A 15-year-old boy is brought to the emergency department by his parents because of lethargy, repeated vomiting, and abdominal pain for 6 hours. Over the past 2 weeks, he has reported increased urinary frequency to his parents that they attributed to his increased oral fluid intake. Examination shows dry mucous membranes and rapid, deep breathing. Laboratory studies show the presence of acetoacetate in the urine. Which of the following cells is unable to use this molecule for energy production?
Q52
An otherwise healthy 13-year-old boy is brought to the physician for the evaluation of severe acne for the last 3 years. Topical retinoic acid and oral tetracycline did not improve his symptoms. He shaves his chin and mustache area every few days. His parents report that he grew 5 cm (2 in) during the last year. The onset of pubic hair growth was at age 8. He is at the 95th percentile for height and weight. Vital signs are within normal limits. Examination shows several pimples and pustules along the skin of the cheeks, chin, and neck. Genitals are Tanner stage 4 and pubic hair is Tanner stage 5. Early morning serum laboratory studies drawn 30 minutes after administration of ACTH show:
Sodium 137 mEq/L
Potassium 3.8 mEq/L
Cortisol (0800 h) 4 μg/dL
Aldosterone 10 ng/dL (N = 7–30)
17OH-Progesterone 230 ng/dL (N = 3–90)
Deoxycorticosterone 2.7 ng/dL (N = 3.5–11.5)
Androstenedione 350 ng/dL (N = 80–240)
Dehydroepiandrosterone sulfate (DHEAS) 420 μg/dL (N = 29–412)
Which of the following is the most likely underlying cause of this patient's symptoms?
Q53
A 12-year-old boy is brought to the emergency department because of acute onset abdominal pain. On arrival, he also complains of nausea and shortness of breath in addition to epigastric pain. He has previously been admitted to the hospital several times for respiratory infections with Pseudomonas species and uses a nebulizer and a chest wall oscillation vest at home. The patient's acute condition is found to be due to premature activation of an enzyme that normally interacts with the brush border. Which of the following describes the activity of this enzyme?
Q54
A 1-year-old girl is brought to the pediatrician because of a 6-month history of diarrhea. She has not received recommended well-child examinations. Her stools are foul-smelling and nonbloody. There is no family history of serious illness. She is at the 15th percentile for height and 5th percentile for weight. Physical examination shows abdominal distension. Her serum triglyceride concentration is 5 mg/dL. Genetic analysis shows a mutation in the gene that encodes microsomal triglyceride transfer protein. Which of the following is the most appropriate treatment for this patient's condition?
Q55
A scientist is trying to design a drug to modulate cellular metabolism in the treatment of obesity. Specifically, he is interested in understanding how fats are processed in adipocytes in response to different energy states. His target is a protein within these cells that catalyzes catabolism of an energy source. The products of this reaction are subsequently used in gluconeogenesis or β-oxidation. Which of the following is true of the most likely protein that is being studied by this scientist?
Q56
A 4-year-old girl is brought to the physician by her mother because of fatigue and generalized weakness for 4 months. Examination shows decreased muscle tone. Her fasting serum glucose concentration is 41 mg/dL. The physician suspects a defect in one of the enzymes involved in the carnitine shuttle. Increased serum concentration of which of the following should most raise suspicion of a different diagnosis?
Q57
A 5-year-old girl is brought in for a routine checkup. She was born at 39 weeks gestation via spontaneous vaginal delivery and is up to date on all vaccines and is meeting all developmental milestones. Upon examination, she is pale with a few petechiae on her chest neck and back. Examination of the abdomen reveals painless hepatosplenomegaly. Liver enzymes are mildly elevated and complete blood cell count shows slight anemia and thrombocytopenia. Iron, B12, and folate are normal. A bone marrow biopsy shows mildly hypocellular marrows with diffuse macrophages with eosinophilic cytoplasm. The cytoplasm looks like wrinkled tissue paper on further inspection. No blasts are observed. What is the most likely diagnosis in the present case?
