A 32-year-old woman is brought to the emergency department after she started convulsing in the office. She has no previous history of seizures and recovers by the time she arrives at the emergency department. She says that over the last 2 days she has also experienced insomnia, abdominal pain, and dark urine. Her past medical history is significant for asthma; however, she says that she has not experienced any of these symptoms previously. She smokes 1 pack of cigarettes per day, drinks a glass of wine with dinner every night, and is currently taking oral contraceptive pills (OCPs). On presentation, her temperature is 99°F (37.2°C), blood pressure is 140/98 mmHg, pulse is 112/min, and respirations are 11/min. Which of the following enzymes is most likely to be defective in this patient?
Q202
An 85-year-old woman presents to her physician with complaints of significant weakness and weight loss. She recently has been diagnosed with stage IV breast cancer for which she currently is receiving treatment. She mentions that, despite taking a diet rich in protein and calories, she continues to lose weight. On physical examination, her vital signs are stable, but muscle wasting is clearly evident in her upper limbs, lower limbs, and face. The physician explains to her that her advanced cancer is the most important cause for the weight loss and muscle wasting. This cachexia is mediated by the proteolysis-inducing factor released from cancer cells. Which of the following effects is produced by this factor?
Q203
A 32-year-old African American man presents to the office for a routine examination. He has no complaints at this time. Records show that his systolic blood pressure was in the 130–138 range and diastolic blood pressure in the 88–95 range despite counseling on lifestyle modification. He admits that he was not compliant with this advice. He takes no medications and works at home as a web designer. He does not drink alcohol but smokes marijuana on a weekly basis. Temperature is 37°C (98.6°F), blood pressure is 138/90 mm Hg, pulse is 76/min, and respirations are 12/min. BMI is 29.8 kg/m2. Physical examination is normal except for truncal obesity, with a waist circumference of 44 inches. Fasting laboratory results are as follows:
Blood glucose 117 mg/dL
Total cholesterol 210 mg/dL
LDL cholesterol 120 mg/dL
HDL cholesterol 38 mg/dL
Triglycerides 240 mg/dL
Which of the following mechanisms contribute to this patient’s condition?
Q204
A 30-year-old woman was found lying down and unresponsive by her parents 2 hours ago. She has no significant medical history. Two years ago, the woman discovered that her husband of 8 years was having an extramarital affair; this revelation subsequently resulted in a drawn-out divorce. After the separation, she moved back in with her parents, who note that she stays in her room, sleeps a lot, and rarely eats. A physical exam shows obtundation. Her temperature is 37.1ºC (98.7ºF), pulse is 110/min, respirations are 24/min, and blood pressure is 126/78 mm Hg. The patient’s admission labs are as follows:
TSH 3.2 µU/mL
Morning cortisol 8 µg/dL
Prolactin 15 ng/mL
FSH 7 mIU/mL
LH 6 mIU/mL
Glucose 22 mg/dL
C-peptide not detected
Beta-hydroxybutyrate ≤ 2.7 mmol/L
Which of the following is most true of the cell type that is likely involved in the production of the molecule causing this patient’s symptoms?
Q205
A 4-year-old boy presents to the ophthalmologist for a down- and inward dislocation of the lens in his left eye. On physical exam, the boy has a marfanoid habitus and intellectual disability. Biochemical tests were performed to locate the exact defect in this boy. It was found that there was a significant deficiency in cystathionine beta-synthase activity with elevated homocysteine levels. Which of the following is the diagnosis?
Q206
A 3-week-old boy is brought to the physician for the evaluation of poor feeding and recurrent episodes of vomiting. He was delivered at term after an uncomplicated pregnancy. He is at the 5th percentile for length and weight. Physical examination shows generalized hypotonia. Urinalysis shows increased propionic acid concentration. The finding on urinalysis is best explained by the breakdown of which of the following substances?
Q207
An investigator is studying the changes that occur in the oxygen-hemoglobin dissociation curve of different types of hemoglobin under various conditions. The blood obtained from a male infant shows decreased affinity for 2,3-bisphosphoglyceric acid. Which of the following is the most likely composition of the hemoglobin molecule in this sample?
