A previously healthy 8-year-old boy is brought to the physician because of increasing visual loss and deterioration of his hearing and speech over the past 2 months. During this period, he has had difficulty walking, using the stairs, and feeding himself. His teachers have noticed that he has had difficulty concentrating. His grades have worsened and his handwriting has become illegible. His maternal male cousin had similar complaints and died at the age of 6 years. Vital signs are within normal limits. Examination shows hyperpigmented skin and nails and an ataxic gait. His speech is dysarthric. Neurologic examination shows spasticity and decreased muscle strength in all extremities. Deep tendon reflexes are 4+ bilaterally. Plantar reflex shows an extensor response bilaterally. Sensation is decreased in the lower extremities. Fundoscopy shows optic atrophy. There is sensorineural hearing loss bilaterally. Which of the following is the most likely cause of this patient's symptoms?
Q12
The human body obtains vitamin D either from diet or from sun exposure. Darker-skinned individuals require more sunlight to create adequate vitamin D stores as the increased melanin in their skin acts like sunscreen; thus, it blocks the necessary UV required for vitamin D synthesis. Therefore, if these individuals spend inadequate time in the light, dietary sources of vitamin D are necessary. Which of the following requires sunlight for its formation?
Q13
A 25-year-old woman presents to you for a routine health checkup. She has no complaints. Family history is significant for 2 of her siblings who have died from Tay-Sachs disease, but she and her parents are phenotypically normal. Which of the following are the chances of this person being a heterozygous carrier of the mutation that causes Tay-Sachs disease?
Q14
A 7-year-old boy is brought to the pediatrician by his parents due to pubic hair growth and changes in his voice. He has been developing in the 98th percentile for his age. His vaccination is up-to-date. The patient’s blood pressure is within the 60th percentile for his age. Physical examination reveals pubic and armpit hair, and Tanner stage 2 characterized by enlarged scrotum and testes. Laboratory findings are significant for the following:
Hemoglobin 13.1 g/dL
Hematocrit 39.7%
Leukocyte count 8,500/mm3
Neutrophils 65%
Lymphocytes 30%
Monocytes 5%
Mean corpuscular volume 82.2 μm3
Platelet count 20,000/mm3
Urine creatinine clearance 98 mL/min
Serum 17-hydroxyprogesterone 313 ng/dL (normal <110 ng/dL)
Which of the following enzymes is most likely to be defective in this patient?
Q15
A 34-year-old woman with Crohn disease comes to the physician because of a 4-week history of nausea, bloating, and epigastric pain that occurs after meals and radiates to the right shoulder. Four months ago, she underwent ileocecal resection for an acute intestinal obstruction. An ultrasound of the abdomen shows multiple echogenic foci with acoustic shadows in the gallbladder. Which of the following mechanisms most likely contributed to this patient’s current presentation?
Q16
A 73-year-old woman is brought to the physician by her son because of a 2-month history of diarrhea and recurrent upper respiratory tract infections. Her son says that she frequently trips over the nightstand when she gets up to go to the bathroom at night. She is 173 cm (5 ft 8 in) tall and weighs 54 kg (120 lb); BMI is 18 kg/m2. Physical examination shows dry skin, multiple bruises on the shins, and triangular keratin plaques on the temporal half of the conjunctiva. A deficiency of which of the following is the most likely underlying cause of these findings?
Q17
A 35-year-old lactose intolerant man presents to the outpatient clinic with complaints of numbness and tingling in his fingers and toes for the past month. He also complains of pain in his calf muscles while sleeping; the pain is severe enough to wake him up in the middle of the night. He is a software engineer and spends most of his time indoors. He has been smoking a pack of cigarettes daily for the past 10 years and occasionally drinks wine with dinner. His current medication regimen includes as needed ibuprofen and calcium supplementation. He does not take any other multivitamins. On examination, his pulse rate is 74/min, blood pressure is 128/67 mm Hg, respiratory rate is 16/min, and temperature is 37.6°C (99.7°F). He has tenderness in the proximal muscles of his upper and lower limbs. Sensory examination is normal. The rest of the physical examination is normal. X-ray imaging of his lower limbs shows features of demineralization. Which of the following sets of abnormalities are most likely occurring in this patient?
