Lipid Metabolism Practice Questions

Q: Esters of fat-soluble vitamins are digested by:

Cholesterol esterase. ### Explanation **Correct Option: B. Cholesterol esterase** **Concept:** Fat-soluble vitamins (A, D, E, and K) are often consumed as esters (e.g., retinyl esters). To be absorbed by the intestinal mucosa, these must be hydrolyzed into free vitamins and fatty acids. **Cholesterol esterase** (also known as **Nonspecific Lipid Esterase** or Bile Salt-Activated Lipase) is the primary enzyme responsible for this process. It has broad specificity and hydrolyzes not only cholesterol esters but also esters of fat-soluble vitamins, monoglycerides, and phospholipids. **Analysis of Incorrect Options:** * **A. Pancreatic lipase:** This is the primary enzyme for the digestion of dietary **triacylglycerols (TAGs)**. It specifically targets the 1 and 3 positions of the glycerol backbone but does not efficiently hydrolyze vitamin esters. * **C. Colipase:** This is a protein co-factor secreted by the pancreas. It does not have enzymatic activity itself; rather, it displaces bile salts from the oil-water interface to allow pancreatic lipase to bind to its substrate. * **D. Carboxypeptidase:** This is a **proteolytic enzyme** (exopeptidase) involved in protein digestion, specifically cleaving amino acids from the C-terminal end of peptides. It has no role in lipid or vitamin metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Absorption Requirement:** Fat-soluble vitamin absorption is entirely dependent on **micelle formation** and adequate bile salt concentration. * **Steatorrhea:** Conditions causing fat malabsorption (e.g., Chronic Pancreatitis, Celiac disease) lead to secondary deficiencies of fat-soluble vitamins. * **Vitamin A Storage:** Once absorbed, Vitamin A is re-esterified and transported in chylomicrons to the liver, where it is stored in **Ito cells** (Stellate cells) as retinyl palmitate.

Q: Which among the following is not a saturated fatty acid?

Linoleic acid. ### Explanation The correct answer is **Linoleic acid**. **1. Why Linoleic acid is the correct answer:** Fatty acids are classified based on the presence of double bonds. **Saturated fatty acids (SFAs)** have no double bonds, while **Unsaturated fatty acids** contain one or more double bonds. Linoleic acid is a **Polyunsaturated Fatty Acid (PUFA)** containing 18 carbon atoms and **two double bonds** (18:2; Δ9,12). It is an essential fatty acid belonging to the Omega-6 (ω-6) family, meaning the body cannot synthesize it and it must be obtained through the diet. **2. Why the other options are incorrect:** * **Myristic acid (A):** A saturated fatty acid with 14 carbon atoms (14:0). * **Palmitic acid (C):** The most common saturated fatty acid in the human body, containing 16 carbon atoms (16:0). It is the primary product of the Fatty Acid Synthase (FAS) complex. * **Stearic acid (B):** A saturated fatty acid with 18 carbon atoms (18:0). **3. High-Yield Clinical Pearls for NEET-PG:** * **Essential Fatty Acids:** There are two—Linoleic acid (ω-6) and Linolenic acid (ω-3). Arachidonic acid becomes essential only if Linoleic acid is deficient. * **Mnemonic for Saturated FAs:** "**L**ittle **M**ice **P**lay **S**tealthily" (**L**auric 12C, **M**yristic 14C, **P**almitic 16C, **S**tearic 18C). * **Prostaglandin Precursor:** Arachidonic acid (20:4; Δ5,8,11,14) is the direct precursor for the synthesis of eicosanoids (prostaglandins, thromboxanes, and leukotrienes). * **Refsum Disease:** A metabolic disorder caused by a deficiency in the α-oxidation of **Phytanic acid** (a branched-chain fatty acid).

Q: Which of the following statements is NOT true regarding the given structure?

