A 38-year-old woman presents to her surgeon 1 year after a surgery for Crohn disease involving the removal of much of her small bowel. She had no major complications during the surgery and recovered as expected. Since then, she has noticed bone pain and weakness throughout her body. She has also had several fractures since the surgery. A panel of labs relevant to bone physiology was obtained and the results are shown below:
Serum:
Phosphate: Decreased
Calcium: Decreased
Alkaline phosphatase: Increased
The factor that is most likely abnormal in this patient can also be synthesized from which of the following?
Q42
A 9-month-old girl is brought to the physician by her parents for multiple episodes of unresponsiveness in which she stares blankly and her eyelids flutter. She has gradually lost control of her neck and ability to roll over during the past 2 months. She is startled by loud noises and does not maintain eye contact. Her parents are of Ashkenazi Jewish descent. Neurological examination shows generalized hypotonia. Deep tendon reflexes are 3+ bilaterally. Fundoscopy shows bright red macular spots bilaterally. Abdominal examination shows no abnormalities. Which of the following metabolites is most likely to accumulate due to this patient's disease?
Q43
A 1-year-old male is found to have high blood pressure on multiple visits to your office. On examination, the patient has normal genitalia. Further laboratory workup reveals low serum aldosterone and high serum testosterone. Which of the following is most likely to be elevated in this patient?
Q44
A newborn is delivered at term to a 38-year-old woman after an uncomplicated pregnancy and delivery. The newborn's blood pressure is 142/85 mm Hg. Examination shows clitoral enlargement and labioscrotal fusion. Serum studies show a sodium of 151 mg/dL and a potassium of 3.2 mg/dL. Karyotype analysis shows a 46, XX karyotype. The patient is most likely deficient in an enzyme that is normally responsible for which of the following reactions?
Q45
A 28-year-old man presents to the emergency department with diffuse abdominal pain and nausea for the past 5 hours. The pain started with a dull ache but is now quite severe. He notes that he “just doesn’t feel like eating” and has not eaten anything for almost a day. Although the nausea is getting worse, the patient has not vomited. He notes no medical issues in the past and is not currently taking any medications. He admits to drinking alcohol (at least 2–3 bottles of beer per day after work and frequent binge-drinking weekends with friends). He says that he does not smoke or use illicit drugs. Vital signs include: pulse rate 120/min, respiratory rate 26/min, and blood pressure 100/70 mm Hg. On examination, the patient’s abdomen is diffusely tender. His breath smells like alcohol, with a fruity tinge to it. Bowel sounds are present. No other findings are noted. Fingerstick glucose is 76mg/dL. After the examination, the patient suddenly and spontaneously vomits. Which of the following is the underlying mechanism of the most likely diagnosis in this patient?
Q46
A 34-year-old male visits the clinic with complaints of intermittent diarrhea over the past 6 months. He has lost 6.8 kg (15 lb) over that time period. His frequent bowel movements are affecting his social life and he would like definitive treatment. Past medical history is significant for chronic type 2 diabetes that is well controlled with insulin. No other family member has a similar condition. He does not smoke tobacco and drinks alcohol only on weekends. Today, his vitals are within normal limits. On physical exam, he appears gaunt and anxious. His heart has a regular rate and rhythm and his lungs are clear to auscultation bilaterally. Additionally, the patient has a red-purple rash on his lower abdomen, groin, and the dorsum of both hands. The rash consists of pruritic annular lesions. He is referred to a dermatologist for core biopsy which is consistent with necrolytic migratory erythema. Further workup reveals a large hormone secreting mass in the tail of his pancreas. Which of the following is the action of the hormone that is in excess in this patient?
Q47
A 51-year-old gentleman presents with new onset bilateral paresthesias of his feet. He also admits that he has not been able to exercise as much as previously and his friends have commented that he looks pale. Upon physical exam you find that he has conjunctival pallor and mildly decreased sensation and proprioception on his feet bilaterally. Based on your suspicions you decide to obtain a blood smear where you see megaloblasts as well as hypersegmented neutrophils. Given these findings you decide to investigate the cause of his disorder by injecting an intramuscular vitamin, then feeding him a radiolabeled version of the same vitamin orally. After waiting 24 hours you see that no radiolabeled vitamin appears in the urine so you repeat the test with intrinsic factor added to the oral mixture, at which point 20% of the radiolabeled vitamin appears in the urine. Which of the following is the most likely etiology of this gentleman's symptoms?
