A 59-year-old man comes to the physician because of a 6-month history of numbness and burning sensation in his feet that is worse at rest. He has not been seen by a physician in several years. He is 178 cm (5 ft 10 in) tall and weighs 118 kg (260 lb); BMI is 37.3 kg/m2. Physical examination shows decreased sensation to pinprick, light touch, and vibration over the soles of both feet. Ankle jerk is 1+ bilaterally. His hemoglobin A1C concentration is 10.2%. Which of the following pathophysiological processes is most likely to be involved in this patient's condition?
Q182
A 7-year-old boy is brought to the hospital for evaluation, he is accompanied by agents from child protective services after he was rescued from a home where he was being neglected. He was found locked in a closet and says that he was fed only once every 2 days for the past month. On presentation, he is found to be extremely emaciated with protruding ribs and prominent joints. He is provided with an appropriate rehydration and nourishment therapy. Despite his prolonged nutritional deprivation, the patient demonstrates appropriate cognitive function for his age. The transporter responsible for preventing cognitive decline in this malnourished patient has which of the following characteristics?
Q183
A 36-year-old woman is fasting prior to a religious ceremony. Her only oral intake in the last 36 hours has been small amounts of water. The metabolic enzyme that is primarily responsible for maintaining normal blood glucose in this patient is located exclusively within the mitochondria. An increase in which of the following substances is most likely to increase the activity of this enzyme?
Q184
A neonate is noted to have very light skin, light blue eyes, and sparse blonde-white hair. The family states that the baby is much lighter in appearance than anyone else in the family. Both parents are Fitzpatrick skin type III with dark brown hair. On further exam, the baby's temperature is 98.4°F (36.9°C), blood pressure is 110/70 mmHg, pulse is 88/min, and respirations are 14/min. The patient is oxygenating well at SpO2 of 97% on room air with no respiratory distress. All reflexes are appropriate, and the APGAR score is 10. A referral is placed with Ophthalmology for a comprehensive eye exam. The condition is believed to be due to an enzyme deficiency, and a hair bulb assay is performed. Which of the following substrates should be incubated with the specimen in order to determine the activity of the enzyme in question for this disease?
Q185
A 40-year-old male with Down syndrome is brought to your clinic by his mother. She reports that over the past few months he has started having difficulty managing his daily routine at his assisted-living facility and no longer seems like himself. She says that last week he wandered away from the facility and was brought back by police. Additionally, he has stopped taking his regular antiepileptic medication, and she is concerned that he might have a seizure. TSH is checked and is normal. Which of the following is most likely to be responsible for this man's current presentation?
Q186
A 66-year-old homeless HIV-positive male presents with numbness in his hands and feet. The patient says that his symptoms started gradually a couple weeks ago and have slowly worsened. He describes numbness initially in just his fingertips and toes but it has now spread to involve his entire hands and feet. Past medical history is significant for HIV diagnosed many years ago, for which the patient has never sought treatment. The patient also has a long history of various illnesses, especially chronic diarrhea, but he is unable to remember any details. He currently takes no medications. The patient has been homeless for years, and he denies any alcohol or drug use. Review of systems is significant for a sore tongue. His temperature is 37°C (98.6°F), blood pressure is 100/65 mm Hg, pulse is 102/min, respiratory rate is 25/min, and oxygen saturation is 97% on room air. On physical exam, the patient is alert and oriented, his body habitus is cachectic, and his BMI is 17 kg/m2. His tongue appears erythematous and smooth with loss of papillae, but no lesions or evidence of infection is noted. Cardiac exam is normal apart from tachycardia. Lungs are clear to auscultation. His abdomen is soft and nontender with no hepatosplenomegaly. There is decreased 2-point discrimination in the hands and feet bilaterally. Strength in the hands and feet is 4/5 bilaterally. Reflexes are absent in the ankles. Gait is slightly wide-based and ataxic, and there is a positive Romberg sign. Which of the following is the most likely cause of this patient’s symptoms?