It is a sphingophospholipid. ***It is a sphingophospholipid*** - **Phosphatidylcholine (lecithin)** is a **glycerophospholipid** with a **glycerol backbone**, not a **sphingosine backbone** characteristic of sphingophospholipids. - **Sphingophospholipids** like **sphingomyelin** contain **sphingosine** as their backbone structure, which is fundamentally different from the **glycerol-based structure** of lecithin. *Phosphatidic acid + choline* - **Phosphatidylcholine** is synthesized via the **CDP-choline pathway** where **phosphatidic acid** reacts with **CDP-choline** to form lecithin. - This is the major biosynthetic pathway for **lecithin production** in hepatocytes and other tissues. *Deficiency can cause respiratory distress in a newborn* - **Lecithin** is the primary component of **pulmonary surfactant**, which reduces surface tension in alveoli. - Deficiency leads to **neonatal respiratory distress syndrome (RDS)** due to **alveolar collapse** and impaired lung expansion. *Largest body store of choline* - **Phosphatidylcholine** represents the **largest reservoir of choline** in the human body, comprising about **95% of total body choline**. - It serves as a source of **free choline** for synthesis of **acetylcholine** and other choline-containing compounds.

Q: What is the enzyme associated with the conversion of androgen to oestrogen in the growing ovarian follicle?

Aromatase. ### Explanation The conversion of androgens to estrogens is a critical step in the ovarian cycle, governed by the **Two-Cell, Two-Gonadotropin Theory**. **1. Why Aromatase is Correct:** In the growing ovarian follicle, **LH** stimulates **Theca cells** to produce androgens (androstenedione and testosterone) from cholesterol. These androgens then diffuse across the basement membrane into the **Granulosa cells**. Under the influence of **FSH**, the enzyme **Aromatase** (a member of the Cytochrome P450 superfamily, CYP19A1) catalyzes the "aromatization" of the A-ring of these androgens to produce estrogens (estrone and estradiol). **2. Analysis of Incorrect Options:** * **Desmolase (Cholesterol Side-Chain Cleavage Enzyme):** This enzyme converts Cholesterol to Pregnenolone. It is the rate-limiting step in overall steroidogenesis but does not perform the final conversion to estrogen. * **Isomerase (3β-Hydroxysteroid Dehydrogenase/Δ5-4 Isomerase):** This enzyme converts Pregnenolone to Progesterone. It is involved in the early stages of the steroid pathway. * **Hydroxylase:** Enzymes like 17α-hydroxylase or 21-hydroxylase are involved in adding hydroxyl groups at specific positions to create precursors for cortisol or androgens, but they do not synthesize the aromatic ring characteristic of estrogens. **3. NEET-PG High-Yield Pearls:** * **Rate-limiting step of Oogenesis:** Cholesterol to Pregnenolone (via Desmolase). * **Aromatase Inhibitors:** Drugs like Letrozole and Anastrozole are used clinically in breast cancer treatment and ovulation induction. * **FSH vs. LH:** Remember: **L**H acts on **T**heca cells (to make androgens); **F**SH acts on **G**ranulosa cells (to stimulate aromatase). * **Estrogen Potency:** Estradiol (E2) > Estrone (E1) > Estriol (E3). Estradiol is the primary estrogen produced by the growing follicle.

Q: The liver removes low-density lipoproteins (LDLs) from the blood by the LDLs binding to which of the following?