Q48
A 35-year-old man comes to the physician because of fatigue and generalized weakness for the past year. He has noticed he has been having fewer bowel movements. He has had pain with defecation and small amounts of blood when wiping. He has not lost weight despite increased efforts to diet and exercise. He has had no fever, throat pain, or difficulty swallowing. His temperature is 36.5°C (97.7°F), pulse is 50/min, blood pressure is 120/90 mm Hg, and BMI is 35 kg/m2. Physical examination shows dry skin and a distended abdomen. There is 1+ pitting edema in the lower extremities. On neurological examination, deep tendon reflexes are 1+. Further evaluation of this patient is most likely to show which of the following findings?
Q49
An 8-year-old boy presents to his primary care pediatrician for routine check-up. During the visit, his mom says that she has noticed yellow bumps on his eyelids and was concerned about whether they were a problem. Upon hearing this concern, the physician inquires about parental health history and learns that both parents have high cholesterol despite adhering to a statin regimen. Furthermore, other family members have suffered early myocardial infarctions in their 30s. Physical exam reveals flat yellow patches on the patient's eyelids bilaterally as well as hard yellow bumps around the patient's ankles. Based on clinical suspicion an LDL level is obtained and shows a level of 300 mg/dL. What protein is most likely defective in this patient causing these findings?
Q50
A 1-week-old infant presents to the ED with seizures that are very difficult to control despite loading with fosphenytoin. The parents note that the child was born at home and has been like this since birth. They note that it has been a difficult week trying to get any response from the infant. Upon examination the infant has poor muscle tone, severe difficulties with sucking and swallowing, corneal opacities, and hepatomegaly. Based on clinical suspicion, a genetic test is performed that reveals the diagnosis. The geneticist counsels that the infant has a rare disorder that causes the accumulation of very long chain fatty acids, adversely affecting myelination and leading to neurological symptoms. Most patients with this disorder die within 1 year. The most likely condition in this patient primarily affects which subcellular compartment?
Lipid metabolism US Medical PG Practice Questions and MCQs
Question 41: A 38-year-old woman presents to her surgeon 1 year after a surgery for Crohn disease involving the removal of much of her small bowel. She had no major complications during the surgery and recovered as expected. Since then, she has noticed bone pain and weakness throughout her body. She has also had several fractures since the surgery. A panel of labs relevant to bone physiology was obtained and the results are shown below:
Serum:
Phosphate: Decreased
Calcium: Decreased
Alkaline phosphatase: Increased
The factor that is most likely abnormal in this patient can also be synthesized from which of the following?
A. Beta-carotene
B. Alpha-tocopherol
C. Calcium-sensing receptor
D. 7-dehydrocholesterol (Correct Answer)
E. Glutamyl carboxylase
Explanation: ***7-dehydrocholesterol***
- The patient's symptoms (bone pain, weakness, fractures) and lab results (decreased calcium and phosphate, increased alkaline phosphatase) suggest **osteomalacia** due to **vitamin D deficiency**.
- **7-dehydrocholesterol** is a precursor in the skin that is converted to **cholecalciferol (vitamin D3)** upon exposure to UV radiation.
*Beta-carotene*
- **Beta-carotene** is a precursor to **vitamin A**, which is important for vision, immune function, and cell growth, but not directly involved in calcium and phosphate metabolism.
- Deficiency of vitamin A would present with different symptoms such as **night blindness** and **xerophthalmia**.
*Alpha-tocopherol*
- **Alpha-tocopherol** is a form of **vitamin E**, a fat-soluble antioxidant.
- Vitamin E deficiency is rare but can lead to **neurological symptoms** and **hemolytic anemia**, not bone disorders.
*Calcium-sensing receptor*
- The **calcium-sensing receptor (CaSR)** regulates parathyroid hormone secretion and renal calcium reabsorption in response to extracellular calcium levels.
- While crucial for calcium homeostasis, it is not a substance that is "synthesized from" the options provided; rather, it is a **protein receptor**.