Q187
A 1-month-old boy is brought to the emergency department 25 minutes after having a seizure. His mother reports that he has become lethargic and does not cry as vigorously anymore. Examination shows muscular hypotonia and hepatomegaly. Arterial blood gas on room air shows metabolic acidosis. Serum studies show elevated levels of methylmalonic acid. A deficiency of which of the following types of enzymes is the most likely cause of this patient's condition?
Q188
A 31-year-old male with cirrhosis, dementia, and Parkinson-like symptoms is diagnosed with a hereditary metabolic disease resulting from the accumulation of a certain metal in various tissues. Impairment of which of the following elimination pathways is most likely responsible?
Q189
Researchers are investigating the effects of an Amazonian plant extract as a novel therapy for certain types of tumors. When applied to tumor cells in culture, the extract causes widespread endoplasmic reticulum stress and subsequent cell death. Further experiments show that the extract acts on an important member of a protein complex that transduces proliferation signals. When this protein alone is exposed to the plant extract, its function is not recovered by the addition of chaperones. Which type of bond is the extract most likely targeting?
Q190
A 66-year-old man is brought to the emergency department because of weakness of his left leg for the past 30 minutes. His pants are soaked with urine. He has hypertension and atrial fibrillation. His temperature is 37°C (98.6°F), pulse is 98/min, and blood pressure is 160/90 mm Hg. Examination shows equal pupils that are reactive to light. Muscle strength is 2/5 in the left lower extremity. Plantar reflex shows an extensor response on the left. Within one minute of the onset of this patient's symptoms, the cells in his right anteromedial cortical surface enlarge significantly. Which of the following is the most likely explanation of the described cellular change?
Metabolism US Medical PG Practice Questions and MCQs
Question 181: A 59-year-old man comes to the physician because of a 6-month history of numbness and burning sensation in his feet that is worse at rest. He has not been seen by a physician in several years. He is 178 cm (5 ft 10 in) tall and weighs 118 kg (260 lb); BMI is 37.3 kg/m2. Physical examination shows decreased sensation to pinprick, light touch, and vibration over the soles of both feet. Ankle jerk is 1+ bilaterally. His hemoglobin A1C concentration is 10.2%. Which of the following pathophysiological processes is most likely to be involved in this patient's condition?
A. Lymphocytic infiltration of islet cells
B. Increased production of adiponectin by adipocytes
C. Complement-mediated destruction of insulin receptors
D. Accumulation of islet amyloid polypeptide (Correct Answer)
E. Expression of human leukocyte antigen subtype DR4
Explanation: ***Accumulation of islet amyloid polypeptide***
- The patient's presentation of **obesity (BMI 37.3 kg/m²)**, **neuropathy** (numbness, burning in feet, decreased sensation), and a **high HbA1C (10.2%)** are classic features of **Type 2 Diabetes Mellitus**.
- **Islet amyloid polypeptide (IAPP)**, or amylin, is co-secreted with insulin from pancreatic β-cells. In Type 2 Diabetes, IAPP can misfold and form **toxic amyloid fibrils** that accumulate in pancreatic islets, contributing to β-cell dysfunction and death.
*Lymphocytic infiltration of islet cells*
- This process is characteristic of **Type 1 Diabetes Mellitus**, where there is autoimmune destruction of pancreatic β-cells.
- The patient's age (59 years) and obesity are more consistent with Type 2 Diabetes, making Type 1 less likely.
*Increased production of adiponectin by adipocytes*
- **Adiponectin** is an adipokine with anti-inflammatory and insulin-sensitizing properties; its levels are typically **decreased** in obesity and Type 2 Diabetes, not increased.
- Increased adiponectin would generally be protective against insulin resistance, which contradicts the patient's presentation.
*Complement-mediated destruction of insulin receptors*
- While there are rare cases of **Type B insulin resistance syndrome** involving autoantibodies to insulin receptors, this is not a typical pathophysiological process in the common form of Type 2 Diabetes.
