Lipid Metabolism Practice Questions

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

Chylomicron. **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.

Q: Hormone-sensitive lipase acts on which substrate?

Triglycerides. **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.

Q: How many carbon atoms are added in fatty acid synthesis?

4 in the first cycle and 2 in the second cycle. ### Explanation The synthesis of fatty acids (Lipogenesis) occurs in the cytoplasm via the **Fatty Acid Synthase (FAS) multienzyme complex**. Understanding the carbon count requires looking at the "priming" step versus the "elongation" steps. **1. Why Option B is Correct:** * **First Cycle:** The process begins with one molecule of **Acetyl-CoA (2C)** and one molecule of **Malonyl-CoA (3C)**. During the condensation reaction, one carbon is lost as $CO_2$. Therefore, $2 + 3 - 1 = 4$ carbons. The first product formed is Butyryl-ACP (a 4-carbon chain). * **Subsequent Cycles:** In every following cycle, only one **Malonyl-CoA (3C)** is added to the existing chain. Again, one carbon is lost as $CO_2$, resulting in a net addition of **2 carbons** per cycle. * Thus, the sequence is 4 carbons in the first cycle and 2 carbons in all subsequent cycles until Palmitate (16C) is formed. **2. Why Other Options are Wrong:** * **Option A & D:** These are incorrect because the first cycle must account for the initial Acetyl-CoA primer plus the first Malonyl-CoA unit, totaling 4 carbons. * **Option C:** This is a common trap. While it is true that 2 carbons are *net* added from Malonyl-CoA in each step, the first cycle results in a 4-carbon molecule because it starts from a 2-carbon Acetyl-CoA base. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Rate-limiting enzyme:** Acetyl-CoA Carboxylase (requires **Biotin**). * **Reductant:** **NADPH** is the essential electron donor (primarily from the HMP Shunt). * **End Product:** The FAS complex specifically releases **Palmitate (16:0)**. Further elongation occurs in the endoplasmic reticulum. * **Citrate Shuttle:** Acetyl-CoA moves from mitochondria to cytosol in the form of Citrate ("Citrate is for Synthesis").

Q: Which enzyme is NOT important in testosterone biosynthesis?

18-hydroxylase. **Explanation:** The biosynthesis of testosterone occurs primarily in the Leydig cells of the testes via the steroidogenic pathway. The correct answer is **18-hydroxylase** because this enzyme is specific to the **adrenal cortex** (specifically the Zona Glomerulosa), where it is required for the final steps of **aldosterone synthesis**. It plays no role in the production of androgens. **Analysis of Options:** * **18-hydroxylase (Correct):** Also known as aldosterone synthase, it converts corticosterone to aldosterone. Its absence in the testes ensures that mineralocorticoids are not produced there. * **Delta 5-4 isomerase:** This enzyme (along with 3β-HSD) is crucial for converting pregnenolone to progesterone or dehydroepiandrosterone (DHEA) to androstenedione, which are intermediate steps in testosterone production. * **Alpha-hydroxylase (17-alpha hydroxylase):** This is a key enzyme in the steroidogenic pathway. It converts progesterone/pregnenolone into 17-hydroxy derivatives, which are essential precursors for both cortisol and testosterone. * **5-alpha reductase:** While testosterone is the primary product of the testes, 5-alpha reductase is the enzyme responsible for converting testosterone into its more potent metabolite, **Dihydrotestosterone (DHT)**, in peripheral tissues and the prostate. In the context of "testosterone biosynthesis" pathways often discussed in exams, it is considered part of the androgenic metabolic machinery. **High-Yield NEET-PG Pearls:** * **Rate-limiting step:** The conversion of cholesterol to pregnenolone by the enzyme **Desmolase** (stimulated by LH). * **17β-HSD:** The final enzyme that converts androstenedione to testosterone. * **Congenital Adrenal Hyperplasia (CAH):** A deficiency in 17α-hydroxylase leads to decreased testosterone and cortisol but increased mineralocorticoids (causing hypertension and sexual infantilism).

Q: Which of the following is an omega-6 fatty acid?

