Lipid Metabolism Practice Questions

Q: A male with hyperpigmentation tanner stage 5 presents with hypertension & precocious puberty. The causative defect is:

11 beta hydroxylase deficiency. ***11 beta hydroxylase deficiency*** - This deficiency leads to an accumulation of **11-deoxycortisol** and **deoxycorticosterone (DOC)**, a potent mineralocorticoid [1]. - **DOC excess** causes **hypertension** and **hypokalemia**, while the shunting of precursors to the androgen pathway results in **precocious puberty** in males and virilization in females, along with **hyperpigmentation** due to increased ACTH [1]. *17 alpha hydroxylase deficiency* - This deficiency impairs the synthesis of **cortisol** and **sex steroids**, leading to an accumulation of **mineralocorticoid precursors (DOC and corticosterone)**. - Patients typically present with **hypertension**, **hypokalemia**, and **absent or rudimentary secondary sexual characteristics** (delayed puberty/sexual infantilism) due to the lack of androgens/estrogens, not precocious puberty. *17 beta hydroxylase deficiency* - This enzyme is crucial for the final step in sex steroid synthesis (e.g., testosterone from androstenedione). - A deficiency would lead to **impaired sexual development** and **ambiguous genitalia or undervirilization** in males, along with delayed puberty, completely contradictory to precocious puberty. *21 beta hydroxylase deficiency* - This is the **most common cause of congenital adrenal hyperplasia (CAH)**, leading to a profound deficiency in cortisol and aldosterone, and an excess in androgens [1]. - Patients typically present with **salt-wasting crises** (due to aldosterone deficiency) or **virilization** (due to androgen excess), but usually **hypotension** (due to salt wasting) or normal blood pressure, not hypertension alongside precocious puberty in this specific manner [1].

Q: Which gene defect causes familial hypercholesterolemia?

LDL Receptor. ***LDL Receptor*** - Familial hypercholesterolemia (FH) is primarily caused by mutations in the **LDL receptor (LDLR) gene**, which leads to impaired clearance of **low-density lipoprotein (LDL)** from the blood. - This defect results in significantly elevated levels of **LDL cholesterol** and an increased risk of premature cardiovascular disease. *Apo E* - Mutations in the **Apo E gene** are associated with **Type III hyperlipoproteinemia (dysbetalipoproteinemia)**, characterized by elevated **chylomicron remnants** and **VLDL remnants**. - This condition presents with xanthomas and premature atherosclerosis, but is distinct from the primary defect in FH. *Apo CII* - **Apo CII** is a cofactor for **lipoprotein lipase (LPL)**, an enzyme essential for the breakdown of **triglycerides** in chylomicrons and VLDL. - Deficiency in Apo CII or LPL causes **Type I hyperlipoproteinemia (familial chylomicronemia syndrome)**, leading to marked **hypertriglyceridemia**, not hypercholesterolemia. *Apo B48* - **Apo B48** is a structural component of **chylomicrons**, which are responsible for transporting dietary fats from the intestines. - It is not directly involved in the primary defect of **LDL clearing** that characterizes familial hypercholesterolemia.

Q: In fatty acid oxidation, what 2-carbon unit is released after each cycle of beta-oxidation?