- The primary defect in Type 2 Diabetes is usually a combination of **insulin resistance** at target tissues and progressive β-cell dysfunction.
*Expression of human leukocyte antigen subtype DR4*
- **HLA-DR4** is a genetic susceptibility factor strongly associated with **Type 1 Diabetes Mellitus**, not Type 2.
- Its presence indicates an increased risk for autoimmune conditions, specifically autoimmune destruction of pancreatic beta cells.
Question 182: A 7-year-old boy is brought to the hospital for evaluation, he is accompanied by agents from child protective services after he was rescued from a home where he was being neglected. He was found locked in a closet and says that he was fed only once every 2 days for the past month. On presentation, he is found to be extremely emaciated with protruding ribs and prominent joints. He is provided with an appropriate rehydration and nourishment therapy. Despite his prolonged nutritional deprivation, the patient demonstrates appropriate cognitive function for his age. The transporter responsible for preventing cognitive decline in this malnourished patient has which of the following characteristics?
A. Has high affinity for fructose
B. Transports glucose against its concentration gradient
C. Facilitates insulin release
D. Responsive to insulin
E. Has high affinity for glucose (Correct Answer)
Explanation: **Has high affinity for glucose**
- The brain relies almost exclusively on **glucose** as its energy source, even during prolonged starvation. The **GLUT1 transporter** in the blood-brain barrier has a high affinity for glucose, ensuring a constant supply to the brain to maintain cognitive function.
- This high affinity is crucial for providing the brain with sufficient glucose even when plasma glucose levels are low due to malnutrition, thereby **preventing cognitive decline**.
*Has high affinity for fructose*
- While fructose is metabolized in the liver, it is not a primary energy source for the brain. The brain utilizes **glucose** almost exclusively.
- Transporters with high affinity for fructose (like **GLUT5**) are primarily found in the small intestine for absorption, not at the blood-brain barrier for brain energy supply.
*Transports glucose against its concentration gradient*
- The transport of glucose into the brain via **GLUT1** occurs by **facilitated diffusion**, which is down its concentration gradient.
- **Active transport** against a concentration gradient typically requires energy (e.g., ATP) and would be mediated by different types of transporters (e.g., SGLT), which are not primarily responsible for glucose uptake into the brain.
*Facilitates insulin release*
- **Insulin release** from pancreatic beta cells is facilitated by **GLUT2 transporters**, which have a low affinity for glucose but a high capacity.
- The transporter responsible for maintaining cognitive function in the brain (**GLUT1**) is largely **insulin-independent** in its function.
*Responsive to insulin*
- The primary glucose transporter in the brain, **GLUT1**, is **not responsive to insulin**. Its expression and activity are constitutive, ensuring constant glucose uptake regardless of insulin levels.
- **Insulin-responsive glucose transporters** (e.g., **GLUT4**) are found in skeletal muscle and adipose tissue, which take up glucose in response to insulin signaling.
Question 183: A 36-year-old woman is fasting prior to a religious ceremony. Her only oral intake in the last 36 hours has been small amounts of water. The metabolic enzyme that is primarily responsible for maintaining normal blood glucose in this patient is located exclusively within the mitochondria. An increase in which of the following substances is most likely to increase the activity of this enzyme?
A. Acetyl coenzyme A (Correct Answer)
B. Citrate
C. Adenosine monophosphate
D. Glucagon
E. Oxidized nicotinamide adenine dinucleotide
Explanation: ***Acetyl coenzyme A***
- The enzyme described is **pyruvate carboxylase**, which is exclusively mitochondrial and plays a crucial anaplerotic role in gluconeogenesis by converting pyruvate to **oxaloacetate**.
- **Acetyl CoA** is an allosteric activator of **pyruvate carboxylase**, signaling a high energy state and readiness for glucose synthesis from non-carbohydrate precursors.
*Citrate*
- **Citrate** is an allosteric inhibitor of **phosphofructokinase-1 (PFK-1)** in glycolysis and can activate **acetyl-CoA carboxylase** in fatty acid synthesis.