*Glutamyl carboxylase*
- **Glutamyl carboxylase** is an enzyme that catalyzes the carboxylation of glutamic acid residues in proteins, a crucial step for the activation of **vitamin K-dependent clotting factors** and bone proteins like osteocalcin.
- Its dysfunction would primarily affect blood clotting and bone matrix formation, but it is not a precursor for vitamin D.
Question 42: A 9-month-old girl is brought to the physician by her parents for multiple episodes of unresponsiveness in which she stares blankly and her eyelids flutter. She has gradually lost control of her neck and ability to roll over during the past 2 months. She is startled by loud noises and does not maintain eye contact. Her parents are of Ashkenazi Jewish descent. Neurological examination shows generalized hypotonia. Deep tendon reflexes are 3+ bilaterally. Fundoscopy shows bright red macular spots bilaterally. Abdominal examination shows no abnormalities. Which of the following metabolites is most likely to accumulate due to this patient's disease?
A. Ceramide trihexoside
B. Sphingomyelin
C. Glucocerebroside
D. Galactocerebroside
E. GM2 ganglioside (Correct Answer)
Explanation: ***GM2 ganglioside***
- This patient presents with classic symptoms of **Tay-Sachs disease**, an **autosomal recessive lysosomal storage disorder** common in Ashkenazi Jewish populations. Key features include **developmental regression**, **hypotonia**, an **exaggerated startle reflex**, and characteristic **cherry-red spots** on fundoscopy.
- Tay-Sachs disease is caused by a deficiency of the enzyme **hexosaminidase A**, leading to the accumulation of **GM2 ganglioside** in neuronal lysosomes.
*Ceramide trihexoside*
- Accumulation of **ceramide trihexoside** (also known as globotriaosylceramide, or Gb3) is characteristic of **Fabry disease**, an X-linked lysosomal storage disorder.
- Symptoms of Fabry disease include **neuropathic pain**, **angiokeratomas**, **corneal opacities**, and **renal and cardiac complications**, which are not seen here.
*Sphingomyelin*
- Accumulation of **sphingomyelin** occurs in **Niemann-Pick disease**, caused by a deficiency of **sphingomyelinase**.
- While Niemann-Pick disease can present with **developmental regression** and **cherry-red spots** (Type A), it typically also involves significant **hepatosplenomegaly**, which is absent in this patient.
*Glucocerebroside*
- **Glucocerebroside** accumulates in **Gaucher disease** due to a deficiency of **glucocerebrosidase**.
- Gaucher disease is characterized by **hepatosplenomegaly**, **bone crises**, and **pancytopenia**, not the neurological regression and cherry-red spots seen in this patient.
*Galactocerebroside*
- Accumulation of **galactocerebroside** is seen in **Krabbe disease** (globoid cell leukodystrophy), an autosomal recessive lysosomal storage disorder.
- Krabbe disease presents with **irritability, developmental regression, hypotonia, and optic atrophy**, but typically does not feature cherry-red spots. Peripheral neuropathy and white matter abnormalities are prominent.
Question 43: A 1-year-old male is found to have high blood pressure on multiple visits to your office. On examination, the patient has normal genitalia. Further laboratory workup reveals low serum aldosterone and high serum testosterone. Which of the following is most likely to be elevated in this patient?
A. 17-hydroxyprogesterone
B. progesterone
C. corticosterone
D. androstenedione
E. 11-deoxycorticosterone (Correct Answer)
Explanation: ***11-deoxycorticosterone***
- This clinical picture describes **11-beta-hydroxylase deficiency**, which blocks the conversion of 11-deoxycorticosterone to corticosterone and 11-deoxycortisol to cortisol.
- The accumulation of **11-deoxycorticosterone** acts as a mineralocorticoid, causing **hypertension** and **low renin/aldosterone levels**.
*17-hydroxyprogesterone*
- This is typically elevated in **21-hydroxylase deficiency**, which presents with **virilization** (due to increased testosterone) but usually causes **hypotension** (due to aldosterone deficiency), not hypertension.
- While testosterone is high, the low aldosterone and hypertension point away from 21-hydroxylase deficiency.
