Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 421: What is the primary role of bile salts?

A. Vitamin B12 absorption
B. Formation of lipid bilayer
C. Emulsification of lipids (Correct Answer)
D. Fatty acid degradation

Explanation: **Explanation:** The primary role of bile salts is the **emulsification of dietary lipids**. Lipids are hydrophobic and tend to aggregate into large globules in the aqueous environment of the small intestine. Bile salts (derivatives of cholesterol like cholate and chenodeoxycholate) are **amphipathic** molecules, meaning they possess both hydrophilic and hydrophobic faces. They coat lipid droplets, reducing surface tension and breaking them down into smaller particles. This increases the surface area available for **pancreatic lipase** to act, which is essential for efficient lipid digestion and subsequent micelle formation. **Analysis of Incorrect Options:** * **A. Vitamin B12 absorption:** This requires **Intrinsic Factor**, secreted by the parietal cells of the stomach, and occurs in the terminal ileum. Bile salts are not directly involved. * **B. Formation of lipid bilayer:** This is the structural role of **phospholipids** (like phosphatidylcholine) and cholesterol within cell membranes, not the functional role of bile salts in digestion. * **C. Fatty acid degradation:** This refers to **Beta-oxidation**, which occurs intracellularly within the mitochondria. Bile salts function extracellularly in the intestinal lumen. **NEET-PG High-Yield Pearls:** * **Rate-limiting step:** Bile acid synthesis is regulated by the enzyme **7-alpha-hydroxylase** (inhibited by bile acids). * **Enterohepatic Circulation:** Approximately 95% of bile salts are reabsorbed in the **terminal ileum** and returned to the liver. * **Clinical Correlation:** Malabsorption of bile salts (e.g., in Crohn’s disease affecting the ileum) leads to **steatorrhea** and deficiency of fat-soluble vitamins (A, D, E, K). * **Composition:** Bile salts are bile acids conjugated with **glycine or taurine**, which lowers their pKa and makes them better emulsifiers at intestinal pH.

Question 422: Which of the following is an allosteric activator of Acetyl-CoA carboxylase?

A. Malonyl-CoA
B. Acetyl-CoA
C. Citrate (Correct Answer)
D. Biotin

Explanation: **Explanation:** **Acetyl-CoA Carboxylase (ACC)** is the rate-limiting enzyme in fatty acid synthesis (lipogenesis). It catalyzes the conversion of Acetyl-CoA to Malonyl-CoA, a process requiring ATP, Biotin, and $\text{CO}_2$. **1. Why Citrate is the Correct Answer:** Citrate acts as a high-energy signal. When the TCA cycle is saturated, citrate levels rise and it is exported from the mitochondria to the cytosol. Here, it acts as a potent **allosteric activator** of ACC. It induces the polymerization of inactive ACC dimers into active long filaments, thereby triggering fatty acid synthesis. **2. Analysis of Incorrect Options:** * **Malonyl-CoA:** This is the immediate product of the reaction. High levels of Malonyl-CoA exert **feedback inhibition** on ACC (and also inhibit Carnitine Palmitoyltransferase-I to prevent simultaneous fatty acid oxidation). * **Acetyl-CoA:** This is the substrate for the reaction, not an allosteric activator. * **Biotin:** This is a mandatory **coenzyme** (prosthetic group) for ACC, not an allosteric regulator. It carries the carboxyl group during the reaction. **High-Yield Facts for NEET-PG:** * **Hormonal Regulation:** ACC is activated by **Insulin** (via dephosphorylation) and inhibited by **Glucagon and Epinephrine** (via phosphorylation by AMPK). * **The "ABC" Enzymes:** ACC belongs to the group of carboxylases (like Pyruvate Carboxylase) that require **A**TP, **B**iotin, and **C**O$_2$. * **Location:** Fatty acid synthesis occurs in the **cytosol**, while the precursor Acetyl-CoA is generated in the mitochondria; Citrate serves as the "shuttle" to move these units into the cytosol.

Question 423: What is the first step in the synthesis of eicosanoids?

