Lipid Metabolism Practice Questions

Q: Which fatty acid is not synthesized in humans?

Linoleic acid. ### Explanation **1. Why Linoleic Acid is the Correct Answer:** Linoleic acid (18:2; Δ9,12) is an **essential fatty acid** because humans lack the enzymes (**Δ12 and Δ15 desaturases**) required to introduce double bonds beyond the Δ9 position of a fatty acid chain. These enzymes are only present in plants. Since humans cannot synthesize double bonds at the ω-6 or ω-3 positions, linoleic acid must be obtained through the diet. **2. Analysis of Incorrect Options:** * **Palmitic acid (16:0):** This is the primary product of the **Fatty Acid Synthase (FAS)** complex in the cytosol. It is the first fatty acid produced during de novo lipogenesis. * **Stearic acid (18:0):** This is synthesized by the elongation of palmitic acid in the mitochondria and endoplasmic reticulum. * **Oleic acid (18:1; Δ9):** Humans possess the **Δ9-desaturase** enzyme, which can introduce a single double bond at the 9th carbon of stearic acid to form oleic acid (an omega-9 fatty acid). **3. NEET-PG High-Yield Clinical Pearls:** * **Essential Fatty Acids (EFAs):** There are two primary EFAs: **Linoleic acid** (ω-6) and **Alpha-linolenic acid** (ω-3). * **Arachidonic acid:** Often called "semi-essential." It can be synthesized from linoleic acid. However, if linoleic acid is deficient in the diet, arachidonic acid becomes essential. * **Clinical Deficiency:** EFA deficiency leads to **Phrynoderma** (follicular hyperkeratosis/toad skin), poor wound healing, and alopecia. * **Key Enzyme:** The conversion of linoleic acid to arachidonic acid involves the enzyme **Δ6-desaturase**, which is the rate-limiting step in this pathway.

Q: Which of the following is NOT an example of a steroid hormone?

Thyroxine. **Explanation:** The classification of hormones based on their chemical structure is a high-yield topic for NEET-PG. Hormones are generally divided into three categories: **Steroids, Peptides/Proteins, and Amino acid derivatives.** **Why Thyroxine (C) is the correct answer:** Thyroxine ($T_4$) is **not** a steroid hormone; it is an **amino acid derivative** synthesized from **Tyrosine**. While it shares a functional similarity with steroids (it is lipophilic and binds to intracellular receptors), its chemical backbone is derived from iodinated tyrosine residues, not cholesterol. **Analysis of Incorrect Options:** * **Aldosterone (A) & Cortisol (D):** These are **Corticosteroids** produced by the adrenal cortex. Aldosterone is a mineralocorticoid (zona glomerulosa), and Cortisol is a glucocorticoid (zona fasciculata). Both are derived from cholesterol. * **Testosterone (B):** This is an **Androgen** (sex steroid) produced primarily by the Leydig cells of the testes. Like all steroid hormones, it is synthesized from cholesterol via the precursor pregnenolone. **High-Yield Clinical Pearls for NEET-PG:** 1. **Cholesterol Precursor:** All steroid hormones (Glucocorticoids, Mineralocorticoids, Androgens, Estrogens, and Progesterone) are derived from **cholesterol**. 2. **Rate-Limiting Step:** The conversion of cholesterol to **pregnenolone** by the enzyme *Desmolase* (CYP11A1) is the rate-limiting step in steroidogenesis. 3. **Mechanism of Action:** Because they are lipid-soluble, both steroid hormones and thyroid hormones cross the cell membrane and bind to **intracellular/nuclear receptors** to alter gene transcription. 4. **The "Tyrosine" Rule:** Tyrosine is the precursor for both **Thyroid hormones** ($T_3, T_4$) and **Catecholamines** (Epinephrine, Norepinephrine, Dopamine). Remember: Thyroid hormones are lipophilic, while Catecholamines are hydrophilic.

Q: Glycerol is the backbone of all of the following phospholipids EXCEPT?

