Lipid Metabolism Practice Questions

Q: Which of the following statements is true about secondary bile acids?

Reabsorbed by enterohepatic circulation.. **Explanation:** Bile acid metabolism is a high-yield topic for NEET-PG. To understand the correct answer, one must distinguish between primary and secondary bile acids. **1. Why Option C is Correct:** Secondary bile acids (Deoxycholic acid and Lithocholic acid) are formed in the **colon** by the action of intestinal bacteria on primary bile acids. Like primary bile acids, they are largely **reabsorbed** (primarily in the ileum) and transported back to the liver via the portal blood. This recycling process is known as **enterohepatic circulation**, which ensures that the body maintains a sufficient bile acid pool without constant de novo synthesis. **2. Why the Other Options are Incorrect:** * **Option A:** Conjugation with taurine or glycine occurs in the **liver** to form bile salts (e.g., Glycocholic acid). This process makes them more water-soluble but does not turn them into "secondary" bile acids. * **Option B:** Deconjugation and dehydroxylation are performed by **intestinal bacteria in the gut**, not in the liver. The liver's role is synthesis and conjugation. * **Option D:** Bile acids are synthesized from **cholesterol**, not bilirubin. Bilirubin is a breakdown product of heme and is a bile pigment, not a bile acid. **Clinical Pearls for NEET-PG:** * **Primary Bile Acids:** Cholic acid and Chenodeoxycholic acid (Mnemonic: **C**ome **C**lean). * **Secondary Bile Acids:** Deoxycholic acid and Lithocholic acid (Mnemonic: **D**irty **L**ittle secrets—formed by bacteria). * **Rate-limiting enzyme:** Cholesterol 7-alpha-hydroxylase (inhibited by bile acids via feedback). * **Cholestyramine:** A bile acid sequestrant that interrupts enterohepatic circulation to lower LDL cholesterol.

Q: The reducing equivalents for cholesterol biosynthesis are provided by which molecule?

NADPH. ### Explanation **1. Why NADPH is the Correct Answer** Cholesterol biosynthesis is a highly reductive anabolic process occurring primarily in the cytosol and endoplasmic reticulum. The synthesis of one molecule of cholesterol requires **28 molecules of NADPH**. * **Mechanism:** NADPH acts as the essential electron donor (reducing equivalent) at multiple steps, most notably the **rate-limiting step** catalyzed by **HMG-CoA Reductase**, which converts HMG-CoA to Mevalonate. * **Sources of NADPH:** In the liver (the primary site of cholesterol synthesis), NADPH is mainly supplied by the **Pentose Phosphate Pathway (HMP Shunt)** and the **Malic Enzyme** reaction. **2. Why Other Options are Incorrect** * **NADH (Option C):** While NADH is a major electron carrier, it is primarily used in **catabolic** pathways (like Glycolysis and the TCA cycle) to generate ATP via the electron transport chain. It is generally not used for reductive biosynthesis. * **FADH2 & FMNH2 (Options A & B):** These are prosthetic groups involved in redox reactions within the mitochondria (e.g., Beta-oxidation, TCA cycle, and Complex II of ETC). They do not provide the reducing power required for the cytosolic enzymes of cholesterol synthesis. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Rate-Limiting Enzyme:** HMG-CoA Reductase (Target of **Statin** drugs). * **Location:** Occurs in the cytosol and microsomal fraction (ER). * **Precursor:** All 27 carbon atoms of cholesterol are derived from **Acetyl-CoA**. * **Key Intermediate:** **Squalene** is the first 30-carbon hydrocarbon formed in the pathway. * **Rule of Thumb:** Remember that **"NADPH is for Building (Anabolism), NADH is for Burning (Catabolism)."** Other pathways requiring NADPH include Fatty Acid synthesis, Steroidogenesis, and Glutathione reduction.

Q: What is the true function of apoprotein CII?

