Lipid Metabolism Practice Questions

Q: The reducing equivalents for fatty acid synthesis are derived from which metabolic pathway?

Pentose phosphate pathway. ### Explanation **Correct Answer: D. Pentose Phosphate Pathway (PPP)** Fatty acid synthesis (lipogenesis) is a reductive process that occurs in the cytosol. It requires **NADPH** as the essential reducing equivalent to reduce the keto groups during the elongation of the fatty acid chain. The primary source of this NADPH (approx. 60%) is the **Pentose Phosphate Pathway** (also known as the Hexose Monophosphate Shunt), specifically through the oxidative reactions catalyzed by **Glucose-6-Phosphate Dehydrogenase (G6PD)** and 6-Phosphogluconate Dehydrogenase. Another significant source is the **Malic Enzyme**, which converts malate to pyruvate in the cytosol. **Why other options are incorrect:** * **A. Glycolysis:** This pathway produces **NADH**, not NADPH. NADH is primarily used for ATP production via the electron transport chain, whereas NADPH is used for reductive biosynthesis. * **B. Kreb’s Cycle:** This mitochondrial pathway generates reducing equivalents in the form of **NADH and FADH₂**, which are utilized for oxidative phosphorylation. * **C. Urea Cycle:** This is a metabolic pathway for nitrogen disposal. It does not generate reducing equivalents; in fact, it consumes ATP. **High-Yield Facts for NEET-PG:** * **Rate-limiting enzyme of Fatty Acid Synthesis:** Acetyl-CoA Carboxylase (requires Biotin). * **Tissues with active PPP:** Tissues active in lipid synthesis (Liver, Adipose tissue, Lactating mammary gland, Adrenal cortex) have high PPP activity to supply NADPH. * **Other uses of NADPH:** Steroid synthesis, maintenance of reduced glutathione in RBCs (preventing hemolysis), and the Respiratory Burst in phagocytes (NADPH Oxidase). * **Key Source Mnemonic:** "Fatty acid synthesis needs **NADPH** from the **HMP** shunt and **Malic** enzyme."

Q: Which of the following hormones has a ketogenic effect on the liver?

Glucagon. **Explanation:** **Glucagon** is the primary hormone responsible for stimulating ketogenesis in the liver. During fasting or starvation, the insulin-to-glucagon ratio decreases. Glucagon acts on the liver to decrease levels of **Malonyl-CoA** (by inhibiting Acetyl-CoA Carboxylase). Since Malonyl-CoA is a potent inhibitor of **Carnitine Palmitoyltransferase-I (CPT-I)**, its decrease allows fatty acids to enter the mitochondria via the carnitine shuttle. Once inside, these fatty acids undergo $\beta$-oxidation to produce Acetyl-CoA, which is then diverted into the synthesis of ketone bodies (Acetoacetate and $\beta$-hydroxybutyrate). **Analysis of Incorrect Options:** * **Insulin:** This is the most potent **anti-ketogenic** hormone. It stimulates lipogenesis, increases Malonyl-CoA levels (inhibiting CPT-I), and promotes the peripheral utilization of glucose, thereby suppressing ketone body formation. * **Growth Hormone (GH):** While GH is lipolytic (breaks down fats in adipose tissue), its primary role in the liver is promoting gluconeogenesis and IGF-1 production. It is considered "diabetogenic" but is not the primary driver of hepatic ketogenesis compared to glucagon. * **Androgens:** These are anabolic steroids that influence protein synthesis and secondary sexual characteristics; they have no significant direct regulatory effect on the ketogenic pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of ketogenesis:** HMG-CoA Synthase (mitochondrial). * **Ketone bodies:** Acetoacetate, $\beta$-hydroxybutyrate, and Acetone (a non-metabolizable byproduct). * **Organ utilization:** The liver **produces** ketone bodies but **cannot use** them because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). * **Diabetic Ketoacidosis (DKA):** Occurs due to absolute insulin deficiency and a relative glucagon excess, leading to unrestrained ketogenesis.

Q: Which of the following is a GM2 gangliosidosis?

