Lipid Metabolism Practice Questions

Q: What is the main component of surfactant?

Dipalmitoyl phosphatidylcholine. **Explanation:** **1. Why Dipalmitoyl phosphatidylcholine (DPPC) is correct:** Lung surfactant is a complex mixture of lipids and proteins, but it is composed of approximately **90% lipids**. The most abundant and functionally significant lipid (accounting for ~50-60% of total surfactant) is **Dipalmitoyl phosphatidylcholine (DPPC)**, also known as **Lecithin**. * **Mechanism:** DPPC is an amphipathic molecule. Its hydrophobic fatty acid tails point toward the air, while the hydrophilic head points toward the alveolar water layer. This orientation significantly **reduces surface tension** at the air-liquid interface, preventing alveolar collapse (atelectasis) during expiration. **2. Why the other options are incorrect:** * **Phosphatidylinositol:** While present in surfactant, it is a minor component (approx. 5-10%) and does not play the primary role in reducing surface tension. * **Dipalmitoyl cephalin (Phosphatidylethanolamine):** Cephalin is a major component of cell membranes (especially nervous tissue) but is not the primary constituent of surfactant. * **Dipalmitoyl serine:** Phosphatidylserine is involved in cell signaling and apoptosis (flipping to the outer membrane leaflet), but it is not a major component of lung surfactant. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **L/S Ratio:** Fetal lung maturity is assessed by the **Lecithin/Sphingomyelin ratio** in amniotic fluid. A ratio **>2.0** indicates mature lungs. * **Type II Pneumocytes:** Surfactant is synthesized and secreted by Type II alveolar cells and stored in **Lamellar bodies**. * **NRDS:** Deficiency of surfactant in premature infants leads to **Neonatal Respiratory Distress Syndrome (Hyaline Membrane Disease)**. * **Glucocorticoids:** These are administered to mothers in preterm labor to stimulate surfactant production by inducing enzymes like *cholinephosphotransferase*.

Q: Which leukodystrophy is characterized by the inability to properly catabolize very-long-chain fatty acids (VLCFA) within peroxisomes, leading to elevated levels of VLCFA in serum?

Adrenoleukodystrophy. **Explanation:** **Adrenoleukodystrophy (ALD)** is an X-linked recessive disorder caused by a mutation in the **ABCD1 gene**, which encodes a peroxisomal membrane transporter protein. This defect prevents the entry of **Very-Long-Chain Fatty Acids (VLCFA)** into the peroxisome for **beta-oxidation**. Consequently, VLCFAs accumulate in the serum and tissues, particularly the adrenal cortex and the white matter of the CNS, leading to demyelination and adrenal insufficiency (Addison’s disease). **Analysis of Incorrect Options:** * **Metachromatic Leukodystrophy:** Caused by a deficiency of **Arylsulfatase A**, leading to the accumulation of **sulfatides**. It is characterized by "metachromasia" of nerves when stained with toluidine blue. * **Krabbe Disease (Globoid Cell Leukodystrophy):** Caused by a deficiency of **Galactocerebrosidase**, leading to the accumulation of galactocerebroside and psychosine. Histology shows characteristic multinucleated **globoid cells**. * **Ortho Leukodystrophy:** A general term for leukodystrophies with normal myelin breakdown products (like Pelizaeus-Merzbacher disease), but it is not specifically associated with VLCFA metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Peroxisomal Disorders:** ALD is the most common peroxisomal disorder. Another high-yield condition is **Zellweger Syndrome** (generalized peroxisome biogenesis defect), which also shows elevated VLCFAs but presents with more severe craniofacial dysmorphism and hepatomegaly. * **Clinical Triad of ALD:** Behavioral changes/cognitive decline, vision/hearing loss, and adrenal failure. * **Biochemical Marker:** Elevated **C24:0 and C26:0** fatty acids in plasma.

Q: HMG-CoA Reductase is associated with which of the following conditions?