LDL receptors and then internalizing them. The liver is the primary organ responsible for clearing Low-Density Lipoprotein (LDL) from systemic circulation, a process critical for maintaining cholesterol homeostasis. ### **Explanation of the Correct Answer** **Option A** is correct because the clearance of LDL is mediated by **LDL receptors (LDLR)** located on the surface of hepatocytes. These receptors specifically recognize **Apolipoprotein B-100**, which is the primary structural protein of LDL. Once the LDL particle binds to the receptor, the entire complex is internalized via **clathrin-mediated endocytosis**. Inside the cell, the LDL is degraded in lysosomes to release free cholesterol, while the LDL receptors are typically recycled back to the cell surface. ### **Why Other Options are Incorrect** * **Option B:** HDL receptors (such as **SR-BI**) are involved in "Reverse Cholesterol Transport," allowing the liver to take up cholesterol esters from High-Density Lipoprotein (HDL), not LDL. * **Option C:** Albumin is a transport protein for free fatty acids and bilirubin, but it is not a structural component of LDL nor a ligand for its clearance. * **Option D:** Transferrin is the transport protein for **iron**. It binds to transferrin receptors, not LDL receptors, and has no role in lipid metabolism. ### **High-Yield Clinical Pearls for NEET-PG** * **Familial Hypercholesterolemia (Type IIa):** Caused by a genetic deficiency or defect in **LDL receptors**, leading to severely elevated serum LDL and premature atherosclerosis. * **PCSK9 Inhibitors:** A modern class of drugs (e.g., Alirocumab) that prevents the degradation of LDL receptors, thereby increasing their density on the liver surface and lowering blood LDL levels. * **Statins:** These drugs inhibit HMG-CoA reductase, which decreases intracellular cholesterol, leading to an **upregulation of LDL receptors** and increased clearance of LDL from the blood.

Q: In which subcellular compartment does fatty acid synthesis occur in a hepatocyte?

Cytoplasm. **Explanation:** Fatty acid synthesis (Lipogenesis) primarily occurs in the **cytoplasm** of hepatocytes, as well as in mammary glands and adipose tissue. The process begins with the conversion of Acetyl-CoA to Malonyl-CoA by the enzyme Acetyl-CoA Carboxylase (the rate-limiting step). The subsequent reactions are catalyzed by the **Fatty Acid Synthase (FAS) multienzyme complex**, which is located exclusively in the cytosol. This spatial localization is essential because the required reducing equivalent, **NADPH**, is generated in the cytoplasm via the Pentose Phosphate Pathway (PPP). **Analysis of Incorrect Options:** * **B. Mitochondria:** While the precursor Acetyl-CoA is produced here, fatty acid *synthesis* does not occur in the mitochondria. Instead, the mitochondria are the primary site for **$\beta$-oxidation** (breakdown of fatty acids) and the Kreb’s cycle. * **C. Nucleus:** The nucleus houses genetic material and is responsible for replication and transcription; it does not contain the enzymatic machinery for lipid synthesis. * **D. Endosomes:** These are involved in the sorting and trafficking of proteins and lipids (endocytosis) rather than the de novo synthesis of fatty acids. **High-Yield Clinical Pearls for NEET-PG:** * **The Citrate Shuttle:** Since Acetyl-CoA cannot cross the mitochondrial membrane, it condenses with oxaloacetate to form **Citrate**, which is transported to the cytoplasm and then cleaved back into Acetyl-CoA for synthesis. * **Key Enzyme:** Acetyl-CoA Carboxylase (ACC) is inhibited by Palmitoyl-CoA and activated by Citrate. * **Cofactor:** NADPH is the essential electron donor for lipogenesis. * **Location Mnemonic:** "Synthesis in the Sol (Cytosol), Breakdown in the Box (Mitochondria)."

Q: 17-alpha hydroxylase is not involved in the pathway for synthesis of?