Acetyl-CoA. ***Acetyl-CoA*** - Each cycle of **beta-oxidation** cleaves a **2-carbon unit** from the fatty acid chain, releasing one molecule of **acetyl-CoA**. - This **acetyl-CoA** then enters the **citric acid cycle** (Krebs cycle) for further oxidation and energy production. - Acetyl-CoA is the primary product representing the carbon units removed from the fatty acid. *FADH2* - **FADH2** is indeed produced during each cycle of beta-oxidation by the enzyme **acyl-CoA dehydrogenase** in the first step. - However, it is an **electron carrier**, not the 2-carbon unit being released from the fatty acid chain. - It feeds electrons into the **electron transport chain** to generate ATP. *NADH* - **NADH** is also produced during each cycle of beta-oxidation by **β-hydroxyacyl-CoA dehydrogenase** in the third step. - Like FADH2, it is an **electron carrier** rather than the carbon product being cleaved off. - It functions in the **electron transport chain** for ATP synthesis. *ATP* - **ATP** is not directly produced during the beta-oxidation cycle itself. - ATP is generated later through **oxidative phosphorylation** when FADH2 and NADH donate electrons to the electron transport chain, and when acetyl-CoA is oxidized in the citric acid cycle. - One complete cycle of beta-oxidation actually requires one ATP equivalent (consumed in the activation step) before producing energy through downstream processes.

Q: How does insulin affect lipid metabolism?

Inhibits lipolysis. ***Inhibits lipolysis*** - Insulin **downregulates hormone-sensitive lipase (HSL)** activity, a key enzyme responsible for breaking down stored triglycerides into fatty acids and glycerol. - By inhibiting lipolysis, insulin promotes the storage of energy and prevents the release of fatty acids into circulation. *Stimulates ketogenesis in adipose tissue* - Ketogenesis primarily occurs in the **liver** during periods of low insulin and high glucagon, not typically in adipose tissue. - Insulin's primary role is to **reduce** ketone body production by promoting glucose utilization and fatty acid storage. *Increases fatty acid oxidation* - Insulin **reduces fatty acid oxidation** (beta-oxidation) by stimulating malonyl-CoA production, which inhibits carnitine palmitoyltransferase 1 (CPT1), preventing fatty acid entry into mitochondria. - Its main effect is to promote **fatty acid synthesis and storage**, not their breakdown for energy. *Activates hormone-sensitive lipase (HSL)* - Insulin has an **inhibitory effect on HSL** through dephosphorylation, while glucagon and catecholamines activate HSL. - The activation of HSL would lead to the **breakdown of triglycerides**, which is contrary to insulin's role in promoting energy storage.

Q: What is the main source of ketone bodies during fasting?

Fatty acids. ***Fatty acids*** - During **fasting**, the body shifts from carbohydrate to fat metabolism to produce energy. - **Fatty acids** are broken down in the liver through **beta-oxidation** to form acetyl-CoA, which is then converted into ketone bodies. *Glucose* - **Glucose** is the primary energy source in the fed state, not during fasting. - During fasting, **glucose levels** decrease, prompting the body to seek alternative fuel sources. *Amino acids* - While some **amino acids** can be converted into glucose (gluconeogenesis) or ketone bodies, they are a secondary source. - **Protein breakdown** for energy is primarily a long-term adaptation to starvation, not the main initial source of ketone bodies. *Glycogen* - **Glycogen stores** (mainly in the liver and muscles) are used during the initial hours of fasting. - Once these stores are depleted, usually within 12-24 hours, the body relies on **fatty acid oxidation** for energy, leading to ketone body production.

Q: Which lysosomal enzyme deficiency is associated with a disease characterized by the accumulation of glucocerebrosides in cells?

Glucocerebrosidase deficiency. **Correct: Glucocerebrosidase deficiency** - This deficiency leads to **Gaucher disease**, specifically characterized by the accumulation of **glucocerebrosides** in macrophages within the lysosome. - The accumulated glucocerebrosides impede normal cellular function, particularly in the **spleen, liver, bone marrow**, and sometimes the nervous system. *Incorrect: Alpha-galactosidase deficiency* - This deficiency causes **Fabry disease**, leading to the accumulation of **globotriaosylceramide (Gb3)**, primarily affecting the kidneys, heart, and nervous system. - Clinical features include **neuropathic pain**, angiokeratomas, and renal failure, which are distinct from the symptoms of glucocerebroside accumulation. *Incorrect: Hexosaminidase A deficiency* - This enzyme deficiency is responsible for **Tay-Sachs disease**, marked by the accumulation of **GM2 gangliosides**, predominantly in neuronal lysosomes. - It results in rapidly progressive neurodegeneration, **developmental regression**, and a characteristic **cherry-red spot** on the retina. *Incorrect: Sphingomyelinase deficiency* - This deficiency is associated with **Niemann-Pick disease**, types A and B, causing the accumulation of **sphingomyelin** in various tissues. - Niemann-Pick disease typically presents with hepatosplenomegaly, **neurodegeneration** (Type A), and often has foam cells.