*Progesterone*
- Elevation of progesterone can occur in **3-beta-hydroxysteroid dehydrogenase deficiency**, which would also lead to virilization but typically causes **salt wasting** and **hypotension**, not hypertension.
- It is not a primary marker for the specific type of hypertension seen here.
*Corticosterone*
- In 11-beta-hydroxylase deficiency, the enzyme responsible for converting 11-deoxycorticosterone to corticosterone is deficient, so **corticosterone levels would be low**, not elevated.
- Corticosterone is a precursor to aldosterone, and its reduced synthesis contributes to the hormonal imbalance.
*Androstenedione*
- While **androstenedione** would be elevated (as it is a precursor to testosterone, which is high), it is not the primary direct cause of the **hypertension** in this specific adrenal enzyme deficiency.
- The direct cause of hypertension is the accumulation of mineralocorticoid precursors like **11-deoxycorticosterone**.
Question 44: A newborn is delivered at term to a 38-year-old woman after an uncomplicated pregnancy and delivery. The newborn's blood pressure is 142/85 mm Hg. Examination shows clitoral enlargement and labioscrotal fusion. Serum studies show a sodium of 151 mg/dL and a potassium of 3.2 mg/dL. Karyotype analysis shows a 46, XX karyotype. The patient is most likely deficient in an enzyme that is normally responsible for which of the following reactions?
A. 11-deoxycorticosterone to corticosterone (Correct Answer)
B. Progesterone to 17-hydroxyprogesterone
C. Testosterone to dihydrotestosterone
D. Progesterone to 11-deoxycorticosterone
E. Androstenedione to estrone
Explanation: ***11-deoxycorticosterone to corticosterone***
- This reaction is catalyzed by **11β-hydroxylase**. Deficiency of this enzyme (second most common cause of congenital adrenal hyperplasia) leads to the clinical triad seen in this patient:
1. **Virilization** in XX individuals (clitoral enlargement, labioscrotal fusion) due to shunting of steroid precursors toward **androgen synthesis**
2. **Hypertension** due to accumulation of **11-deoxycorticosterone (DOC)**, which has potent mineralocorticoid activity
3. **Hypokalemia and hypernatremia** resulting from the mineralocorticoid excess caused by accumulated DOC
- The 46,XX karyotype with ambiguous genitalia confirms this is a disorder of **androgen excess** affecting a genetic female
- 11β-hydroxylase deficiency accounts for approximately 5-8% of CAH cases
*Progesterone to 11-deoxycorticosterone*
- This reaction is catalyzed by **21-hydroxylase** (most common cause of CAH, ~90-95% of cases)
- However, 21-hydroxylase deficiency typically presents with **salt-wasting** (hypotension, hyponatremia, hyperkalemia) in 75% of cases, or simple virilizing form with normal blood pressure and electrolytes in 25% of cases
- The **hypertension with hypokalemia and hypernatremia** in this case is NOT consistent with 21-hydroxylase deficiency, which causes **decreased** mineralocorticoid production
- While virilization occurs in 21-hydroxylase deficiency, the cardiovascular and electrolyte findings rule this out
*Progesterone to 17-hydroxyprogesterone*
- This reaction is catalyzed by **17α-hydroxylase**
- Deficiency leads to **decreased androgens and estrogens**, causing **undervirilization** in XY individuals and absent secondary sexual characteristics
- Would cause hypertension due to mineralocorticoid excess, but **cannot explain the virilization** seen in this XX patient
- The clinical picture of masculinization directly contradicts 17α-hydroxylase deficiency
*Androstenedione to estrone*
- This reaction is catalyzed by **aromatase**
- Aromatase deficiency causes accumulation of androgens and deficiency of estrogens
- While this could explain virilization, it **does not cause hypertension** or the electrolyte abnormalities (hypernatremia, hypokalemia) seen in this patient
- Aromatase deficiency would present with virilization but normal blood pressure
*Testosterone to dihydrotestosterone*
- This conversion is mediated by **5α-reductase**
- Deficiency causes **undervirilization** in XY individuals with ambiguous genitalia at birth
- **Would not cause virilization** in an XX individual, as this enzyme affects androgen potency rather than androgen production
- Does not explain the hypertension or electrolyte abnormalities
- 5α-reductase deficiency presents in genetic males (46,XY), not genetic females (46,XX)
Question 45: A 28-year-old man presents to the emergency department with diffuse abdominal pain and nausea for the past 5 hours. The pain started with a dull ache but is now quite severe. He notes that he “just doesn’t feel like eating” and has not eaten anything for almost a day. Although the nausea is getting worse, the patient has not vomited. He notes no medical issues in the past and is not currently taking any medications. He admits to drinking alcohol (at least 2–3 bottles of beer per day after work and frequent binge-drinking weekends with friends). He says that he does not smoke or use illicit drugs. Vital signs include: pulse rate 120/min, respiratory rate 26/min, and blood pressure 100/70 mm Hg. On examination, the patient’s abdomen is diffusely tender. His breath smells like alcohol, with a fruity tinge to it. Bowel sounds are present. No other findings are noted. Fingerstick glucose is 76mg/dL. After the examination, the patient suddenly and spontaneously vomits. Which of the following is the underlying mechanism of the most likely diagnosis in this patient?