Gamma-Linolenic acid. **Explanation:** Fatty acids are classified based on the position of the first double bond from the methyl ($\omega$) end. **Omega-6 ($\omega$-6) fatty acids** have their first double bond at the sixth carbon atom. **Gamma-Linolenic Acid (GLA)** is an 18-carbon polyunsaturated fatty acid (18:3; $\omega$-6) with double bonds at positions 6, 9, and 12. It is synthesized from Linoleic acid (the essential parent $\omega$-6) by the enzyme $\Delta^6$-desaturase. **Analysis of Incorrect Options:** * **Alpha-Linolenic acid (ALA):** This is an **$\omega$-3** fatty acid (18:3; $\omega$-3). It is the essential precursor for the $\omega$-3 series. * **Timnodonic acid:** Also known as **Eicosapentaenoic acid (EPA)**, it is a 20-carbon **$\omega$-3** fatty acid (20:5; $\omega$-3) found in fish oil. * **Cervonic acid:** Also known as **Docosahexaenoic acid (DHA)**, it is a 22-carbon **$\omega$-3** fatty acid (22:6; $\omega$-3) vital for retinal and brain function. **High-Yield Clinical Pearls for NEET-PG:** * **Essential Fatty Acids (EFA):** Humans lack $\Delta^{12}$ and $\Delta^{15}$ desaturases, making Linoleic acid ($\omega$-6) and Alpha-Linolenic acid ($\omega$-3) nutritionally essential. * **Arachidonic Acid:** A 20-carbon $\omega$-6 fatty acid (20:4; $\omega$-6) that serves as the precursor for pro-inflammatory prostaglandins and leukotrienes. It becomes "essential" only if Linoleic acid is deficient. * **Deficiency Sign:** EFA deficiency leads to **Phrynoderma** (toad skin), characterized by follicular hyperkeratosis on the extensor surfaces of limbs.

Q: Fredrickson's type II hyperlipoproteinemia is due to excess amounts of which lipoprotein?

VLDL. **Explanation:** **Fredrickson Classification** (also known as the WHO classification) categorizes hyperlipoproteinemias based on the specific lipoprotein pattern observed in the plasma. **Type IV Hyperlipoproteinemia** is characterized by an isolated elevation of **Very Low-Density Lipoproteins (VLDL)**. The underlying pathophysiology usually involves the overproduction of VLDL by the liver or a decrease in its peripheral clearance. Since VLDL is rich in endogenous triglycerides, patients typically present with significant **hypertriglyceridemia**, while cholesterol levels remain normal or only slightly elevated. **Analysis of Options:** * **VLDL (Correct):** This is the hallmark of Type IV. It is often associated with obesity, diabetes mellitus, and alcohol consumption. * **VLDL Remnants (Incorrect):** Accumulation of VLDL remnants (IDL) is the characteristic feature of **Type III** hyperlipoproteinemia (Dysbetalipoproteinemia), caused by Apo-E deficiency. * **Floating LDL (Incorrect):** This is another term for IDL (Broad-beta band), which is specific to **Type III**, not Type IV. * **HDL (Incorrect):** HDL is the "good cholesterol." High levels are generally cardioprotective and are not part of the Fredrickson classification of hyperlipoproteinemias. **High-Yield Clinical Pearls for NEET-PG:** * **Type I:** Elevated Chylomicrons (Deficiency of Lipoprotein Lipase or Apo C-II). * **Type IIa:** Elevated LDL (LDL receptor deficiency). * **Type IIb:** Elevated LDL + VLDL. * **Type III:** Elevated IDL (Apo-E deficiency; presents with palmar xanthomas). * **Type IV:** Elevated VLDL (Most common type; associated with pancreatitis risk if TGs >1000 mg/dL). * **Type V:** Elevated VLDL + Chylomicrons.

Q: Slow-reacting substance of anaphylaxis (SRS-A) is?

Leukotriene. **Explanation:** The correct answer is **Leukotriene**. **Slow-reacting substance of anaphylaxis (SRS-A)** is a mixture of specific cysteinyl leukotrienes, primarily **LTC4, LTD4, and LTE4**. These are synthesized via the **Lipoxygenase (LOX) pathway** from arachidonic acid. They are called "slow-reacting" because, unlike histamine which causes immediate bronchoconstriction, SRS-A induces a slower, more prolonged, and significantly more potent contraction of smooth muscle in the airways. **Why other options are incorrect:** * **Prostaglandins:** These are products of the Cyclooxygenase (COX) pathway. While they mediate inflammation and pain (e.g., PGD2 causes bronchoconstriction), they do not constitute SRS-A. * **Serotonin:** A monoamine neurotransmitter and vasoactive amine. While it plays a role in platelet aggregation and GI motility, it is not a component of SRS-A. * **Histamine:** This is a pre-formed mediator stored in mast cell granules. It is responsible for the **immediate** (Type I) hypersensitivity reaction but is rapidly metabolized, unlike the sustained action of SRS-A. **High-Yield Clinical Pearls for NEET-PG:** * **Potency:** Cysteinyl leukotrienes (SRS-A) are **100 to 1000 times more potent** than histamine in inducing bronchoconstriction and increasing vascular permeability. * **Aspirin-Exacerbated Respiratory Disease (AERD):** Inhibiting the COX pathway with Aspirin shunts arachidonic acid toward the LOX pathway, increasing leukotriene production and triggering asthma. * **Pharmacology Link:** **Montelukast and Zafirlukast** are leukotriene receptor antagonists (LTRA) used in chronic asthma management to block the effects of SRS-A. * **LTB4** is primarily involved in **chemotaxis** (neutrophil recruitment) and is not part of SRS-A.