- It does not directly activate pyruvate carboxylase.
*Adenosine monophosphate*
- **AMP** is a marker of low energy status, activating **AMP-activated protein kinase (AMPK)** and **phosphofructokinase-1 (PFK-1)**, thereby stimulating glycolysis.
- Its role is to increase glucose utilization, not glucose synthesis.
*Glucagon*
- **Glucagon** is a hormone that *regulates* gluconeogenesis by signaling through GPCRs and increasing cAMP, leading to phosphorylation and activation of key gluconeogenic enzymes.
- However, glucagon itself is a signaling molecule, not a direct positive allosteric modulator of pyruvate carboxylase activity.
*Oxidized nicotinamide adenine dinucleotide*
- **NAD+** is a coenzyme primarily involved in oxidative reactions, acting as an electron acceptor and is crucial for the function of enzymes like **glyceraldehyde-3-phosphate dehydrogenase** in glycolysis or **isocitrate dehydrogenase** in the TCA cycle.
- It is a substrate for various dehydrogenases, but not a direct allosteric activator of pyruvate carboxylase.
Question 184: A neonate is noted to have very light skin, light blue eyes, and sparse blonde-white hair. The family states that the baby is much lighter in appearance than anyone else in the family. Both parents are Fitzpatrick skin type III with dark brown hair. On further exam, the baby's temperature is 98.4°F (36.9°C), blood pressure is 110/70 mmHg, pulse is 88/min, and respirations are 14/min. The patient is oxygenating well at SpO2 of 97% on room air with no respiratory distress. All reflexes are appropriate, and the APGAR score is 10. A referral is placed with Ophthalmology for a comprehensive eye exam. The condition is believed to be due to an enzyme deficiency, and a hair bulb assay is performed. Which of the following substrates should be incubated with the specimen in order to determine the activity of the enzyme in question for this disease?
A. Dihydroxyphenylalanine (Correct Answer)
B. Pyridoxine
C. Tetrahydrobiopterin
D. Dopamine
E. Homogentisic Acid
Explanation: ***Dihydroxyphenylalanine***
- The clinical presentation of **hypopigmentation** (light skin, blue eyes, blonde-white hair), particularly in contrast to the parents' darker features, points towards **oculocutaneous albinism**.
- **Oculocutaneous albinism** is most commonly caused by a deficiency in **tyrosinase**, an enzyme that converts **tyrosine** to **dihydroxyphenylalanine (DOPA)**, which is essential for melanin synthesis. Therefore, incubating with DOPA helps assess tyrosinase activity.
*Pyridoxine*
- **Pyridoxine (vitamin B6)** is a cofactor for many enzymes, particularly those involved in amino acid metabolism, but it is not directly involved in the melanin synthesis pathway or used to test for albinism.
- Deficiency can lead to neurological symptoms and anemia, which are not suggested by the neonate's presentation.
*Tetrahydrobiopterin*
- **Tetrahydrobiopterin (BH4)** is a cofactor for several hydroxylase enzymes, including phenylalanine hydroxylase, which is deficient in **phenylketonuria (PKU)**.
- While PKU can cause hypopigmentation due to a secondary decrease in tyrosine, the primary enzyme deficiency in albinism is tyrosinase itself, not a defect in BH4 metabolism.
*Dopamine*
- **Dopamine** is a neurotransmitter and a product of DOPA decarboxylation, further downstream from the tyrosinase reaction in the metabolic pathway.
- It is not used as a substrate to test for tyrosinase activity in the diagnosis of albinism.
*Homogentisic Acid*
- **Homogentisic acid** is an intermediate in the catabolism of tyrosine and phenylalanine, primarily associated with **alkaptonuria**.
- Alkaptonuria is characterized by dark urine upon standing and eventually ochronosis, which are entirely different from the hypopigmentation seen in albinism.
