A 3-year-old boy is brought to the physician for a follow-up examination. He has lactose intolerance. His family emigrated from Somalia 6 months ago. He is at the 30th percentile for height and 15th percentile for weight. Vital signs are within normal limits. Examination shows pale conjunctivae, an erythematous throat, and swollen tongue. There is inflammation of the perioral and labial mucosa, and peeling and cracking of the skin at the corners of the mouth. Cardiopulmonary examination shows no abnormalities. His hemoglobin concentration is 9.8 g/dL and mean corpuscular volume is 87 μm3. If left untreated, this child is also most likely to develop which of the following?
Q42
A 64-year-old man who is post-op day 4 following a radical nephrectomy is noted to have a temperature of 103.4F, pulse of 115, blood pressure of 86/44, and respiratory rate of 26. Arterial blood gas shows a pH of 7.29 and pCO2 of 28. Chemistry panel shows: Na+ 136, Cl- 100, HCO3- 14. CBC is significant for a significant leukocytosis with bandemia. The laboratory reports that blood cultures are growing gram positive cocci.
Which of the following is true about this patient's biochemical state?
Q43
A 5-year-old boy is brought to the physician by his parents for evaluation of easy bruising. He has met all developmental milestones. Vital signs are within normal limits. He is at the 50th percentile for height and weight. Physical examination shows velvety, fragile skin that can be stretched further than normal and multiple ecchymoses. Joint range of motion is increased. A defect in which of the following is the most likely cause of this patient's condition?
Q44
A 48-year-old homeless man presents to a free clinic complaining of several weeks of bleeding gums. He states that he has not seen dental or medical care for several years and that the bleeding has been painless. He normally sleeps on the street or occasionally stays at a shelter and typically eats 1-2 fast food meals per day. On exam, his temperature is 98.4°F (36.9°C), blood pressure is 122/76 mmHg, pulse is 64/min, and respirations are 12/min. He has poor dentition and significant periodontal disease with notable areas of bleeding gingiva. The patient is found to have coarse hair and on further questioning, the patient endorses the recent loss of 2 of his teeth and several weeks of fatigue. Which of the following processes is most immediately affected by his condition?
Q45
A 4-year-old boy is brought to the physician for a well-child examination. He started walking at 20 months of age. He can use a cup to drink but cannot use silverware. He speaks in 2-word sentences and can build a tower of 4 blocks. He can scribble but cannot draw a circle. He is above the 99th percentile for height and at the 15th percentile for weight. Vital signs are within normal limits. Examination shows bilateral inferior lens dislocation. His fingers are long and slender. He has a high-arched palate. The thumb and 5th finger overlap when he grips a wrist with the opposite hand. The skin over the neck can be extended and stretched easily. Which of the following is the most likely cause of these findings?
Q46
A 26-year-old man from India visits the clinic with complaints of feeling tired all the time and experiencing lack of energy for the past couple of weeks. He also complains of weakness and numbness of his lower limbs. He has been strictly vegan since the age of 18, including not consuming eggs and milk. He does not take any vitamin or dietary supplements. Physical examination reveals a smooth, red beefy tongue along with lower extremity sensory and motor deficits. What other finding is most likely to accompany this patient’s condition?
Q47
A 6-year-old boy is brought to the pediatrician by his mother complaining of abdominal pain and constipation. She reports that his appetite has been reduced and that he has not had a bowel movement in 2 days. Prior to this, he had a regular bowel movement once a day. She also reports that he has appeared to be more tired than usual. The family recently moved into a house built in the 1940s and have just begun renovations. The child was born via spontaneous vaginal delivery at 39 weeks gestation. He is up to date on all vaccinations and meeting all developmental milestones. Today, his blood pressure is 120/80 mm Hg, heart rate is 95/min, respiratory rate is 25/min, and temperature is 37.0°C (98.6°F). A physical exam is only significant for moderate conjunctival pallor. A peripheral blood smear shows red blood cells with basophilic stippling. What is the most likely mechanism causing this patient’s symptoms?