Q208
A 5-month-old boy is brought to the pediatrician by his parents, who began noticing that the infant was not able to hold his head upright when sitting or in a prone position. Upon examination, the infant has a musty odor, fair skin with patches of eczema, and blue eyes. The pediatrician orders laboratory tests and prescribes a special diet. Which of the following amino acids becomes essential and must be supplemented in this patient's condition?
Q209
A 22-year-old primigravid woman comes to the physician for her initial prenatal visit at 12 weeks' gestation. She has had generalized fatigue and shortness of breath over the past 2 months. She has also had a tingling sensation in her toes for the past month. Three years ago, she was treated for gonorrhea. She follows a strict vegan diet since the age of 13 years. Her temperature is 37°C (98.6°F), pulse is 111/min, and blood pressure is 122/80 mm Hg. Examination shows pale conjunctivae and a shiny tongue. Muscle tone and strength is normal. Deep tendon reflexes are 2+ bilaterally. Sensation to vibration and position is decreased over the upper and lower extremities. When asked to stand, hold her arms in front of her, and close her eyes, she loses her balance and takes a step backward. Which of the following is most likely to have prevented this patient's condition?
Q210
A 2-month-old Middle Eastern female infant from a consanguinous marriage presents with seizures, anorexia, failure to thrive, developmental delay, and vomiting and fatigue after eating. Blood work demonstrated levels of methylmalonic acid nearly 500 times normal levels. A carbon-14 propionate incorporation assay was performed on the fibroblasts of the patient and compared to a healthy, normal individual. Little to none of the radiolabeled carbons of the propionate appeared in any of the intermediates of the Krebs cycle. Which of the following reactions is not taking place in this individual?
Metabolism US Medical PG Practice Questions and MCQs
Question 201: A 32-year-old woman is brought to the emergency department after she started convulsing in the office. She has no previous history of seizures and recovers by the time she arrives at the emergency department. She says that over the last 2 days she has also experienced insomnia, abdominal pain, and dark urine. Her past medical history is significant for asthma; however, she says that she has not experienced any of these symptoms previously. She smokes 1 pack of cigarettes per day, drinks a glass of wine with dinner every night, and is currently taking oral contraceptive pills (OCPs). On presentation, her temperature is 99°F (37.2°C), blood pressure is 140/98 mmHg, pulse is 112/min, and respirations are 11/min. Which of the following enzymes is most likely to be defective in this patient?
A. Aminolevulinate synthase
B. Ferrochelatase
C. Uroporphyrinogen decarboxylase
D. Porphobilinogen deaminase (Correct Answer)
E. Aminolevulinate dehydratase
Explanation: ***Porphobilinogen deaminase***
- This patient's symptoms, including **seizures**, **insomnia**, **abdominal pain**, and **dark urine**, are classic for an **acute intermittent porphyria (AIP)** crisis.
- **Porphobilinogen deaminase (PBG deaminase)**, also known as hydroxymethylbilane synthase, is the enzyme deficient in AIP, leading to the accumulation of neurotoxic ALA and PBG.
*Aminolevulinate synthase*
- This enzyme is the **rate-limiting step** in heme synthesis and is often upregulated in porphyrias but is not the primary enzyme defect in AIP.
- A defect in aminolevulinate synthase itself (or increased activity) would typically lead to **X-linked sideroblastic anemia**, not an acute porphyric crisis with neurological and abdominal symptoms.
*Ferrochelatase*
- A deficiency in **ferrochelatase** causes **erythropoietic protoporphyria**, characterized by **photosensitivity** and **liver damage**, but not acute neurovisceral symptoms like seizures or abdominal pain.
- This enzyme inserts **iron into protoporphyrin IX** to form heme.
*Uroporphyrinogen decarboxylase*
- A deficiency in **uroporphyrinogen decarboxylase** causes **porphyria cutanea tarda (PCT)**, which is the most common porphyria.
- PCT primarily presents with **photosensitivity** (blisters, fragile skin), hyperpigmentation, and hypertrichosis, and does not typically cause acute neurological or abdominal symptoms.