A. Increased acetyl CoA levels (Correct Answer)
B. Inadequate insulin production
C. Increased osmolal gap
D. Starvation
E. Thiamine deficiency
Explanation: ***Increased acetyl CoA levels***
- This patient presents with **alcoholic ketoacidosis (AKA)**, and the underlying biochemical mechanism is the accumulation of **acetyl CoA** that is shunted into ketone body synthesis.
- In AKA, starvation depletes glycogen stores and **oxaloacetate** (needed for the TCA cycle). Simultaneously, alcohol metabolism increases the **NADH/NAD+ ratio**, which further impairs gluconeogenesis and reduces oxaloacetate availability.
- Enhanced lipolysis (due to low insulin and high counter-regulatory hormones) produces large amounts of fatty acids, which undergo β-oxidation to generate **acetyl CoA**.
- With insufficient oxaloacetate to enter the TCA cycle, acetyl CoA accumulates and is diverted to **ketogenesis** (producing acetoacetate, β-hydroxybutyrate, and acetone).
- The **fruity breath** is from acetone, and the metabolic acidosis causes the patient's symptoms (abdominal pain, nausea, tachycardia, tachypnea).
*Starvation*
- While **starvation** is a critical **precipitating factor** in AKA (it depletes glycogen and triggers lipolysis), it is not the underlying biochemical mechanism itself.
- Starvation creates the metabolic conditions (low insulin, depleted oxaloacetate) that lead to the accumulation of acetyl CoA and subsequent ketogenesis, but the question asks for the mechanism, not the trigger.
*Inadequate insulin production*
- **Inadequate insulin production** is the primary defect in **diabetic ketoacidosis (DKA)**, which is ruled out by this patient's normal blood glucose (76 mg/dL).
- In AKA, insulin levels are typically low (due to starvation), but this is secondary to metabolic stress rather than a primary pancreatic failure. The key mechanism is still the acetyl CoA accumulation and shunting to ketogenesis.
*Increased osmolal gap*
- An **increased osmolal gap** suggests unmeasured osmolytes, typically seen with toxic alcohol ingestions (methanol, ethylene glycol) or severe lactic acidosis.
- While ethanol itself can transiently increase the osmolal gap, this is not the mechanism explaining the ketoacidosis, fruity breath, and clinical presentation in this case.
*Thiamine deficiency*
- **Thiamine deficiency** is common in chronic alcoholics and causes Wernicke-Korsakoff syndrome and other neurological complications.
- Thiamine is a cofactor for pyruvate dehydrogenase and α-ketoglutarate dehydrogenase, but its deficiency does not directly cause the ketoacidosis seen here, which results from altered lipid metabolism and acetyl CoA accumulation.
Question 46: A 34-year-old male visits the clinic with complaints of intermittent diarrhea over the past 6 months. He has lost 6.8 kg (15 lb) over that time period. His frequent bowel movements are affecting his social life and he would like definitive treatment. Past medical history is significant for chronic type 2 diabetes that is well controlled with insulin. No other family member has a similar condition. He does not smoke tobacco and drinks alcohol only on weekends. Today, his vitals are within normal limits. On physical exam, he appears gaunt and anxious. His heart has a regular rate and rhythm and his lungs are clear to auscultation bilaterally. Additionally, the patient has a red-purple rash on his lower abdomen, groin, and the dorsum of both hands. The rash consists of pruritic annular lesions. He is referred to a dermatologist for core biopsy which is consistent with necrolytic migratory erythema. Further workup reveals a large hormone secreting mass in the tail of his pancreas. Which of the following is the action of the hormone that is in excess in this patient?