Q48
A 43-year-old woman visits her primary care provider complaining of fatigue. Although she has had it for several months, her fatigue has been worsening over the past few weeks. She has no other symptoms. Past medical history is significant for hypertension. She takes chlorthalidone, an oral contraceptive pill, and a multivitamin every day. Family history is noncontributory. She drinks about 1 bottle of wine every day and started taking a shot or two of whisky or vodka every morning before work to “clear out the cobwebs”. She was recently fired from her job. Today, her heart rate is 90/min, respiratory rate is 17/min, blood pressure is 110/65 mm Hg, and temperature is 36.7°C (98.1°F). On physical exam, she appears malnourished and anxious. Her conjunctiva are pale, and glossitis is noted on oral exam. Her heart has a regular rate and rhythm and her lungs are clear to auscultation bilaterally. She has no gait or balance abnormalities. Lab results show a hemoglobin of 10 g/dL, with a mean corpuscular volume (MCV) of 108 fl. Elevated level of which of the following will most likely to be found in this patient?
Q49
A 28-year-old man is brought in by ambulance to the ER, barely conscious, after feeling drowsy and falling to the floor during a presentation several hours ago. His colleague who accompanied him says he has had similar episodes 5 times in the past 3 months. No significant past medical history. His blood pressure is 110/80 mm Hg and pulse is 114/min. His capillary blood glucose is 15 mg/dL. Immediate IV dextrose with thiamine is started, and he rapidly regains consciousness. A contrast CT of the abdomen is performed which reveals a tumor in the pancreas. Which of the following relative laboratory findings would you most likely expect to find in this patient?
Q50
An investigator is studying the activity level of several different enzymes in human subjects from various demographic groups. An elevated level of activity of phosphoribosyl pyrophosphate synthetase is found in one of the study subjects. This patient is most likely to have which of the following conditions?
Metabolism US Medical PG Practice Questions and MCQs
Question 41: A 3-year-old boy is brought to the physician for a follow-up examination. He has lactose intolerance. His family emigrated from Somalia 6 months ago. He is at the 30th percentile for height and 15th percentile for weight. Vital signs are within normal limits. Examination shows pale conjunctivae, an erythematous throat, and swollen tongue. There is inflammation of the perioral and labial mucosa, and peeling and cracking of the skin at the corners of the mouth. Cardiopulmonary examination shows no abnormalities. His hemoglobin concentration is 9.8 g/dL and mean corpuscular volume is 87 μm3. If left untreated, this child is also most likely to develop which of the following?
A. Dilated cardiomyopathy
B. Keratomalacia
C. Ataxia
D. Corneal vascularization (Correct Answer)
E. Hypersegmented neutrophils
Explanation: ***Corneal vascularization***
- The clinical presentation of **angular cheilitis** (cracking/peeling at mouth corners), **glossitis** (swollen, erythematous tongue), **perioral and labial inflammation**, and **normocytic anemia** in a child with **lactose intolerance** (limiting dairy intake) strongly suggests **riboflavin (vitamin B2) deficiency**.
- If left untreated, riboflavin deficiency progresses to **corneal vascularization**, characterized by blood vessel invasion into the normally avascular cornea, often accompanied by photophobia and conjunctival injection.
- **Dairy products are the primary dietary source of riboflavin**, making lactose-intolerant individuals particularly vulnerable to deficiency.
*Hypersegmented neutrophils*
- This is the hallmark hematologic finding in **megaloblastic anemia** caused by **folate or vitamin B12 deficiency**.
- These deficiencies cause **macrocytic anemia** (MCV >100 μm³), but this patient has **normocytic anemia** (MCV 87 μm³), ruling out megaloblastic causes.
- The normal MCV excludes folate/B12 deficiency as the primary diagnosis.
*Dilated cardiomyopathy*
- This is the classic cardiac manifestation of **thiamine (vitamin B1) deficiency** (wet beriberi).
- While severe malnutrition can affect cardiac function, the specific clinical features presented (angular cheilitis, glossitis) are not characteristic of thiamine deficiency.