A. Activation of lipoxygenase
B. Activation of PGH2 synthetase
C. Activation of hydrolase (Correct Answer)
D. None of the above

Explanation: **Explanation:** The synthesis of eicosanoids (prostaglandins, leukotrienes, and thromboxanes) begins with the release of the precursor fatty acid, typically **arachidonic acid**, from the cell membrane. **1. Why "Activation of hydrolase" is correct:** Arachidonic acid is not found free in the cytosol; it is esterified in membrane phospholipids. The **rate-limiting and first step** in eicosanoid synthesis is the liberation of arachidonic acid by the enzyme **Phospholipase A2 (PLA2)**. Since Phospholipase A2 is a type of **hydrolase** (it uses water to cleave the ester bond at the sn-2 position of phospholipids), its activation is the mandatory initial step before any downstream processing can occur. **2. Why the other options are incorrect:** * **Option A (Lipoxygenase):** This enzyme acts on free arachidonic acid to produce leukotrienes. It is part of the "linear pathway" but occurs only *after* the hydrolase has released the substrate. * **Option B (PGH2 synthetase):** Also known as Cyclooxygenase (COX), this enzyme converts free arachidonic acid into prostaglandins and thromboxanes (the "cyclic pathway"). Like lipoxygenase, it cannot act until the hydrolase step is complete. **High-Yield Clinical Pearls for NEET-PG:** * **Glucocorticoids** inhibit eicosanoid synthesis by inducing **Lipocortin (Annexin A1)**, which inhibits Phospholipase A2. * **Arachidonic acid** is an omega-6 fatty acid (20:4, Δ5,8,11,14) derived from the essential fatty acid **linoleic acid**. * **Rate-limiting step:** Release of arachidonic acid by PLA2. * **Key Inhibitors:** NSAIDs and Aspirin inhibit the COX pathway, but not the PLA2 or lipoxygenase pathways.

Question 424: Krabbe's disease is caused by a defect of which enzyme?

A. Ceramidase
B. Beta-galactosidase
C. Alpha-galactosidase
D. Galactosyl ceramidase (Correct Answer)

Explanation: **Explanation:** **Krabbe’s disease** (also known as Globoid Cell Leukodystrophy) is an autosomal recessive lysosomal storage disorder. It is caused by a deficiency of the enzyme **Galactosyl ceramidase** (also called galactocerebrosidase). Under normal conditions, this enzyme breaks down galactocerebroside into galactose and ceramide. When deficient, galactocerebroside and its toxic derivative, **psychosine**, accumulate. Psychosine is particularly toxic to oligodendrocytes, leading to widespread demyelination in the central and peripheral nervous systems. **Analysis of Options:** * **Option A: Ceramidase** – Deficiency leads to **Farber disease**, characterized by painful joint swelling, hoarseness (laryngeal involvement), and subcutaneous nodules. * **Option B: Beta-galactosidase** – Deficiency leads to **GM1 Gangliosidosis**, which presents with hepatosplenomegaly and skeletal deformities (dysostosis multiplex). * **Option C: Alpha-galactosidase** – Deficiency leads to **Fabry disease**, an X-linked disorder characterized by angiokeratomas, peripheral neuropathy (burning pain), and renal failure. **High-Yield Clinical Pearls for NEET-PG:** * **Pathognomonic Feature:** Presence of **Globoid cells** (multinucleated macrophages) in the brain white matter. * **Clinical Presentation:** Irritability, severe developmental delay, limb rigidity, and optic atrophy in early infancy. * **Inheritance:** Autosomal Recessive. * **Mnemonic:** "The **Krab** is **Galactic**" (Krabbe = Galactosyl ceramidase).

Question 425: Dietary triglyceride is a major source of nutrient for the human body. It is digested mostly in the intestinal lumen by pancreatic lipase to release what?