A. Activation of glycogen synthase
B. Inhibition of gluconeogenesis
C. Inhibition of insulin secretion
D. Stimulation of lipolysis (Correct Answer)
E. Inhibition of acetone production
Explanation: ***Stimulation of lipolysis***
- The patient's symptoms (diarrhea, weight loss, necrolytic migratory erythema, and a pancreatic mass) are highly suggestive of a **glucagonoma**.
- **Glucagon** is a catabolic hormone that **stimulates gluconeogenesis and glycogenolysis** to raise blood glucose, and it also promotes **lipolysis** and **ketogenesis**.
*Activation of glycogen synthase*
- **Glycogen synthase** is activated by **insulin**, which promotes glycogen synthesis, storing glucose.
- Glucagon opposes the action of insulin; thus, it would **inactivate glycogen synthase**, not activate it.
*Inhibition of gluconeogenesis*
- **Glucagon's primary role** is to increase blood glucose levels, and it does so by **stimulating gluconeogenesis and glycogenolysis**.
- **Inhibition of gluconeogenesis** would occur with insulin, not glucagon.
*Inhibition of insulin secretion*
- While glucagon can have complex effects on insulin, its primary metabolic actions are independent of direct inhibition of insulin secretion.
- Pancreatic somatostatin inhibits both insulin and glucagon secretion.
*Inhibition of acetone production*
- Glucagon promotes **ketogenesis**, which can lead to the production of **ketone bodies**, including acetone, especially in states of insulin deficiency.
- Therefore, glucagon would **stimulate**, not inhibit, processes that can lead to acetone production.
Question 47: A 51-year-old gentleman presents with new onset bilateral paresthesias of his feet. He also admits that he has not been able to exercise as much as previously and his friends have commented that he looks pale. Upon physical exam you find that he has conjunctival pallor and mildly decreased sensation and proprioception on his feet bilaterally. Based on your suspicions you decide to obtain a blood smear where you see megaloblasts as well as hypersegmented neutrophils. Given these findings you decide to investigate the cause of his disorder by injecting an intramuscular vitamin, then feeding him a radiolabeled version of the same vitamin orally. After waiting 24 hours you see that no radiolabeled vitamin appears in the urine so you repeat the test with intrinsic factor added to the oral mixture, at which point 20% of the radiolabeled vitamin appears in the urine. Which of the following is the most likely etiology of this gentleman's symptoms?
A. Bacterial overgrowth
B. Pancreatic insufficiency
C. Insufficient vitamin intake
D. Pernicious anemia (Correct Answer)
E. Folate deficiency
Explanation: ***Pernicious anemia***
- The combination of **neurological symptoms** (paresthesias, decreased proprioception), **anemia symptoms** (pallor, fatigue), **megaloblastic anemia** on blood smear (megaloblasts, hypersegmented neutrophils), and the specific Schilling test results (vitamin B12 malabsorption corrected by added **intrinsic factor**) is diagnostic for pernicious anemia.
- Pernicious anemia is an autoimmune condition targeting **gastric parietal cells** or **intrinsic factor** itself, leading to **vitamin B12 deficiency** due to impaired absorption.
*Bacterial overgrowth*
- **Bacterial overgrowth in the small intestine** can consume vitamin B12, leading to deficiency, but the Schilling test would show improvement with **antibiotics**, not intrinsic factor.
- While it can cause B12 deficiency and megaloblastic anemia, improvement upon adding intrinsic factor points away from this diagnosis.
*Pancreatic insufficiency*
- **Pancreatic enzymes** are required to cleave vitamin B12 from its binding proteins, but the **Schilling test** would show improvement with **pancreatic enzyme supplements**, not intrinsic factor.
- While pancreatic insufficiency can lead to B12 malabsorption, the specific Schillings test results rule it out.