- Thiamine deficiency typically presents with peripheral neuropathy, muscle weakness, and cardiovascular collapse, not the oral/mucosal lesions seen here.
*Keratomalacia*
- This condition results from **severe vitamin A deficiency** and involves corneal softening, ulceration, and blindness.
- Vitamin A deficiency presents with **night blindness, xerophthalmia, and Bitot's spots**, not the oral mucosal lesions described.
- The clinical picture does not support vitamin A deficiency.
*Ataxia*
- **Ataxia** is associated with **vitamin B12 deficiency** (subacute combined degeneration affecting posterior columns and corticospinal tracts) or **vitamin E deficiency**.
- Again, the **normal MCV** rules out B12 deficiency as the primary diagnosis.
- The prominent oral and skin findings are not typical of neurological vitamin deficiencies.
Question 42: A 64-year-old man who is post-op day 4 following a radical nephrectomy is noted to have a temperature of 103.4F, pulse of 115, blood pressure of 86/44, and respiratory rate of 26. Arterial blood gas shows a pH of 7.29 and pCO2 of 28. Chemistry panel shows: Na+ 136, Cl- 100, HCO3- 14. CBC is significant for a significant leukocytosis with bandemia. The laboratory reports that blood cultures are growing gram positive cocci.
Which of the following is true about this patient's biochemical state?
A. Increased flux through the electron transport chain
B. Decreased activity of lactate dehydrogenase
C. Increased activity of isocitrate dehydrogenase
D. Decreased activity of pyruvate dehydrogenase (Correct Answer)
E. Increased activity of alcohol dehydrogenase
Explanation: ***Decreased activity of pyruvate dehydrogenase***
- The patient presents with **septic shock** (fever, hypotension, tachycardia, leukocytosis with bandemia, positive blood cultures) and signs of **lactic acidosis** (pH 7.29, HCO3- 14, pCO2 28 consistent with metabolic acidosis with respiratory compensation).
- In situations of tissue hypoperfusion and hypoxia, the **pyruvate dehydrogenase complex (PDHC)** activity decreases. This shunts pyruvate away from the Krebs cycle and towards **lactate production** via lactate dehydrogenase, contributing to lactic acidosis.
*Increased flux through the electron transport chain*
- **Septic shock** causes tissue hypoperfusion and cellular hypoxia, impairing **oxidative phosphorylation** and the electron transport chain (ETC).
- Rather than increased flux, the ETC is likely **dysfunctional** due to insufficient oxygen and mitochondrial damage, leading to decreased ATP production and increased reliance on anaerobic glycolysis.
*Decreased activity of lactate dehydrogenase*
- In lactic acidosis, **lactate dehydrogenase** activity is generally **increased** as it converts pyruvate to lactate to regenerate NAD+, which is crucial for glycolysis to continue in anaerobic conditions.
- A decrease in lactate dehydrogenase activity would hinder lactate production and worsen the accumulation of pyruvate, which is not what's observed.
*Increased activity of isocitrate dehydrogenase*
- **Isocitrate dehydrogenase** is a key enzyme in the **Krebs cycle (citric acid cycle)**, which is part of aerobic respiration.
- In septic shock with hypoxia and impaired mitochondrial function, the overall activity of the Krebs cycle, including isocitrate dehydrogenase, would likely be **decreased** due to reduced oxygen availability and cellular stress.
*Increased activity of alcohol dehydrogenase*
- **Alcohol dehydrogenase** is primarily involved in the metabolism of **ethanol**.
- There is no clinical indication or biochemical evidence (e.g., elevated ethanol levels) to suggest increased activity of alcohol dehydrogenase in this patient's presentation.
Question 43: A 5-year-old boy is brought to the physician by his parents for evaluation of easy bruising. He has met all developmental milestones. Vital signs are within normal limits. He is at the 50th percentile for height and weight. Physical examination shows velvety, fragile skin that can be stretched further than normal and multiple ecchymoses. Joint range of motion is increased. A defect in which of the following is the most likely cause of this patient's condition?