Q: In a patient with CPT I deficiency, what best describes the primary consequence on fatty acid metabolism?

Inhibits FA transport to mitochondria. ***Inhibits FA transport to mitochondria*** - **CPT I (Carnitine Palmitoyltransferase I)** is the rate-limiting enzyme in the **carnitine shuttle system**, which is essential for transporting long-chain fatty acids into the mitochondrial matrix for **beta-oxidation**. - A deficiency in CPT I directly prevents the formation of **acylcarnitine**, thereby blocking the entry of fatty acids into the mitochondria for energy production. *Enhances FA synthesis* - Fatty acid synthesis and fatty acid oxidation are distinct metabolic pathways that are generally regulated inversely. - A CPT I deficiency would lead to an accumulation of fatty acids in the cytoplasm, which might *downregulate* synthesis through feedback mechanisms, rather than enhance it. *Increases ketogenesis* - Ketogenesis is the process of producing ketone bodies from fatty acids in the liver, primarily when glucose is scarce. - Since CPT I deficiency impairs the transport of fatty acids into the mitochondria, it would *decrease* the availability of substrates for beta-oxidation, thus reducing ketogenesis. *Reduces lipolysis* - Lipolysis is the breakdown of stored triglycerides into free fatty acids and glycerol, primarily occurring in adipose tissue. - CPT I deficiency affects the *utilization* of fatty acids, not their release from storage, so it would not directly reduce lipolysis.

Q: How does a defect in the enzyme 21-hydroxylase affect the synthesis of adrenal hormones?

Decreases aldosterone and cortisol synthesis. ***Decreases aldosterone and cortisol synthesis*** - 21-hydroxylase is a crucial enzyme in the adrenal steroid synthesis pathway, necessary for converting **progesterone** to **11-deoxycorticosterone** (a precursor to aldosterone) and **17-hydroxyprogesterone** to **11-deoxycortisol** (a precursor to cortisol). - A defect in this enzyme directly impairs the production of both **cortisol** and **aldosterone**, leading to their deficiency. - This is the hallmark of **congenital adrenal hyperplasia (CAH)** due to 21-hydroxylase deficiency. *Decreases cortisol production* - While cortisol production is decreased, this option is incomplete as it fails to mention the concurrent decrease in **aldosterone** synthesis, which is also a significant consequence of 21-hydroxylase deficiency. - The deficiency impacts multiple steroid pathways, not just the cortisol pathway in isolation. *Leads to decreased estrogen production* - Estrogen synthesis primarily occurs in the **gonads** and **peripheral tissues** from androgens, not directly regulated by 21-hydroxylase in the adrenal cortex. - A 21-hydroxylase defect leads to an **accumulation of androgen precursors** (like DHEA), which can be converted to estrogens peripherally, sometimes even increasing estrogen levels in certain forms of congenital adrenal hyperplasia. *Decreases androgen synthesis* - This is **incorrect** - a 21-hydroxylase defect causes a shunting of precursors towards the **androgen pathway**, leading to an **increase in androgen synthesis**, not a decrease. - This excess androgen production is responsible for the **virilization** observed in individuals with congenital adrenal hyperplasia due to 21-hydroxylase deficiency, including ambiguous genitalia in females and precocious puberty in males.

Q: Which enzyme deficiency leads to the accumulation of ceramide trihexoside in Fabry disease?