Sphingomyelin. ### Explanation The core concept tested here is the structural classification of phospholipids. Phospholipids are divided into two main categories based on their alcohol backbone: **Glycerophospholipids** (glycerol backbone) and **Sphingophospholipids** (sphingosine backbone). **Why Sphingomyelin is the Correct Answer:** Sphingomyelin is the only human phospholipid that is **not** a glycerophospholipid. Instead of glycerol, it contains **sphingosine**, an 18-carbon amino alcohol. A fatty acid is attached to the amino group of sphingosine to form a **ceramide**, and a phosphorylcholine group is attached to the primary hydroxyl group. Therefore, it lacks a glycerol backbone. **Analysis of Incorrect Options:** * **A & C (Phosphatidylethanolamine & Phosphatidylcholine):** These are the most abundant glycerophospholipids in cell membranes. They consist of a glycerol-3-phosphate backbone esterified with two fatty acids and a nitrogenous base (ethanolamine or choline). * **B (Cardiolipin):** Also known as diphosphatidylglycerol, it consists of two molecules of phosphatidic acid linked by a **glycerol** bridge. It is unique to the inner mitochondrial membrane. **NEET-PG High-Yield Pearls:** * **Sphingomyelinase Deficiency:** Leads to **Niemann-Pick Disease**, characterized by hepatosplenomegaly and "cherry-red spots" on the macula. * **Cardiolipin Clinical Link:** It is the antigen used in the **VDRL test** for Syphilis. Antibodies against it (Anti-cardiolipin antibodies) are seen in **Antiphospholipid Syndrome (APS)**. * **Dipalmitoylphosphatidylcholine (DPPC):** Also known as Lecithin, it is the major constituent of **lung surfactant**. A Lecithin/Sphingomyelin (L/S) ratio > 2 in amniotic fluid indicates fetal lung maturity.

Q: Prostaglandins are derived from:

Arachidonic acid. **Explanation:** Prostaglandins belong to a group of biologically active lipid compounds called **Eicosanoids** (20-carbon fatty acids). They are synthesized from **Arachidonic acid**, a polyunsaturated fatty acid (PUFA) containing 20 carbons and four double bonds (C20:4, ω-6). 1. **Why Arachidonic acid is correct:** Arachidonic acid is released from membrane phospholipids by the enzyme **Phospholipase A₂**. It then enters the **Cyclooxygenase (COX) pathway** to produce Prostaglandins, Prostacyclins, and Thromboxanes, or the **Lipoxygenase (LOX) pathway** to produce Leukotrienes. 2. **Why other options are incorrect:** * **Stearic acid:** A 18-carbon saturated fatty acid (C18:0). It does not have the double bonds or carbon length required for eicosanoid synthesis. * **Glutamic acid & Aspartic acid:** These are acidic amino acids involved in protein synthesis and the urea cycle, not lipid metabolism or prostaglandin synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The release of arachidonic acid by **Phospholipase A₂** is the rate-limiting step in prostaglandin synthesis. This enzyme is inhibited by **Corticosteroids** (via Annexin A1/Lipocortin). * **NSAIDs Mechanism:** Aspirin and other NSAIDs act by irreversibly or reversibly inhibiting **COX-1 and COX-2**, thereby blocking prostaglandin synthesis. * **Essential Fatty Acid Link:** Arachidonic acid is considered semi-essential; it can be synthesized from the essential fatty acid **Linoleic acid** (C18:2, ω-6). * **Prostaglandin E1 (Alprostadil):** Used clinically to keep the Ductus Arteriosus patent in neonates with cyanotic heart disease.

Q: A young man presents for a physical exam concerned about his family history of premature heart attacks. His father died of a heart attack in his late 40s. The patient has elevated serum cholesterol and LDL levels, with normal VLDL and triglycerides. Further investigation reveals an LDL receptor deficiency. Which type of hyperlipidemia does this patient have?