A. α-collagen triple helix formation (Correct Answer)
B. Lysine-hydroxylysine cross-linking
C. α1-antitrypsin production
D. Fibrillin-1 glycoprotein production
E. Proline and lysine hydroxylation
Explanation: ***Correct: α-collagen triple helix formation***
- The patient's symptoms of **velvety, fragile, hyperextensible skin**, **easy bruising (ecchymoses)**, and **increased joint range of motion (hypermobility)** are classic signs of **Ehlers-Danlos syndrome (EDS)**, particularly the **classical type**.
- Classical EDS is caused by mutations in **COL5A1 or COL5A2 genes**, which encode **type V collagen**. These mutations result in defects in **collagen triple helix formation and structure**, leading to abnormal collagen fibrils with reduced tensile strength.
- Type V collagen regulates the assembly and diameter of type I collagen fibrils, so defects compromise overall connective tissue integrity.
*Incorrect: Lysine-hydroxylysine cross-linking*
- Defects in **lysyl hydroxylase** (which catalyzes hydroxylysine formation needed for cross-linking) cause **kyphoscoliotic EDS (type VI)**, not classical EDS.
- Kyphoscoliotic EDS presents with **severe progressive kyphoscoliosis**, **ocular fragility**, and **hypotonia**, which are not described in this patient.
*Incorrect: α1-antitrypsin production*
- Defects in **α1-antitrypsin production** cause **emphysema** (especially early-onset panacinar) and **liver disease** (cirrhosis), not connective tissue hyperextensibility.
- This does not cause skin fragility or joint hypermobility.
*Incorrect: Fibrillin-1 glycoprotein production*
- A defect in **fibrillin-1** causes **Marfan syndrome**, which can include joint hypermobility and affects connective tissue.
- However, Marfan syndrome typically presents with **tall stature**, **arachnodactyly**, **lens dislocation**, and **cardiovascular abnormalities** (aortic root dilatation/dissection), none of which are present in this patient.
*Incorrect: Proline and lysine hydroxylation*
- Deficiency in **proline and lysine hydroxylation** occurs in **scurvy** (vitamin C deficiency), as vitamin C is a cofactor for prolyl and lysyl hydroxylases.
- Scurvy presents with **bleeding gums**, **perifollicular hemorrhages**, **poor wound healing**, and **petechiae**, but not the characteristic skin hyperextensibility and joint hypermobility seen in EDS.
Question 44: A 48-year-old homeless man presents to a free clinic complaining of several weeks of bleeding gums. He states that he has not seen dental or medical care for several years and that the bleeding has been painless. He normally sleeps on the street or occasionally stays at a shelter and typically eats 1-2 fast food meals per day. On exam, his temperature is 98.4°F (36.9°C), blood pressure is 122/76 mmHg, pulse is 64/min, and respirations are 12/min. He has poor dentition and significant periodontal disease with notable areas of bleeding gingiva. The patient is found to have coarse hair and on further questioning, the patient endorses the recent loss of 2 of his teeth and several weeks of fatigue. Which of the following processes is most immediately affected by his condition?
A. Proteolytic cleavage
B. Cross-linking
C. Glycosylation
D. Hydroxylation (Correct Answer)
E. Translation
Explanation: ***Hydroxylation***
- The patient's symptoms (bleeding gums, poor dentition, fatigue, coarse hair) in the context of poor nutrition (homelessness, fast food diet) are classic for **scurvy**, caused by **vitamin C deficiency**.
- **Vitamin C** is a crucial cofactor for the **hydroxylation of proline and lysine residues** in procollagen, a vital step in **collagen synthesis**. Without hydroxylation, collagen is unstable, leading to impaired wound healing, fragile blood vessels, and connective tissue defects.
*Proteolytic cleavage*
- This process involves the breakdown of proteins and is not directly impaired by vitamin C deficiency in a way that would cause these specific symptoms.
- While collagen maturation involves some proteolytic steps (e.g., removal of propeptides), the primary defect in scurvy is not in the cleavage itself but in the structural integrity due to failed hydroxylation.