Q: Bile salts act as all except:

Help in fat digestion. **Explanation:** The question asks for the exception regarding the functions of bile salts. While bile salts are essential for the overall process of lipid absorption, they do **not** possess enzymatic activity and therefore do not directly "digest" fat. **1. Why Option D is the Correct Answer (The Exception):** Digestion is a chemical process involving the hydrolysis of bonds. This is performed by enzymes, specifically **Pancreatic Lipase**. Bile salts are biological detergents, not enzymes. Their role is purely mechanical/physical: they break down large fat globules into smaller droplets (micelles), increasing the surface area for lipase to act upon. Without lipase, bile salts alone cannot digest fat. **2. Analysis of Incorrect Options:** * **A & B (Emulsifying agent/Detergent):** Bile salts are amphipathic (containing both hydrophobic and hydrophilic regions). They surround lipid droplets, preventing them from coalescing. This detergent action creates an **emulsion**, which is vital for the subsequent action of water-soluble lipases. * **C (Surface tension lowering agent):** By acting at the oil-water interface, bile salts significantly lower the surface tension of lipid droplets. This property is the basis of the **Hay’s Test** used to detect bile salts in urine (where they cause sulfur powder to sink). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Primary Bile Acids:** Cholic acid and Chenodeoxycholic acid (synthesized from cholesterol in the liver). * **Rate-limiting enzyme:** Cholesterol 7-alpha-hydroxylase. * **Secondary Bile Acids:** Deoxycholic acid and Lithocholic acid (formed by bacterial action in the colon). * **Enterohepatic Circulation:** 95% of bile salts are reabsorbed in the **terminal ileum**. * **Colipase:** A protein secreted by the pancreas that displaces bile salts from the fat droplet surface to allow Pancreatic Lipase to bind and begin digestion.

Question 1

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

Accepted Answer

Chylomicron

Answer

Chylomicron remnants

Answer

LDL

Answer

HDL

Question 2

Hormone-sensitive lipase acts on which substrate?

Accepted Answer

Triglycerides

Answer

Cholesterol esters

Answer

Phospholipids

Answer

Gangliosides

Question 3

How many carbon atoms are added in fatty acid synthesis?

Accepted Answer

4 in the first cycle and 2 in the second cycle

Answer

2 in the first cycle and 4 in the second cycle

Answer

2 in the first cycle and 2 in the second cycle

Answer

4 in the first cycle and 4 in the second cycle

Question 4

Which enzyme is NOT important in testosterone biosynthesis?

Accepted Answer

18-hydroxylase

Answer

5-alpha reductase

Answer

Delta 5-4 isomerase

Answer

Alpha-hydroxylase

Question 5

Beta-oxidation of an odd-carbon fatty acid chain produces which of the following?

Accepted Answer

Propionyl CoA

Answer

Succinyl CoA

Answer

Acetyl CoA

Answer

Malonyl CoA

Question 6

Sphingolipids are chiefly accumulated in which of the following tissues?

Accepted Answer

Central nervous system (CNS)

Answer

Cardiac muscle

Answer

Adipose tissue

Answer

Skeletal muscle

Question 7

Which of the following is an omega-6 fatty acid?

Accepted Answer

Gamma-Linolenic acid

Answer

Alpha-Linolenic acid

Answer

Timnodonic acid

Answer

Cervonic acid

Question 8

Fredrickson's type II hyperlipoproteinemia is due to excess amounts of which lipoprotein?

Accepted Answer

VLDL

Answer

VLDL remnants

Answer

Floating LDL

Answer

HDL

Question 9

Slow-reacting substance of anaphylaxis (SRS-A) is?

Accepted Answer

Leukotriene

Answer

Prostaglandin

Answer

Serotonin

Answer

Histamine

Question 10

Bile salts act as all except:

Accepted Answer

Help in fat digestion

Answer

Emulsifying agent

Answer

Detergent

Answer

Surface tension lowering agent

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?