Q18
A researcher is investigating the relationship between inflammatory mediators and omega-3 fatty acids, namely docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), in post-MI patients. IL-1ß is an important pro-inflammatory cytokine involved in fibrosis and arrhythmias in the post-MI period. Research indicates that it causes loss of function in the gap junction connexin 43 (Cx43), resulting in an arrhythmogenic state. They perform an experiment investigating the cardioprotective effect of DHA on patients after a recent MI. Their results are shown in a Western blot analysis. Which of the following is the most accurate conclusion from these results?
Q19
A 14-year-old girl presents with her mother to a physician's office. They are both concerned with the amount of hair growing on the girl's upper lip and cheeks. There are also sparse hairs on her chest. The mother reports that her daughter has not started menstruating either. The girl was born at 39 weeks gestation via spontaneous vaginal delivery. She is up to date on all vaccines and is meeting all developmental milestones. On examination, the patient is in the 55th percentile for her height. Her blood pressure is 90/50 mm Hg, pulse is 75/min, and respirations are 15/min. There is thin dark hair on her upper lip and on her cheeks. She also has pustular acne on her face and shoulders. Her breasts are in the initial stages of development and she speaks with a deep voice describing her concerns to the physician. Based on her clinical history, which of the following enzymes are most likely deficient?
Q20
A 42-year-old man presents for evaluation of vitamin D deficiency with possible osteomalacia. The patient had a pathologic fracture 3 weeks ago and was found to have dangerously low levels of vitamin D with normal serum calcium levels. Bone density has been drastically affected, leading to the fracture this patient experienced. The lack of what compound is most responsible for the formation of this disease?
Lipid metabolism US Medical PG Practice Questions and MCQs
Question 11: A previously healthy 8-year-old boy is brought to the physician because of increasing visual loss and deterioration of his hearing and speech over the past 2 months. During this period, he has had difficulty walking, using the stairs, and feeding himself. His teachers have noticed that he has had difficulty concentrating. His grades have worsened and his handwriting has become illegible. His maternal male cousin had similar complaints and died at the age of 6 years. Vital signs are within normal limits. Examination shows hyperpigmented skin and nails and an ataxic gait. His speech is dysarthric. Neurologic examination shows spasticity and decreased muscle strength in all extremities. Deep tendon reflexes are 4+ bilaterally. Plantar reflex shows an extensor response bilaterally. Sensation is decreased in the lower extremities. Fundoscopy shows optic atrophy. There is sensorineural hearing loss bilaterally. Which of the following is the most likely cause of this patient's symptoms?
A. β-Glucocerebrosidase deficiency
B. α-Galactosidase A deficiency
C. Lysosomal galactocerebrosidase deficiency
D. Arylsulfatase A deficiency
E. ATP-binding cassette transporter dysfunction (Correct Answer)
Explanation: ***ATP-binding cassette transporter dysfunction***
- The patient's symptoms, including progressive visual loss, hearing loss, speech deterioration, ataxia, spasticity, hyperpigmentation, and neurological decline, are highly suggestive of **adrenoleukodystrophy (ALD)**. This X-linked disorder is caused by a defect in an **ATP-binding cassette (ABC) transporter** protein (specifically, the ABCD1 gene) that normally transports very long-chain fatty acids (VLCFAs) into peroxisomes for degradation.
- The accumulation of **VLCFAs** in the brain, adrenal glands, and Schwann cells leads to **demyelination**, adrenal insufficiency (manifested by hyperpigmentation), and progressive neurological dysfunction. The family history of a male cousin with similar complaints and early death further supports an X-linked recessive inheritance pattern.
*β-Glucocerebrosidase deficiency*
- This deficiency causes **Gaucher disease**, which is characterized by **visceromegaly** (hepatosplenomegaly), **bone crises**, and variable neurological involvement (in neuronopathic forms).
- The clinical presentation in this patient, particularly the prominent demyelination and adrenal insufficiency signs, is not typical for Gaucher disease.