Increases the activity of lipoprotein lipase. **Explanation:** **Apoprotein CII (Apo C-II)** is a crucial cofactor in lipid metabolism. It is primarily found on the surface of **Chylomicrons** and **Very Low-Density Lipoproteins (VLDL)**. 1. **Why Option C is Correct:** The primary physiological role of Apo C-II is to act as a potent **activator of Lipoprotein Lipase (LPL)**. LPL is an enzyme located on the capillary endothelial walls of adipose tissue and muscle. When Apo C-II binds to LPL, it triggers the hydrolysis of triglycerides within the core of chylomicrons and VLDL into free fatty acids and glycerol, allowing tissues to utilize or store the fat. 2. **Why Other Options are Incorrect:** * **Option A & D:** Cholesterol and fatty acid synthesis are intracellular enzymatic processes (primarily in the liver and cytoplasm) regulated by enzymes like HMG-CoA reductase and Fatty Acid Synthase, respectively. Apoproteins are structural or regulatory components of circulating lipoproteins, not enzymes for synthesis. * **Option B:** While Apo C-II is *carried* on transport particles, the specific function of "transport" is attributed to the lipoprotein particle as a whole (and structural proteins like Apo B-48 or B-100), not specifically to the C-II cofactor. **High-Yield Clinical Pearls for NEET-PG:** * **Deficiency:** A genetic deficiency of either Apo C-II or LPL leads to **Type I Hyperlipoproteinemia (Familial Chylomicronemia Syndrome)**, characterized by severe hypertriglyceridemia, eruptive xanthomas, and recurrent pancreatitis. * **Source:** Apo C-II is donated to nascent chylomicrons and VLDL by **HDL** in the circulation. * **Antagonist:** **Apo C-III** acts as an inhibitor of LPL, opposing the action of Apo C-II.

Q: Which of the following is NOT a constituent of glycosphingolipid?

Glycerol. **Explanation:** The core concept tested here is the structural classification of lipids. Lipids are broadly divided into **glycerophospholipids** (glycerol-based) and **sphingolipids** (sphingosine-based). **Why Glycerol is the correct answer:** Glycosphingolipids belong to the sphingolipid family. Their structural backbone is **Ceramide**, which consists of a long-chain amino alcohol called **sphingosine** attached to a **fatty acid** via an amide bond. In glycosphingolipids, a **carbohydrate** (monosaccharide or oligosaccharide) is attached to the primary hydroxyl group of ceramide. **Glycerol** is the backbone for triglycerides and phospholipids (like lecithin), but it is entirely absent in sphingolipids. **Analysis of Incorrect Options:** * **A. Sphingosine:** This is the 18-carbon amino alcohol that serves as the fundamental structural framework for all sphingolipids. * **B. Fatty Acid:** A long-chain fatty acid is attached to the amino group of sphingosine to form Ceramide, the parent compound of glycosphingolipids. * **D. Carbohydrate:** By definition, "glyco-" refers to the sugar component (e.g., glucose in glucocerebroside or complex oligosaccharides in gangliosides) attached to the ceramide. **High-Yield Clinical Pearls for NEET-PG:** * **Ceramide** = Sphingosine + Fatty acid. * **Cerebroside** = Ceramide + Single sugar (Glucose or Galactose). * **Ganglioside** = Ceramide + Oligosaccharide + **Sialic acid (NANA)**. * **Sphingomyelin** is the only sphingolipid that is a phospholipid (contains phosphate) but NOT a glycolipid. * **Clinical Correlation:** Deficiencies in lysosomal enzymes that degrade glycosphingolipids lead to **Sphingolipidoses** (e.g., Gaucher’s, Niemann-Pick, and Tay-Sachs disease).

Q: A female child presents with ambiguous genitalia and hyperpigmentation of the skin. She has increased blood pressure and hypokalemia. Which of the following enzymes is deficient in this child?

11 beta hydroxylase. ### Explanation The clinical presentation of **ambiguous genitalia** in a female child (virilization) combined with **hypertension** and **hypokalemia** is diagnostic of **11β-hydroxylase deficiency**, the second most common cause of Congenital Adrenal Hyperplasia (CAH). **1. Why 11β-hydroxylase is correct:** A deficiency in this enzyme blocks the conversion of 11-deoxycortisol to cortisol and 11-deoxycorticosterone (DOC) to corticosterone. This leads to: * **Androgen Excess:** Shunting of precursors into the androgen pathway causes virilization (ambiguous genitalia in females). * **Mineralocorticoid Excess:** While aldosterone is low, there is a massive buildup of **11-deoxycorticosterone (DOC)**. DOC is a potent mineralocorticoid that causes sodium retention and potassium excretion, leading to **hypertension** and **hypokalemia**. * **Hyperpigmentation:** Low cortisol leads to increased ACTH, which stimulates melanocytes. **2. Why other options are incorrect:** * **21-hydroxylase deficiency:** The most common CAH. It causes virilization but presents with **hypotension** (salt-wasting) and **hyperkalemia** due to a total lack of mineralocorticoids. * **17α-hydroxylase deficiency:** Presents with hypertension and hypokalemia, but results in a **lack of sex hormones**. This causes delayed puberty or primary amenorrhea in females, not ambiguous genitalia. * **17, 20 Lyase deficiency:** Affects only sex hormone synthesis. It leads to a lack of androgens (undervirilization in males) but does not affect mineralocorticoids or cause hypertension. **Clinical Pearls for NEET-PG:** * **Rule of "1":** If the enzyme starts with **1** (11β or 17α), it causes **Hypertension**. * **Rule of "Virilization":** If the enzyme ends with **1** (11β or 21), it causes **Virilization** in females. * **11β-hydroxylase** is unique because it is the only CAH that causes **both** virilization and hypertension.

