Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 601: What is the role of insulin in lipid metabolism?

A. Increases lipolysis
B. Activates lipoprotein lipase (Correct Answer)
C. Activates hormone sensitive lipase
D. All of the options

Explanation: ***Activates lipoprotein lipase*** - Insulin stimulates **lipoprotein lipase (LPL)** activity, leading to the uptake of **triglycerides** from chylomicrons and VLDL into adipocytes for storage. - This action promotes **lipid storage** in adipose tissue after a meal. *Increase lipolysis* - Insulin actually **inhibits lipolysis**, preventing the breakdown of stored triglycerides into free fatty acids. - The primary effect of insulin on lipid metabolism is to promote **lipid storage**, not breakdown. *Activate hormone sensitive lipase* - Insulin **inhibits hormone-sensitive lipase (HSL)**, which is responsible for breaking down stored triglycerides. - Inhibition of HSL is a key mechanism by which insulin prevents the release of **free fatty acids** from adipose tissue. *All of the options* - This option is incorrect because insulin **inhibits lipolysis** and **inactivates hormone-sensitive lipase**, directly contradicting two of the other statements. - Insulin's primary role in lipid metabolism is to **promote storage** and inhibit the breakdown and release of fats.

Question 602: All are required for conversion of progesterone to estrogen except

A. Isomerase
B. Reductase
C. 11-hydroxylase (Correct Answer)
D. Lyase

Explanation: ***11-hydroxylase*** - **11-hydroxylase** is involved in the synthesis of **cortisol** and **aldosterone** in the adrenal glands, converting 11-deoxycortisol to cortisol and 11-deoxycorticosterone to corticosterone. - It is **not involved** in the metabolic pathway for converting progesterone to estrogen. - This enzyme functions in the **glucocorticoid and mineralocorticoid pathways**, which are distinct from the sex steroid pathway. *Isomerase* - **3β-hydroxysteroid dehydrogenase/Δ5-Δ4-isomerase** is required for converting **pregnenolone to progesterone**, which occurs upstream before progesterone is formed. - Once progesterone is already synthesized, this isomerase is **not required** for the subsequent conversion of progesterone to estrogen. - The pathway from progesterone to estrogen proceeds via hydroxylation, lyase cleavage, and aromatization—not isomerization. *Reductase* - **17β-hydroxysteroid dehydrogenase** (which has reductase activity) is involved in the conversion of **androstenedione to testosterone** in the androgen synthesis pathway. - Since androgens serve as immediate precursors for estrogen synthesis, this reductase activity is **involved** in the overall pathway from progesterone to estrogen. - However, **5α-reductase** (testosterone → DHT) is not part of the progesterone-to-estrogen pathway. *Lyase* - **17,20-lyase** (part of the CYP17A1 enzyme complex) is **critical** for converting **17-hydroxyprogesterone to androstenedione** by cleaving the C17-C20 bond. - These **androgens** (androstenedione and testosterone) then serve as direct precursors for **estrogen synthesis** via the **aromatase enzyme** (CYP19A1). - Lyase is absolutely required in the progesterone-to-estrogen pathway.

Question 603: Alcohol is found in all except :

A. Sphingomyelin
B. DHA (Docosahexaenoic Acid) (Correct Answer)
C. Glucocerebroside
D. Lecithin

Explanation: ***DHA (Docosahexaenoic Acid)*** - **DHA** is an **omega-3 fatty acid**, not an alcohol. It contains a carboxyl group, characterizing it as a fatty acid. - Its structure consists of a long hydrocarbon chain with 22 carbons and 6 double bonds, making it highly unsaturated but lacking a hydroxyl group to be classified as an alcohol. *Sphingomyelin* - **Sphingomyelin** is a **sphingolipid** that contains the alcohol **sphingosine** as its backbone. - It also contains a phosphate group and choline, but the backbone alcohol is essential to its structure. *Glucocerebroside* - **Glucocerebroside** is a **glycosphingolipid** that contains the alcohol **sphingosine** linked to a fatty acid and a single glucose molecule. - The **sphingosine** component is the alcohol in its structure. *Lecithin* - **Lecithin** (phosphatidylcholine) is a **glycerophospholipid** that contains **glycerol** as its alcohol backbone. - It consists of a glycerol molecule esterified with two fatty acids and a phosphate group linked to choline.