*Cross-linking*
- **Collagen cross-linking** primarily involves **lysyl oxidase**, an enzyme that requires **copper** as a cofactor, not vitamin C.
- While proper collagen structure eventually supports effective cross-linking, the immediate and direct process affected by vitamin C deficiency is hydroxylation, which occurs earlier in collagen synthesis.
*Glycosylation*
- **Glycosylation** is the enzymatic addition of carbohydrates to proteins or lipids.
- This process is not directly dependent on vitamin C and is not the primary mechanism compromised in scurvy.
*Translation*
- **Translation** is the process by which mRNA is decoded to produce a specific protein during protein synthesis.
- Vitamin C deficiency does not directly impair the ribosomal machinery or the general process of translation. The defect in scurvy occurs post-translationally, specifically affecting the modification of certain amino acids in collagen.
Question 45: A 4-year-old boy is brought to the physician for a well-child examination. He started walking at 20 months of age. He can use a cup to drink but cannot use silverware. He speaks in 2-word sentences and can build a tower of 4 blocks. He can scribble but cannot draw a circle. He is above the 99th percentile for height and at the 15th percentile for weight. Vital signs are within normal limits. Examination shows bilateral inferior lens dislocation. His fingers are long and slender. He has a high-arched palate. The thumb and 5th finger overlap when he grips a wrist with the opposite hand. The skin over the neck can be extended and stretched easily. Which of the following is the most likely cause of these findings?
A. Hypoxanthine-guanine-phosphoribosyl transferase deficiency
B. Galactokinase deficiency
C. Fibrillin 1 deficiency
D. Cystathionine synthase deficiency (Correct Answer)
E. Type V collagen deficiency
Explanation: ***Cystathionine synthase deficiency***
- The combination of **inferior lens dislocation**, **marfanoid habitus** (tall stature, long slender fingers, high-arched palate), **developmental delay** (late walking, speech delay), and **hyperelastic skin** is highly suggestive of **homocystinuria** due to cystathionine synthase deficiency.
- **Homocystinuria** is an autosomal recessive disorder causing accumulation of **homocysteine**, leading to multisystem involvement.
*Hypoxanthine-guanine-phosphoribosyl transferase deficiency*
- This deficiency causes **Lesch-Nyhan syndrome**, characterized by **gout, intellectual disability, choreoathetosis, and self-mutilation**.
- It does not present with lens dislocation or marfanoid features.
*Galactokinase deficiency*
- This is a rare form of **galactosemia** primarily causing **cataracts**.
- It does not explain the developmental delay, marfanoid features, or lens dislocation.
*Fibrillin 1 deficiency*
- This causes **Marfan syndrome**, which shares features like **tall stature, long slender fingers, high-arched palate, and lens dislocation**.
- However, in Marfan syndrome, lens dislocation is typically **superior**, while in this case, it is **inferior**, pointing towards homocystinuria.
*Type V collagen deficiency*
- This can be associated with **Ehlers-Danlos syndrome**, which features **hyperelastic skin** and joint hypermobility.
- However, it does not typically cause lens dislocation or the specific marfanoid habitus described, and developmental delay is not a primary feature.
Question 46: A 26-year-old man from India visits the clinic with complaints of feeling tired all the time and experiencing lack of energy for the past couple of weeks. He also complains of weakness and numbness of his lower limbs. He has been strictly vegan since the age of 18, including not consuming eggs and milk. He does not take any vitamin or dietary supplements. Physical examination reveals a smooth, red beefy tongue along with lower extremity sensory and motor deficits. What other finding is most likely to accompany this patient’s condition?
A. Upper limb weakness
B. Psychiatric symptoms
C. Decreased visual acuity
D. Microcytic anemia
E. Ataxia (Correct Answer)
Explanation: **Ataxia**
- The patient's history of being a strict vegan, fatigue, weakness, numbness, and neurological deficits (sensory and motor) along with a **smooth, red beefy tongue** are classic signs of **vitamin B12 deficiency**.