*Aminolevulinate dehydratase*
- A deficiency in **aminolevulinate dehydratase** (also known as ALA dehydratase) causes **ALA dehydratase deficiency porphyria** (ADDP).
- While ADDP can cause neurovisceral symptoms similar to AIP, it is much rarer, and the patient's presentation is more classic for AIP due to its prevalence and common precipitants.
Question 202: An 85-year-old woman presents to her physician with complaints of significant weakness and weight loss. She recently has been diagnosed with stage IV breast cancer for which she currently is receiving treatment. She mentions that, despite taking a diet rich in protein and calories, she continues to lose weight. On physical examination, her vital signs are stable, but muscle wasting is clearly evident in her upper limbs, lower limbs, and face. The physician explains to her that her advanced cancer is the most important cause for the weight loss and muscle wasting. This cachexia is mediated by the proteolysis-inducing factor released from cancer cells. Which of the following effects is produced by this factor?
A. Activation of hormone-sensitive lipase in adipose tissue
B. Suppression of the appetite center in the hypothalamus
C. Stimulation of apoptosis
D. Increased release of tumor necrosis factor (TNF) from macrophages
E. Activation of NF-κB (Correct Answer)
Explanation: ***Activation of NF-κB***
- Proteolysis-inducing factor (PIF) released from cancer cells leads to the **activation of NF-κB** in muscle cells.
- This activation results in an increased expression of **ubiquitin-proteasome pathway** components, which promotes protein degradation and muscle wasting in **cancer cachexia**.
*Activation of hormone-sensitive lipase in adipose tissue*
- While **cachexia** involves fat loss, PIF's primary effect on muscle wasting is through protein degradation, not direct activation of hormone-sensitive lipase.
- **Hormone-sensitive lipase** is mainly activated by catecholamines and cortisol during stress.
*Suppression of the appetite center in the hypothalamus*
- **Anorexia** (loss of appetite) is a component of cachexia, but PIF directly mediates muscle breakdown rather than directly suppressing hypothalamic appetite centers.
- Various cytokines like IL-1, IL-6, and TNF-α, as well as neurotransmitters, influence appetite.
*Stimulation of apoptosis*
- While **apoptosis** can contribute to cell loss in various contexts, PIF's main mechanism for muscle wasting is through the **ubiquitin-proteasome system**, not primarily by inducing apoptosis.
- Muscle atrophy in cachexia is often characterized more by protein breakdown than widespread myocyte death via apoptosis.
*Increased release of tumor necrosis factor (TNF) from macrophages*
- **TNF-α** is a major cytokine implicated in cachexia, but PIF itself is an independent factor released by tumor cells.
- PIF can act synergistically with cytokines like TNF-α to exacerbate muscle wasting, but it does not primarily cause the release of TNF-α from macrophages.
Question 203: A 32-year-old African American man presents to the office for a routine examination. He has no complaints at this time. Records show that his systolic blood pressure was in the 130–138 range and diastolic blood pressure in the 88–95 range despite counseling on lifestyle modification. He admits that he was not compliant with this advice. He takes no medications and works at home as a web designer. He does not drink alcohol but smokes marijuana on a weekly basis. Temperature is 37°C (98.6°F), blood pressure is 138/90 mm Hg, pulse is 76/min, and respirations are 12/min. BMI is 29.8 kg/m2. Physical examination is normal except for truncal obesity, with a waist circumference of 44 inches. Fasting laboratory results are as follows:
Blood glucose 117 mg/dL
Total cholesterol 210 mg/dL
LDL cholesterol 120 mg/dL
HDL cholesterol 38 mg/dL
Triglycerides 240 mg/dL
Which of the following mechanisms contribute to this patient’s condition?
A. Excessive cortisol secretion and activity
B. Granulomatous inflammation in medium-sized vessels
C. LDL receptor gene mutation
D. Insulin receptor resistance (Correct Answer)
E. Autoimmune destruction of pancreatic beta cells
Explanation: ***Insulin receptor resistance***
- The patient exhibits features of **metabolic syndrome**, including **truncal obesity** (BMI 29.8 kg/m², waist circumference 44 inches), **elevated blood pressure** (138/90 mm Hg), **impaired fasting glucose** (117 mg/dL), **high triglycerides** (240 mg/dL), and **low HDL cholesterol** (38 mg/dL). These are all key manifestations of insulin resistance.