Sandhoff's disease. ### Explanation **GM2 Gangliosidoses** are a group of lysosomal storage disorders caused by the inability to degrade GM2 gangliosides. This occurs due to a deficiency in the enzyme **Hexosaminidase** or its activator protein. **1. Why Sandhoff’s Disease is Correct:** Sandhoff’s disease is caused by a deficiency in **Hexosaminidase A and B** (due to a mutation in the *HEXB* gene). This leads to the massive accumulation of GM2 gangliosides and globosides in the brain and other organs. It is clinically similar to Tay-Sachs disease (which is also a GM2 gangliosidosis) but often involves visceral organomegaly. **2. Why the Other Options are Incorrect:** * **Niemann-Pick Disease:** This is a **Sphingomyelinosis** caused by a deficiency of the enzyme **Sphingomyelinase**, leading to the accumulation of sphingomyelin. * **Gaucher’s Disease:** This is a **Glucosylceramide lipidosis** caused by a deficiency of **$\beta$-Glucosidase** (Glucocerebrosidase), leading to the accumulation of glucocerebroside. * **Fabry’s Disease:** This is an **X-linked** glycosphingolipidosis caused by a deficiency of **$\alpha$-Galactosidase A**, leading to the accumulation of ceramide trihexoside. **3. High-Yield Clinical Pearls for NEET-PG:** * **Tay-Sachs vs. Sandhoff:** Both present with a **Cherry-red spot** on the macula and progressive neurodegeneration. However, Sandhoff’s disease may present with **hepatosplenomegaly**, whereas Tay-Sachs does not. * **Enzyme mnemonic:** * Tay-Sachs = Hexosaminidase **A** (Missing **A**lpha subunit). * Sandhoff = Hexosaminidase **A & B** (Missing **B**eta subunit). * **Histology:** Look for "onion-skin" lysosomes in electron microscopy for GM2 gangliosidoses.

Q: What is the precursor of all steroid hormones?

Pregnenolone. **Explanation:** The synthesis of all steroid hormones (mineralocorticoids, glucocorticoids, and sex steroids) begins with **Cholesterol**. However, the first committed steroid intermediate in this biosynthetic pathway is **Pregnenolone**. **Why Pregnenolone is correct:** Cholesterol (a 27-carbon molecule) is transported into the mitochondria by the **StAR protein**. There, the enzyme **Cholesterol side-chain cleavage enzyme (P450scc / Desmolase)** converts cholesterol into Pregnenolone (a 21-carbon molecule). Because all subsequent steroid pathways—whether in the adrenal cortex or gonads—diverge from this single molecule, Pregnenolone is recognized as the universal precursor or "grandparent" of all steroid hormones. **Analysis of Incorrect Options:** * **B. Deoxycorticosterone:** This is an intermediate specifically in the mineralocorticoid pathway (leading to Aldosterone). It is formed downstream from pregnenolone. * **C. Androstenedione:** This is an intermediate in the androgen pathway, synthesized from either DHEA or progesterone. It serves as a precursor to testosterone and estrone, but not to corticosteroids. * **D. Dehydroepiandrosterone (DHEA):** This is a weak androgen produced from 17-hydroxypregnenolone. While it is a precursor for sex steroids, it is not a precursor for mineralocorticoids or glucocorticoids. **High-Yield NEET-PG Pearls:** * **Rate-limiting step:** The conversion of cholesterol to pregnenolone by **Desmolase** is the rate-limiting step in steroidogenesis. * **ACTH Action:** ACTH stimulates steroid synthesis primarily by increasing the activity of Desmolase and the StAR protein. * **Location:** This initial step occurs in the **mitochondria**, while subsequent steps occur in the Smooth Endoplasmic Reticulum (SER).

Q: Which of the following is the most characteristic finding for Type III hyperlipoproteinemia?

Palmar xanthomas. **Explanation:** **Type III Hyperlipoproteinemia**, also known as **Familial Dysbetalipoproteinemia** or Broad Beta Disease [1], is an autosomal recessive disorder caused by a deficiency in **Apolipoprotein E (ApoE)**. Since ApoE is essential for the hepatic uptake of chylomicron remnants and VLDL remnants (IDL), its deficiency leads to the accumulation of these "remnant particles" in the plasma [3]. 1. **Why Option A is correct:** **Palmar xanthomas** (specifically *Xanthoma striatum palmare*) are orange-yellow discolorations or plaques in the creases of the palms and fingers. They are considered **pathognomonic** (highly characteristic) for Type III hyperlipoproteinemia. 2. **Why Option B is incorrect:** While triglycerides are elevated in Type III (usually 300–600 mg/dL), levels >1000 mg/dL are more characteristic of **Type I (Familial Chylomicronemia)** or **Type V** hyperlipidemia, which carry a high risk of acute pancreatitis. 3. **Why Option C is incorrect:** Subcutaneous extensor tendon xanthomas (especially on the Achilles tendon) are the hallmark of **Type IIa (Familial Hypercholesterolemia)**, caused by LDL receptor defects [1]. 4. **Why Option D is incorrect:** In Type III, both serum cholesterol and triglycerides are typically elevated (often in a 1:1 ratio) due to the accumulation of IDL [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Genetics:** Associated with the **ApoE2/E2 genotype** (ApoE2 has low affinity for the LDL receptor). * **Electrophoresis:** Shows a **"Broad Beta Band"** (due to IDL). * **Clinical Presentation:** Patients have a high risk of premature **Peripheral Vascular Disease (PVD)** and Coronary Artery Disease [1]. * **Treatment:** Highly responsive to **Fibrates** and lifestyle modifications [2].