Q58
A 6-month-old girl is brought to the emergency department by her father after he observed jerking movements of her arms and legs earlier in the day. She appears lethargic. Physical examination shows generalized hypotonia. The liver edge is palpable 3 cm below the right costophrenic angle. Her fingerstick glucose shows hypoglycemia. Serum levels of acetone, acetoacetate, and β-hydroxybutyrate are undetectable. Molecular genetic testing shows a mutation in the carnitine palmitoyltransferase II gene. This patient will most likely benefit from supplementation with which of the following?
Q59
An investigator is studying the effect of a high-lipid diet on glucose metabolism in Wistar rats. The experimental rat group is fed a high-lipid diet while the control group is fed a low-lipid diet. Two months after initiation of the experiment, the rats in both groups are injected with insulin and serum glucose measurements are obtained. Compared to the control group, the high-lipid diet group has a significantly higher average serum glucose after receiving insulin. Which of the following intracellular changes is most likely involved in the pathogenesis of this finding?
Q60
A mother brings her 1-year-old daughter who has had several seizures in the past 2 weeks to the pediatrician. The mother explains that the child is unable to crawl, sit, or even hold up her own head. She thinks the weakness is getting worse. The parents of the child are first cousins, and the mother's sister had one child who died before the age of 3 with similar symptoms. Hexosaminidase A activity was assayed in the blood and found to be absent. Which of the following will be found on fundoscopic examination of the child?
Lipid metabolism US Medical PG Practice Questions and MCQs
Question 51: A 15-year-old boy is brought to the emergency department by his parents because of lethargy, repeated vomiting, and abdominal pain for 6 hours. Over the past 2 weeks, he has reported increased urinary frequency to his parents that they attributed to his increased oral fluid intake. Examination shows dry mucous membranes and rapid, deep breathing. Laboratory studies show the presence of acetoacetate in the urine. Which of the following cells is unable to use this molecule for energy production?
A. Myocyte
B. Adipocyte
C. Neuron
D. Thrombocyte
E. Hepatocyte (Correct Answer)
Explanation: ***Hepatocyte***
- **Hepatocytes** are the primary site of **ketone body synthesis** during fasting, starvation, or poorly controlled diabetes, producing acetoacetate and β-hydroxybutyrate from fatty acid oxidation.
- However, hepatocytes **cannot utilize ketone bodies for energy** because they lack the enzyme **succinyl-CoA:3-ketoacid CoA transferase (thiophorase)**, which is essential for converting acetoacetate to acetoacetyl-CoA.
- This enzymatic deficiency ensures that ketone bodies produced by the liver are exported to **peripheral tissues** (brain, muscle, kidney) for energy utilization during periods of glucose scarcity.
- This represents a critical metabolic concept: the liver makes ketones for other organs but cannot use them itself.
*Thrombocyte*
- **Thrombocytes** (platelets) lack **mitochondria** and rely exclusively on **anaerobic glycolysis** for ATP production.
- While platelets technically cannot use ketone bodies due to the absence of mitochondria, this is not the primary educational focus when discussing ketone body metabolism in biochemistry.
- Platelets also cannot perform oxidative phosphorylation or utilize fatty acids.
*Adipocyte*
- **Adipocytes** can utilize **acetoacetate** for energy, particularly during fasting states when ketone body levels are elevated.
- They possess mitochondria and the enzyme **succinyl-CoA:3-ketoacid CoA transferase**, allowing conversion of acetoacetate to acetoacetyl-CoA and subsequent entry into the citric acid cycle.
*Myocyte*
- **Myocytes** (cardiac and skeletal muscle) are major consumers of **ketone bodies** during prolonged fasting, starvation, or extended exercise.
- The heart preferentially uses ketone bodies over glucose when available, making them highly efficient at ketone body oxidation.
- Muscle cells contain all necessary enzymes to convert acetoacetate to acetyl-CoA for **oxidative phosphorylation**.
*Neuron*
- **Neurons** adapt to use **ketone bodies** as an alternative fuel to glucose during prolonged fasting (after 3-4 days), providing up to 60-70% of the brain's energy needs.