Q: What is the toxin found in unripe apple fruit that causes Jamaican vomiting sickness?

Hypoglycin A. **Explanation:** **Jamaican Vomiting Sickness** is caused by the ingestion of the unripe fruit of the **Ackee tree** (*Blighia sapida*), which contains the potent toxin **Hypoglycin A**. **Why Hypoglycin A is the correct answer:** Hypoglycin A is a non-proteinogenic amino acid that is metabolized into **methylenecyclopropylacetyl-CoA (MCPA-CoA)**. This metabolite acts as a suicide inhibitor of **Acyl-CoA Dehydrogenase**, the first enzyme in the **$\beta$-oxidation of fatty acids**. By blocking fatty acid oxidation, the body cannot generate acetyl-CoA or NADH required for gluconeogenesis. This leads to profound **non-ketotic hypoglycemia**, as the liver cannot produce glucose or ketone bodies to meet metabolic demands. **Analysis of Incorrect Options:** * **Ricin (A):** A potent cytotoxin found in **castor beans** (*Ricinus communis*). It acts by inhibiting the 60S ribosomal subunit, halting protein synthesis. * **Muscarine (B):** A toxin found in certain **mushrooms** (*Amanita muscaria*). it acts as a parasympathomimetic, causing cholinergic crisis (SLUDGE syndrome). * **Amygdalin (C):** A cyanogenic glycoside found in **apricot kernels** and apple seeds. It is metabolized into hydrogen cyanide, which inhibits cytochrome c oxidase in the electron transport chain. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Presentation:** Severe vomiting, altered consciousness, and seizures due to hypoglycemia, occurring 2–6 hours after ingestion. * **Biochemical Hallmark:** **Non-ketotic hypoglycemia** (unlike starvation, where ketones are elevated). * **Enzyme Inhibited:** Primarily **Short-chain and Medium-chain Acyl-CoA Dehydrogenases (MCAD)**. * **Treatment:** Aggressive IV glucose administration and supportive care.

Q: What is mainly composed of triacylglycerols synthesized in intestinal epithelial cells?

Chylomicrons. **Explanation:** The correct answer is **Chylomicrons**. **1. Why Chylomicrons are correct:** Chylomicrons are the largest and least dense lipoproteins. They are synthesized exclusively in the **intestinal mucosal cells** (enterocytes) following the absorption of dietary fats. Their primary function is the transport of **exogenous (dietary) triacylglycerols (TAGs)** from the intestine to peripheral tissues like adipose tissue and muscle. They consist of approximately 85-90% triacylglycerols, making TAGs their predominant lipid component. **2. Why other options are incorrect:** * **VLDL (Very Low-Density Lipoprotein):** While also rich in TAGs (approx. 60%), VLDL is synthesized in the **liver**, not the intestine. It transports **endogenous** triacylglycerols. * **LDL (Low-Density Lipoprotein):** Formed from VLDL metabolism, LDL is the primary carrier of **cholesterol** to peripheral tissues. It contains very little TAG. * **HDL (High-Density Lipoprotein):** Synthesized in both the liver and intestine, HDL is the smallest lipoprotein. It is primarily composed of **protein** and phospholipids and is involved in **reverse cholesterol transport**. **3. NEET-PG High-Yield Pearls:** * **Apolipoprotein Marker:** **Apo B-48** is the unique structural protein for chylomicrons (synthesized via RNA editing), whereas **Apo B-100** is the marker for VLDL and LDL. * **Rate-Limiting Enzyme:** Lipoprotein Lipase (LPL), activated by **Apo C-II**, clears TAGs from chylomicrons in the capillaries. * **Clinical Correlation:** **Abetalipoproteinemia** is a condition where a deficiency in Microsomal Triglyceride Transfer Protein (MTP) prevents the assembly of chylomicrons, leading to fat malabsorption and steatorrhea. * **Appearance:** Chylomicrons cause the "milky" appearance of plasma (chylous) after a fatty meal.

