Connective tissue cells and fibers — MCQs

10 questions

A 30-year-old man comes to the physician for a follow-up examination 1 month after sustaining a chemical burn over the dorsum of his right hand and forearm. Physical examination shows hyperextension of the hand at the wrist. The skin over the dorsum of the wrist is tense and there is a thick, epithelialized scar. Range of motion of the right wrist is restricted. This patient's contracture is most likely due to activity of which of the following cells?

A 15-year-old boy presents with shortness of breath on exertion for the past 2 weeks. Although he does not have any other complaints, he is concerned about not gaining much weight despite a good appetite. His height is 188 cm (6 ft 2 in) and weight is 58 kg (124 lb). His blood pressure is 134/56 mm Hg and his pulse rate is 78/min. On cardiac auscultation, his apex beat is displaced laterally with a diastolic murmur lateral to the left sternal border. Slit-lamp examination shows an upward and outward displacement of both lenses. Synthesis of which of the following proteins is most likely defective in this patient?

A 15-year-old female presents to her family physician for an annual school physical exam and check-up. She is accompanied by her mother to the visit and is present in the exam room. The patient has no complaints, and she does not have any past medical problems. She takes no medications. The patient reports that she remains active, exercising 5 times a week, and eats a healthy and varied diet. Which of the following would be the best way for the physician to obtain a more in-depth social history, including sexual history and use of alcohol, tobacco, or recreational drugs?

A 5-year-old girl accompanied by her mother presents to the emergency department after suffering a fall on the elementary school playground. Her mother reports that a child on the playground pushed her daughter who fell on her right side, after which she screamed and was found clutching her right leg. The girl's past medical history is significant for a fracture of the left femur and right radius over the past 2 years and an auditory deficit requiring hearing aid use starting 6 months ago. Inspection reveals a relatively short girl in moderate distress. She has brown opalescent teeth. She refuses to bear weight on her right lower extremity. Radiography of the right lower extremity reveals a femoral midshaft fracture. Which of the following is the most likely etiology of the patient's condition?

A 25-year-old man presents to the emergency department with sudden-onset severe pain in the right shoulder that occurred when he threw a bowling ball 2 hours ago. He has a history of dislocations in both shoulders and subluxation of the right knee and left wrist. There is no history of fractures. On physical examination, the right arm is slightly abducted and externally rotated. An anterior bulge is seen near the shoulder joint. The neurovascular examination of the right arm shows no abnormalities. The skin examination shows multiple widened atrophic scars around the knees, elbows, and ankles. The skin of the neck and around the elbow can easily be extended up to 4 cm. The sclera is white. The remainder of the physical examination shows no abnormalities. A defect in which of the following proteins is the most likely cause of the findings in this patient?

A 5-year-old boy is brought to the emergency room by his parents after slipping on a rug at home and experiencing exquisite pain and swelling of his arms. Radiographs reveal a new supracondylar fracture of the humerus, as well as indications of multiple, old fractures that have healed. His parents note that an inherited disorder is present in their family history. A comprehensive physical exam also reveals blue-tinted sclera and yellow-brown, discolored teeth. What is the etiology of the patient’s disorder?

A 62-year-old man with prostate cancer comes to the physician because of low back pain for 2 weeks and a 4.5-kg (10-lb) weight loss. Physical examination shows localized tenderness over the lumbar spine. An x-ray of the lumbar spine shows several osteoblastic lesions at the level of L2 and L4 vertebrae. Microscopic examination of a bone biopsy specimen from the L4 vertebra shows irregular bone trabeculae and star-shaped cells with long, cytoplasmic processes located deep within the lacunae. Exchange of nutrients and waste products between these cells most likely occurs through which of the following structures?

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?

Research is being conducted on embryoblasts. The exact date of fertilization is unknown. There is the presence of a cytotrophoblast and syncytiotrophoblast, marking the time when implantation into the uterus would normally occur. Within the embryoblast, columnar and cuboidal cells are separated by a membrane. Which of these cell layers begins to line the yolk sac cavity?