α-galactosidase A deficiency. ***α-galactosidase A deficiency*** - **Fabry disease** is an **X-linked recessive** lysosomal storage disorder caused by a deficiency of the enzyme **α-galactosidase A**. - This enzyme deficiency leads to the accumulation of its substrate, **globotriaosylceramide (Gb3)**, also known as **ceramide trihexoside**, in various tissues. *β-glucosidase deficiency* - This deficiency is characteristic of **Gaucher disease**, leading to the accumulation of **glucocerebroside**. - Patients typically present with **hepatosplenomegaly**, **bone pain**, and **pancytopenia**, which are not features of Fabry disease. *Hexosaminidase A deficiency* - This enzyme deficiency is associated with **Tay-Sachs disease**, which results in the accumulation of **GM2 gangliosides**. - Tay-Sachs disease primarily affects the **nervous system**, causing neurodegeneration in infancy. *Sphingomyelinase deficiency* - This deficiency causes **Niemann-Pick disease**, leading to the accumulation of **sphingomyelin** in lysosomes. - Clinical features include **hepatosplenomegaly**, **neurodegeneration** (especially in Type A), and characteristic **foam cells**.

Question 1

A male with hyperpigmentation tanner stage 5 presents with hypertension & precocious puberty. The causative defect is:

Accepted Answer

11 beta hydroxylase deficiency

Answer

17 alpha hydroxylase deficiency

Answer

17 beta hydroxylase deficiency

Answer

21 beta hydroxylase deficiency

Question 2

Which gene defect causes familial hypercholesterolemia?

Accepted Answer

LDL Receptor

Answer

Apo E

Answer

Apo CII

Answer

Apo B48

Question 3

In fatty acid oxidation, what 2-carbon unit is released after each cycle of beta-oxidation?

Accepted Answer

Acetyl-CoA

Answer

FADH2

Answer

NADH

Answer

ATP

Question 4

How does insulin affect lipid metabolism?

Accepted Answer

Inhibits lipolysis

Answer

Stimulates ketogenesis in adipose tissue

Answer

Increases fatty acid oxidation

Answer

Activates hormone-sensitive lipase (HSL)

Question 5

What is the main source of ketone bodies during fasting?

Accepted Answer

Fatty acids

Answer

Glucose

Answer

Amino acids

Answer

Glycogen

Question 6

A patient with muscle weakness and hypoketotic hypoglycemia is diagnosed with CPT II deficiency. How does this deficiency affect fatty acid metabolism?

Accepted Answer

Inhibits FA transport into mitochondria; reduced β-oxidation

Answer

Enhances FA synthesis; lipid accumulation

Answer

Increases ketogenesis; ketoacidosis

Answer

Reduces lipolysis; decreased free FAs

Question 7

Which lysosomal enzyme deficiency is associated with a disease characterized by the accumulation of glucocerebrosides in cells?

Accepted Answer

Glucocerebrosidase deficiency

Answer

Alpha-galactosidase deficiency

Answer

Hexosaminidase A deficiency

Answer

Sphingomyelinase deficiency

Question 8

In a patient with CPT I deficiency, what best describes the primary consequence on fatty acid metabolism?

Accepted Answer

Inhibits FA transport to mitochondria

Answer

Enhances FA synthesis

Answer

Increases ketogenesis

Answer

Reduces lipolysis

Question 9

How does a defect in the enzyme 21-hydroxylase affect the synthesis of adrenal hormones?

Accepted Answer

Decreases aldosterone and cortisol synthesis

Answer

Decreases androgen synthesis

Answer

Decreases cortisol production

Answer

Leads to decreased estrogen production

Question 10

Which enzyme deficiency leads to the accumulation of ceramide trihexoside in Fabry disease?

Accepted Answer

α-galactosidase A deficiency

Answer

β-glucosidase deficiency

Answer

Hexosaminidase A deficiency

Answer

Sphingomyelinase deficiency

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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