- **Vitamin B12 deficiency** often leads to **subacute combined degeneration of the spinal cord**, which can manifest as **ataxia**, spasticity, and paresthesias due to demyelination.
*Upper limb weakness*
- While B12 deficiency can cause generalized weakness, the question specifically highlights **lower extremity sensory and motor deficits**, with **ataxia** being a more characteristic and often earlier neurological sign of spinal cord involvement than upper limb weakness.
- Upper limb weakness might develop in advanced stages, but it is not the **most likely** accompanying finding in the early or moderate stages often described with lower limb involvement and ataxia.
*Psychiatric symptoms*
- **Psychiatric symptoms** such as depression, irritability, and cognitive impairment can occur in **vitamin B12 deficiency**, but **ataxia** is a more direct and common neurological consequence stemming from the demyelination in the spinal cord.
- While possible, the question asks for the **most likely** additional finding given the specific neurological presentation.
*Decreased visual acuity*
- **Optic neuropathy** and **decreased visual acuity** can occur in some cases of **vitamin B12 deficiency**, but it is less common than the spinal cord and peripheral nerve manifestations like ataxia and paresthesias.
- The presented symptoms directly point to spinal cord involvement, making **ataxia** a more prominent associated neurological finding.
*Microcytic anemia*
- **Vitamin B12 deficiency** typically causes **megaloblastic (macrocytic) anemia**, not microcytic anemia.
- **Microcytic anemia** is primarily associated with **iron deficiency**, lead poisoning, or thalassemia.
Question 47: A 6-year-old boy is brought to the pediatrician by his mother complaining of abdominal pain and constipation. She reports that his appetite has been reduced and that he has not had a bowel movement in 2 days. Prior to this, he had a regular bowel movement once a day. She also reports that he has appeared to be more tired than usual. The family recently moved into a house built in the 1940s and have just begun renovations. The child was born via spontaneous vaginal delivery at 39 weeks gestation. He is up to date on all vaccinations and meeting all developmental milestones. Today, his blood pressure is 120/80 mm Hg, heart rate is 95/min, respiratory rate is 25/min, and temperature is 37.0°C (98.6°F). A physical exam is only significant for moderate conjunctival pallor. A peripheral blood smear shows red blood cells with basophilic stippling. What is the most likely mechanism causing this patient’s symptoms?
A. Inhibition of ALA synthase
B. Activation of glutathione
C. Inhibition of ferrochelatase (Correct Answer)
D. Activation of ALA dehydratase
E. Inactivation of uroporphyrinogen III cosynthase
Explanation: ***Inhibition of ferrochelatase***
- This patient's symptoms (abdominal pain, constipation, reduced appetite, fatigue, conjunctival pallor, and basophilic stippling on a blood smear) in the context of recent home renovations in an old house (built in the 1940s) are highly suggestive of **lead poisoning**.
- **Lead** primarily inhibits two key enzymes in the heme synthesis pathway: **ferrochelatase** and **ALA dehydratase**. Inhibition of ferrochelatase prevents the incorporation of iron into protoporphyrin IX, leading to an accumulation of protoporphyrin and iron.
*Inhibition of ALA synthase*
- **ALA synthase** is the **rate-limiting enzyme** in heme synthesis, and its inhibition would lead to a reduction in heme production.
- While lead poisoning does impair heme synthesis, its primary and most impactful targets are ALA dehydratase and ferrochelatase, rather than ALA synthase, which is generally not directly inhibited by lead.
*Activation of glutathione*
- **Glutathione** is a critical antioxidant that helps protect cells from **oxidative damage**.
- While glutathione plays a role in cellular defense against toxins, lead poisoning typically *reduces* glutathione levels and causes oxidative stress, rather than activating glutathione in a manner that would explain the symptoms.
*Activation of ALA dehydratase*
- **ALA dehydratase** is an enzyme in the heme synthesis pathway that converts delta-aminolevulinic acid (ALA) to porphobilinogen.
- Lead actually **inhibits** ALA dehydratase, not activates it. Inhibition of this enzyme leads to an accumulation of ALA, contributing to the neurotoxic effects of lead.