Q10

An 8-year old boy is brought into clinic for evaluation of possible scoliosis that was newly found on a routine exam at school. On exam, he is also noted to be in the 99th percentile for height and 70th percentile for weight. He appears to have abnormally long extremities as well as an upward lens dislocation on ophthalmologic exam. A mutation leading to a defect in which of the following proteins is the most likely cause of his condition?

Connective tissue cells and fibers US Medical PG Practice Questions and MCQs

Question 1: A 30-year-old man comes to the physician for a follow-up examination 1 month after sustaining a chemical burn over the dorsum of his right hand and forearm. Physical examination shows hyperextension of the hand at the wrist. The skin over the dorsum of the wrist is tense and there is a thick, epithelialized scar. Range of motion of the right wrist is restricted. This patient's contracture is most likely due to activity of which of the following cells?

A. Fibroblasts
B. Myofibroblasts (Correct Answer)
C. Neutrophils
D. Macrophages
E. Endothelial cells

Explanation: ***Myofibroblasts*** - The patient presents with a **contracture** (restricted range of motion and hyperextension of the hand due to a thick scar) after a burn, which is characteristic of the action of **myofibroblasts**. - Myofibroblasts are specialized cells that possess features of both fibroblasts (synthesizing **collagen** and extracellular matrix) and smooth muscle cells (containing **actin microfilaments**), allowing them to exert contractile force which leads to scar contraction. *Fibroblasts* - While fibroblasts are crucial for **wound healing** by producing collagen and other extracellular matrix components, they primarily lay down the foundation for scar tissue. - They lack the strong contractile capabilities that lead to significant **tissue retraction** and contracture. *Neutrophils* - Neutrophils are **acute inflammatory cells** that primarily function in the early stages of wound healing to phagocytose debris and pathogens. - They do not play a direct role in the formation of **scar tissue** or **contractures**. *Macrophages* - Macrophages are important **immune cells** involved in clearing debris, presenting antigens, and releasing growth factors during the later stages of wound healing. - They also influence fibroblast activity, but they do not directly cause **tissue contraction** or scar formation. *Endothelial cells* - Endothelial cells line **blood vessels** and are essential for **angiogenesis** (formation of new blood vessels) during wound healing. - They are not involved in the **contractile process** that leads to scar contracture.

Question 2: A 15-year-old boy presents with shortness of breath on exertion for the past 2 weeks. Although he does not have any other complaints, he is concerned about not gaining much weight despite a good appetite. His height is 188 cm (6 ft 2 in) and weight is 58 kg (124 lb). His blood pressure is 134/56 mm Hg and his pulse rate is 78/min. On cardiac auscultation, his apex beat is displaced laterally with a diastolic murmur lateral to the left sternal border. Slit-lamp examination shows an upward and outward displacement of both lenses. Synthesis of which of the following proteins is most likely defective in this patient?

A. Fibronectin
B. Elastin
C. Fibrillin (Correct Answer)
D. Reticular fibers
E. Laminin

Explanation: ***Fibrillin*** - The patient's presentation with **tall stature**, **arachnodactyly** (implied by tall, thin build), **ectopia lentis** (upward and outward lens displacement), and a **diastolic murmur** (suggesting aortic root dilation or dissection, or mitral valve prolapse) are classic features of **Marfan syndrome**. - **Marfan syndrome** is caused by a defect in the gene encoding **fibrillin-1**, a glycoprotein essential for the formation of elastic fibers and connective tissue integrity. *Fibronectin* - **Fibronectin** is involved in cell adhesion, growth, migration, and differentiation, and plays a crucial role in wound healing and embryonic development. - While essential for connective tissue, defects in fibronectin are not typically associated with the constellation of symptoms seen in Marfan syndrome. *Elastin* - **Elastin** works in conjunction with fibrillin to provide elasticity to tissues like the skin, lungs, and blood vessels. - While Marfan syndrome affects elastic fibers, the primary defect is in fibrillin, which then impairs the proper formation and function of elastin-containing microfibrils. *Reticular fibers* - **Reticular fibers** are fine collagen fibers (primarily type III collagen) that form a delicate supporting network in various tissues and organs. - Defects in reticular fibers are not characteristic of Marfan syndrome; Marfan syndrome is specifically linked to fibrillin defects. *Laminin* - **Laminins** are major proteins of the **basal lamina**, essential for cell adhesion and differentiation in epithelial and endothelial tissues. - Genetic defects in laminin components are often associated with muscular dystrophies or epidermolysis bullosa, not the Marfanoid features presented.