Aldosterone. **Explanation:** The adrenal cortex is divided into three zones, each producing specific steroid hormones based on the presence or absence of specific enzymes. The enzyme **17-alpha hydroxylase** is the key "branch point" enzyme that diverts precursors away from the mineralocorticoid pathway. **1. Why Aldosterone is the correct answer:** Aldosterone is synthesized in the **Zona Glomerulosa**. This zone lacks the enzyme 17-alpha hydroxylase. Therefore, Pregnenolone is converted to Progesterone and then eventually to Aldosterone via 21-hydroxylase and 11-beta hydroxylase. Because 17-alpha hydroxylase is absent here, the mineralocorticoid pathway remains "17-deoxy." **2. Why the other options are incorrect:** * **Cortisol (Option A):** Synthesized in the **Zona Fasciculata**. This zone requires 17-alpha hydroxylase to convert Progesterone into 17-OH Progesterone, the essential precursor for cortisol. * **Androstenedione & Testosterone (Options C & D):** Synthesized in the **Zona Reticularis** (and gonads). 17-alpha hydroxylase is mandatory to convert C21 steroids into C19 androgenic precursors (via its 17,20-lyase activity). **Clinical Pearls for NEET-PG:** * **17-alpha Hydroxylase Deficiency:** Results in a "shunting" of precursors toward the Mineralocorticoid pathway. Clinical presentation includes **Hypertension** and **Hypokalemia** (due to excess 11-deoxycorticosterone) and **Delayed Puberty/Sexual Infantilism** (due to lack of sex steroids). * **Mnemonic:** "The deeper you go, the sweeter it gets" (Salt → Sugar → Sex). * *Glomerulosa:* Salt (Aldosterone) - No 17-α hydroxylase. * *Fasciculata:* Sugar (Cortisol) - Has 17-α hydroxylase. * *Reticularis:* Sex (Androgens) - Has 17-α hydroxylase.

Question 1

Esters of fat-soluble vitamins are digested by:

Accepted Answer

Cholesterol esterase

Answer

Pancreatic lipase

Answer

Colipase

Answer

Carboxypeptidase

Question 2

Which among the following is not a saturated fatty acid?

Accepted Answer

Linoleic acid

Answer

Myristic acid

Answer

Stearic acid

Answer

Palmitic acid

Question 3

Which of the following statements is NOT true regarding the given structure?

Accepted Answer

It is a sphingophospholipid

Answer

Phosphatidic acid + choline

Answer

Deficiency can cause respiratory distress in a newborn

Answer

Largest body store of choline

Question 4

A patient is on treatment for his abnormal lipid profile blood tests. All of the following statements are true about familial hypercholesterolemia, except?

Accepted Answer

Patients with receptor-defective LDL receptor defects have the worst prognosis.

Answer

Cutaneous xanthomas on the hands, wrists, elbows present.

Answer

Total cholesterol levels can be higher than 1000 mg/dL.

Answer

Accelerated atherosclerosis is a devastating complication.

Question 5

What is the role of carnitine in lipid metabolism?

Accepted Answer

Transfer of activated long-chain free fatty acids into mitochondria

Answer

Catalyzation of the cyclization sequence

Answer

Essential for extracellular transfer of fatty acids

Answer

Essential for biosynthesis of fatty acids

Question 6

What dietary component should be administered to manage chyluria?

Accepted Answer

Medium-chain fatty acids

Answer

Short-chain fatty acids

Answer

Long-chain fatty acids

Answer

Omega-3 unsaturated fatty acids

Question 7

What is the enzyme associated with the conversion of androgen to oestrogen in the growing ovarian follicle?

Accepted Answer

Aromatase

Answer

Desmolase

Answer

Isomerase

Answer

Hydroxylase

Question 8

The liver removes low-density lipoproteins (LDLs) from the blood by the LDLs binding to which of the following?

Accepted Answer

LDL receptors and then internalizing them

Answer

HDL receptors and then internalizing them

Answer

The albumin present on LDLs and then internalizing them

Answer

The transferrin present on LDL and then internalizing them

Question 9

In which subcellular compartment does fatty acid synthesis occur in a hepatocyte?

Accepted Answer

Cytoplasm

Answer

Mitochondria

Answer

Nucleus

Answer

Endosomes

Question 10

17-alpha hydroxylase is not involved in the pathway for synthesis of?

Accepted Answer

Aldosterone

Answer

Cortisol

Answer

Androstenedione

Answer

Testosterone

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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