*Inactivation of uroporphyrinogen III cosynthase*
- **Uroporphyrinogen III cosynthase** is an enzyme involved in converting hydroxymethylbilane to uroporphyrinogen III in the heme synthesis pathway.
- While defects in this enzyme cause **congenital erythropoietic porphyria**, it is not a direct target or primary mechanism of lead toxicity.
Question 48: A 43-year-old woman visits her primary care provider complaining of fatigue. Although she has had it for several months, her fatigue has been worsening over the past few weeks. She has no other symptoms. Past medical history is significant for hypertension. She takes chlorthalidone, an oral contraceptive pill, and a multivitamin every day. Family history is noncontributory. She drinks about 1 bottle of wine every day and started taking a shot or two of whisky or vodka every morning before work to “clear out the cobwebs”. She was recently fired from her job. Today, her heart rate is 90/min, respiratory rate is 17/min, blood pressure is 110/65 mm Hg, and temperature is 36.7°C (98.1°F). On physical exam, she appears malnourished and anxious. Her conjunctiva are pale, and glossitis is noted on oral exam. Her heart has a regular rate and rhythm and her lungs are clear to auscultation bilaterally. She has no gait or balance abnormalities. Lab results show a hemoglobin of 10 g/dL, with a mean corpuscular volume (MCV) of 108 fl. Elevated level of which of the following will most likely to be found in this patient?
A. Methionine
B. Phenylalanine
C. Cysteine
D. Homocysteine
E. Methylmalonic acid (Correct Answer)
Explanation: ***Methylmalonic acid***
- The patient's presentation with **macrocytic anemia** (MCV 108 fl, Hb 10 g/dL), **glossitis**, and **fatigue** suggests a **vitamin B12 deficiency**.
- **Elevated methylmalonic acid (MMA)** is a sensitive and specific marker for **vitamin B12 deficiency**, as **B12** acts as a cofactor for the enzyme **methylmalonyl-CoA mutase**, which converts MMA to succinyl-CoA.
*Methionine*
- **Methionine** is an essential amino acid, and its levels are not directly elevated in **vitamin B12 deficiency**; rather, **B12** is involved in the conversion of **homocysteine to methionine**.
- In **B12 deficiency**, the conversion of **homocysteine to methionine** is impaired, leading to **elevated homocysteine**, not necessarily methionine.
*Phenylalanine*
- **Phenylalanine** is an essential amino acid whose metabolism is primarily linked to **phenylketonuria (PKU)** if its levels are elevated due to deficient **phenylalanine hydroxylase**, which is unrelated to **vitamin B12 deficiency**.
- Elevated phenylalanine would not be an expected finding in this patient's clinical scenario.
*Cysteine*
- **Cysteine** is a non-essential amino acid, and its levels are not typically elevated as a direct consequence or marker of **vitamin B12 deficiency**.
- While involved in various metabolic pathways, it doesn't serve as a diagnostic indicator for B12 status.
*Homocysteine*
- While **homocysteine levels are elevated in vitamin B12 deficiency**, as B12 is a cofactor for **methionine synthase**, which converts homocysteine to methionine, **methylmalonic acid** is a more specific marker for **B12 deficiency**.
- **Elevated homocysteine** can also be caused by **folate deficiency** or **vitamin B6 deficiency**, making it less specific than **MMA** for isolating B12 deficiency.
Question 49: A 28-year-old man is brought in by ambulance to the ER, barely conscious, after feeling drowsy and falling to the floor during a presentation several hours ago. His colleague who accompanied him says he has had similar episodes 5 times in the past 3 months. No significant past medical history. His blood pressure is 110/80 mm Hg and pulse is 114/min. His capillary blood glucose is 15 mg/dL. Immediate IV dextrose with thiamine is started, and he rapidly regains consciousness. A contrast CT of the abdomen is performed which reveals a tumor in the pancreas. Which of the following relative laboratory findings would you most likely expect to find in this patient?