Question 604: Hypothyroid state is characterized by

A. Increased cholesterol (Correct Answer)
B. Increased lipolysis
C. Increased protein synthesis
D. Increased glycolysis

Explanation: ***Increased cholesterol*** - In a hypothyroid state, there is decreased activity of **lipoprotein lipase** and reduced expression of **LDL receptors** in the liver. - This leads to impaired clearance of **LDL cholesterol** and triglycerides from the bloodstream, causing elevated cholesterol levels. - Hypercholesterolemia is a **hallmark clinical feature** of hypothyroidism. *Increased lipolysis* - **Lipolysis** (the breakdown of fats) is typically *reduced* in hypothyroidism because thyroid hormones normally stimulate this process. - Decreased thyroid hormone levels result in less breakdown of fat stores and decreased lipolysis. *Increased protein synthesis* - Thyroid hormones generally have an **anabolic effect** on protein synthesis at physiological levels and are critical for normal growth and development. - In hypothyroidism, there is a *reduction* in overall metabolic activity, including protein synthesis. *Increased glycolysis* - **Glycolysis** (the breakdown of glucose) is generally *reduced* in hypothyroidism due to lower metabolic rates and reduced enzyme activity. - Thyroid hormones promote various metabolic processes, including glucose utilization, so increased glycolysis would not characterize hypothyroidism.

Question 605: Rothera's test is used for detection of:

A. Proteins
B. Fatty acid
C. Glucose
D. Ketones (Correct Answer)

Explanation: ***Ketones*** - **Rothera's test** is a qualitative test used to detect the presence of **ketone bodies**, primarily **acetoacetate and acetone**, in urine. - A positive test typically indicates **ketosis**, which can occur in conditions like **diabetic ketoacidosis** or prolonged fasting. *Proteins* - **Proteins** in urine are typically detected using tests such as the **sulfosalicylic acid (SSA) test** or **urine dipstick protein tests**. - **Rothera's test** does not react with proteins. *Fatty acid* - **Fatty acids** in urine are not routinely tested for specific detection with a single spot test like **Rothera's test**. - **Rothera's test** is specific for ketone bodies, not fatty acids. *Glucose* - **Glucose** in urine is commonly detected using **urine dipstick tests** which rely on the **glucose oxidase enzymatic reaction**. - **Rothera's test** does not detect glucose.

Question 606: Which of the following is a 17β-OH steroid?

A. Estrogen
B. Androgen (Correct Answer)
C. Progesterone
D. None of the options

Explanation: ***Androgen*** - **Testosterone** is the classic example of a **17β-hydroxysteroid**, containing a hydroxyl group (-OH) at the **17β-position** of its steroid nucleus. - Other androgens like **androstenediol** and **dehydroepiandrosterone (DHEA)** derivatives also possess the **17β-OH configuration**. - This structural feature is essential for **androgen receptor binding** and biological activity in target tissues. - The 17β-hydroxyl group distinguishes active androgens from their inactive 17-ketosteroid metabolites. *Estrogen* - **Estradiol**, the primary estrogen, also contains a **17β-hydroxyl group**, making it technically a 17β-OH steroid. - However, in the context of steroid chemistry and clinical biochemistry, **testosterone** (androgen) is more commonly referenced as the prototypical **17β-hydroxysteroid**. - Both androgens and estrogens can be 17β-OH steroids, but androgens are the primary class associated with this terminology in medical literature. *Progesterone* - **Progesterone** is a C21 steroid with a **ketone group at C3 and C20**, not a hydroxyl group. - At position 17, progesterone has an **acetyl side chain** (CH3CO-), not a hydroxyl group. - It lacks the 17β-OH configuration that characterizes androgenic and estrogenic steroids. *None of the options* - This is incorrect because **androgen** (specifically testosterone) is a well-established **17β-hydroxysteroid**. - The measurement of **17-ketosteroids** (oxidized metabolites of 17β-OH steroids) in urine was historically used to assess androgen production.

Question 607: Acid that is decreased in acne comedones is?

A. Palmitic acid
B. Linolenic acid
C. Acetic acid
D. Linoleic acid (Correct Answer)

Explanation: ***Linoleic acid*** - A decrease in **linoleic acid** (an essential fatty acid) within the sebum leads to increased **comedone formation** in acne. - Reduced linoleic acid alters the **sebum composition**, making it more pro-inflammatory and less fluid, which contributes to follicular plugging. *Palmitic acid* - **Palmitic acid** is a common **saturated fatty acid** found in sebum, and its levels are generally not decreased in acne comedones; rather, the *ratio* of fatty acids is altered. - It is a major component of **triglycerides** and is often found in *higher proportions* relative to essential fatty acids in acne-prone skin. *Acetic acid* - **Acetic acid** is a **short-chain fatty acid** and is not a primary component of human sebum in significant quantities, nor is its decrease implicated in acne pathogenesis. - It is more commonly associated with microbial metabolism or certain skin infections rather than sebaceous gland dysfunction in acne. *Linolenic acid* - **Linolenic acid** (alpha-linolenic acid) is another **essential fatty acid**, but it is **linoleic acid** (omega-6) that is specifically found to be decreased in acne comedones and is more directly implicated in the pathology. - While important for skin health, its role in acne is generally less prominent than that of linoleic acid.

Question 608: What is the primary mechanism of action of 5-α reductase?