Question 3: A 15-year-old female presents to her family physician for an annual school physical exam and check-up. She is accompanied by her mother to the visit and is present in the exam room. The patient has no complaints, and she does not have any past medical problems. She takes no medications. The patient reports that she remains active, exercising 5 times a week, and eats a healthy and varied diet. Which of the following would be the best way for the physician to obtain a more in-depth social history, including sexual history and use of alcohol, tobacco, or recreational drugs?

A. Disallow the mother to be present in the examination room throughout the entirety of the visit
B. Give the patient a social history questionnaire to fill out in the exam room
C. Ask the mother to step outside into the hall for a portion of the visit (Correct Answer)
D. Ask the patient the questions directly, with her mother still in the exam room
E. Speak softly to the patient so that the mother does not hear and the patient is not embarrassed

Explanation: ***Ask the mother to step outside into the hall for a portion of the visit*** - This approach allows the physician to speak with the adolescent **privately and confidentially**, which is crucial for obtaining sensitive information such as sexual history, drug use, and mental health concerns. - Adolescents are more likely to disclose personal information when their parents are not present, fostering trust and ensuring **comprehensive history-taking** vital for their well-being. *Disallow the mother to be present in the examination room throughout the entirety of the visit* - This is an **overly restrictive** approach that might create tension or distrust between the physician, patient, and parent, especially at the start of the visit. - While privacy is essential for sensitive topics, parental presence can be valuable for discussing general health, family history, and **treatment plans**, especially for younger adolescents. *Give the patient a social history questionnaire to fill out in the exam room* - While questionnaires can be useful for gathering basic information, they often **lack the nuance** of a direct conversation and may not prompt the patient to elaborate on sensitive issues. - Furthermore, having the mother present while the patient fills out a questionnaire on sensitive topics still **compromises confidentiality** and may lead to incomplete or dishonest answers. *Ask the patient the questions directly, with her mother still in the exam room* - Asking sensitive questions with a parent present is **unlikely to yield truthful and complete answers**, as adolescents may feel embarrassed, judged, or fear parental disapproval. - This approach compromises the **confidentiality** that is fundamental to building trust with adolescent patients. *Speak softly to the patient so that the mother does not hear and the patient is not embarrassed* - Speaking softly is **unprofessional** and still does not guarantee privacy, as the mother might still overhear parts of the conversation. - This method also **fails to establish true confidentiality**, which is central to building rapport and encouraging open communication with adolescent patients about sensitive topics.

Question 4: A 5-year-old girl accompanied by her mother presents to the emergency department after suffering a fall on the elementary school playground. Her mother reports that a child on the playground pushed her daughter who fell on her right side, after which she screamed and was found clutching her right leg. The girl's past medical history is significant for a fracture of the left femur and right radius over the past 2 years and an auditory deficit requiring hearing aid use starting 6 months ago. Inspection reveals a relatively short girl in moderate distress. She has brown opalescent teeth. She refuses to bear weight on her right lower extremity. Radiography of the right lower extremity reveals a femoral midshaft fracture. Which of the following is the most likely etiology of the patient's condition?

A. Decreased cystathionine beta synthase activity
B. Vitamin D deficiency
C. Fibrillin gene defect
D. Defective type I collagen production (Correct Answer)
E. Type III collagen gene defect