- **Insulin resistance** is central to metabolic syndrome, leading to compensatory hyperinsulinemia, which contributes to hypertension, dyslipidemia, and impaired glucose tolerance, eventually progressing to type 2 diabetes if pancreatic beta cells fail.
*Excessive cortisol secretion and activity*
- **Cushing's syndrome**, characterized by excessive cortisol, also causes truncal obesity, hypertension, and hyperglycemia, but typically presents with additional features like **moon facies**, **buffalo hump**, and **skin striae**, which are not mentioned here.
- The patient's blood pressure and glucose levels, while elevated, are not severe enough to strongly suggest Cushing's syndrome in the absence of other characteristic signs.
*Granulomatous inflammation in medium-sized vessels*
- This description is characteristic of various forms of **vasculitis**, such as **Polyarteritis Nodosa** or **Giant Cell Arteritis**.
- Vasculitis typically presents with constitutional symptoms, organ ischemia, and specific inflammatory markers, none of which are present in this patient's routine examination.
*LDL receptor gene mutation*
- An **LDL receptor gene mutation** is associated with **familial hypercholesterolemia**, primarily causing greatly elevated LDL cholesterol levels and premature cardiovascular disease.
- While this patient has elevated LDL, his overall lipid profile with high triglycerides and low HDL is more consistent with metabolic syndrome than a primary LDL receptor defect.
*Autoimmune destruction of pancreatic beta cells*
- This is the underlying mechanism of **type 1 diabetes mellitus**, which usually presents with profound hyperglycemia, polyuria, polydipsia, and weight loss, typically in younger patients.
- This patient's mild hyperglycemia and features of metabolic syndrome are more indicative of **insulin resistance (type 2 diabetes)** rather than autoimmune beta-cell destruction.
Question 204: A 30-year-old woman was found lying down and unresponsive by her parents 2 hours ago. She has no significant medical history. Two years ago, the woman discovered that her husband of 8 years was having an extramarital affair; this revelation subsequently resulted in a drawn-out divorce. After the separation, she moved back in with her parents, who note that she stays in her room, sleeps a lot, and rarely eats. A physical exam shows obtundation. Her temperature is 37.1ºC (98.7ºF), pulse is 110/min, respirations are 24/min, and blood pressure is 126/78 mm Hg. The patient’s admission labs are as follows:
TSH 3.2 µU/mL
Morning cortisol 8 µg/dL
Prolactin 15 ng/mL
FSH 7 mIU/mL
LH 6 mIU/mL
Glucose 22 mg/dL
C-peptide not detected
Beta-hydroxybutyrate ≤ 2.7 mmol/L
Which of the following is most true of the cell type that is likely involved in the production of the molecule causing this patient’s symptoms?
A. Located in the periphery of islets of Langerhans and produce glucagon
B. Located in the center of islets of Langerhans and produce insulin (Correct Answer)
C. Located in the adrenal medulla and produce epinephrine
D. Located in the anterior pituitary and produce growth hormone
E. Located in zona fasciculata of the adrenal cortex and produce cortisol
Explanation: ***Located in the center of islets of Langerhans and produce insulin***
- This patient presents with **severe hypoglycemia** (glucose 22 mg/dL), **undetectable C-peptide**, and **inappropriately low beta-hydroxybutyrate** (≤ 2.7 mmol/L). The clinical context (depression, isolation) combined with undetectable C-peptide strongly suggests **exogenous insulin administration** (factitious hypoglycemia or suicide attempt).
- The molecule **causing this patient's symptoms** is **insulin**, which drives glucose into cells and suppresses counterregulatory mechanisms, resulting in profound hypoglycemia and neuroglycopenic symptoms (obtundation).
- **Beta cells** located in the **center of the islets of Langerhans** are the cell type that normally produces insulin. Even though this patient's hypoglycemia is caused by exogenous insulin (not endogenous production, as evidenced by undetectable C-peptide), the question asks about the cell type involved in producing the causative molecule - which is insulin, produced by pancreatic beta cells.