Familial Hypercholesterolemia. ### Explanation **Correct Option: B. Familial Hypercholesterolemia** HMG-CoA Reductase is the **rate-limiting enzyme** in the de novo synthesis of cholesterol, converting HMG-CoA to Mevalonate. In **Familial Hypercholesterolemia (FH)**, there is a primary defect in LDL receptors (Type IIa hyperlipidemia). This leads to decreased cellular uptake of LDL-cholesterol. Under normal physiological conditions, intracellular cholesterol exerts **negative feedback inhibition** on HMG-CoA Reductase. In FH, because cholesterol cannot enter the cell effectively, this feedback mechanism fails, leading to the constitutive over-activation of HMG-CoA Reductase and excessive endogenous cholesterol production. Statins, the mainstay of treatment, work by competitively inhibiting this specific enzyme. **Analysis of Incorrect Options:** * **A. Type 2 Diabetes:** Primarily associated with insulin resistance and dysregulation of enzymes like Glucokinase or PEPCK, rather than a primary defect involving HMG-CoA Reductase. * **C. Gaucher's disease:** This is a Lysosomal Storage Disorder caused by a deficiency of **Glucocerebrosidase** (Acid β-glucosidase), leading to the accumulation of glucosylceramide. * **D. Abetalipoproteinemia:** Caused by a mutation in the **Microsomal Triglyceride Transfer Protein (MTP)**, resulting in an inability to assemble or secrete ApoB-containing lipoproteins (VLDL, LDL, and Chylomicrons). **High-Yield Clinical Pearls for NEET-PG:** * **Location:** HMG-CoA Reductase is located in the **Endoplasmic Reticulum (ER)** membrane. * **Regulation:** It is inhibited by **Glucagon** (via phosphorylation) and activated by **Insulin** (via dephosphorylation). * **Statins:** These are structural analogs of HMG-CoA and act as reversible competitive inhibitors. * **SREBP Path:** Low intracellular cholesterol triggers SREBP (Sterol Regulatory Element Binding Protein) to increase the transcription of the HMG-CoA Reductase gene.

Q: Saturated fatty acids containing up to 16 carbon atoms are assembled in which cellular location?

Cytoplasm. **Explanation:** The correct answer is **Cytoplasm** because the **De novo synthesis of fatty acids** (specifically Palmitate, a 16-carbon saturated fatty acid) occurs primarily in the cytosol. This process is catalyzed by the **Fatty Acid Synthase (FAS) multienzyme complex**. The primary substrate is Acetyl-CoA, which is transported from the mitochondria to the cytoplasm via the **Citrate-Malate shuttle**. The rate-limiting step of this pathway is the conversion of Acetyl-CoA to Malonyl-CoA by the enzyme Acetyl-CoA Carboxylase (ACC). **Analysis of Incorrect Options:** * **Mitochondria:** While the mitochondria are the primary site for **$\beta$-oxidation** (breakdown) of fatty acids and the synthesis of Acetyl-CoA, they are not the site for assembly of fatty acids up to 16 carbons. A separate system for fatty acid elongation exists here, but it is minor. * **Smooth Endoplasmic Reticulum (SER):** The SER is the site for **chain elongation** (beyond 16 carbons) and **desaturation** (adding double bonds) of pre-existing fatty acids. It is also the primary site for triacylglycerol and phospholipid synthesis. * **Rough Endoplasmic Reticulum (RER):** This organelle is primarily involved in protein synthesis and post-translational modifications, not lipid assembly. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Acetyl-CoA Carboxylase (requires **Biotin** as a cofactor). * **Reducing equivalent:** **NADPH** is the essential electron donor for fatty acid synthesis, primarily supplied by the Hexose Monophosphate (HMP) Shunt. * **Key Shuttle:** The **Citrate shuttle** moves Acetyl-CoA out of the mitochondria; Citrate acts as an allosteric activator of Acetyl-CoA Carboxylase. * **Inhibitor:** Palmitoyl-CoA (the end product) provides feedback inhibition.

Q: Enzyme imiglucerase (Cerezyme) is used in the treatment of which condition?

Gaucher's disease. **Explanation:** **Imiglucerase (Cerezyme)** is a recombinant form of the human lysosomal enzyme **beta-glucocerebrosidase**. It is the mainstay of **Enzyme Replacement Therapy (ERT)** for **Gaucher’s disease**, the most common lysosomal storage disorder. 1. **Why Gaucher’s Disease is Correct:** Gaucher’s disease is caused by a deficiency of the enzyme glucocerebrosidase, leading to the accumulation of glucosylceramide (glucocerebroside) in macrophages. These "Gaucher cells" infiltrate the bone marrow, liver, and spleen. Imiglucerase replaces the deficient enzyme, reducing organomegaly and improving hematological parameters. 2. **Why Other Options are Incorrect:** * **Galactosemia:** This is a metabolic disorder of carbohydrate metabolism (deficiency of GALT, GALK, or GALE). Treatment is dietary restriction of galactose/lactose, not ERT. * **Niemann-Pick Disease:** This is caused by a deficiency of **sphingomyelinase** (Types A and B). While ERT (Olipudase alfa) exists for Type B, Imiglucerase is specific only to the glucocerebrosidase deficiency in Gaucher’s. * **Trans-maxillary approach:** This is a surgical technique (often used for skull base tumors), not a biochemical condition or a target for pharmacological therapy. **High-Yield Clinical Pearls for NEET-PG:** * **Gaucher Cells:** Described as having a **"wrinkled paper"** or "crumpled silk" cytoplasm. * **Clinical Triad:** Hepatosplenomegaly, bone pain/crises (Erlenmeyer flask deformity of femur), and cytopenias. * **Biochemical Marker:** Elevated levels of **Chitotriosidase** are used to monitor disease activity and response to ERT. * **Alternative Treatment:** **Miglustat** or **Eliglustat** (Substrate Reduction Therapy) can be used for patients who cannot tolerate ERT.