*α-Galactosidase A deficiency*
- This deficiency causes **Fabry disease**, an X-linked lysosomal storage disorder. It is characterized by neurological symptoms such as **neuropathic pain** (acroparesthesias), **renal failure**, **cardiovascular disease**, and distinct skin lesions called **angiokeratomas**.
- While it is X-linked, the specific constellation of symptoms, including hyperpigmentation and profound demyelinating neurological decline, does not fit Fabry disease.
*Lysosomal galactocerebrosidase deficiency*
- This deficiency leads to **Krabbe disease** (globoid cell leukodystrophy). Krabbe disease typically presents in infancy with **irritability, developmental regression, spasticity**, and **optic atrophy**.
- While it causes demyelination and some similar neurological features, the later onset (8 years old) and the specific symptom of hyperpigmentation (suggesting adrenal involvement) are less characteristic of Krabbe disease.
*Arylsulfatase A deficiency*
- This deficiency causes **metachromatic leukodystrophy (MLD)**. MLD is characterized by progressive demyelination, leading to **ataxia, spasticity, peripheral neuropathy**, and **cognitive decline**.
- While MLD shares features of demyelination and neurological deterioration, it does not typically present with the prominent **hyperpigmentation** indicative of adrenal involvement, which is a key distinguishing feature pointing towards ALD in this case.
Question 12: The human body obtains vitamin D either from diet or from sun exposure. Darker-skinned individuals require more sunlight to create adequate vitamin D stores as the increased melanin in their skin acts like sunscreen; thus, it blocks the necessary UV required for vitamin D synthesis. Therefore, if these individuals spend inadequate time in the light, dietary sources of vitamin D are necessary. Which of the following requires sunlight for its formation?
A. 1,25-dihydroxyvitamin D
B. 25-hydroxyvitamin D
C. 7-dehydrocholesterol
D. Cholecalciferol (D3) (Correct Answer)
E. Ergocalciferol (D2)
Explanation: ***Cholecalciferol (D3)***
- **Cholecalciferol** (vitamin D3) is synthesized in the skin when **7-dehydrocholesterol** is exposed to **ultraviolet B (UVB) radiation** from sunlight.
- This is the initial step in the body's natural production of vitamin D, which then undergoes further hydroxylation in the liver and kidneys to become its active form.
*1,25-dihydroxyvitamin D*
- This is the **active form of vitamin D**, also known as **calcitriol**, produced in the **kidneys** from 25-hydroxyvitamin D via 1-alpha-hydroxylase.
- Its formation requires prior synthesis of cholecalciferol and subsequent hydroxylation, but it does not directly require sunlight.
*25-hydroxyvitamin D*
- This compound, also known as **calcidiol**, is formed in the **liver** from cholecalciferol (or ergocalciferol) through **25-hydroxylation**.
- While its precursor, cholecalciferol, is sunlight-dependent, 25-hydroxyvitamin D itself is not directly formed by sunlight.
*7-dehydrocholesterol*
- **7-dehydrocholesterol** is a **precursor molecule** found in the skin that is converted to cholecalciferol upon exposure to sunlight.
- It is not "formed" by sunlight; rather, it's the substrate upon which sunlight acts.
*Ergocalciferol (D2)*
- **Ergocalciferol** (vitamin D2) is primarily obtained from **plant-based sources** and fortified foods.
- It is not synthesized in the human skin through exposure to sunlight.
Question 13: A 25-year-old woman presents to you for a routine health checkup. She has no complaints. Family history is significant for 2 of her siblings who have died from Tay-Sachs disease, but she and her parents are phenotypically normal. Which of the following are the chances of this person being a heterozygous carrier of the mutation that causes Tay-Sachs disease?
A. 50%
B. 66% (Correct Answer)
C. 33%
D. 25%
E. 0%
Explanation: ***66%***
- Tay-Sachs disease is an **autosomal recessive** condition, meaning affected individuals inherit two copies of the mutated gene (aa). Since her two siblings died from Tay-Sachs disease, her parents must both be carriers of the mutation (Aa x Aa).