- The key distinction: C-peptide is co-secreted with endogenous insulin, so its absence confirms an exogenous source, but insulin itself (whether endogenous or exogenous) is still the molecule causing the pathophysiology.
*Located in the adrenal medulla and produce epinephrine*
- **Chromaffin cells** in the adrenal medulla produce **epinephrine**, a key counterregulatory hormone that responds to hypoglycemia by promoting glycogenolysis and gluconeogenesis to raise blood glucose.
- While epinephrine is involved in the physiologic response to this patient's condition, it is not the molecule **causing** the hypoglycemia - it's attempting to correct it.
*Located in the anterior pituitary and produce growth hormone*
- The anterior pituitary produces **growth hormone (GH)**, which has anti-insulin effects and promotes gluconeogenesis.
- Like epinephrine, GH is a counterregulatory hormone that opposes hypoglycemia rather than causing it.
*Located in zona fasciculata of the adrenal cortex and produce cortisol*
- The **zona fasciculata** produces **cortisol**, a glucocorticoid that raises blood glucose through gluconeogenesis.
- The patient's morning cortisol of 8 µg/dL is within normal range. Cortisol is another counterregulatory hormone, not the cause of hypoglycemia.
*Located in the periphery of islets of Langerhans and produce glucagon*
- **Alpha cells** at the periphery of islets produce **glucagon**, which opposes insulin action by promoting glycogenolysis and gluconeogenesis.
- In exogenous insulin overdose, excess insulin suppresses glucagon secretion and overwhelms counterregulatory mechanisms, but glucagon itself is not causing the hypoglycemia.
Question 205: A 4-year-old boy presents to the ophthalmologist for a down- and inward dislocation of the lens in his left eye. On physical exam, the boy has a marfanoid habitus and intellectual disability. Biochemical tests were performed to locate the exact defect in this boy. It was found that there was a significant deficiency in cystathionine beta-synthase activity with elevated homocysteine levels. Which of the following is the diagnosis?
A. Marfan syndrome
B. Homocystinuria (Correct Answer)
C. Alkaptonuria
D. Phenylketonuria
E. Maple syrup urine disease
Explanation: ***Homocystinuria***
- The combination of **ectopia lentis** (lens dislocation), **marfanoid habitus**, and **intellectual disability** is characteristic of homocystinuria.
- The **deficiency in cystathionine beta-synthase** leading to **elevated homocysteine levels** is the biochemical hallmark of this disorder.
*Marfan syndrome*
- While it presents with **marfanoid habitus** and **ectopia lentis**, the lens dislocation is typically **upward and outward**, unlike the down- and inward dislocation seen here.
- Marfan syndrome is caused by a defect in **fibrillin-1**, and biochemical tests would not show elevated homocysteine.
*Alkaptonuria*
- This disorder is characterized by **dark urine** upon standing, **ochronosis** (darkening of cartilage and connective tissue), and **arthropathy**.
- It results from a deficiency in **homogentisate oxidase** and does not present with lens dislocation or intellectual disability.
*Phenylketonuria*
- PKU is caused by a deficiency in **phenylalanine hydroxylase**, leading to an accumulation of phenylalanine.
- It primarily causes **severe intellectual disability**, seizures, and a musty odor, but not ectopia lentis or marfanoid habitus.
*Maple syrup urine disease*
- This is a metabolic disorder affecting the metabolism of **branched-chain amino acids** (leucine, isoleucine, valine).
- It presents with a characteristic **maple syrup odor** in urine, poor feeding, lethargy, and developmental delay, but not the specific features seen in this case.
Question 206: A 3-week-old boy is brought to the physician for the evaluation of poor feeding and recurrent episodes of vomiting. He was delivered at term after an uncomplicated pregnancy. He is at the 5th percentile for length and weight. Physical examination shows generalized hypotonia. Urinalysis shows increased propionic acid concentration. The finding on urinalysis is best explained by the breakdown of which of the following substances?
