

Answer

Type II B Hyperlipoproteinemia

Answer

Type III Hyperlipoproteinemia

Question 7

Under metabolic conditions associated with a high rate of fatty acid oxidation, what does the liver produce?

Accepted Answer

Acetoacetate

Answer

Glutamate

Answer

Cholesterol

Answer

Glycine

Question 8

A 10-year-old boy admitted for cleft lip surgery suddenly developed acute abdominal pain and was found to have xanthomas and milky plasma on investigation. Which lipoprotein is increased?

Accepted Answer

Chylomicron

Answer

VLDL Remnant

Answer

Triglycerides

Answer

Cholesterol

Question 9

HDL3 is converted to HDL2 by which enzyme?

Accepted Answer

LCAT

Answer

CETP

Answer

Both

Answer

PLTP

Question 10

All of the following statements about LDL are true EXCEPT:

Accepted Answer

It contains Apo-B48

Answer

It delivers cholesterol to cells

Answer

It contains only one apolipoprotein

Answer

It is a marker for cardiovascular disease

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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