Q: Stored triacylglycerol and cholesterol are released by which enzyme?

Hormone sensitive lipase. **Explanation:** **1. Why Hormone-Sensitive Lipase (HSL) is correct:** Hormone-Sensitive Lipase is the rate-limiting enzyme for **lipolysis** in adipose tissue. It hydrolyzes stored triacylglycerols (TAGs) into free fatty acids and glycerol. Crucially, HSL also possesses **cholesteryl esterase** activity, allowing it to release free cholesterol from stored cholesteryl esters. It is activated by glucagon and epinephrine (via cAMP/Protein Kinase A phosphorylation) during fasting or stress, and inhibited by insulin. **2. Why the other options are incorrect:** * **Lysosomal Lipase:** This enzyme (acid lipase) degrades lipids that enter the cell via endocytosis (e.g., LDL particles). While it breaks down TAGs and cholesterol esters, it is not the primary regulator for mobilizing systemic energy stores from adipocytes. * **Lipoprotein Lipase (LPL):** Found on the endothelial surface of capillaries, LPL acts on **circulating** lipids (Chylomicrons and VLDL) to provide fatty acids to peripheral tissues. It does not act on stored intracellular lipids. * **LCAT (Lecithin-Cholesterol Acyltransferase):** This enzyme is involved in cholesterol **esterification** within HDL particles in the plasma (converting free cholesterol to cholesterol esters). It does not release stored lipids. **Clinical Pearls for NEET-PG:** * **Perilipin:** In resting adipocytes, lipids are protected by the protein perilipin. Phosphorylation of perilipin by PKA allows HSL to access the lipid droplet. * **Insulin’s Role:** Insulin is the most potent inhibitor of HSL; this is why diabetic ketoacidosis (DKA) occurs in insulin deficiency—uninhibited HSL leads to massive fatty acid release and subsequent ketone body formation. * **Product of Lipolysis:** Glycerol released by HSL cannot be reused by adipocytes (due to lack of **glycerol kinase**) and must go to the liver for gluconeogenesis.

Q: Deficiency in alpha-oxidation of fatty acids leads to which of the following conditions?

Refsum disease. **Explanation:** **Correct Answer: D. Refsum disease** Alpha-oxidation is a specialized pathway occurring in peroxisomes, primarily required for the breakdown of **Phytanic acid** (a branched-chain fatty acid found in dairy and chlorophyll). Unlike most fatty acids, phytanic acid has a methyl group at the beta-carbon, which blocks beta-oxidation. Alpha-oxidation removes one carbon atom from the carboxyl end to bypass this block. Refsum disease is caused by a deficiency in the enzyme **Phytanoyl-CoA hydroxylase**. This leads to the toxic accumulation of phytanic acid in tissues, manifesting clinically as retinitis pigmentosa, peripheral neuropathy, cerebellar ataxia, and ichthyosis. **Analysis of Incorrect Options:** * **A. Dicarboxylic aciduria:** This occurs when **beta-oxidation** is impaired (e.g., MCAD deficiency). The body resorts to **omega-oxidation** in the endoplasmic reticulum, producing dicarboxylic acids that are excreted in the urine. * **B. Zellweger syndrome:** This is a **peroxisomal biogenesis disorder** where peroxisomes are absent or non-functional. While it affects alpha and beta oxidation, it is a generalized defect rather than a specific deficiency of the alpha-oxidation pathway itself. * **C. Jamaican vomiting sickness:** Caused by **Hypoglycin A** (from unripe ackee fruit), which inhibits **Acyl-CoA dehydrogenase**, thereby blocking beta-oxidation and leading to profound hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **Dietary Management:** The primary treatment for Refsum disease is the strict avoidance of green leafy vegetables (chlorophyll) and ruminant fats. * **Location:** Alpha-oxidation occurs exclusively in **peroxisomes**. * **Key Enzyme:** Phytanoyl-CoA hydroxylase (Deficient in Classic Refsum). * **Refsum vs. Zellweger:** Refsum is a single enzyme defect; Zellweger is a total organelle (peroxisome) failure.

Q: Fatty acids are not utilized by which of the following?