Q: In congenital adrenal hyperplasia, precocious puberty in males is due to which of the following?

3 beta hydroxysteroid dehydrogenase deficiency. **Explanation:** In Congenital Adrenal Hyperplasia (CAH), precocious puberty in males is categorized as **Peripheral Precocious Puberty** (GnRH-independent). This occurs because the enzyme block leads to the shunting of steroid precursors into the androgen synthesis pathway. **Why Option C is Correct:** **3-beta hydroxysteroid dehydrogenase (3β-HSD) deficiency** is a rare form of CAH. In this condition, there is a block in the conversion of Pregnenolone to Progesterone and DHEA to Androstenedione. While it results in incomplete virilization (ambiguous genitalia) in males due to low testosterone, the accumulation of **Dehydroepiandrosterone (DHEA)**—a weak androgen—is significant. This excess DHEA can lead to premature pubarche, accelerated bone age, and signs of precocious puberty in early childhood. **Why Other Options are Incorrect:** * **Option A (21-alpha hydroxylase deficiency):** This is the most common cause of CAH (90%). While it causes virilization in females and pseudoprecocious puberty in males, the question specifically targets the mechanism where 3β-HSD is the identified answer in this specific MCQ context. (Note: In many standard texts, 21-OH and 11-β deficiency also cause precocious puberty; however, 3β-HSD is unique because it presents with DHEA excess). * **Option B (11-beta hydroxylase deficiency):** This leads to excess androgens and mineralocorticoid precursors (11-deoxycorticosterone), causing virilization and **hypertension**. **High-Yield Clinical Pearls for NEET-PG:** * **Most Common CAH:** 21-alpha hydroxylase deficiency (presents with salt-wasting and hypotension). * **CAH with Hypertension:** 11-beta hydroxylase and 17-alpha hydroxylase deficiency. * **Ambiguous Genitalia in Males:** Seen in 3β-HSD and 17-alpha hydroxylase deficiency. * **Diagnostic Marker:** 17-OH Progesterone is elevated in 21-alpha hydroxylase deficiency.

Q: Acetyl CoA carboxylase is activated by which molecule?

Citrate. **Explanation:** **Acetyl CoA Carboxylase (ACC)** is the **rate-limiting enzyme** in de novo fatty acid synthesis (lipogenesis). It catalyzes the conversion of Acetyl CoA to Malonyl CoA. **1. Why Citrate is Correct:** Citrate acts as a **feed-forward allosteric activator**. Fatty acid synthesis occurs in the cytoplasm, but Acetyl CoA is produced in the mitochondria. Since Acetyl CoA cannot cross the mitochondrial membrane, it condenses with oxaloacetate to form Citrate, which is then shuttled into the cytoplasm. High levels of cytoplasmic Citrate signal an energy surplus, triggering the **polymerization** of inactive ACC dimers into active long filaments, thereby stimulating lipogenesis. **2. Analysis of Incorrect Options:** * **Malonyl CoA (A):** This is the immediate product of the ACC reaction. It does not activate ACC; rather, it serves as a potent inhibitor of *Carnitine Palmitoyltransferase-I (CPT-1)*, preventing the entry of fatty acids into mitochondria for beta-oxidation. * **Palmitoyl CoA (C):** This is the end-product of fatty acid synthesis. It acts as a **feedback allosteric inhibitor**, causing the active ACC filaments to dissociate back into inactive dimers. * **Acetoacetate (D):** This is a ketone body. While it is related to lipid metabolism, it does not play a direct regulatory role in the activation of ACC. **High-Yield Clinical Pearls for NEET-PG:** * **Hormonal Regulation:** ACC is activated by **Insulin** (via dephosphorylation) and inhibited by **Glucagon/Epinephrine** (via phosphorylation by AMPK). * **Cofactor Requirement:** ACC requires **Biotin (Vitamin B7)**, ATP, and $CO_2$ (ABC carboxylase rule). * **Location:** Occurs in the "Liver, Lactating mammary gland, and Adipose tissue" (Mnemonic: **LLA**).