Explanation: **Defective type I collagen production** - The patient's history of **recurrent fractures** from minor trauma, **auditory deficit**, and **brown opalescent teeth** (dentinogenesis imperfecta) are classic manifestations of **osteogenesis imperfecta**, a genetic disorder characterized by **defective type I collagen production**. - **Type I collagen** is a major component of bone, dentin, and the sclera, and its deficiency leads to brittle bones, hearing loss due to ossicle abnormalities, and dental defects. *Decreased cystathionine beta synthase activity* - This condition describes **homocystinuria**, which can present with skeletal abnormalities and developmental delays. - However, typical features include **marfanoid habitus**, **lens luxation**, and **thromboembolic events**, which are not reported in this patient. *Vitamin D deficiency* - **Vitamin D deficiency** leads to **rickets** in children, characterized by bone pain, muscle weakness, and skeletal deformities but typically presents with **bowing of legs**, **rachitic rosary**, and **craniotabes**, not recurrent fractures from minor trauma or opalescent teeth. - While it weakens bones, the constellation of symptoms here, especially dental and auditory deficits, points away from simple vitamin D deficiency. *Fibrillin gene defect* - A **fibrillin gene defect** is characteristic of **Marfan syndrome**, which presents with **long limbs**, **arachnodactyly**, **lens luxation**, and **aortic root dilation**. - While it involves connective tissue, it does not typically cause the bone fragility and dental findings seen in this patient. *Type III collagen gene defect* - A **type III collagen gene defect** is associated with **Ehlers-Danlos syndrome, vascular type**, which features **easy bruising**, **fragile blood vessels**, and **organ rupture**. - This condition leads to hypermobility and skin hyperextensibility, but not the specific combination of brittle bones, hearing loss, and dental defects seen in this patient.

Question 5: A 25-year-old man presents to the emergency department with sudden-onset severe pain in the right shoulder that occurred when he threw a bowling ball 2 hours ago. He has a history of dislocations in both shoulders and subluxation of the right knee and left wrist. There is no history of fractures. On physical examination, the right arm is slightly abducted and externally rotated. An anterior bulge is seen near the shoulder joint. The neurovascular examination of the right arm shows no abnormalities. The skin examination shows multiple widened atrophic scars around the knees, elbows, and ankles. The skin of the neck and around the elbow can easily be extended up to 4 cm. The sclera is white. The remainder of the physical examination shows no abnormalities. A defect in which of the following proteins is the most likely cause of the findings in this patient?

A. Collagen (Correct Answer)
B. Keratin
C. Fibrillin-1
D. Tau
E. Elastin

Explanation: ***Collagen*** - This patient's symptoms, including **recurrent dislocations**, easy **skin extensibility**, and **atrophic scars**, are classic signs of **Ehlers-Danlos syndrome (EDS)**, which is primarily caused by defects in **collagen synthesis or processing**. - The shoulder dislocation from a minor activity (throwing a bowling ball), along with multisystem hypermobility (shoulders, knee, wrist), strongly points to a **connective tissue disorder** affecting collagen. *Keratin* - **Keratin** is a structural protein primarily found in **epithelial cells**, forming hair, skin, and nails. - Defects in keratin typically manifest as disorders of **skin fragility**, such as **epidermolysis bullosa**, not joint hypermobility or tissue extensibility. *Fibrillin-1* - **Fibrillin-1** defects cause **Marfan syndrome**, characterized by **tall stature**, **arachnodactyly**, **aortic root dilation**, and **lens dislocation**. - While Marfan syndrome involves connective tissue, the patient's presentation of easy skin extensibility and atrophic scars is not characteristic of fibrillin-1 defects, and there is no mention of cardiovascular or ocular issues typical of Marfan. *Tau* - **Tau protein** is a **microtubule-associated protein** primarily found in neurons and is involved in stabilizing microtubules. - Defects or abnormal aggregation of tau protein are associated with **neurodegenerative diseases** like **Alzheimer's disease** and **frontotemporal dementia**, not connective tissue disorders. *Elastin* - **Elastin** provides elasticity to tissues, allowing them to stretch and recoil. Defects can lead to conditions like **cutis laxa** or **Williams syndrome**. - While some features of increased skin extensibility might overlap, the constellation of recurrent joint dislocations and specific atrophic scars seen in this patient is more indicative of **collagen defects** common in Ehlers-Danlos syndrome.

Question 6: A 5-year-old boy is brought to the emergency room by his parents after slipping on a rug at home and experiencing exquisite pain and swelling of his arms. Radiographs reveal a new supracondylar fracture of the humerus, as well as indications of multiple, old fractures that have healed. His parents note that an inherited disorder is present in their family history. A comprehensive physical exam also reveals blue-tinted sclera and yellow-brown, discolored teeth. What is the etiology of the patient’s disorder?