- The cross between two carriers (Aa x Aa) yields offspring genotypes in a 1:2:1 ratio: 1 AA (unaffected, non-carrier), 2 Aa (unaffected, carriers), and 1 aa (affected). Since the woman is phenotypically normal (not affected), she cannot be 'aa'. Therefore, among the unaffected offspring (AA and Aa), the probability of being a carrier (Aa) is 2/3, or **66%**.
*50%*
- This probability would apply if one parent was a carrier and the other was homozygous dominant (Aa x AA), but this is not the case here as affected siblings indicate both parents are carriers.
- A 50% chance would also be relevant if we were considering the chance of one specific gamete from a carrier parent carrying the mutation.
*33%*
- This probability is not directly derived from the Mendelian ratios for autosomal recessive inheritance in this specific scenario.
- It does not represent the conditional probability of being a carrier given that the individual is phenotypically normal.
*25%*
- This is the overall probability of any child from two carrier parents being affected with an autosomal recessive disease (aa).
- It is also the probability of being homozygous dominant (AA) among all offspring, but not the probability of being a carrier among unaffected offspring.
*0%*
- This is incorrect because, given that her siblings had Tay-Sachs disease and her parents are phenotypically normal, both parents must be carriers. This means there is a significant chance that she also inherited one copy of the mutated gene.
- Absence of symptoms does not rule out carrier status; carriers are phenotypically normal but carry one copy of the mutated gene.
Question 14: A 7-year-old boy is brought to the pediatrician by his parents due to pubic hair growth and changes in his voice. He has been developing in the 98th percentile for his age. His vaccination is up-to-date. The patient’s blood pressure is within the 60th percentile for his age. Physical examination reveals pubic and armpit hair, and Tanner stage 2 characterized by enlarged scrotum and testes. Laboratory findings are significant for the following:
Hemoglobin 13.1 g/dL
Hematocrit 39.7%
Leukocyte count 8,500/mm3
Neutrophils 65%
Lymphocytes 30%
Monocytes 5%
Mean corpuscular volume 82.2 μm3
Platelet count 20,000/mm3
Urine creatinine clearance 98 mL/min
Serum 17-hydroxyprogesterone 313 ng/dL (normal <110 ng/dL)
Which of the following enzymes is most likely to be defective in this patient?
A. Aromatase
B. 21-hydroxylase (Correct Answer)
C. 11ß-hydroxylase
D. 17-α-hydroxylase
E. 5-α-reductase
Explanation: ***21-hydroxylase***
- The elevated 17-hydroxyprogesterone level is a hallmark of **21-hydroxylase deficiency**, the most common cause of **congenital adrenal hyperplasia (CAH)**.
- Deficiency in this enzyme leads to the shunting of steroid precursors towards **androgen production**, causing premature pubic hair, voice changes, and accelerated growth in males, as seen in this patient.
*Aromatase*
- **Aromatase** converts androgens to estrogens; a deficiency would lead to masculinization in females and tall stature with delayed epiphyseal closure in males due to lack of estrogen for bone maturation.
- This enzyme deficiency would not lead to elevated 17-hydroxyprogesterone.
*11ß-hydroxylase*
- **11ß-hydroxylase deficiency** also causes CAH and leads to androgen excess but is characterized by an accumulation of **11-deoxycorticosterone** and **11-deoxycortisol**, which have mineralocorticoid activity, leading to **hypertension**.
- This patient has normal blood pressure and an elevated 17-hydroxyprogesterone, not 11-deoxycorticosterone or 11-deoxycortisol.
*17-α-hydroxylase*
- **17-α-hydroxylase deficiency** impairs the production of cortisol and sex steroids, leading to an accumulation of **mineralocorticoids** (like corticosterone and deoxycorticosterone), causing **hypertension** and **hypokalemia**.
- This condition would result in **decreased androgen production** and **delayed puberty**, which is contrary to the clinical presentation of this patient.
*5-α-reductase*
- **5-α-reductase** converts testosterone to the more potent dihydrotestosterone (DHT), which is crucial for external male virilization.
- A deficiency in this enzyme would lead to **undervirilization of external genitalia** in 46, XY individuals, typically not premature puberty with significant androgenic effects.