A. Lysophosphatidylcholines and fatty acids
B. Glycerol and fatty acids
C. Diglyceride and fatty acids
D. 2-Monoglyceride and fatty acids (Correct Answer)

Explanation: **Explanation:** The digestion of dietary triglycerides (TGs) primarily occurs in the small intestine through the action of **Pancreatic Lipase**. This enzyme is highly specific for the hydrolysis of ester bonds at the **1 and 3 positions** of the triglyceride molecule. 1. **Why Option D is Correct:** Pancreatic lipase acts on the primary ester linkages (C1 and C3) of the triglyceride. This sequential removal of fatty acids results in the production of **two free fatty acids (FFAs)** and one **2-monoacylglycerol (2-MAG)**. These products, along with bile salts and cholesterol, form mixed micelles for absorption into the intestinal mucosal cells. 2. **Why Other Options are Incorrect:** * **Option A:** Lysophosphatidylcholines are products of phospholipid digestion (via Phospholipase A2), not triglyceride digestion. * **Option B:** Complete hydrolysis into glycerol and three fatty acids is rare in the intestinal lumen; it typically occurs inside the enterocytes or via lipoprotein lipase in peripheral tissues. * **Option C:** Diglycerides are transient intermediates in the reaction but are not the final major end-products of pancreatic lipase action. **High-Yield Clinical Pearls for NEET-PG:** * **Colipase:** Pancreatic lipase requires a protein cofactor called **Colipase** (secreted as pro-colipase) to anchor it to the lipid-water interface and prevent inhibition by bile salts. * **Orlistat:** An anti-obesity drug that works by inhibiting gastric and pancreatic lipases, thereby decreasing fat absorption. * **Steatorrhea:** Failure of TG digestion (e.g., chronic pancreatitis) or absorption (e.g., Celiac disease) leads to bulky, foul-smelling, fatty stools. * **Short & Medium Chain Fatty Acids:** Unlike long-chain FAs, these do not require micelle formation and are absorbed directly into the portal blood.

Question 426: What is the full form of LCAT?

A. Lecithin cholesterol acyltransferase (Correct Answer)
B. Lecithin cholesterol alkyltransferase
C. Lecithin choline acetyltransferase
D. Lecithin choline alcohol transferase

Explanation: **Explanation:** **Lecithin-cholesterol acyltransferase (LCAT)** is a crucial enzyme in lipid metabolism, specifically in the process of **Reverse Cholesterol Transport (RCT)**. 1. **Why Option A is correct:** LCAT is synthesized by the liver and circulates in the plasma associated with High-Density Lipoprotein (HDL). It catalyzes the transfer of a fatty acid (acyl group) from the second position of **Lecithin** (phosphatidylcholine) to the free **Cholesterol** present on the surface of HDL. This reaction produces **Cholesterol Esters** and Lysolecithin. Because cholesterol esters are more hydrophobic, they move into the core of the HDL particle, transforming nascent discoid HDL into mature spherical HDL. 2. **Why other options are incorrect:** * **Option B (Alkyltransferase):** An alkyl group is a saturated hydrocarbon chain; LCAT specifically transfers an *acyl* (fatty acid) group. * **Option C & D (Choline/Alcohol):** The substrate is cholesterol, not choline or alcohol. While lecithin contains choline, the enzyme's primary function is the esterification of cholesterol. **Clinical Pearls for NEET-PG:** * **Activator:** LCAT is activated by **Apo A-I** (found on HDL). * **Biological Significance:** It maintains the concentration gradient for cholesterol, allowing HDL to "scavenge" more cholesterol from peripheral tissues. * **LCAT Deficiency:** * *Familial LCAT Deficiency:* Leads to the "Fish-eye disease" (corneal opacities), hemolytic anemia, and renal failure. * *Lab finding:* Presence of **Lipoprotein X** and very low levels of HDL. * **CETP Connection:** Once LCAT forms cholesterol esters, the **Cholesterol Ester Transfer Protein (CETP)** helps exchange these esters for triglycerides with VLDL and LDL.

Question 427: A patient presents with eruptive xanthomas and blood that appears milky upon drawing. Which lipoprotein is elevated in the plasma?