A. Defect in the glycoprotein that forms a sheath around elastin
B. Deficiency of type 3 procollagen
C. Defect in the hydroxylation step of collagen synthesis
D. Deficiency of type 1 collagen (Correct Answer)
E. Deficiency of type 5 collagen

Explanation: ***Deficiency of type 1 collagen*** - This patient presents with a history of **multiple fractures**, **blue sclera**, and **discolored teeth**, which are classic signs of **osteogenesis imperfecta (OI)**. - OI is most commonly caused by **mutations in COL1A1 or COL1A2 genes**, leading to **quantitative or qualitative defects in type I collagen synthesis**, which is a major structural component of bone, sclera, and dentin. - This represents the primary molecular defect in the majority of OI cases. *Defect in the glycoprotein that forms a sheath around elastin* - This defect is characteristic of **Marfan syndrome**, which typically presents with **tall stature**, **arachnodactyly**, and **cardiovascular abnormalities** (e.g., aortic root dilation). - The clinical presentation with multiple fractures and blue sclera does not align with Marfan syndrome. *Deficiency of type 3 procollagen* - A deficiency in type III procollagen is associated with **Ehlers-Danlos syndrome, vascular type**, which primarily involves **fragile blood vessels**, **rupture of internal organs**, and **easy bruising**. - This does not explain the patient's severe bone fragility or blue sclera. *Defect in the hydroxylation step of collagen synthesis* - Defects in collagen hydroxylation can occur due to **vitamin C deficiency (scurvy)** or mutations in hydroxylase enzymes (e.g., **LEPRE1** in some rare OI types). - While certain rare forms of OI can involve hydroxylation defects, the **classic presentation** with blue sclera and dentinogenesis imperfecta is most characteristic of **primary structural mutations in type I collagen genes** (COL1A1/COL1A2), making "deficiency of type 1 collagen" the more precise answer for this patient's etiology. *Deficiency of type 5 collagen* - Deficiency of type V collagen is associated with the **classical type of Ehlers-Danlos syndrome**, characterized primarily by **skin hyperextensibility** and **joint hypermobility**. - While it is a collagen disorder, it does not typically present with the severe bone fragility and blue sclera seen in this patient.

Question 7: A 62-year-old man with prostate cancer comes to the physician because of low back pain for 2 weeks and a 4.5-kg (10-lb) weight loss. Physical examination shows localized tenderness over the lumbar spine. An x-ray of the lumbar spine shows several osteoblastic lesions at the level of L2 and L4 vertebrae. Microscopic examination of a bone biopsy specimen from the L4 vertebra shows irregular bone trabeculae and star-shaped cells with long, cytoplasmic processes located deep within the lacunae. Exchange of nutrients and waste products between these cells most likely occurs through which of the following structures?

A. Zonula adherens
B. Macula adherens
C. Macula communicans (Correct Answer)
D. Zonula occludens
E. Hemidesmosomes

Explanation: ***Macula communicans*** - The description of **star-shaped cells with long, cytoplasmic processes** located deep within lacunae refers to **osteocytes**. These cells communicate and exchange nutrients/waste products via **gap junctions**, also known as **macula communicans**. - **Gap junctions** are essential for maintaining the viability of osteocytes embedded in the mineralized bone matrix by allowing the passage of small molecules and ions. *Zonula adherens* - This is an **adherens junction** that provides strong cell-to-cell adhesion, typically forming a continuous belt-like structure around the cell. - Its primary role is mechanical adhesion, not the direct exchange of nutrients and waste products between cells. *Macula adherens* - Also known as a **desmosome**, this junction provides strong focal adhesion between cells, often in tissues subjected to mechanical stress. - Like adherens junctions, its main function is structural integrity, not transepithelial transport or communication. *Zonula occludens* - This is a **tight junction**, which forms a seal between adjacent cells, preventing paracellular transport of molecules and maintaining cell polarity. - While crucial for creating barriers, it does not facilitate direct communication or nutrient exchange between the cytoplasm of neighboring cells. *Hemidesmosomes* - These junctions anchor epithelial cells to the **basement membrane**, providing strong adhesion between the cell and the extracellular matrix. - Their function is cell-matrix adhesion, distinct from cell-to-cell communication for nutrient exchange.