Question 15: A 34-year-old woman with Crohn disease comes to the physician because of a 4-week history of nausea, bloating, and epigastric pain that occurs after meals and radiates to the right shoulder. Four months ago, she underwent ileocecal resection for an acute intestinal obstruction. An ultrasound of the abdomen shows multiple echogenic foci with acoustic shadows in the gallbladder. Which of the following mechanisms most likely contributed to this patient’s current presentation?
A. Decreased biliary concentration of bile acids (Correct Answer)
B. Increased hepatic cholesterol secretion
C. Decreased fat absorption
D. Increased bilirubin production
E. Decreased motility of the gallbladder
Explanation: ***Decreased biliary concentration of bile acids***
- The patient's **Crohn disease** and recent **ileocecal resection** impair the reabsorption of bile acids in the terminal ileum, leading to reduced bile acid concentration in the enterohepatic circulation.
- This **reduced bile acid pool** causes supersaturation of cholesterol in bile, leading to the formation of **cholesterol gallstones**, consistent with the ultrasound findings of echogenic foci with acoustic shadows.
*Increased hepatic cholesterol secretion*
- While increased hepatic cholesterol secretion can contribute to gallstone formation, the primary driver in this patient's case is the **impaired enterohepatic recirculation of bile acids** due to Crohn disease and ileocecal resection.
- The specific history points to a problem with bile acid reabsorption, not necessarily an isolated increase in cholesterol synthesis or secretion.
*Decreased fat absorption*
- Decreased fat absorption (malabsorption) can occur in Crohn disease due to inflammation or resection, but it primarily leads to **steatorrhea** and **fat-soluble vitamin deficiencies**.
- While malabsorption of fat can indirectly impact bile acid metabolism, the more direct and relevant mechanism for gallstone formation here is the **disruption of bile acid reabsorption** in the ileum.
*Increased bilirubin production*
- Increased bilirubin production primarily leads to **pigment gallstones** (black or brown stones), which are more commonly associated with conditions causing hemolysis or biliary infections.
- The ultrasound findings of **echogenic foci with acoustic shadows** are characteristic of **cholesterol gallstones**, not bilirubin stones.
*Decreased motility of the gallbladder*
- Poor gallbladder motility (stasis) can contribute to gallstone formation, as it allows bile to become more concentrated and cholesterol to precipitate.
- However, in the context of Crohn disease and ileocecal resection, the most significant and *primary* mechanism for gallstone development is the **disruption of bile acid enterohepatic circulation**, rather than primarily gallbladder dysmotility.
Question 16: A 73-year-old woman is brought to the physician by her son because of a 2-month history of diarrhea and recurrent upper respiratory tract infections. Her son says that she frequently trips over the nightstand when she gets up to go to the bathroom at night. She is 173 cm (5 ft 8 in) tall and weighs 54 kg (120 lb); BMI is 18 kg/m2. Physical examination shows dry skin, multiple bruises on the shins, and triangular keratin plaques on the temporal half of the conjunctiva. A deficiency of which of the following is the most likely underlying cause of these findings?
A. Retinol (Correct Answer)
B. Zinc
C. Riboflavin
D. Niacin
E. Pantothenic acid
Explanation: ***Retinol***
- The constellation of symptoms including **night blindness** (frequently trips at night), **dry skin**, **recurrent infections**, and **Bitot's spots** (triangular keratin plaques on conjunctiva) are classic signs of **Vitamin A (retinol) deficiency**.
- **Diarrhea** can lead to malabsorption, further exacerbating the deficiency, and Vitamin A plays a crucial role in **immune function**, explaining the recurrent infections.
*Zinc*
- Zinc deficiency can cause **diarrhea**, **recurrent infections**, and skin changes, but it does not typically present with **night blindness** or **Bitot's spots**.
- Other common symptoms of zinc deficiency include **poor wound healing** and **taste disturbances**, which are not mentioned here.
*Riboflavin*
- Riboflavin (Vitamin B2) deficiency is associated with **cheilosis** (cracking at mouth corners), **glossitis** (inflamed tongue), and **angular stomatitis**, none of which are described in the patient.
- It does not cause night blindness or Bitot's spots.