Type IIa. **Explanation:** The patient presents with a classic case of **Familial Hypercholesterolemia (Type IIa Hyperlipoproteinemia)**. This condition is characterized by a genetic deficiency or defect in the **LDL receptors**, leading to impaired clearance of LDL from the plasma. **Why Type IIa is correct:** In Type IIa, there is a selective elevation of **Low-Density Lipoprotein (LDL)**. Since LDL is the primary carrier of cholesterol, the lipid profile shows significantly elevated serum cholesterol with **normal triglycerides and VLDL**. The clinical history of premature coronary artery disease (CAD) and autosomal dominant inheritance (suggested by the father’s early death) are hallmark features. **Why other options are incorrect:** * **Type I (Familial Chylomicronemia):** Caused by a deficiency in Lipoprotein Lipase (LPL) or Apo C-II. It presents with severely elevated **Triglycerides** and Chylomicrons, not isolated cholesterol. * **Type IIb (Combined Hyperlipidemia):** Involves elevation of both **LDL and VLDL**. Consequently, both cholesterol and triglycerides are elevated, unlike this patient who has normal VLDL/Triglycerides. * **Type III (Dysbetalipoproteinemia):** Caused by **Apo E deficiency**, leading to the accumulation of IDL and Chylomicron remnants (Broad-beta band). Both cholesterol and triglycerides are typically elevated. **High-Yield NEET-PG Pearls:** * **Clinical Signs:** Look for **Xanthomas** (Tendon xanthoma is pathognomonic for Type II) and **Corneal arcus** at a young age. * **Friedewald Formula:** LDL = Total Cholesterol – [HDL + (TG/5)]. Note: This is invalid if TG >400 mg/dL. * **Statins** are the first-line treatment as they upregulate LDL receptor expression.

Q: Beta-oxidation of fatty acids occurring in peroxisomes is differentiated from that occurring in mitochondria by which of the following?

Formation of H2O2. **Explanation:** The primary difference between mitochondrial and peroxisomal beta-oxidation lies in the **initial step of the pathway**, specifically how electrons are transferred from the substrate. 1. **Why Option B is Correct:** In mitochondria, the first step is catalyzed by *Acyl-CoA dehydrogenase*, which transfers electrons to FAD, eventually entering the electron transport chain to produce ATP. In **peroxisomes**, the first step is catalyzed by **Acyl-CoA oxidase**. This enzyme transfers electrons directly to molecular oxygen ($O_2$), resulting in the formation of **Hydrogen Peroxide ($H_2O_2$)**. This $H_2O_2$ is subsequently degraded by the enzyme **catalase**. 2. **Why Other Options are Incorrect:** * **Option A:** Both pathways result in the production of **Acetyl CoA** (though peroxisomal oxidation stops at shorter chain lengths, usually octanoyl-CoA, which is then exported to mitochondria). * **Option C:** While it is true that different enzymes are used (e.g., bifunctional protein in peroxisomes), the question asks for the specific biochemical product that *differentiates* the two processes. $H_2O_2$ formation is the hallmark metabolic distinction. * **Option D:** Both pathways require $NAD^+$ and produce **NADH** during the 3-hydroxyacyl-CoA dehydrogenase step. **High-Yield NEET-PG Pearls:** * **Substrate Specificity:** Peroxisomes are specialized for the oxidation of **Very Long Chain Fatty Acids (VLCFA)** (C22 or longer) and branched-chain fatty acids (e.g., phytanic acid via alpha-oxidation). * **Energy Yield:** Peroxisomal oxidation is **less energy-efficient** because the first step bypasses the ETC, dissipating energy as heat rather than capturing it as ATP. * **Clinical Correlation:** Defects in peroxisomal beta-oxidation or biogenesis lead to **Zellweger Syndrome** (accumulation of VLCFA in the brain and liver) and **X-linked Adrenoleukodystrophy**.

Q: What is the number of double bonds in arachidonic acid?

4. **Explanation:** Arachidonic acid is a **20-carbon** polyunsaturated fatty acid (PUFA) belonging to the **Omega-6 (ω-6)** family. Its chemical notation is **20:4 (Δ5, 8, 11, 14)**, which indicates that it contains **4 double bonds** located at the 5th, 8th, 11th, and 14th carbon atoms. **Why Option D is Correct:** Arachidonic acid is synthesized from the essential fatty acid, linoleic acid. It serves as the primary precursor for the synthesis of **eicosanoids**, including prostaglandins, thromboxanes, and leukotrienes, via the cyclooxygenase (COX) and lipoxygenase (LOX) pathways. **Why Other Options are Incorrect:** * **Option A (1):** Represents monounsaturated fatty acids (MUFA) like **Oleic acid** (18:1). * **Option B (2):** Represents **Linoleic acid** (18:2), an essential ω-6 fatty acid. * **Option C (3):** Represents **α-Linolenic acid** (18:3), an essential ω-3 fatty acid. **High-Yield NEET-PG Pearls:** 1. **Essentiality:** Arachidonic acid becomes "essential" only if its precursor, linoleic acid, is deficient in the diet. 2. **Pro-inflammatory role:** Most eicosanoids derived from arachidonic acid (like PGE2 and LTB4) are mediators of inflammation. 3. **Membrane source:** It is typically esterified in membrane phospholipids at the **sn-2 position** and is released by the enzyme **Phospholipase A2**. 4. **Mnemonic:** Remember the "4" in the name "Arachidonic" (A-R-A-C) to recall the 4 double bonds.