- This metabolic flexibility is crucial for survival during starvation and is the basis for therapeutic ketogenic diets in certain neurological conditions.
- Brain tissue efficiently metabolizes both acetoacetate and β-hydroxybutyrate.
Question 52: An otherwise healthy 13-year-old boy is brought to the physician for the evaluation of severe acne for the last 3 years. Topical retinoic acid and oral tetracycline did not improve his symptoms. He shaves his chin and mustache area every few days. His parents report that he grew 5 cm (2 in) during the last year. The onset of pubic hair growth was at age 8. He is at the 95th percentile for height and weight. Vital signs are within normal limits. Examination shows several pimples and pustules along the skin of the cheeks, chin, and neck. Genitals are Tanner stage 4 and pubic hair is Tanner stage 5. Early morning serum laboratory studies drawn 30 minutes after administration of ACTH show:
Sodium 137 mEq/L
Potassium 3.8 mEq/L
Cortisol (0800 h) 4 μg/dL
Aldosterone 10 ng/dL (N = 7–30)
17OH-Progesterone 230 ng/dL (N = 3–90)
Deoxycorticosterone 2.7 ng/dL (N = 3.5–11.5)
Androstenedione 350 ng/dL (N = 80–240)
Dehydroepiandrosterone sulfate (DHEAS) 420 μg/dL (N = 29–412)
Which of the following is the most likely underlying cause of this patient's symptoms?
A. Constitutive activation of adenylyl cyclase
B. Leydig-cell tumor production of androgens
C. Exposure to exogenous steroids
D. 17α-hydroxylase deficiency
E. 21-hydroxylase deficiency (Correct Answer)
Explanation: ***21-hydroxylase deficiency***
- The very high **17-OH progesterone** (230 ng/dL) is characteristic of **21-hydroxylase deficiency**, as the enzyme block causes a buildup of precursors proximal to the block.
- This leads to increased shunting towards **androgen production** (**androstenedione** and **DHEAS** are elevated), explaining the severe acne, early puberty, and advanced physical development, while **cortisol** and **aldosterone** are low or normal leading to mineralocorticoid deficiency, though it's not a salt-wasting crisis in this non-classical presentation.
*Constitutive activation of adenylyl cyclase*
- This condition, in the context of adrenal hyperplasia, would lead to **ACTH-independent cortisol production**, not the low cortisol observed here after ACTH administration, nor the specific steroid precursor elevations.
- It is associated with conditions like **primary pigmented nodular adrenocortical disease** (PPNAD) or McCune-Albright syndrome, which presents differently, primarily with Cushing features.
*Leydig-cell tumor production of androgens*
- While a Leydig-cell tumor would cause **precocious puberty** and elevated androgens, it would typically **suppress LH and FSH** and would not explain the specific pattern of elevated adrenal precursors like **17-OH progesterone**.
- **Adrenal steroid levels** (like 17-OH progesterone, DHEA-S) would not be elevated from a testicular Leydig cell tumor.
*Exposure to exogenous steroids*
- Exogenous androgen exposure would lead to **suppression of endogenous androgen production** and would likely cause **adrenal suppression**, resulting in low cortisol levels, but not the elevated **17-OH progesterone** and other adrenal androgen precursors seen in this patient.
- Long-term exogenous corticosteroid use causes **Cushingoid features** and adrenal suppression, not directly increased androgen production.
*17α-hydroxylase deficiency*
- This deficiency would lead to **low cortisol and androgens** but markedly **elevated mineralocorticoids** like deoxycorticosterone and corticosterone.
- The patient's **deoxycorticosterone** is below the normal range, and **aldosterone** is normal, ruling out 17α-hydroxylase deficiency.
Question 53: A 12-year-old boy is brought to the emergency department because of acute onset abdominal pain. On arrival, he also complains of nausea and shortness of breath in addition to epigastric pain. He has previously been admitted to the hospital several times for respiratory infections with Pseudomonas species and uses a nebulizer and a chest wall oscillation vest at home. The patient's acute condition is found to be due to premature activation of an enzyme that normally interacts with the brush border. Which of the following describes the activity of this enzyme?