Question 8: 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 9: Research is being conducted on embryoblasts. The exact date of fertilization is unknown. There is the presence of a cytotrophoblast and syncytiotrophoblast, marking the time when implantation into the uterus would normally occur. Within the embryoblast, columnar and cuboidal cells are separated by a membrane. Which of these cell layers begins to line the yolk sac cavity?

A. Hypoblast (Correct Answer)
B. Epiblast
C. Syncytiotrophoblast
D. Inner cell mass
E. Endoderm

Explanation: ***Hypoblast*** - The **hypoblast** is a layer of cuboidal cells that forms from the inner cell mass around day 8 post-fertilization. - It plays a crucial role in forming the **primary yolk sac** by migrating to line the exocoelomic cavity. *Epiblast* - The **epiblast** is composed of columnar cells located dorsal to the hypoblast and forms the floor of the **amniotic cavity**. - It is the source of the **three primary germ layers** during gastrulation (ectoderm, mesoderm, and endoderm), not the yolk sac lining itself. *Syncytiotrophoblast* - The **syncytiotrophoblast** is the outer, invasive layer of the trophoblast that facilitates implantation and forms the fetal component of the placenta. - It is not involved in lining the yolk sac cavity but rather in **invading the uterine endometrium** and producing hCG. *Inner cell mass* - The **inner cell mass (ICM)** is the cluster of cells within the blastocyst that gives rise to the embryoblast (which further differentiates into epiblast and hypoblast). - The ICM itself does not line the yolk sac; rather, its derivative, the hypoblast, does. *Endoderm* - The **endoderm** is one of the three primary germ layers that forms during gastrulation from the epiblast derivative. - It ultimately forms the linings of the **gastrointestinal and respiratory tracts**, not the primary yolk sac lining.

Question 10: An 8-year old boy is brought into clinic for evaluation of possible scoliosis that was newly found on a routine exam at school. On exam, he is also noted to be in the 99th percentile for height and 70th percentile for weight. He appears to have abnormally long extremities as well as an upward lens dislocation on ophthalmologic exam. A mutation leading to a defect in which of the following proteins is the most likely cause of his condition?

A. Type IV collagen
B. Type I collagen
C. Elastin
D. Fibrillin (Correct Answer)
E. ATP7A

Explanation: ***Fibrillin*** - The patient's clinical features, including **scoliosis**, being in the **99th percentile for height**, having **abnormally long extremities** (arachnodactyly), and **upward lens dislocation**, are classic signs of **Marfan syndrome**. - **Marfan syndrome** is an autosomal dominant disorder caused by a mutation in the *FBN1* gene, which codes for **fibrillin-1**, a glycoprotein essential for the formation of elastic fibers in connective tissue. *Type I collagen* - Defects in **Type I collagen** are primarily associated with **osteogenesis imperfecta**, characterized by **bone fragility**, multiple fractures, blue sclera, and hearing loss. - While it can present with skeletal abnormalities, it does not typically cause the extreme height, arachnodactyly, or lens dislocation seen in this patient. *Type IV collagen* - Defects in **Type IV collagen** are linked to conditions like **Alport syndrome**, which primarily affects the kidneys (glomerulonephritis), ears (hearing loss), and eyes (ocular defects including lenticonus), but not typically the skeletal features described. - It is a major component of **basement membranes**, important for filtration and structural support in various organs. *Elastin* - Mutations in **elastin** are associated with conditions like **supravalvular aortic stenosis** (Williams syndrome) or cutis laxa, which affect the skin and cardiovascular system. - It does not explain the characteristic skeletal and ocular findings of Marfan syndrome. *ATP7A* - A mutation in the *ATP7A* gene, which codes for an ATPase involved in copper transport, is responsible for **Menkes disease**. - **Menkes disease** is characterized by **sparse, kinky hair**, failure to thrive, neurological degeneration, and connective tissue abnormalities due to copper deficiency, which does not align with the patient's presentation.

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