*Insufficient vitamin intake*
- **Insufficient dietary intake** of vitamin B12 would lead to B12 deficiency, but in such a case, the **Schilling test** would show normal absorption of radiolabeled B12 even without intrinsic factor in the second stage.
- The initial B12 malabsorption indicates an absorption problem, not solely a dietary lack.
*Folate deficiency*
- **Folate deficiency** also causes **megaloblastic anemia** and can present with fatigue and pallor, but it does **not cause neurological symptoms** like paresthesias or proprioception deficits.
- The Schilling test specifically evaluates **vitamin B12 absorption**, not folate levels, so it would be irrelevant for diagnosing pure folate deficiency.
Question 48: A 35-year-old man comes to the physician because of fatigue and generalized weakness for the past year. He has noticed he has been having fewer bowel movements. He has had pain with defecation and small amounts of blood when wiping. He has not lost weight despite increased efforts to diet and exercise. He has had no fever, throat pain, or difficulty swallowing. His temperature is 36.5°C (97.7°F), pulse is 50/min, blood pressure is 120/90 mm Hg, and BMI is 35 kg/m2. Physical examination shows dry skin and a distended abdomen. There is 1+ pitting edema in the lower extremities. On neurological examination, deep tendon reflexes are 1+. Further evaluation of this patient is most likely to show which of the following findings?
A. Hyperglycemia
B. Decreased serum creatinine
C. Elevated serum low-density lipoprotein (Correct Answer)
D. Decreased plasma homocysteine concentrations
E. Decreased serum creatine kinase
Explanation: ***Elevated serum low-density lipoprotein***
- The patient's symptoms (fatigue, generalized weakness, constipation, weight gain despite efforts, cold intolerance implied by dry skin, bradycardia, and edema) are highly suggestive of **hypothyroidism**.
- **Hypothyroidism** classically leads to **hyperlipidemia**, specifically elevated **LDL cholesterol**, due to decreased catabolism of cholesterol.
*Decreased serum creatine kinase*
- **Hypothyroidism** typically causes **myopathy**, which can manifest as elevated, not decreased, serum **creatine kinase** levels.
- Muscle weakness and fatigue in hypothyroidism are often associated with muscle damage and elevated CK.
*Hyperglycemia*
- While hypothyroidism can impact glucose metabolism, it more commonly leads to **insulin resistance** and can contribute to **hyperglycemia**, but it is not a direct or consistent consequence, and other factors are often more significant.
- However, hypothyroidism is not primarily characterized by hyperglycemia as a direct pathognomonic finding.
*Decreased serum creatinine*
- **Hypothyroidism** is typically associated with **decreased glomerular filtration rate (GFR)**, which can lead to **increased serum creatinine**, not decreased.
- A decreased GFR would imply reduced clearance of creatinine, thus elevating its serum concentration.
*Decreased plasma homocysteine concentrations*
- **Hypothyroidism** is linked to **elevated plasma homocysteine** concentrations, likely due to impaired metabolism (e.g., reduced activity of methionine synthase).
- High homocysteine levels are a risk factor for cardiovascular disease.
Question 49: An 8-year-old boy presents to his primary care pediatrician for routine check-up. During the visit, his mom says that she has noticed yellow bumps on his eyelids and was concerned about whether they were a problem. Upon hearing this concern, the physician inquires about parental health history and learns that both parents have high cholesterol despite adhering to a statin regimen. Furthermore, other family members have suffered early myocardial infarctions in their 30s. Physical exam reveals flat yellow patches on the patient's eyelids bilaterally as well as hard yellow bumps around the patient's ankles. Based on clinical suspicion an LDL level is obtained and shows a level of 300 mg/dL. What protein is most likely defective in this patient causing these findings?
A. Apoprotein B48
B. Lipoprotein lipase
C. LDL receptor (Correct Answer)
D. VLDL receptor
E. Apoprotein E
Explanation: ***LDL receptor***
- The patient's presentation with **xanthomas** (yellow bumps on eyelids and ankles), a very high **LDL level (300 mg/dL)**, and a strong family history of **early myocardial infarctions** and hypercholesterolemia in both parents is characteristic of **familial hypercholesterolemia**.