*Niacin*
- Niacin (Vitamin B3) deficiency causes **pellagra**, characterized by the "3 Ds": **dermatitis**, **diarrhea**, and **dementia**.
- While diarrhea is present, the key ophthalmological and night blindness symptoms are absent, and the skin findings are not typical for pellagra's characteristic rash.
*Pantothenic acid*
- Pantothenic acid (Vitamin B5) deficiency is extremely rare and its symptoms are largely non-specific, including **fatigue**, **headaches**, and **numbness**.
- It is not associated with night blindness, dry skin, or Bitot's spots.
Question 17: A 35-year-old lactose intolerant man presents to the outpatient clinic with complaints of numbness and tingling in his fingers and toes for the past month. He also complains of pain in his calf muscles while sleeping; the pain is severe enough to wake him up in the middle of the night. He is a software engineer and spends most of his time indoors. He has been smoking a pack of cigarettes daily for the past 10 years and occasionally drinks wine with dinner. His current medication regimen includes as needed ibuprofen and calcium supplementation. He does not take any other multivitamins. On examination, his pulse rate is 74/min, blood pressure is 128/67 mm Hg, respiratory rate is 16/min, and temperature is 37.6°C (99.7°F). He has tenderness in the proximal muscles of his upper and lower limbs. Sensory examination is normal. The rest of the physical examination is normal. X-ray imaging of his lower limbs shows features of demineralization. Which of the following sets of abnormalities are most likely occurring in this patient?
A. Low alkaline phosphatase levels with low serum PTH levels
B. High serum 25-OH D levels with high serum parathyroid hormone (PTH) levels
C. Low serum 25-OH D with low serum calcium levels (Correct Answer)
D. High serum 25-OH D with high phosphate levels
E. Low serum 25-OH D with high serum calcium levels
Explanation: ***Low serum 25-OH D with low serum calcium levels***
- The patient's presentation with **numbness, tingling, muscle pain, bone demineralization**, and risk factors (lactose intolerance, indoor lifestyle, smoking) are highly suggestive of **vitamin D deficiency** (low 25-OH D).
- Severe vitamin D deficiency impairs calcium absorption, leading to **hypocalcemia**, which directly causes neuromuscular symptoms like **paresthesias** and **muscle cramps (tetany)**.
- The hypocalcemia typically triggers **secondary hyperparathyroidism** (elevated PTH) as a compensatory mechanism to maintain calcium homeostasis.
*Low alkaline phosphatase levels with low serum PTH levels*
- **Low alkaline phosphatase** is typically not associated with vitamin D deficiency or osteomalacia; rather, alkaline phosphatase is often **normal or elevated** due to increased osteoblastic activity.
- **Low PTH levels** would contradict the body's compensatory response to hypocalcemia from vitamin D deficiency, where PTH levels usually **rise** (secondary hyperparathyroidism) to maintain calcium homeostasis.
*High serum 25-OH D levels with high serum parathyroid hormone (PTH) levels*
- **High serum 25-OH D levels** would indicate vitamin D sufficiency or toxicity, which contradicts the clinical picture of symptoms and risk factors for deficiency.
- While **high PTH levels** can be seen in primary hyperparathyroidism, the accompanying high 25-OH D level and the patient's symptoms are inconsistent with this diagnosis.
*High serum 25-OH D with high phosphate levels*
- **High serum 25-OH D levels** are inconsistent with the patient's presentation of symptoms and risk factors for vitamin D deficiency.
- **High phosphate levels** are not typically seen with vitamin D deficiency; rather, hypophosphatemia can occur due to secondary hyperparathyroidism induced by vitamin D deficiency.
*Low serum 25-OH D with high serum calcium levels*
- While **low serum 25-OH D** is consistent with the patient's presentation, **high serum calcium levels** would be contradictory.
- Vitamin D deficiency directly leads to impaired calcium absorption and **hypocalcemia**, not hypercalcemia.
Question 18: A researcher is investigating the relationship between inflammatory mediators and omega-3 fatty acids, namely docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), in post-MI patients. IL-1ß is an important pro-inflammatory cytokine involved in fibrosis and arrhythmias in the post-MI period. Research indicates that it causes loss of function in the gap junction connexin 43 (Cx43), resulting in an arrhythmogenic state. They perform an experiment investigating the cardioprotective effect of DHA on patients after a recent MI. Their results are shown in a Western blot analysis. Which of the following is the most accurate conclusion from these results?