Question 185: A 40-year-old male with Down syndrome is brought to your clinic by his mother. She reports that over the past few months he has started having difficulty managing his daily routine at his assisted-living facility and no longer seems like himself. She says that last week he wandered away from the facility and was brought back by police. Additionally, he has stopped taking his regular antiepileptic medication, and she is concerned that he might have a seizure. TSH is checked and is normal. Which of the following is most likely to be responsible for this man's current presentation?
A. Premature degradation of a protein
B. Abnormal protein metabolism (Correct Answer)
C. Hormone deficiency
D. Nutritional deficiency
E. Expansion of trinucleotide repeats
Explanation: ***Abnormal protein metabolism***
- Individuals with **Down syndrome (trisomy 21)** are at a significantly higher risk of developing early-onset **Alzheimer's disease** due to an extra copy of the **APP gene** on chromosome 21.
- **Amyloid Precursor Protein (APP)** metabolism is altered, leading to excessive production and aggregation of **beta-amyloid plaques**, which is a hallmark of Alzheimer's pathology and causes the cognitive decline observed.
*Premature degradation of a protein*
- This option points to a general protein issue but doesn't specifically address the **pathophysiology of Alzheimer's** in Down syndrome, which is primarily about abnormal production and aggregation, not premature degradation.
- While protein degradation pathways are involved in neurodegenerative diseases, the core problem in Alzheimer's relates to the **accumulation of misfolded proteins**.
*Hormone deficiency*
- The patient's **TSH is normal**, ruling out **hypothyroidism**, which can cause cognitive changes and is more common in Down syndrome.
- While other hormonal imbalances could affect cognitive function, the significant and progressive decline described is more consistent with a **neurodegenerative process** than a general hormone deficiency.
*Nutritional deficiency*
- While nutritional deficiencies can certainly impact cognitive function and behavior, the patient's age, underlying condition (**Down syndrome**), and the specific pattern of progressive decline strongly suggest a **neurodegenerative cause** rather than a primary nutritional deficiency.
- There is no specific information in the vignette to point toward a nutritional deficiency.
*Expansion of trinucleotide repeats*
- **Trinucleotide repeat expansion disorders** (e.g., Huntington's disease, Fragile X syndrome) cause a variety of neurological and psychiatric symptoms.
- However, the patient's presentation in the context of Down syndrome is characteristic of **early-onset Alzheimer's disease**, which is not caused by trinucleotide repeat expansion.
Question 186: A 66-year-old homeless HIV-positive male presents with numbness in his hands and feet. The patient says that his symptoms started gradually a couple weeks ago and have slowly worsened. He describes numbness initially in just his fingertips and toes but it has now spread to involve his entire hands and feet. Past medical history is significant for HIV diagnosed many years ago, for which the patient has never sought treatment. The patient also has a long history of various illnesses, especially chronic diarrhea, but he is unable to remember any details. He currently takes no medications. The patient has been homeless for years, and he denies any alcohol or drug use. Review of systems is significant for a sore tongue. His temperature is 37°C (98.6°F), blood pressure is 100/65 mm Hg, pulse is 102/min, respiratory rate is 25/min, and oxygen saturation is 97% on room air. On physical exam, the patient is alert and oriented, his body habitus is cachectic, and his BMI is 17 kg/m2. His tongue appears erythematous and smooth with loss of papillae, but no lesions or evidence of infection is noted. Cardiac exam is normal apart from tachycardia. Lungs are clear to auscultation. His abdomen is soft and nontender with no hepatosplenomegaly. There is decreased 2-point discrimination in the hands and feet bilaterally. Strength in the hands and feet is 4/5 bilaterally. Reflexes are absent in the ankles. Gait is slightly wide-based and ataxic, and there is a positive Romberg sign. Which of the following is the most likely cause of this patient’s symptoms?
A. Autoimmune reaction
B. Elevated levels of methylmalonic acid (MMA) (Correct Answer)
C. Disseminated bacterial infection
D. Deposition of an insoluble protein
E. Poorly controlled, undiagnosed diabetes
Explanation: ***Elevated levels of methylmalonic acid (MMA)***
- The patient's symptoms, including **numbness in hands and feet** (stocking-glove neuropathy), **ataxic gait**, positive **Romberg sign**, and **glossitis** (sore, smooth tongue with loss of papillae), are classic signs of **vitamin B12 deficiency**.