Q: Respiratory distress syndrome is due to a defect in the biosynthesis of which substance?

Dipalmitoyl lecithin. ### Explanation **Correct Option: A. Dipalmitoyl lecithin** **Mechanism and Concept:** Respiratory Distress Syndrome (RDS), also known as Hyaline Membrane Disease, is primarily caused by a deficiency of **pulmonary surfactant**. Surfactant is a surface-active lipoprotein complex secreted by **Type II pneumocytes**. Its primary function is to reduce surface tension at the air-liquid interface of the alveoli, preventing alveolar collapse (atelectasis) during expiration. The most critical functional component of surfactant is **Dipalmitoylphosphatidylcholine (DPPC)**, commonly known as **Dipalmitoyl lecithin**. It accounts for approximately 50–60% of the surfactant composition. Chemically, it is a phospholipid with two palmitic acid chains. A defect in its biosynthesis or premature birth (before 34 weeks) leads to high surface tension, decreased lung compliance, and subsequent respiratory failure. **Why Other Options are Incorrect:** * **B, C, and D:** While Cephalin (Phosphatidylethanolamine), Serine (Phosphatidylserine), and Inositol (Phosphatidylinositol) are all phospholipids found in cell membranes and in minor quantities within surfactant, they do not possess the unique surface-tension-reducing properties of lecithin. They are not the primary functional molecules whose deficiency leads to RDS. **High-Yield Clinical Pearls 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. * **Glucocorticoids:** Antenatal administration of steroids (e.g., Betamethasone or Dexamethasone) accelerates surfactant synthesis by inducing enzymes in Type II pneumocytes. * **Surfactant Proteins:** SP-A and SP-D are involved in innate immunity, while **SP-B and SP-C** are crucial for the physical properties of surfactant. * **Radiology:** RDS typically presents with a "ground-glass appearance" and air bronchograms on a chest X-ray.

Question 1

Which of the following statements is true about secondary bile acids?

Accepted Answer

Reabsorbed by enterohepatic circulation.

Answer

Primary bile acids conjugated with taurine and glycine form secondary bile acids.

Answer

Deconjugated by bacteria in the liver.

Answer

They are formed from bilirubin.

Question 2

The reducing equivalents for cholesterol biosynthesis are provided by which molecule?

Accepted Answer

NADPH

Answer

FADH2

Answer

FMNH2

Answer

NADH

Question 3

What is the true function of apoprotein CII?

Accepted Answer

Increases the activity of lipoprotein lipase

Answer

Cholesterol synthesis

Answer

Triglyceride transport

Answer

Fatty acid synthesis

Question 4

Increased formation of ketone bodies during fasting is a result of?

Accepted Answer

Increased levels of free fatty acids in blood

Answer

Decreased level of circulating glucagon

Answer

Decreased formation of Acetyl CoA in the liver

Answer

Inhibition of beta-oxidation of fatty acids in the liver

Question 5

All of the following statements are true regarding lipoproteins except:

Accepted Answer

Increased blood cholesterol is associated with increased LDL receptors.

Answer

VLDL transports endogenous lipids.

Answer

LDL transports lipids to the tissues.

Answer

Increased HDL is associated with decreased risk of coronary disease.

Question 6

Which of the following is NOT a constituent of glycosphingolipid?

Accepted Answer

Glycerol

Answer

Sphingosine

Answer

Fatty acid

Answer

Carbohydrate

Question 7

A female child presents with ambiguous genitalia and hyperpigmentation of the skin. She has increased blood pressure and hypokalemia. Which of the following enzymes is deficient in this child?

Accepted Answer

11 beta hydroxylase

Answer

21 hydroxylase

Answer

17 alpha hydroxylase

Answer

17, 20 Lyase

Question 8

What is the toxin found in unripe apple fruit that causes Jamaican vomiting sickness?

Accepted Answer

Hypoglycin A

Answer

Ricin

Answer

Muscarine

Answer

Amaydin

Question 9

What is mainly composed of triacylglycerols synthesized in intestinal epithelial cells?

Accepted Answer

Chylomicrons

Answer

VLDL

Answer

LDL

Answer

HDL

Question 10

Respiratory distress syndrome is due to a defect in the biosynthesis of which substance?

Accepted Answer

Dipalmitoyl lecithin

Answer

Dipalmitoyl cephalin

Answer

Dipalmitoyl serine

Answer

Dipalmitoyl inositol

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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