- Familial hypercholesterolemia is most commonly caused by a defective or deficient **LDL receptor**, which impairs the uptake of LDL from the blood, leading to significantly elevated LDL levels.
*Apoprotein B48*
- **Apoprotein B48** is primarily found on **chylomicrons** and is essential for their formation and secretion from the intestines.
- A defect in apoB48 would lead to **abetalipoproteinemia**, characterized by very low levels of chylomicrons, VLDL, and LDL, and would present with fat malabsorption and neurological symptoms, which is inconsistent with this patient's findings.
*Lipoprotein lipase*
- **Lipoprotein lipase (LPL)** is an enzyme responsible for hydrolyzing triglycerides in circulating **chylomicrons** and **VLDLs**.
- A deficiency in LPL would cause severe **hypertriglyceridemia** and the accumulation of chylomicrons, leading to eruptive xanthomas and pancreatitis, rather than isolated high LDL and tendinous xanthomas.
*VLDL receptor*
- The **VLDL receptor** is involved in the clearance of VLDL remnants and chylomicrons, especially in peripheral tissues.
- While it plays a role in lipid metabolism, its primary role in familial hypercholesterolemia, which is characterized by profoundly elevated LDL, is less significant than the **LDL receptor**.
*Apoprotein E*
- **Apoprotein E** is crucial for the uptake of **chylomicron remnants** and **VLDL remnants** by the liver.
- Defects in apoE are associated with **Type III hyperlipoproteinemia (familial dysbetalipoproteinemia)**, characterized by elevated levels of chylomicron and VLDL remnants, and typically presents with palmar xanthomas and tuberous xanthomas, which is different from the presentation here.
Question 50: A 1-week-old infant presents to the ED with seizures that are very difficult to control despite loading with fosphenytoin. The parents note that the child was born at home and has been like this since birth. They note that it has been a difficult week trying to get any response from the infant. Upon examination the infant has poor muscle tone, severe difficulties with sucking and swallowing, corneal opacities, and hepatomegaly. Based on clinical suspicion, a genetic test is performed that reveals the diagnosis. The geneticist counsels that the infant has a rare disorder that causes the accumulation of very long chain fatty acids, adversely affecting myelination and leading to neurological symptoms. Most patients with this disorder die within 1 year. The most likely condition in this patient primarily affects which subcellular compartment?
A. Mitochondria
B. Endoplasmic reticulum
C. Peroxisome (Correct Answer)
D. Golgi apparatus
E. Lysosome
Explanation: ***Peroxisome***
- The clinical description of **seizures**, poor muscle tone, **corneal opacities**, and **hepatomegaly** in a neonate, along with the accumulation of **very long chain fatty acids (VLCFAs)** due to impaired myelination, is characteristic of a peroxisomal disorder like **Zellweger syndrome**.
- **Peroxisomes** are organelles responsible for the beta-oxidation of VLCFAs; their dysfunction leads to VLCFA accumulation.
*Mitochondria*
- **Mitochondrial disorders** present with a wide range of symptoms, including neurological dysfunction, but typically involve deficits in energy production, not specifically VLCFA accumulation.
- While VLCFAs are also metabolized in mitochondria, the primary defect in **Zellweger spectrum disorders** lies in peroxisomal assembly or function.
*Endoplasmic reticulum*
- The **endoplasmic reticulum (ER)** is involved in protein synthesis and lipid metabolism, but its primary dysfunction does not typically lead to the specific accumulation of VLCFAs or the constellation of symptoms observed.
- ER stress can contribute to cellular pathology, but it is not the primary site of metabolic defect in this case.
*Golgi apparatus*
- The **Golgi apparatus** is crucial for modifying, sorting, and packaging proteins and lipids for secretion or delivery to other organelles.
- Defects in the Golgi apparatus would present with different clinical manifestations, such as glycoprotein processing errors, rather than VLCFA accumulation.
*Lysosome*
- **Lysosomes** are involved in the degradation of various macromolecules, and their dysfunction leads to **lysosomal storage disorders**.
- While these can cause neurological symptoms and hepatomegaly, they are characterized by the accumulation of specific substrates like mucopolysaccharides or sphingolipids, not VLCFAs.