A. Chylomicron (Correct Answer)
B. Chylomicron remnants
C. LDL
D. HDL

Explanation: **Explanation:** The clinical presentation of **eruptive xanthomas** and **milky (lipemic) plasma** is a classic hallmark of **Type I Hyperlipoproteinemia** (Familial Chylomicronemia Syndrome). 1. **Why Chylomicrons are correct:** Chylomicrons are the largest and least dense lipoproteins, primarily composed of dietary triglycerides (TGs). When TG levels exceed 1000 mg/dL, they scatter light, giving the blood a characteristic "milky" or "creamy" appearance. Eruptive xanthomas (small, yellow-orange papules) occur due to the deposition of these lipids in the skin. This condition is typically caused by a deficiency in **Lipoprotein Lipase (LPL)** or its cofactor, **Apo C-II**, preventing the clearance of chylomicrons from the blood. 2. **Why other options are incorrect:** * **Chylomicron remnants:** These are smaller and do not cause milky plasma; they are elevated in Type III Hyperlipoproteinemia (Dysbetalipoproteinemia). * **LDL:** Elevated LDL (Type IIa) leads to tendon xanthomas and xanthelasma, but the plasma remains clear because LDL does not scatter light like TGs. * **HDL:** Known as "good cholesterol," elevated HDL is cardioprotective and does not cause milky plasma or xanthomas. **NEET-PG High-Yield Pearls:** * **Refrigeration Test:** If the plasma is left overnight at 4°C, chylomicrons form a **creamy layer on top**, while the underlying plasma remains clear. * **Pancreatitis Risk:** Extremely high chylomicron levels are a major risk factor for **acute pancreatitis**. * **Apo B-48:** This is the characteristic apoprotein found on chylomicrons.

Question 428: The aromatic enzyme complex is involved in the biosynthesis of which of the following?

A. Cholesterol
B. Adrenal hormones
C. Vitamin D3
D. Estradiol/estrogens (Correct Answer)

Explanation: **Explanation:** The correct answer is **Estradiol/estrogens**. The "aromatic enzyme complex" refers to **Aromatase** (also known as Estrogen Synthase or CYP19A1), a member of the Cytochrome P450 superfamily. **Why Estradiol is Correct:** The biosynthesis of estrogens involves the conversion of androgens (specifically testosterone and androstenedione) into estrogens (estradiol and estrone). This process requires the **aromatization of the 'A' ring** of the steroid nucleus. The aromatase enzyme complex catalyzes three sequential hydroxylation steps that result in the loss of a methyl group and the formation of a stable aromatic benzene ring, which is the defining structural feature of estrogens. **Why Other Options are Incorrect:** * **Cholesterol:** Synthesized from Acetyl-CoA via the HMG-CoA reductase pathway. It serves as the precursor for all steroid hormones but does not require an aromatization step for its own synthesis. * **Adrenal Hormones (Cortisol/Aldosterone):** These are synthesized in the adrenal cortex. While they share the steroid nucleus, their synthesis involves hydroxylations and dehydrogenations that do not result in an aromatic ring. * **Vitamin D3:** Synthesized from 7-dehydrocholesterol in the skin via UV light exposure, followed by hydroxylations in the liver and kidney. It does not involve the aromatase enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Aromatase Inhibitors (e.g., Letrozole, Anastrozole):** These are first-line treatments for Estrogen Receptor-positive (ER+) breast cancer in postmenopausal women. * **Location:** Aromatase is highly expressed in the ovaries (granulosa cells), placenta, adipose tissue, and brain. * **Polycystic Ovary Syndrome (PCOS):** Often involves a functional deficiency or imbalance in aromatase activity, leading to hyperandrogenism.

Question 429: Hormone-sensitive lipase acts on which substrate?