A. Reduction of C4-C5 double bond (Correct Answer)
B. Breakage of amide bond
C. Breakage of C-N bond
D. Breakage of N-N bond

Explanation: ***Reduction of C4-C5 double bond*** - 5-α reductase is a **NADPH-dependent reductase enzyme** that catalyzes the **reduction (saturation) of the C4-C5 double bond** in the A-ring of testosterone to form **dihydrotestosterone (DHT)**. - This reduction involves **adding two hydrogen atoms** across the double bond, converting it to a single bond with **5-α stereochemistry**. - DHT is a more potent androgen crucial for **prostate development, external genitalia formation, and male pattern baldness**, making 5-α reductase inhibitors (like finasteride) clinically important for treating benign prostatic hyperplasia and androgenetic alopecia. *Breakage of amide bond* - Breaking **amide bonds (C-N bonds with a carbonyl)** is the function of **proteases and amidases**, not reductases. - This process involves **hydrolysis** and is fundamental to protein degradation and peptide metabolism. *Breakage of C-N bond* - **Carbon-nitrogen bond cleavage** occurs in reactions like **deamination** (catalyzed by deaminases) or metabolism of nitrogenous compounds. - Reductases perform **electron transfer reactions**, not bond cleavage reactions. *Breakage of N-N bond* - **Nitrogen-nitrogen bond** cleavage is rare in human biochemistry and may occur in hydrazine metabolism or by specialized enzymes. - Steroid hormones do not contain N-N bonds, making this mechanism irrelevant to 5-α reductase function.

Question 609: Progesterone has how many carbons?

A. 21 (Correct Answer)
B. 20
C. 19
D. 18

Explanation: ***21*** - Progesterone is a **C21 steroid hormone**, meaning it has 21 carbon atoms in its complete structure. - The steroid nucleus contains **17 carbons** arranged in four fused rings (A, B, C, D), plus **two methyl groups** (C-18 and C-19) and a **2-carbon side chain** at position 17 (an acetyl group: -COCH₃). - This C21 structure classifies it as a **progestogen** along with other pregnancy-related hormones. *20* - No major physiologically significant steroid hormones contain exactly 20 carbons. - This count does not correspond to progesterone's actual molecular structure. *19* - **Androgens** such as testosterone and androstenedione are **C19 steroid hormones**, characterized by 19 carbon atoms. - These lack the 2-carbon side chain at C-17 that is present in progesterone, having only a keto or hydroxyl group at that position. *18* - **Estrogens** (e.g., estradiol, estrone) are **C18 steroid hormones**, characterized by an aromatic A-ring and 18 carbon atoms. - They lack both the C-19 methyl group and the C-17 side chain, resulting in fewer carbons than progesterone.

Question 610: Which of the following is the PRIMARY physiological action of insulin in the fed state?

A. Activation of key enzymes of glycolysis (Correct Answer)
B. Stimulation of gluconeogenesis
C. Increased amino acid entry in the cell
D. Increased lipogenesis

Explanation: ***Activation of key enzymes of glycolysis*** - In the fed state, **insulin's primary and immediate action** is to promote **glucose utilization** through activation of glycolytic enzymes including **hexokinase, phosphofructokinase (PFK-1), and pyruvate kinase**. - This represents the **most direct and immediate metabolic response** to elevated blood glucose after a meal, allowing cells to use glucose for **immediate energy production**. - Insulin also promotes **GLUT4 translocation** to cell membranes in muscle and adipose tissue, enhancing glucose uptake, which directly feeds into **glycolysis**. - **Glycogen synthesis** (glycogenesis) occurs simultaneously as another primary action for glucose storage in liver and muscle. *Increased lipogenesis* - While insulin does stimulate **lipogenesis** (fatty acid and triglyceride synthesis) in the fed state, this is a **secondary action** that becomes significant primarily when **glycogen stores are replete**. - Lipogenesis represents **long-term energy storage**, but the primary immediate concern in the fed state is handling the glucose load through **direct utilization and glycogen storage**. - Only the excess glucose beyond immediate energy needs and glycogen storage capacity is converted to fat via lipogenesis. *Stimulation of gluconeogenesis* - This is **incorrect** - insulin **inhibits gluconeogenesis** in the fed state by suppressing key enzymes like **PEPCK and glucose-6-phosphatase**. - Stimulating gluconeogenesis would counteract insulin's primary function of **lowering blood glucose levels**. *Increased amino acid entry in the cell* - Insulin does promote **amino acid uptake** and **protein synthesis**, which is an important anabolic action. - However, in the context of the fed state's primary metabolic challenge (handling elevated blood glucose), **glucose utilization through glycolysis** takes precedence as the primary action.

Practice by Chapter

Lipid Classification and Chemistry

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Fatty Acid Oxidation

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Ketone Body Metabolism

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Fatty Acid Synthesis

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Metabolism of Triacylglycerols

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Phospholipid Metabolism

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Cholesterol Metabolism and Biosynthesis

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Bile Acids and Bile Salts

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Lipoprotein Metabolism and Transport

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Dyslipidemias and Atherosclerosis

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Prostaglandins and Eicosanoids

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Fatty Liver and Lipotropic Factors

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