Q: What is the only part of an odd-chain fatty acid that is glucogenic?

Propionyl CoA. **Explanation:** In the metabolism of fatty acids, even-chain fatty acids undergo β-oxidation to produce **Acetyl CoA**, which enters the TCA cycle but cannot be used for net glucose synthesis (gluconeogenesis) because the two carbons lost as $CO_2$ in the cycle balance the two carbons entering. **Why Propionyl CoA is correct:** Odd-chain fatty acids undergo β-oxidation until a final three-carbon fragment remains: **Propionyl CoA**. This is the only part of the fatty acid that is glucogenic because it follows a specific three-step pathway: 1. **Propionyl CoA** is carboxylated to D-methylmalonyl CoA (requires **Biotin/B7**). 2. It is isomerized to L-methylmalonyl CoA. 3. L-methylmalonyl CoA is converted to **Succinyl CoA** (requires **Vitamin B12**). Succinyl CoA is a TCA cycle intermediate that can be converted to oxaloacetate and subsequently enter the gluconeogenic pathway to form glucose. **Why the other options are incorrect:** * **B. Acetyl CoA:** Produced by even-chain fatty acids; it cannot be converted back to pyruvate or oxaloacetate for net glucose synthesis in humans. * **C. Malonyl CoA:** An intermediate in fatty acid *synthesis* (lipogenesis) and a potent inhibitor of Carnitine Palmitoyltransferase-1 (CPT-1); it is not a product of β-oxidation. * **D. Acyl CoA:** This is a general term for a fatty acid attached to Coenzyme A at any stage of oxidation; it is not a specific glucogenic end-product. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin B12 Deficiency:** Leads to the accumulation of **Methylmalonic acid (MMA)** in urine, a specific diagnostic marker that distinguishes B12 deficiency from Folate deficiency. * **Enzyme Deficiency:** Propionic acidemia is caused by a deficiency of Propionyl CoA carboxylase. * **Mnemonic:** "Odd-chain fatty acids are the exception to the rule that fats don't make sugar."

Q: What is the common precursor of mineralocorticoids, glucocorticoids, and sex steroids?

Pregnenolone. **Explanation:** The synthesis of all steroid hormones (steroidogenesis) begins with **cholesterol**. The first and rate-limiting step occurs in the mitochondria, where the enzyme **Cholesterol side-chain cleavage enzyme (P450scc/Desmolase)** converts cholesterol into **Pregnenolone**. Pregnenolone serves as the "master precursor" or the common trunk of the steroidogenic pathway. From Pregnenolone, the pathway branches out to produce: 1. **Mineralocorticoids** (e.g., Aldosterone) via Progesterone. 2. **Glucocorticoids** (e.g., Cortisol) via 17-hydroxypregnenolone. 3. **Sex Steroids** (e.g., Testosterone, Estrogen) via Dehydroepiandrosterone (DHEA). **Analysis of Incorrect Options:** * **B. 17-alpha-hydroxyprogesterone:** This is an intermediate specifically involved in the synthesis of glucocorticoids and sex steroids, but it is formed *after* pregnenolone. * **C. Dehydrotestosterone (DHT):** This is a potent androgen derived from testosterone via the action of 5-alpha-reductase; it is a terminal product, not a common precursor. * **D. 11-Deoxycortisol:** This is a direct precursor to cortisol in the glucocorticoid pathway, formed by the action of 21-hydroxylase. **High-Yield NEET-PG Pearls:** * **Rate-limiting step:** Conversion of Cholesterol to Pregnenolone by **Desmolase**. This step is stimulated by ACTH in the adrenal cortex and LH in the gonads. * **StAR Protein:** The Steroidogenic Acute Regulatory (StAR) protein is responsible for transporting cholesterol into the mitochondria; its deficiency leads to Congenital Lipoid Adrenal Hyperplasia. * **Congenital Adrenal Hyperplasia (CAH):** The most common enzyme deficiency is **21-hydroxylase**, which leads to a buildup of 17-hydroxyprogesterone and shunting of precursors toward androgen synthesis (virilization).