A. Triglycerides (Correct Answer)
B. Cholesterol esters
C. Phospholipids
D. Gangliosides

Explanation: **Explanation:** **Hormone-Sensitive Lipase (HSL)** is the key regulatory enzyme involved in the mobilization of stored energy from adipose tissue. 1. **Why Triglycerides are correct:** HSL catalyzes the hydrolysis of **Triglycerides (TAGs)** stored in adipocytes into free fatty acids and glycerol. Specifically, it is the rate-limiting step for the conversion of diacylglycerol (DAG) to monoacylglycerol (MAG), though it acts on triacylglycerols as well. This process, known as lipolysis, provides fuel for peripheral tissues during fasting or exercise. 2. **Why other options are incorrect:** * **Cholesterol esters:** While HSL can exhibit some activity against cholesterol esters in certain tissues (like the adrenal cortex), its primary physiological substrate in the context of lipid metabolism and energy mobilization is Triglycerides. * **Phospholipids:** These are hydrolyzed by **Phospholipases** (e.g., PLA2), not HSL. * **Gangliosides:** These are complex glycosphingolipids degraded by specific lysosomal enzymes (e.g., Hexosaminidase A); deficiencies here lead to sphingolipidoses like Tay-Sachs disease. **High-Yield Clinical Pearls for NEET-PG:** * **Regulation:** HSL is **activated by phosphorylation** via Protein Kinase A. Therefore, it is stimulated by "stress" hormones like **Glucagon, Epinephrine, and ACTH**. * **Inhibition:** **Insulin** dephosphorylates and inhibits HSL, promoting fat storage instead of mobilization. * **Location:** It is found in the cytosol of adipocytes. * **Distinction:** Do not confuse HSL with **Lipoprotein Lipase (LPL)**. LPL acts on circulating chylomurons/VLDL in the capillary endothelium, whereas HSL acts on stored fat inside the cell.

Question 430: Which is the most essential fatty acid?

A. Linolenic acid
B. Linoleic acid (Correct Answer)
C. Arachidonic acid
D. Eicosapentanoic acid

Explanation: **Explanation:** **1. Why Linoleic Acid is the Correct Answer:** Essential fatty acids (EFAs) are those that the human body cannot synthesize de novo because humans lack the enzymes (**$\Delta^{12}$ and $\Delta^{15}$ desaturases**) required to introduce double bonds beyond the $\Delta^9$ position. **Linoleic acid (18:2; $\omega$-6)** is considered the **most essential** because it serves as the primary precursor for the synthesis of other $\omega$-6 fatty acids, including Arachidonic acid. If Linoleic acid is provided in the diet, the body can meet its requirements for other $\omega$-6 derivatives. **2. Analysis of Incorrect Options:** * **Linolenic acid (18:3; $\omega$-3):** While also an essential fatty acid, it is the precursor for the $\omega$-3 series (EPA/DHA). In the context of "most essential" for human growth and preventing deficiency symptoms like dermatitis, Linoleic acid takes precedence. * **Arachidonic acid (20:4; $\omega$-6):** This is considered **semi-essential**. It can be synthesized from Linoleic acid. It only becomes essential if Linoleic acid is deficient in the diet. * **Eicosapentaenoic acid (EPA):** This is a long-chain $\omega$-3 fatty acid synthesized from $\alpha$-Linolenic acid. It is not a primary essential fatty acid. **3. NEET-PG High-Yield Clinical Pearls:** * **Deficiency Manifestation:** EFA deficiency leads to **Phrynoderma** (Toad skin), characterized by follicular hyperkeratosis on the extensor surfaces of limbs. * **Triene:Tetraene Ratio:** A ratio > 0.4 in plasma is a biochemical marker of EFA deficiency. * **Functions:** EFAs are structural components of cell membranes and precursors for **Eicosanoids** (Prostaglandins, Leukotrienes, and Thromboxanes). * **Memory Aid:** Remember **"LL"** (Linoleic and Linolenic) as the true essentials. If only one must be chosen, Linoleic acid is the biochemical priority.

Practice by Chapter

Lipid Classification and Chemistry

Practice Questions

Fatty Acid Oxidation

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Ketone Body Metabolism

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Fatty Acid Synthesis

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Metabolism of Triacylglycerols

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Phospholipid Metabolism

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Cholesterol Metabolism and Biosynthesis

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Bile Acids and Bile Salts

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Lipoprotein Metabolism and Transport

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Dyslipidemias and Atherosclerosis

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Prostaglandins and Eicosanoids

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Fatty Liver and Lipotropic Factors

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