A. Fatty acids with double bonds in the 3rd position adjacent to the carboxy-terminus are cardioprotective against the effects of IL-1β in post-MI cells.
B. Fatty acids with double bonds in the 3rd position adjacent to the terminal carbon are cardioprotective against the effects of IL-1β in post-MI cells. (Correct Answer)
C. Fatty acids with double bonds in the 3rd position adjacent to the terminal carbon provide minimal benefits against arrhythmias after myocardial infarctions.
D. Fatty acids with 3 cis-double bonds provide minimal benefits against arrhythmias after myocardial infarctions.
E. Fatty acids with double bonds in the 3rd position adjacent to the carboxy-terminus provide minimal benefits against arrhythmias after myocardial infarctions.
Explanation: ***Fatty acids with double bonds in the 3rd position adjacent to the terminal carbon are cardioprotective against the effects of IL-1β in post-MI cells.***
- **DHA** and **EPA** are omega-3 fatty acids, meaning their first double bond is located at the third carbon atom from the **methyl (omega) end** of the fatty acid chain (the terminal carbon).
- The problem states that the experiment investigates the **cardioprotective effects of DHA** after MI and implies that the given Western blot results support this, indicating that these specific fatty acids mitigate the arrhythmogenic effects of IL-1β on Cx43.
*Fatty acids with double bonds in the 3rd position adjacent to the carboxy-terminus are cardioprotective against the effects of IL-1β in post-MI cells.*
- The **carboxy-terminus** refers to the COOH group at one end of the fatty acid chain; numbering from this end would not define omega-3 fatty acids. Such a description is chemically incorrect for classifying omega-3 fatty acids.
*Fatty acids with double bonds in the 3rd position adjacent to the terminal carbon provide minimal benefits against arrhythmias after myocardial infarctions.*
- The question implies that DHA (an omega-3 fatty acid) has a **cardioprotective effect**, which contradicts the idea of minimal benefits.
- The core premise is to demonstrate protection against IL-1β-induced Cx43 dysfunction, which reduces arrhythmogenic potential.
*Fatty acids with 3 cis-double bonds provide minimal benefits against arrhythmias after myocardial infarctions.*
- While DHA has 6 double bonds, this option refers to fatty acids with only **3 cis-double bonds**, which describes a different type of fatty acid, not specifically DHA or EPA.
- Furthermore, the conclusion that they provide minimal benefits contradicts the context of DHA's cardioprotective role.
*Fatty acids with double bonds in the 3rd position adjacent to the carboxy-terminus provide minimal benefits against arrhythmias after myocardial infarctions.*
- As mentioned, describing the location of the double bond from the **carboxy-terminus** is an incorrect way to classify omega-3 fatty acids.
- This option also incorrectly states minimal benefits, contradicting the implied cardioprotective role of DHA in the given scenario.
Question 19: A 14-year-old girl presents with her mother to a physician's office. They are both concerned with the amount of hair growing on the girl's upper lip and cheeks. There are also sparse hairs on her chest. The mother reports that her daughter has not started menstruating either. The girl was born at 39 weeks gestation via spontaneous vaginal delivery. She is up to date on all vaccines and is meeting all developmental milestones. On examination, the patient is in the 55th percentile for her height. Her blood pressure is 90/50 mm Hg, pulse is 75/min, and respirations are 15/min. There is thin dark hair on her upper lip and on her cheeks. She also has pustular acne on her face and shoulders. Her breasts are in the initial stages of development and she speaks with a deep voice describing her concerns to the physician. Based on her clinical history, which of the following enzymes are most likely deficient?
A. 17-α-hydroxylase
B. 5-α-reductase
C. 21-hydroxylase (Correct Answer)
D. Aromatase
E. 11-β-hydroxylase
Explanation: ***21-hydroxylase***
- **21-hydroxylase deficiency** is the most common cause of congenital adrenal hyperplasia (CAH), leading to a buildup of **androgen precursors**.