A. Glucose: ↑, Insulin: ↓, C-Peptide: ↓, Ketoacidosis: Present
B. Glucose: Normal, Insulin: Normal, C-Peptide: Normal, Ketoacidosis: Absent
D. Glucose: ↓, Insulin: ↑, C-Peptide: ↓, Ketoacidosis: Absent
E. Glucose: ↑, Insulin: ↑/Normal, C-Peptide: ↑/Normal, Ketoacidosis: Absent
Explanation: ***Glucose: ↓, Insulin: ↑, C-Peptide: ↑, Ketoacidosis: Absent***
- The patient's **hypoglycemia (15 mg/dL)**, coupled with a pancreatic tumor and recurrent episodes, strongly suggests an **insulinoma**.
- An **insulinoma** is an insulin-secreting tumor, leading to **high insulin** and **C-peptide** levels in the presence of low glucose, and typically no ketoacidosis because insulin inhibits ketogenesis.
*Glucose: ↑, Insulin: ↓, C-Peptide: ↓, Ketoacidosis: Present*
- This profile describes **Type 1 Diabetes Mellitus** or severe insulin deficiency, where high glucose is due to lack of insulin production and subsequent diabetic ketoacidosis.
- The patient's symptoms (hypoglycemia) and the presence of a pancreatic tumor producing insulin are contradictory to this profile.
*Glucose: Normal, Insulin: Normal, C-Peptide: Normal, Ketoacidosis: Absent*
- This profile represents a **healthy individual** with normal metabolic function, which is inconsistent with the patient's severe hypoglycemia and recurrent collapses.
- It would not explain the patient's symptoms or the pancreatic tumor's function.
*Glucose: ↓, Insulin: ↑, C-Peptide: ↓, Ketoacidosis: Absent*
- This finding would be typical of **exogenous insulin administration** (e.g., insulin overdose) where insulin levels are high, but C-peptide (which is co-secreted with endogenous insulin) is low.
- While hypoglycemia is present, the low C-peptide contradicts the presence of an endogenous insulin-secreting pancreatic tumor.
*Glucose: ↑, Insulin: ↑/Normal, C-Peptide: ↑/Normal, Ketoacidosis: Absent*
- These findings could be seen in conditions like **Type 2 Diabetes** with **insulin resistance** or Cushing's syndrome where glucose and insulin might be elevated, but the patient's primary presentation is severe hypoglycemia.
- This profile does not align with the patient's profound hypoglycemia and clinical picture of an insulinoma.
Question 50: An investigator is studying the activity level of several different enzymes in human subjects from various demographic groups. An elevated level of activity of phosphoribosyl pyrophosphate synthetase is found in one of the study subjects. This patient is most likely to have which of the following conditions?
A. Phenylketonuria
B. Homocystinuria
C. Gout (Correct Answer)
D. Alkaptonuria
E. Maple syrup urine disease
Explanation: ***Gout***
- **Elevated phosphoribosyl pyrophosphate (PRPP) synthetase activity** leads to increased production of PRPP, a precursor for **purine biosynthesis**.
- Increased purine synthesis results in overproduction of **uric acid**, which can precipitate as monosodium urate crystals in joints, causing **gout**.
*Phenylketonuria*
- Caused by a deficiency in **phenylalanine hydroxylase**, leading to an accumulation of **phenylalanine**.
- Not directly related to increased PRPP synthetase activity or purine metabolism.
*Homocystinuria*
- Primarily due to a deficiency in **cystathionine beta-synthase**, leading to elevated levels of **homocysteine**.
- This condition involves methionine metabolism, not purine metabolism or PRPP synthetase.
*Alkaptonuria*
- Results from a deficiency in **homogentisate 1,2-dioxygenase**, causing the accumulation of **homogentisic acid**.
- It is an inborn error of tyrosine metabolism and is unrelated to PRPP synthetase activity.
*Maple syrup urine disease*
- Caused by a deficiency in the **branched-chain alpha-keto acid dehydrogenase complex**, leading to accumulation of **leucine, isoleucine, and valine**.
- This condition affects branched-chain amino acid metabolism, not purine metabolism.