- **Vitamin B12 deficiency** leads to elevated **methylmalonic acid (MMA)** and **homocysteine** levels, which are neurotoxic and impair myelin synthesis, causing subacute combined degeneration of the spinal cord and peripheral neuropathy.
*Autoimmune reaction*
- While autoimmune conditions can cause neuropathy, the constellation of symptoms, especially **glossitis** and the specific neurological findings like **ataxic gait** and **absent ankle reflexes**, points more directly to a nutritional deficiency in this **HIV-positive, cachectic patient** with a history of chronic diarrhea.
- There are no specific indicators in the patient's presentation, such as recent vaccination or infection, to suggest an acute autoimmune demyelinating condition like Guillain-Barré syndrome, and chronic inflammatory demyelinating polyneuropathy (CIDP) is less likely given the prominent glossitis.
*Disseminated bacterial infection*
- A disseminated bacterial infection would typically present with more prominent systemic signs of infection, such as **fever, chills, and organ dysfunction**, which are not a primary focus in this patient's presentation.
- While some bacterial infections (e.g., Lyme disease) can cause neuropathy, they do not explain the **glossitis** and specific pattern of neurological deficits observed.
*Deposition of an insoluble protein*
- This option refers to conditions like **amyloidosis**, where abnormal proteins deposit in tissues and organs, including nerves. While amyloidosis can cause neuropathy, it typically presents with other systemic symptoms like **cardiomyopathy, nephropathy, or hepatomegaly**, which are not described.
- The patient's **glossitis** and specific neurological pattern are not characteristic of amyloid neuropathy.
*Poorly controlled, undiagnosed diabetes*
- Diabetic neuropathy commonly presents as **stocking-glove sensory loss** and can cause **ataxia**, but it does not cause **glossitis** or a prominent **positive Romberg sign** in the absence of significant motor weakness.
- Although the patient is homeless and has poor health, there is no mention of classic diabetic symptoms like **polyuria, polydipsia, or unexplained weight loss**, or risk factors like obesity, although HIV does increase the risk of diabetes.
Question 187: A 1-month-old boy is brought to the emergency department 25 minutes after having a seizure. His mother reports that he has become lethargic and does not cry as vigorously anymore. Examination shows muscular hypotonia and hepatomegaly. Arterial blood gas on room air shows metabolic acidosis. Serum studies show elevated levels of methylmalonic acid. A deficiency of which of the following types of enzymes is the most likely cause of this patient's condition?
A. Phosphatase
B. Carboxylase
C. Hydroxylase
D. Mutase (Correct Answer)
E. Phosphorylase
Explanation: ***Mutase***
- Elevated **methylmalonic acid** levels are characteristic of a deficiency in **methylmalonyl-CoA mutase**, a **vitamin B12 (adenosylcobalamin)-dependent enzyme** responsible for converting methylmalonyl-CoA to succinyl-CoA in the TCA cycle.
- This deficiency leads to **methylmalonic acidemia**, a metabolic disorder presenting with **seizures**, **lethargy**, **hypotonia**, **hepatomegaly**, and **metabolic acidosis** in infancy.
*Phosphatase*
- **Phosphatases** remove phosphate groups from molecules and are involved in various metabolic pathways and signaling processes.
- Deficiencies in phosphatases are associated with conditions like **glycogen storage diseases** (e.g., glucose-6-phosphatase deficiency in von Gierke's disease), which primarily manifest with **hypoglycemia** and hepatomegaly, not typically isolated high methylmalonic acid.
*Carboxylase*
- **Carboxylases** are enzymes that add a carboxyl group to a molecule, often using **biotin** as a cofactor.
- Deficiencies in carboxylases (e.g., pyruvate carboxylase, propionyl-CoA carboxylase) can cause metabolic acidosis and other neurological symptoms, but they typically result in elevated levels of different metabolites such as **pyruvate** or **propionic acid**, not primarily methylmalonic acid.