A. Even-chain fatty acids
B. Bicyclic nitrogenous bases
C. Branched-chain amino acids (Correct Answer)
D. Hexose monosaccharides
E. Catechol-containing monoamines
Explanation: ***Branched-chain amino acids***
- The elevated **propionic acid** in the urine (propionic acidemia) is a hallmark of **propionyl-CoA carboxylase deficiency**.
- This enzyme is crucial for metabolizing **propionyl-CoA**, which is produced from the breakdown of **valine and isoleucine** (both branched-chain amino acids).
- Note: Propionyl-CoA is also generated from **methionine and threonine** (non-branched-chain amino acids) and **odd-chain fatty acids**, but among the given options, branched-chain amino acids is the most relevant answer.
- Clinical features include poor feeding, vomiting, hypotonia, and metabolic acidosis in early infancy.
*Even-chain fatty acids*
- The breakdown of even-chain fatty acids primarily generates **acetyl-CoA** through beta-oxidation.
- Even-chain fatty acids produce only acetyl-CoA units and do **not** produce propionyl-CoA or propionic acid.
*Bicyclic nitrogenous bases*
- Bicyclic nitrogenous bases refer to **purines** (adenine, guanine) in nucleotide metabolism.
- Their catabolism produces **uric acid**, not propionic acid.
*Hexose monosaccharides*
- Hexose monosaccharides like **glucose** are metabolized through glycolysis to **pyruvate**, then to acetyl-CoA.
- Their breakdown does not result in propionyl-CoA or propionic acid accumulation.
*Catechol-containing monoamines*
- These include neurotransmitters like **dopamine, norepinephrine, and epinephrine**.
- Their metabolism involves **monoamine oxidase (MAO)** and **catechol-O-methyltransferase (COMT)**, producing metabolites such as **vanillylmandelic acid (VMA)** and **homovanillic acid (HVA)**, not propionic acid.
Question 207: An investigator is studying the changes that occur in the oxygen-hemoglobin dissociation curve of different types of hemoglobin under various conditions. The blood obtained from a male infant shows decreased affinity for 2,3-bisphosphoglyceric acid. Which of the following is the most likely composition of the hemoglobin molecule in this sample?
A. α2βS2
B. α2β2
C. α2δ2
D. α2γ2 (Correct Answer)
E. β4
Explanation: ***α2γ2***
- This formula represents **fetal hemoglobin (HbF)**, which is the predominant hemoglobin in infants.
- HbF has **decreased affinity for 2,3-bisphosphoglyceric acid (2,3-BPG)** compared to adult hemoglobin (HbA) because 2,3-BPG binds less avidly to the gamma chains.
- This decreased 2,3-BPG binding results in HbF having **higher oxygen affinity** than HbA (left-shifted oxygen-hemoglobin dissociation curve).
- The higher oxygen affinity allows fetal blood to efficiently extract oxygen from maternal blood across the placenta.
*α2βS2*
- This represents **hemoglobin S (HbS)**, found in **sickle cell disease**.
- HbS has similar 2,3-BPG binding to HbA, not decreased affinity.
- Its primary characteristic is polymerization and red blood cell sickling under deoxygenated conditions.
*α2β2*
- This represents **adult hemoglobin (HbA)**, the most common type of hemoglobin in adults.
- HbA has **higher affinity for 2,3-BPG** compared to HbF because 2,3-BPG binds strongly to the beta chains.
- The binding of 2,3-BPG to HbA decreases oxygen affinity, facilitating oxygen release to tissues.
*α2δ2*
- This represents **hemoglobin A2 (HbA2)**, a minor component of adult hemoglobin (typically <3.5%).
- HbA2 has normal 2,3-BPG binding similar to HbA, not decreased affinity.
- This doesn't fit the clinical description of an infant with decreased 2,3-BPG affinity.
*β4*
- This represents **hemoglobin H (HbH)**, which occurs in **alpha-thalassemia** where there is an excess of beta chains that form tetramers.
- HbH has **extremely high oxygen affinity** and does not release oxygen well to tissues.
- While HbH also has decreased 2,3-BPG binding, it is not found in healthy infants and represents a pathological condition.