A. Activates pancreatic enzyme precursors (Correct Answer)
B. Breaks down elastin molecules
C. Hydrolyzes phospholipids
D. Digests triglycerides
E. Activates phospholipase A2
Explanation: ***Activates pancreatic enzyme precursors***
- The patient's history of **recurrent respiratory infections with Pseudomonas** and use of a **nebulizer/chest wall oscillation vest** strongly suggests **cystic fibrosis (CF)**.
- In cystic fibrosis, **thickened secretions** can obstruct the pancreatic ducts, leading to **autodigestion of the pancreas** due to obstruction preventing the release of pancreatic enzymes. The enzyme being referred to is **trypsin**, which, when prematurely activated, activates other pancreatic enzyme precursors, leading to **pancreatitis**.
*Breaks down elastin molecules*
- This activity is characteristic of **elastase**, an enzyme produced by the pancreas. While elastase is involved in the overall digestive process and can be prematurely activated, its primary role is not the one alluded to in the clinical presentation, which points to **pancreatitis** from premature activation of the cascade.
- Damage to elastin is more classically associated with conditions like **emphysema** (due to alpha-1 antitrypsin deficiency) rather than acute abdominal pain secondary to autodigestion.
*Hydrolyzes phospholipids*
- This is the function of **phospholipase**, another pancreatic enzyme. While also capable of contributing to pancreatic autodigestion if prematurely activated, it is typically activated by **trypsin**, making trypsin the primary enzyme responsible for initiating the cascade of activation.
- **Phospholipase A2** acts on phospholipids, but the question describes an enzyme that *normally interacts with the brush border* before activation of the precursors begins.
*Digests triglycerides*
- This is the function of **pancreatic lipase**. Premature activation of lipase can contribute to the fat necrosis seen in pancreatitis.
- However, lipase, like many other pancreatic enzymes, is activated by **trypsin**, which is the initial enzyme in the cascade of activation leading to autodigestion.
*Activates phospholipase A2*
- This describes the action of **trypsinogen turning into trypsin**, which then activates other proenzymes like **prophospholipase A2**.
- While correct that trypsin activates phospholipase A2, the question asks about the primary enzyme whose *premature activation* causes the issue, which is **trypsin** itself, as it activates *multiple* pancreatic enzyme precursors, initiating a cascade.
Question 54: A 1-year-old girl is brought to the pediatrician because of a 6-month history of diarrhea. She has not received recommended well-child examinations. Her stools are foul-smelling and nonbloody. There is no family history of serious illness. She is at the 15th percentile for height and 5th percentile for weight. Physical examination shows abdominal distension. Her serum triglyceride concentration is 5 mg/dL. Genetic analysis shows a mutation in the gene that encodes microsomal triglyceride transfer protein. Which of the following is the most appropriate treatment for this patient's condition?
A. Nicotinic acid supplementation
B. Pancreatic enzyme replacement
C. Long-term antibiotic therapy
D. Restriction of long-chain fatty acids (Correct Answer)
E. Avoidance of dietary gluten
Explanation: ***Restriction of long-chain fatty acids***
- This patient has **abetalipoproteinemia** due to the mutation in the **microsomal triglyceride transfer protein (MTP)**, causing defective chylomicron formation and severe fat malabsorption. Restricting **long-chain fatty acids (LCFAs)** minimizes the substrate for chylomicron synthesis, reducing gastrointestinal symptoms.
- **Medium-chain triglycerides (MCTs)** can be supplemented as they do not require chylomicron formation for absorption, bypassing the defective MTP pathway and providing a source of energy.
- Additional treatment includes **fat-soluble vitamin supplementation** (vitamins A, D, E, K) since fat malabsorption leads to deficiency of these vitamins.
*Nicotinic acid supplementation*
- **Nicotinic acid (niacin)** is typically used to lower **LDL cholesterol** and **triglyceride** levels in patients with **hyperlipidemia**.