*Hydroxylase*
- **Hydroxylases** add a hydroxyl group (-OH) to a compound and are critical in many metabolic pathways, including steroid synthesis and detoxification.
- Deficiencies in hydroxylase enzymes (e.g., **21-hydroxylase deficiency** in congenital adrenal hyperplasia) lead to distinct syndromes with hormonal imbalances rather than the specific biochemical profile seen here.
*Phosphorylase*
- **Phosphorylases** cleave a bond by adding a phosphate group and are crucial in carbohydrate metabolism, particularly glycogenolysis.
- **Glycogen phosphorylase** deficiency (e.g., McArdle's disease or Hers disease) causes exercise intolerance or hepatomegaly and hypoglycemia, but not the elevated methylmalonic acid or comprehensive symptoms observed in this case.
Question 188: A 31-year-old male with cirrhosis, dementia, and Parkinson-like symptoms is diagnosed with a hereditary metabolic disease resulting from the accumulation of a certain metal in various tissues. Impairment of which of the following elimination pathways is most likely responsible?
A. Glomerular filtration
B. Bleeding
C. Loop of Henle secretion into lumen of kidney
D. Secretion into bile (Correct Answer)
E. Duodenal secretion
Explanation: ***Secretion into bile***
- The constellation of **cirrhosis**, **dementia**, and **Parkinson-like symptoms** in a young male strongly suggests **Wilson's disease**, an autosomal recessive disorder of **copper metabolism**.
- Wilson's disease results from a defect in the **ATP7B gene**, leading to impaired incorporation of copper into **ceruloplasmin** and, critically, impaired **biliary excretion of copper**, causing its accumulation in the liver, brain, and other organs.
*Glomerular filtration*
- While essential for waste removal, **glomerular filtration** primarily handles water-soluble small molecules and is not the primary physiological pathway for the elimination of larger, protein-bound metal ions like copper.
- Impairment of glomerular filtration would typically lead to symptoms of **renal failure**, which are not the primary features described here.
*Bleeding*
- While severe bleeding could lead to the loss of blood components including some metals, it is not a physiological or primary elimination pathway for systemic metal excess.
- Furthermore, **significant blood loss** would manifest with acute symptoms like hypovolemia and anemia, unrelated to the chronic accumulation seen in Wilson's disease.
*Loop of Henle secretion into lumen of kidney*
- The Loop of Henle is primarily involved in **water and salt reabsorption** and concentration of urine, not active secretion of metals.
- While the kidneys do play a role in copper excretion, the primary defect in Wilson's disease is in **biliary excretion**, not renal tubular secretion.
*Duodenal secretion*
- The duodenum is primarily involved in **absorption of nutrients** and regulation of gastric emptying, not active secretion of metals as a major elimination pathway.
- While some gastrointestinal excretion can occur, **biliary excretion** via the liver is the predominant route for copper elimination in the normal physiological state.
Question 189: Researchers are investigating the effects of an Amazonian plant extract as a novel therapy for certain types of tumors. When applied to tumor cells in culture, the extract causes widespread endoplasmic reticulum stress and subsequent cell death. Further experiments show that the extract acts on an important member of a protein complex that transduces proliferation signals. When this protein alone is exposed to the plant extract, its function is not recovered by the addition of chaperones. Which type of bond is the extract most likely targeting?
A. Hydrophobic interactions
B. Covalent bonds between carboxyl and amino groups
C. Ionic bonds
D. Hydrogen bonds
E. Covalent bond between two sulfur atoms (disulfide bonds) (Correct Answer)
Explanation: ***Covalent bond between two sulfur atoms (disulfide bonds)***
- The extract causes **widespread endoplasmic reticulum (ER) stress** and **cell death**, which is consistent with the disruption of protein folding and stability, a primary role of disulfide bonds in many extracellular and secreted proteins processed in the ER.