Question 208: A 5-month-old boy is brought to the pediatrician by his parents, who began noticing that the infant was not able to hold his head upright when sitting or in a prone position. Upon examination, the infant has a musty odor, fair skin with patches of eczema, and blue eyes. The pediatrician orders laboratory tests and prescribes a special diet. Which of the following amino acids becomes essential and must be supplemented in this patient's condition?
A. Thiamine
B. Large neutral amino acids
C. Arginine
D. Phenylalanine-free amino acid formula
E. Tyrosine (Correct Answer)
Explanation: ***Tyrosine***
- This patient presents with **phenylketonuria (PKU)**, characterized by the classic triad of developmental delay, musty odor (from phenylketones in urine), and hypopigmentation (fair skin, blue eyes, eczema).
- PKU is caused by **phenylalanine hydroxylase deficiency**, which prevents the conversion of phenylalanine to tyrosine.
- Because tyrosine cannot be synthesized from phenylalanine, **tyrosine becomes a conditionally essential amino acid** in PKU patients and must be supplemented in the diet.
- Treatment consists of a phenylalanine-restricted diet with **tyrosine supplementation** to ensure adequate protein synthesis and melanin production.
*Arginine*
- Arginine becomes conditionally essential in **urea cycle disorders**, where it helps drive nitrogen excretion.
- However, this patient's presentation (musty odor, hypopigmentation, eczema) is classic for PKU, not a urea cycle disorder.
- Urea cycle disorders typically present with hyperammonemia, vomiting, lethargy, and encephalopathy—not the findings described here.
*Phenylalanine-free amino acid formula*
- This is a **dietary intervention**, not an amino acid that becomes essential.
- While phenylalanine-free formulas are crucial in PKU management, the question specifically asks which amino acid becomes essential and must be supplemented.
- Phenylalanine itself is *restricted*, not supplemented.
*Thiamine*
- Thiamine (vitamin B1) is a **vitamin**, not an amino acid.
- It is a cofactor for enzymes in carbohydrate metabolism and is deficient in beriberi and Wernicke-Korsakoff syndrome.
- It is not relevant to PKU management.
*Large neutral amino acids*
- Large neutral amino acids (LNAAs) can be used as an **adjunctive therapy** in PKU to compete with phenylalanine for transport across the blood-brain barrier.
- However, they are not the amino acid that becomes conditionally essential due to the metabolic block.
- The primary nutritional requirement is **tyrosine supplementation** to replace what cannot be synthesized.
Question 209: A 22-year-old primigravid woman comes to the physician for her initial prenatal visit at 12 weeks' gestation. She has had generalized fatigue and shortness of breath over the past 2 months. She has also had a tingling sensation in her toes for the past month. Three years ago, she was treated for gonorrhea. She follows a strict vegan diet since the age of 13 years. Her temperature is 37°C (98.6°F), pulse is 111/min, and blood pressure is 122/80 mm Hg. Examination shows pale conjunctivae and a shiny tongue. Muscle tone and strength is normal. Deep tendon reflexes are 2+ bilaterally. Sensation to vibration and position is decreased over the upper and lower extremities. When asked to stand, hold her arms in front of her, and close her eyes, she loses her balance and takes a step backward. Which of the following is most likely to have prevented this patient's condition?
A. Calcium supplementation
B. Thyroxine supplementation
C. Vitamin B12 supplementation (Correct Answer)
D. Iron supplementation
E. Penicillin G therapy
Explanation: ***Vitamin B12 supplementation***
- The patient's symptoms, including **fatigue**, **shortness of breath**, **tingling sensation in toes** (paresthesias), **pale conjunctivae**, **shiny tongue** (glossitis), decreased **vibration and position sensation**, and **ataxia** (losing balance with eyes closed), are classic signs of **vitamin B12 deficiency**.
- A **strict vegan diet** since age 13 significantly increases the risk of vitamin B12 deficiency, as B12 is primarily found in animal products. Supplementation would have prevented these symptoms.
*Calcium supplementation*
- **Calcium** is essential for bone health, muscle function, and nerve transmission, but deficiency typically causes symptoms like **osteoporosis**, **muscle cramps**, or **tetany**, not the specific neurological and hematological signs seen here.