- It would not address the underlying issue of fat malabsorption or the inability to form chylomicrons in abetalipoproteinemia.
*Pancreatic enzyme replacement*
- **Pancreatic enzyme replacement therapy** is used for conditions like **cystic fibrosis** or **chronic pancreatitis**, where there is insufficient production of lipase and other digestive enzymes.
- In this case, the issue is not enzyme deficiency but rather the inability to package absorbed fats into chylomicrons for transport.
*Long-term antibiotic therapy*
- Long-term antibiotics are primarily used to treat **chronic bacterial infections** or **small intestinal bacterial overgrowth (SIBO)**.
- The patient's symptoms are due to a genetic metabolic defect, not an infection, making antibiotics ineffective for the primary condition.
*Avoidance of dietary gluten*
- **Gluten-free diets** are prescribed for **celiac disease**, an autoimmune condition triggered by gluten, leading to villous atrophy and malabsorption.
- The patient's symptoms, low triglyceride levels, and genetic mutation point to abetalipoproteinemia, not celiac disease.
Question 55: A scientist is trying to design a drug to modulate cellular metabolism in the treatment of obesity. Specifically, he is interested in understanding how fats are processed in adipocytes in response to different energy states. His target is a protein within these cells that catalyzes catabolism of an energy source. The products of this reaction are subsequently used in gluconeogenesis or β-oxidation. Which of the following is true of the most likely protein that is being studied by this scientist?
A. It is stimulated by epinephrine (Correct Answer)
B. It is inhibited by glucagon
C. It is inhibited by acetylcholine
D. It is inhibited by cortisol
E. It is stimulated by insulin
Explanation: ***It is stimulated by epinephrine***
- The protein described is likely **hormone-sensitive lipase (HSL)**, which catabolizes **triglycerides** in adipocytes to **glycerol** and **fatty acids**.
- **Epinephrine** (and norepinephrine) stimulates HSL activity via a **cAMP-dependent protein kinase A (PKA)** pathway, leading to increased fatty acid release for energy.
*It is inhibited by glucagon*
- **Glucagon primarily acts on the liver** to promote gluconeogenesis and glycogenolysis, but it does **not directly inhibit HSL** in adipocytes.
- While glucagon has a lipolytic effect, it doesn't inhibit the enzyme that releases fatty acids.
*It is inhibited by acetylcholine*
- **Acetylcholine** is a neurotransmitter involved in the **parasympathetic nervous system**, which generally promotes energy storage.
- It does **not directly inhibit HSL**; its effects on lipid metabolism are indirect and typically involve other pathways.
*It is inhibited by cortisol*
- **Cortisol**, a glucocorticoid, generally **promotes lipolysis** (breakdown of fats) in certain contexts, particularly during stress to provide energy substrates.
- Therefore, it would **not inhibit HSL**; rather, it often enhances its activity or provides a permissive effect for other lipolytic hormones.
*It is stimulated by insulin*
- **Insulin** is an **anabolic hormone** that promotes energy storage, including **lipogenesis** (fat synthesis) and inhibits lipolysis.
- Insulin **inhibits HSL activity** by activating phosphodiesterase, which reduces cAMP levels, thus deactivating PKA and preventing HSL phosphorylation.
Question 56: A 4-year-old girl is brought to the physician by her mother because of fatigue and generalized weakness for 4 months. Examination shows decreased muscle tone. Her fasting serum glucose concentration is 41 mg/dL. The physician suspects a defect in one of the enzymes involved in the carnitine shuttle. Increased serum concentration of which of the following should most raise suspicion of a different diagnosis?
A. Ammonia (Correct Answer)
B. Creatine kinase
C. Alanine aminotransferase
D. Uric acid
E. β-hydroxybutyrate
Explanation: ***Ammonia***
- An elevated **ammonia** level in the context of hypoglycemia and muscle weakness in a child suggests an **inborn error of metabolism** that affects the **urea cycle** or **organic acidemia**, not primarily the carnitine shuttle.