- The fact that the protein's function is **not recovered by chaperones** suggests a strong, irreversible alteration of its structure, which is characteristic of the breaking or formation of **covalent disulfide bonds** that permanently change the protein's conformation.
*Hydrophobic interactions*
- While important for protein folding, disruption of **hydrophobic interactions** might be partially reversible by chaperones, as they assist in refolding proteins by shielding exposed hydrophobic regions.
- Their disruption alone is unlikely to lead to the **irreversible loss of function** described unless severe aggregation occurs, which isn't directly implied as the primary target.
*Covalent bonds between carboxyl and amino groups*
- These are **peptide bonds**, the fundamental links forming the **polypeptide backbone**. Breaking these bonds would lead to protein fragmentation, not just a loss of function in a folded protein, which would be a much more drastic and non-specific effect.
- If targeting peptide bonds, chaperones would be irrelevant as the protein itself would be destroyed.
*Ionic bonds*
- **Ionic bonds** are electrostatic interactions between charged groups on amino acids, crucial for tertiary and quaternary structure.
- While their disruption can denature a protein, it's often reversible, and chaperones can assist in refolding proteins even after significant ionic bond changes.
*Hydrogen bonds*
- **Hydrogen bonds** are weaker, non-covalent interactions vital for secondary (alpha-helices, beta-sheets) and tertiary protein structure.
- Their disruption is a common feature of denaturation, but the process is frequently reversible, and **chaperone proteins** are specifically designed to help reform and stabilize these bonds to correct misfolded proteins.
Question 190: A 66-year-old man is brought to the emergency department because of weakness of his left leg for the past 30 minutes. His pants are soaked with urine. He has hypertension and atrial fibrillation. His temperature is 37°C (98.6°F), pulse is 98/min, and blood pressure is 160/90 mm Hg. Examination shows equal pupils that are reactive to light. Muscle strength is 2/5 in the left lower extremity. Plantar reflex shows an extensor response on the left. Within one minute of the onset of this patient's symptoms, the cells in his right anteromedial cortical surface enlarge significantly. Which of the following is the most likely explanation of the described cellular change?
A. Breakdown of the cell membrane
B. Release of pro-apoptotic proteins
C. Intracellular depletion of ATP (Correct Answer)
D. Influx of extracellular calcium
E. Rupture of lysosomes
Explanation: ***Intracellular depletion of ATP***
- The patient's symptoms (sudden left leg weakness, incontinence, hypertension, atrial fibrillation) suggest an **acute ischemic stroke**. Ischemia rapidly leads to a lack of oxygen and nutrients, impairing oxidative phosphorylation and thus causing a significant and rapid **depletion of ATP**.
- Without sufficient ATP, the **Na+/K+-ATPase pump** fails, leading to an intracellular accumulation of sodium and water, causing cellular swelling (enlargement) within minutes of the insult.
*Breakdown of the cell membrane*
- While cell membrane breakdown is a feature of irreversible cell injury and **necrosis**, it typically occurs later in the ischemic cascade, not within the first minute of the insult.
- Initial changes involve membrane pump dysfunction due to ATP depletion, followed by membrane damage.
*Release of pro-apoptotic proteins*
- The release of **pro-apoptotic proteins** is characteristic of apoptosis, a form of programmed cell death, which is a slower process than the rapid cellular swelling seen in acute ischemia.
- Apoptosis generally takes hours to days to become evident, unlike the immediate cellular changes due to ATP depletion.
*Influx of extracellular calcium*
- An **influx of extracellular calcium** into the cell is a critical event in ischemic injury, activating various enzymes and contributing to cell damage.
- However, this influx is a consequence of ATP depletion and membrane pump failure, occurring slightly downstream of the initial ATP deficit.
*Rupture of lysosomes*
- **Lysosomal rupture** releases hydrolytic enzymes that digest cellular components, contributing to irreversible cell damage and necrosis.
- This event also occurs later in the ischemic process, typically after significant membrane damage and prolonged ischemia, not within the first minute.