- While important in pregnancy, calcium deficiency does not explain the patient's array of symptoms, especially the neurological presentation.
*Thyroxine supplementation*
- **Thyroxine** (thyroid hormone) deficiency (hypothyroidism) can cause fatigue, but symptoms typically also include **weight gain**, constipation, cold intolerance, and dry skin, which are not mentioned.
- It does not cause the specific neurological signs like **paresthesias**, loss of **vibration sensation**, or **ataxia**, nor does it explain the **shiny tongue**.
*Iron supplementation*
- **Iron deficiency** is a common cause of fatigue, shortness of breath, and pale conjunctivae (due to anemia). However, it does not typically cause the **neurological symptoms** described (tingling, sensory loss, ataxia) or the **shiny tongue**.
- Although iron deficiency anemia might coexist, the neurological findings strongly point away from it as the primary cause of all symptoms.
*Penicillin G therapy*
- **Penicillin G** is used to treat bacterial infections, including syphilis and gonorrhea. While the patient was treated for gonorrhea, this current presentation is not indicative of an active infection.
- The symptoms are characteristic of a nutritional deficiency, not a bacterial infection or a complication of prior gonorrhea without current active infection.
Question 210: A 2-month-old Middle Eastern female infant from a consanguinous marriage presents with seizures, anorexia, failure to thrive, developmental delay, and vomiting and fatigue after eating. Blood work demonstrated levels of methylmalonic acid nearly 500 times normal levels. A carbon-14 propionate incorporation assay was performed on the fibroblasts of the patient and compared to a healthy, normal individual. Little to none of the radiolabeled carbons of the propionate appeared in any of the intermediates of the Krebs cycle. Which of the following reactions is not taking place in this individual?
A. Pyruvate --> acetyl-CoA
B. Acetyl-CoA + CO2 --> Malonyl-CoA
C. Methylmalonyl-CoA --> Succinyl-CoA (Correct Answer)
D. Propionyl-CoA --> Methylmalonyl-CoA
E. Acetyl-CoA + Oxaloacetate --> Citrate
Explanation: ***Methylmalonyl-CoA --> Succinyl-CoA***
- The significantly elevated levels of **methylmalonic acid** and the failure of **radio-labeled propionate** (which is metabolized to methylmalonyl-CoA) to enter the Krebs cycle strongly indicate a defect in the succinyl-CoA mutase enzyme.
- This reaction, catalyzed by **methylmalonyl-CoA mutase** (with vitamin B12 as a cofactor), is essential for converting methylmalonyl-CoA to succinyl-CoA, a key intermediate in the Krebs cycle.
*Pyruvate --> acetyl-CoA*
- This reaction is catalyzed by the **pyruvate dehydrogenase complex** and is a critical step for glucose metabolism entering the Krebs cycle.
- The patient's symptoms and lab findings are related to **propionate metabolism**, not primarily glucose, and the experiment directly tests propionate, not pyruvate.
*Acetyl-CoA + CO2 --> Malonyl-CoA*
- This reaction, catalyzed by **acetyl-CoA carboxylase**, is the committed step in **fatty acid synthesis**.
- While important for metabolism, it is not directly related to the breakdown of propionate or the accumulation of methylmalonic acid.
*Propionyl-CoA --> Methylmalonyl-CoA*
- This reaction, catalyzed by **propionyl-CoA carboxylase**, converts propionyl-CoA to D-methylmalonyl-CoA, eventually isomerized to L-methylmalonyl-CoA.
- The fact that methylmalonic acid is nearly 500 times normal suggests that **propionyl-CoA carboxylase is functioning**, leading to the accumulation of its product.
*Acetyl-CoA + Oxaloacetate --> Citrate*
- This reaction is the first step of the **Krebs cycle**, catalyzed by **citrate synthase**.
- While propionate metabolism feeds into the Krebs cycle, the immediate block is upstream at succinyl-CoA formation from methylmalonyl-CoA, preventing its entry into the cycle, rather than the initial step of the cycle itself.