- Urea cycle disorders lead to **hyperammonemia**, which can cause neurological symptoms, fatigue, and muscle weakness, often exacerbated by catabolic states.
- This finding would **strongly suggest a different diagnosis** from a carnitine shuttle defect.
*Creatine kinase*
- **Creatine kinase (CK)** levels are typically **elevated in carnitine shuttle defects** due to muscle damage and myopathy.
- Elevated CK would **support** the suspected diagnosis of a carnitine shuttle defect rather than suggest an alternative.
- This is an **expected finding** in fatty acid oxidation disorders.
*Alanine aminotransferase*
- **Alanine aminotransferase (ALT)** can be elevated in **carnitine shuttle defects** due to liver involvement and hepatic dysfunction.
- While elevated ALT indicates liver damage, it can occur in fatty acid oxidation disorders and would not necessarily point away from a carnitine shuttle defect.
- This finding is **consistent with** rather than against the suspected diagnosis.
*Uric acid*
- **Uric acid** levels are not directly affected by defects in the **carnitine shuttle**.
- While an elevated uric acid level might prompt investigation into conditions like **glycogen storage diseases** or purine metabolism disorders, it is not a strong discriminator for alternative diagnoses in this clinical context.
*β-hydroxybutyrate*
- **β-hydroxybutyrate** is a **ketone body** produced from fatty acid oxidation during fasting states.
- In carnitine shuttle defects, the body **cannot effectively oxidize fatty acids** to produce ketones, resulting in **hypoketotic hypoglycemia** (low or inappropriately low ketones despite low glucose).
- If β-hydroxybutyrate is **elevated** during fasting hypoglycemia, this indicates **intact fatty acid oxidation** and would suggest a different diagnosis such as **hyperinsulinism**, **glycogen storage disease**, or other causes of hypoglycemia where ketogenesis is preserved.
- However, **ammonia elevation** is a stronger indicator of an alternative diagnosis (urea cycle disorder) compared to the scenario presented.
Question 57: A 5-year-old girl is brought in for a routine checkup. She was born at 39 weeks gestation via spontaneous vaginal delivery and is up to date on all vaccines and is meeting all developmental milestones. Upon examination, she is pale with a few petechiae on her chest neck and back. Examination of the abdomen reveals painless hepatosplenomegaly. Liver enzymes are mildly elevated and complete blood cell count shows slight anemia and thrombocytopenia. Iron, B12, and folate are normal. A bone marrow biopsy shows mildly hypocellular marrows with diffuse macrophages with eosinophilic cytoplasm. The cytoplasm looks like wrinkled tissue paper on further inspection. No blasts are observed. What is the most likely diagnosis in the present case?
A. Autoimmune disorder
B. Gaucher disease type I (Correct Answer)
C. Biliary obstruction
D. Acute lymphoblastic leukemia
E. Viral hepatitis
Explanation: **Gaucher disease type I**
- The characteristic finding of **diffuse macrophages with eosinophilic cytoplasm** that resembles **"wrinkled tissue paper"** on bone marrow biopsy is pathognomonic for **Gaucher cells**, confirming **Gaucher disease type I**.
- **Painless hepatosplenomegaly**, **anemia**, **thrombocytopenia**, and **petechiae** are common clinical manifestations resulting from the accumulation of glucocerebroside in macrophages within the reticuloendothelial system.
*Autoimmune disorder*
- While autoimmune disorders can cause anemia and hepatosplenomegaly, the distinct **"wrinkled tissue paper" macrophages** found in the bone marrow biopsy are not characteristic of autoimmune conditions.
- Autoimmune disorders like lupus or autoimmune hemolytic anemia would present with different serological markers and histological findings.
*Biliary obstruction*
- **Biliary obstruction** typically presents with **jaundice**, dark urine, pale stools, and significant elevation of **liver enzymes** (especially direct bilirubin and alkaline phosphatase), which are not prominent features here.
- It would not explain the hematopoietic abnormalities (anemia, thrombocytopenia) or the presence of Gaucher cells in the bone marrow.