Red blood cells. **Explanation:** The correct answer is **Red Blood Cells (RBCs)**. The utilization of fatty acids for energy occurs via **Beta-oxidation**, a metabolic pathway that takes place exclusively within the **mitochondria**. **1. Why Red Blood Cells (RBCs) cannot utilize fatty acids:** Mature RBCs lack mitochondria. Consequently, they are incapable of performing beta-oxidation or the TCA cycle. RBCs are entirely dependent on **anaerobic glycolysis** in the cytosol for their ATP requirements, converting glucose to lactate. **2. Analysis of Incorrect Options:** * **Skeletal Muscle:** These cells contain abundant mitochondria. During rest and low-intensity exercise, fatty acids are the preferred fuel source for skeletal muscle. * **Liver:** The liver is the primary site for fatty acid metabolism. It oxidizes fatty acids to generate ATP and produces ketone bodies (ketogenesis) during fasting states. * **Heart:** The myocardium is highly aerobic and has a very high mitochondrial density. Under normal physiological conditions, **60–80% of the heart's energy** is derived from the oxidation of long-chain fatty acids. **High-Yield Clinical Pearls for NEET-PG:** * **Brain Paradox:** Although the brain has mitochondria, it cannot utilize fatty acids because they are bound to albumin and cannot cross the **Blood-Brain Barrier (BBB)**. The brain uses glucose or ketone bodies (during starvation). * **Essentiality of Glucose:** Because RBCs and the Brain cannot use fatty acids, the body must maintain blood glucose levels via gluconeogenesis during fasting. * **Key Enzyme:** Carnitine Palmitoyltransferase-1 (CPT-1) is the rate-limiting enzyme for fatty acid entry into the mitochondria.

Question 1

The reducing equivalents for fatty acid synthesis are derived from which metabolic pathway?

Accepted Answer

Pentose phosphate pathway

Answer

Glycolysis

Answer

Kreb's cycle

Answer

Urea cycle

Question 2

Which of the following hormones has a ketogenic effect on the liver?

Accepted Answer

Glucagon

Answer

Insulin

Answer

Growth Hormone (GH)

Answer

Androgen

Question 3

A 6-day-old neonate presents to the NICU with respiratory depression, poor feeding, and persistent seizures. The child was born at 39 weeks gestation by spontaneous vaginal delivery to healthy parents. On examination, the neonate has a high forehead with unslanting palpebral fissures, hypoplastic supraorbital ridges, epicanthal folds, a depressed nasal bridge with a long philtrum, and hearing loss. Liver dysfunction, polycystic kidneys, and a patent ductus arteriosus were also present. Which of the following entities will be found to be increased in the serum of the baby?

Accepted Answer

Very long-chain fatty acids (VLCFAs)

Answer

Phytanic acid

Answer

Pipecolic acid

Answer

Pristanic acid

Question 4

Which of the following is a GM2 gangliosidosis?

Accepted Answer

Sandhoff's disease

Answer

Niemann-Pick disease

Answer

Gaucher's disease

Answer

Fabry's disease

Question 5

What is the precursor of all steroid hormones?

Accepted Answer

Pregnenolone

Answer

Deoxycorticosterone

Answer

Androstenedione

Answer

Dehydroepiandrosterone

Question 6

Which of the following is the most characteristic finding for Type III hyperlipoproteinemia?

Accepted Answer

Palmar xanthomas

Answer

Triglycerides > 1000 mg/dL

Answer

Subcutaneous extensor tendon xanthomas

Answer

Low serum cholesterol

Question 7

Stored triacylglycerol and cholesterol are released by which enzyme?

Accepted Answer

Hormone sensitive lipase

Answer

Lysosomal lipase

Answer

Lipoprotein lipase

Answer

LCAT

Question 8

A 38-year-old man presents with a history of an infection. Ocular examination reveals small opaque rings on the lower edge of the iris in the anterior chamber. Nodular lesions are found on his Achilles tendon. Successful therapy should be aimed at increasing which of the following gene products in hepatocyte cell membranes?

Accepted Answer

Apolipoprotein B-100 receptor

Answer

Apolipoprotein B-100

Answer

Apolipoprotein E

Answer

Apolipoprotein E receptor

Question 9

Deficiency in alpha-oxidation of fatty acids leads to which of the following conditions?

Accepted Answer

Refsum disease

Answer

Dicarboxylic aciduria

Answer

Zellweger syndrome

Answer

Jamaican vomiting sickness

Question 10

Fatty acids are not utilized by which of the following?

Accepted Answer

Red blood cells

Answer

Skeletal muscle

Answer

Liver

Answer

Heart

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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