Q: Which enzyme is deficient in Refsum disease?

Phytanoyl-CoA a-hydroxylase. **Explanation:** **Refsum Disease** is a rare autosomal recessive peroxisomal disorder characterized by the inability to degrade **phytanic acid**, a 20-carbon branched-chain fatty acid derived from chlorophyll in the diet. 1. **Why Option B is Correct:** Phytanic acid has a methyl group at the beta-carbon, which blocks normal $\beta$-oxidation. To bypass this, the body uses **$\alpha$-oxidation** to remove the first carbon atom as $CO_2$. The key enzyme for this process is **Phytanoyl-CoA $\alpha$-hydroxylase (PhyH)**. In Refsum disease, a deficiency of this enzyme leads to the toxic accumulation of phytanic acid in tissues and plasma, particularly affecting the nervous system and retina. 2. **Why Other Options are Incorrect:** * **A. Sphingomyelinase:** Deficient in **Niemann-Pick disease**, leading to sphingomyelin accumulation. * **C. Alpha-galactosidase A:** Deficient in **Fabry disease**, an X-linked lysosomal storage disorder. * **D. N-acetylglucosaminidase:** Deficient in **Sanfilippo syndrome Type B** (a Mucopolysaccharidosis). **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Retinitis pigmentosa, peripheral neuropathy, and cerebellar ataxia. Other features include sensorineural deafness and ichthyosis (scaly skin). * **Diagnosis:** Elevated serum levels of phytanic acid. * **Management:** Dietary restriction of chlorophyll-containing foods (e.g., green leafy vegetables, dairy, and ruminant fats). * **Zellweger Syndrome vs. Refsum:** While both involve peroxisomes, Zellweger is a defect in peroxisome *biogenesis*, whereas Refsum is a specific enzyme defect in $\alpha$-oxidation.

Question 1

Which fatty acid is not synthesized in humans?

Accepted Answer

Linoleic acid

Answer

Oleic acid

Answer

Palmitic acid

Answer

Stearic acid

Question 2

Which of the following is NOT an example of a steroid hormone?

Accepted Answer

Thyroxine

Answer

Aldosterone

Answer

Testosterone

Answer

Cortisol

Question 3

Glycerol is the backbone of all of the following phospholipids EXCEPT?

Accepted Answer

Sphingomyelin

Answer

Phosphatidylethanolamine

Answer

Cardiolipin

Answer

Phosphatidylcholine

Question 4

Prostaglandins are derived from:

Accepted Answer

Arachidonic acid

Answer

Stearic acid

Answer

Glutamic acid

Answer

Aspartic acid

Question 5

A young man presents for a physical exam concerned about his family history of premature heart attacks. His father died of a heart attack in his late 40s. The patient has elevated serum cholesterol and LDL levels, with normal VLDL and triglycerides. Further investigation reveals an LDL receptor deficiency. Which type of hyperlipidemia does this patient have?

Accepted Answer

Type IIa

Answer

Type I

Answer

Type IIb

Answer

Type III

Question 6

Beta-oxidation of fatty acids occurring in peroxisomes is differentiated from that occurring in mitochondria by which of the following?

Accepted Answer

Formation of H2O2

Answer

Formation of Acetyl CoA

Answer

Presence of different enzymes in different sites

Answer

Requirement of NADH

Question 7

What is the number of double bonds in arachidonic acid?

Accepted Answer

4

Answer

1

Answer

2

Answer

3

Question 8

What is the only part of an odd-chain fatty acid that is glucogenic?

Accepted Answer

Propionyl CoA

Answer

Acetyl CoA

Answer

Malonyl CoA

Answer

Acyl CoA

Question 9

What is the common precursor of mineralocorticoids, glucocorticoids, and sex steroids?

Accepted Answer

Pregnenolone

Answer

alpha-hydroxyprogesterone

Answer

Dehydrotestosterone

Answer

Deoxycortisol

Question 10

Which enzyme is deficient in Refsum disease?

Accepted Answer

Phytanoyl-CoA a-hydroxylase

Answer

Sphingomyelinase

Answer

Alpha galactosidase-A

Answer

N-acetylglucosaminidase

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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