*Acute lymphoblastic leukemia*
- **Acute lymphoblastic leukemia** would typically show a significant presence of **blasts** (immature white blood cells) in the bone marrow, which are explicitly noted as absent in this case.
- While it can cause anemia and thrombocytopenia, the key diagnostic feature of **blasts** is missing, and the characteristic macrophages would not be present.
*Viral hepatitis*
- **Viral hepatitis** primarily causes **liver inflammation** and can lead to significant elevation of **liver enzymes** (transaminases), often much higher than "mildly elevated."
- It does not explain the **painless hepatosplenomegaly**, **anemia**, **thrombocytopenia**, or the pathognomonic **Gaucher cells** in the bone marrow.
Question 58: A 6-month-old girl is brought to the emergency department by her father after he observed jerking movements of her arms and legs earlier in the day. She appears lethargic. Physical examination shows generalized hypotonia. The liver edge is palpable 3 cm below the right costophrenic angle. Her fingerstick glucose shows hypoglycemia. Serum levels of acetone, acetoacetate, and β-hydroxybutyrate are undetectable. Molecular genetic testing shows a mutation in the carnitine palmitoyltransferase II gene. This patient will most likely benefit from supplementation with which of the following?
A. Thiamine
B. Coenzyme A
C. Methionine
D. Tetrahydrobiopterin
E. Medium-chain triglycerides (Correct Answer)
Explanation: ***Medium-chain triglycerides***
- This patient presents with symptoms consistent with a **fatty acid oxidation disorder** (lethargy, hypotonia, hepatomegaly, hypoglycemia, and undetectable ketones in the setting of hypoglycemia). A **carnitine palmitoyltransferase II (CPT II) deficiency** is a specific fatty acid oxidation disorder.
- **Medium-chain triglycerides (MCTs)** can be directly used for energy in the mitochondria without requiring the carnitine shuttle system, bypassing the defective CPT II enzyme and providing an alternative energy source.
*Thiamine*
- **Thiamine (vitamin B1)** is a coenzyme important for carbohydrate metabolism (e.g., pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase) and can be deficient in conditions like Wernicke-Korsakoff syndrome.
- It is not directly involved in fatty acid oxidation or carnitine palmitoyltransferase II function.
*Coenzyme A*
- **Coenzyme A (CoA)** is a crucial cofactor in many metabolic pathways, including fatty acid oxidation, but its deficiency is not the primary issue in CPT II deficiency.
- Supplementation with CoA would not address the specific defect in the carnitine shuttle system, which prevents long-chain fatty acids from entering the mitochondria.
*Methionine*
- **Methionine** is an essential amino acid involved in protein synthesis and as a precursor for other molecules like S-adenosylmethionine (SAM).
- It is not directly involved in fatty acid oxidation or CPT II function, and its supplementation would not alleviate the metabolic crisis caused by CPT II deficiency.
*Tetrahydrobiopterin*
- **Tetrahydrobiopterin (BH4)** is a cofactor for several enzymes, including phenylalanine hydroxylase (deficient in PKU) and tyrosine hydroxylase.
- It is not involved in fatty acid metabolism, and its deficiency would not explain the symptoms or the underlying genetic defect in CPT II.
Question 59: An investigator is studying the effect of a high-lipid diet on glucose metabolism in Wistar rats. The experimental rat group is fed a high-lipid diet while the control group is fed a low-lipid diet. Two months after initiation of the experiment, the rats in both groups are injected with insulin and serum glucose measurements are obtained. Compared to the control group, the high-lipid diet group has a significantly higher average serum glucose after receiving insulin. Which of the following intracellular changes is most likely involved in the pathogenesis of this finding?
A. Decreased activation of caspase 8
B. Decreased production of protein kinase A
C. Decreased expression of TP53
D. Increased exposure of nuclear localization signal
E. Increased activity of serine kinases (Correct Answer)
Explanation: ***Increased activity of serine kinases***
- High-fat diets lead to **increased levels of intracellular fatty acids** and their metabolites, which activate **serine kinases** (e.g., JNK, IKK).