Q: All of the following sphingolipidoses have an autosomal recessive inheritance pattern except?

Fabry's disease. **Explanation:** The correct answer is **Fabry’s disease**. In medical genetics and biochemistry, most enzyme deficiencies (including most lysosomal storage disorders) follow an **Autosomal Recessive (AR)** inheritance pattern. However, Fabry’s disease and Hunter syndrome (a mucopolysaccharidosis) are the two major exceptions that follow an **X-linked Recessive (XLR)** pattern. **1. Why Fabry’s disease is the correct answer:** Fabry’s disease is caused by a deficiency of the enzyme **$\alpha$-galactosidase A**, leading to the accumulation of **ceramide trihexoside**. Because the gene encoding this enzyme is located on the X chromosome, it is inherited in an X-linked recessive manner, primarily affecting males. **2. Why the other options are incorrect:** * **Niemann-Pick disease:** Caused by sphingomyelinase deficiency; it follows a classic AR pattern. * **Tay-Sachs disease:** Caused by Hexosaminidase A deficiency; it is a classic AR disorder, notably prevalent in the Ashkenazi Jewish population. * **Gaucher’s disease:** The most common lysosomal storage disorder, caused by $\beta$-glucocerebrosidase deficiency; it follows an AR pattern. **NEET-PG High-Yield Pearls:** * **Mnemonic for X-linked Recessive Lysosomal Disorders:** "The **Hunter** aimed for the **Fabry** (Fabric)" $\rightarrow$ **Hunter Syndrome** and **Fabry’s Disease** are XLR. * **Clinical Triad of Fabry’s:** Episodic peripheral neuropathy (burning pain), angiokeratomas, and hypohidrosis. Late-stage complications include renal failure and cardiovascular disease. * **Biopsy Finding:** "Maltese crosses" (lipid globules) in urine sediment or "Zebra bodies" on electron microscopy are characteristic of Fabry's.

Question 1

What is the main component of surfactant?

Accepted Answer

Dipalmitoyl phosphatidylcholine

Answer

Phosphatidylinositol

Answer

Dipalmitoyl cephalin

Answer

Dipalmitoyl serine

Question 2

Which leukodystrophy is characterized by the inability to properly catabolize very-long-chain fatty acids (VLCFA) within peroxisomes, leading to elevated levels of VLCFA in serum?

Accepted Answer

Adrenoleukodystrophy

Answer

Ortho leukodystrophy

Answer

Metachromatic leukodystrophy

Answer

Krabbe disease

Question 3

HMG-CoA Reductase is associated with which of the following conditions?

Accepted Answer

Familial Hypercholesterolemia

Answer

Type 2 Diabetes

Answer

Gaucher's disease

Answer

Abetalipoproteinemia

Question 4

Saturated fatty acids containing up to 16 carbon atoms are assembled in which cellular location?

Accepted Answer

Cytoplasm

Answer

Mitochondria

Answer

Rough endoplasmic reticulum

Answer

Smooth endoplasmic reticulum

Question 5

Accumulation of sphingomyelin in the spleen and liver is found in which condition?

Accepted Answer

Niemann-Pick disease

Answer

Gaucher's disease

Answer

Obstructive jaundice

Answer

Von Gierke's disease

Question 6

Enzyme imiglucerase (Cerezyme) is used in the treatment of which condition?

Accepted Answer

Gaucher's disease

Answer

Galactosemia

Answer

Niemann Pick disease

Answer

Trans-maxillary approach

Question 7

In congenital adrenal hyperplasia, precocious puberty in males is due to which of the following?

Accepted Answer

3 beta hydroxysteroid dehydrogenase deficiency

Answer

21 alpha hydroxylase deficiency

Answer

11 beta hydroxylase deficiency

Answer

None of the above

Question 8

Acetyl CoA carboxylase is activated by which molecule?

Accepted Answer

Citrate

Answer

Malonyl CoA

Answer

Palmitoyl CoA

Answer

Acetoacetate

Question 9

All of the following sphingolipidoses have an autosomal recessive inheritance pattern except?

Accepted Answer

Fabry's disease

Answer

Niemann-Pick disease

Answer

Tay-Sachs disease

Answer

Gaucher's disease

Question 10

Multiple sclerosis is characterized by a loss of which lipids?

Accepted Answer

Phospholipids and sphingolipids

Answer

Phospholipids and ceramide

Answer

Sphingolipids and ceramide

Answer

Sphingolipids and gangliosides

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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