- The patient's symptoms of **hirsutism** (upper lip, cheeks, chest hair), **acne**, **deep voice**, and **primary amenorrhea** are all consistent with **virilization** due to excess androgens.
*17-α-hydroxylase*
- **17-α-hydroxylase deficiency** impairs the production of all sex steroids and cortisol, leading to **hypertension**, **hypokalemia**, and **female pseudohermaphroditism** in XY individuals or **lack of secondary sexual characteristics** in XX individuals.
- This patient presents with signs of **androgen excess**, not deficiency, and her blood pressure is normal, making 17-α-hydroxylase deficiency an unlikely diagnosis.
*5-α-reductase*
- **5-α-reductase deficiency** prevents the conversion of **testosterone to dihydrotestosterone (DHT)**, affecting external male genitalia development but not adrenal steroid synthesis.
- This typically presents as **ambiguous genitalia** in males (XY individuals) but would not cause the signs of **virilization** and **amenorrhea** observed in this female patient.
*Aromatase*
- **Aromatase deficiency** prevents the conversion of androgens to estrogens, leading to **virilization** in XX individuals due to unopposed androgen action.
- While it can manifest with similar symptoms of **hirsutism** and **amenorrhea**, it is much rarer than 21-hydroxylase deficiency and doesn't explain the potential for **adrenal crisis** issues associated with CAH.
*11-β-hydroxylase*
- **11-β-hydroxylase deficiency** leads to an accumulation of **11-deoxycorticosterone (DOC)**, causing **hypertension** and **virilization** due to increased androgen production.
- While virilization is present, the patient's **normal blood pressure** (90/50 mm Hg) makes this diagnosis less likely compared to 21-hydroxylase deficiency, which typically presents with normal or low blood pressure (salt-wasting forms) unless a salt-retaining form is present.
Question 20: A 42-year-old man presents for evaluation of vitamin D deficiency with possible osteomalacia. The patient had a pathologic fracture 3 weeks ago and was found to have dangerously low levels of vitamin D with normal serum calcium levels. Bone density has been drastically affected, leading to the fracture this patient experienced. The lack of what compound is most responsible for the formation of this disease?
A. Cholecalciferol
B. 25-hydroxycholecalciferol
C. Vitamin D binding protein
D. Calcitriol (Correct Answer)
E. PTH
Explanation: ***Calcitriol***
- **Calcitriol** (1,25-dihydroxycholecalciferol) is the active form of vitamin D, crucial for intestinal **calcium and phosphate absorption** and **bone mineralization**. Its deficiency directly leads to osteomalacia.
- The patient's low vitamin D levels, normal serum calcium (often maintained by compensatory PTH), and pathologic fracture due to compromised bone density align with a primary lack of calcitriol function, whether from insufficient synthesis or action.
*Cholecalciferol*
- **Cholecalciferol** (vitamin D3) is a precursor; while its deficiency can lead to osteomalacia, it must be metabolized to 25-hydroxycholecalciferol and then to calcitriol to be active.
- The direct lack of the *active form* is more central to the impaired bone mineralization in osteomalacia than the initial precursor.
*25-hydroxycholecalciferol*
- **25-hydroxycholecalciferol** (calcifediol) is the main circulating form of vitamin D, reflecting overall vitamin D stores, but it is not the directly active form.
- While low levels indicate vitamin D deficiency, the final conversion to **calcitriol** is the critical step for its bone-mineralizing effects.
*Vitamin D binding protein*
- **Vitamin D binding protein (DBP)** transports vitamin D metabolites in the blood but does not directly cause osteomalacia with its deficiency.
- While altered DBP levels can affect vitamin D bioavailability, the core problem in osteomalacia is the lack of the active vitamin D metabolite itself.
*PTH*
- **Parathyroid hormone (PTH)** regulates calcium and phosphate homeostasis and stimulates renal 1-alpha-hydroxylase to produce calcitriol.
- While PTH levels can be affected in vitamin D deficiency (often elevated), its lack is not the direct cause of osteomalacia; rather, it plays a role in the *synthesis* of the compound responsible for bone health.