- These serine kinases phosphorylate the **insulin receptor substrate (IRS) proteins** at serine residues, rather than tyrosine residues, thereby **inhibiting insulin signal transduction** and leading to insulin resistance.
*Decreased activation of caspase 8*
- **Caspase 8** is primarily involved in the **extrinsic apoptotic pathway** and is not directly implicated in a decrease in glucose uptake in the context of insulin resistance.
- While prolonged insulin resistance and lipotoxicity can eventually lead to β-cell apoptosis, decreased caspase 8 activation would generally *reduce* apoptosis, not directly cause impaired glucose metabolism in this acute scenario.
*Decreased production of protein kinase A*
- **Protein kinase A (PKA)** is activated by cAMP and plays a role in various metabolic processes, including glucose homeostasis, but its *decreased* production is not the primary mechanism by which a high-lipid diet causes insulin resistance.
- In some contexts, PKA activity can even be involved in counter-regulatory responses to insulin or may play complex roles, but the direct inhibition of insulin signaling via IRS serine phosphorylation is more central to lipid-induced resistance.
*Decreased expression of TP53*
- **TP53** (p53) is a tumor suppressor gene involved in cell cycle arrest, DNA repair, and apoptosis.
- While p53 can indirectly influence metabolism in various cancer-related contexts, its decreased expression is not a direct or primary mechanism for high-lipid diet-induced **insulin resistance** in this setting.
*Increased exposure of nuclear localization signal*
- An **increased exposure of a nuclear localization signal (NLS)** would facilitate the transport of proteins into the nucleus, which is relevant for transcription factors or nuclear processes.
- This is not a direct or well-established mechanism for the development of **insulin resistance** caused by a high-lipid diet, which primarily affects early insulin signaling pathways at the cell membrane and in the cytoplasm.
Question 60: A mother brings her 1-year-old daughter who has had several seizures in the past 2 weeks to the pediatrician. The mother explains that the child is unable to crawl, sit, or even hold up her own head. She thinks the weakness is getting worse. The parents of the child are first cousins, and the mother's sister had one child who died before the age of 3 with similar symptoms. Hexosaminidase A activity was assayed in the blood and found to be absent. Which of the following will be found on fundoscopic examination of the child?
A. Cotton wool spots
B. Arteriovenous nicking
C. Cherry red spot (Correct Answer)
D. Papilledema
E. Hollenhorst plaque
Explanation: The patient's presentation with **developmental regression**, **seizures**, worsening weakness, and a family history of a consanguineous marriage (first cousins) and a sibling with similar symptoms strongly suggests a **lysosomal storage disease**. The finding of **absent hexosaminidase A activity** confirms the diagnosis of **Tay-Sachs disease**.
***Cherry red spot***
- A **cherry red spot** on fundoscopic examination is a classic and highly characteristic finding in **Tay-Sachs disease** and other gangliosidoses.
- It is caused by the accumulation of **GM2 ganglioside** in the ganglion cells of the retina, making the fovea appear red in contrast to the surrounding opaque retina.
*Cotton wool spots*
- **Cotton wool spots** are seen in conditions like **hypertensive retinopathy** and **diabetic retinopathy**, representing areas of retinal nerve fiber layer ischemia.
- They are not associated with lysosomal storage diseases like Tay-Sachs disease.
*Arteriovenous nicking*
- **Arteriovenous nicking** is a sign of **hypertensive retinopathy**, where stiffened arteries compress underlying veins.
- This finding is unrelated to the pathophysiology of Tay-Sachs disease.
*Papilledema*
- **Papilledema** is swelling of the optic disc due to **increased intracranial pressure**.
- While increased intracranial pressure can occur in some metabolic disorders, a **cherry red spot** is a more specific and direct ocular manifestation of Tay-Sachs disease.
*Hollenhorst plaque*
- A **Hollenhorst plaque** is a **cholesterol embolus** in a retinal artery, typically indicating a source of embolism from the carotid arteries.
- This finding is indicative of atherosclerotic disease and is not associated with Tay-Sachs disease.
