Metaphyseal Skeletal Dysplasias MCQs | Arab Board Ortho

Question 1

A 4-year-old boy is evaluated for short stature. Examination reveals rhizomelic shortening of the limbs, frontal bossing, and a trident hand. Radiographs show narrowing of the interpedicular distances in the lumbar spine. Which zone of the physis is primarily affected by the genetic mutation responsible for this condition?

Explanation

Correct Answer: Proliferative zone

The patient has achondroplasia, the most common form of short-limbed dwarfism. It is caused by an autosomal dominant gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. This mutation leads to constitutive activation of the receptor, which paradoxically inhibits chondrocyte proliferation in the proliferative zone of the physis, leading to decreased endochondral ossification and shortened long bones.

Question 2

A 5-year-old girl presents with a waddling gait, short stature, and bowing of the lower extremities. Radiographs demonstrate coxa vara and flaring of the metaphyses with normal epiphyses. Laboratory studies, including calcium, phosphorus, and alkaline phosphatase, are normal. A mutation in which of the following genes is most likely responsible?

Explanation

Correct Answer: COL10A1

The clinical and radiographic presentation is classic for Schmid type metaphyseal chondrodysplasia. It is characterized by short stature, coxa vara, bowed legs, and a waddling gait. The epiphyses are normal, but the metaphyses are flared and irregular. It is caused by mutations in the COL10A1 gene, which encodes type X collagen, a protein exclusively expressed by hypertrophic chondrocytes in the growth plate.

Question 3

An infant is diagnosed with a rare skeletal dysplasia characterized by severe shortening of the limbs and prominent, irregular, 'cupped' metaphyses. Laboratory evaluation reveals significant hypercalcemia and hypophosphatemia despite normal parathyroid hormone levels. What is the underlying genetic defect?

Explanation

Correct Answer: Activating mutation of the PTH/PTHrP receptor (PTH1R)

Jansen type metaphyseal chondrodysplasia is a rare autosomal dominant disorder caused by an activating mutation in the PTH1R gene (parathyroid hormone/parathyroid hormone-related peptide receptor). This leads to ligand-independent activation of the receptor, resulting in severe hypercalcemia and hypophosphatemia, mimicking hyperparathyroidism, along with severe metaphyseal changes and short stature.

Question 4

A 35-year-old male with a known history of a skeletal dysplasia caused by an FGFR3 mutation presents with neurogenic claudication and bilateral lower extremity weakness. Which of the following radiographic findings is the primary anatomical basis for his symptoms?

Explanation

Correct Answer: Decreased interpedicular distance from L1 to L5 with short pedicles

The patient has achondroplasia. A hallmark radiographic feature of achondroplasia is the narrowing (rather than the normal widening) of the interpedicular distance from L1 to L5, combined with congenitally short pedicles. This severely compromises the spinal canal diameter, predisposing these patients to symptomatic spinal stenosis in adulthood, often presenting as neurogenic claudication.

Question 5

A 3-year-old boy with short-limbed dwarfism is noted to have fine, sparse, light-colored hair. Radiographs show metaphyseal flaring and irregularity. He has a history of recurrent severe respiratory infections. Which of the following is the most critical systemic association to screen for in this patient?

Explanation

Correct Answer: Cellular immunodeficiency

The presentation is highly suggestive of Cartilage-Hair Hypoplasia (McKusick type metaphyseal chondrodysplasia). It is an autosomal recessive disorder caused by mutations in the RMRP gene. Patients present with short-limbed dwarfism, fine sparse hair, and notably, a cellular immunodeficiency (T-cell defect) that makes them highly susceptible to severe, recurrent infections. They also have an increased risk of malignancies such as lymphomas.

Question 6

A 7-year-old child with achondroplasia presents with progressive bowing of the lower extremities. Clinical examination and standing radiographs confirm severe genu varum. What is the primary biomechanical and anatomical driver of this specific deformity in achondroplasia?

Explanation

Correct Answer: Relative overgrowth of the fibula compared to the tibia

Genu varum is a common orthopedic manifestation in achondroplasia. It is primarily driven by the disproportionate growth between the tibia and the fibula. The fibula is relatively less affected by the FGFR3 mutation than the tibia, leading to fibular overgrowth. This overgrowth exerts a tethering effect and biomechanical force that drives the tibia into a varus deformity.

Question 7

In a patient with Schmid type metaphyseal chondrodysplasia, the genetic defect primarily affects a specific collagen type. Which zone of the growth plate is predominantly abnormal as a direct result of this specific collagen deficiency?

Explanation

Correct Answer: Hypertrophic zone

Schmid type metaphyseal chondrodysplasia is caused by a mutation in the COL10A1 gene, which encodes type X collagen. Type X collagen is a short-chain collagen that is synthesized exclusively by hypertrophic chondrocytes. Therefore, the hypertrophic zone of the physis is the primary site of pathology, leading to the characteristic metaphyseal flaring and irregularity seen on radiographs.

Question 8

A 6-month-old infant with achondroplasia is brought to the clinic. The parents report episodes of apnea and profound hypotonia. Which of the following is the most appropriate next step in management?

Explanation

Correct Answer: MRI of the cervicomedullary junction to evaluate for foramen magnum stenosis

Infants with achondroplasia are at high risk for foramen magnum stenosis due to abnormal endochondral ossification of the skull base. This can lead to cervicomedullary compression, presenting as central apnea, profound hypotonia, hyperreflexia, or even sudden death. An MRI of the craniocervical junction is urgently indicated to assess the degree of stenosis and determine if surgical decompression is required.

Question 9

A 10-year-old boy presents with short stature and mild disproportionate shortening of the limbs. Unlike classic achondroplasia, his facial features are normal without frontal bossing or midface hypoplasia. Radiographs show mild narrowing of the lumbar interpedicular distances. Which of the following statements regarding his condition is most accurate?

Explanation

Correct Answer: It is caused by a different mutation in the same gene responsible for achondroplasia

The patient's presentation is consistent with hypochondroplasia, which is clinically similar to but milder than achondroplasia. Patients typically lack the characteristic facial features (frontal bossing, midface hypoplasia) seen in achondroplasia. Like achondroplasia, hypochondroplasia is caused by mutations in the FGFR3 gene, but it involves different specific point mutations (most commonly N540K) that result in a less severe phenotype.

Question 10

When evaluating a child for a suspected skeletal dysplasia with predominantly metaphyseal involvement, which of the following radiographic patterns is the hallmark of this specific group of disorders?

Explanation

Correct Answer: Flared and irregular metaphyses with normal epiphyseal ossification centers

Skeletal dysplasias are often categorized by the region of the bone most affected. In predominantly metaphyseal dysplasias (such as Schmid, Jansen, and McKusick types), the hallmark radiographic finding is flaring, widening, and irregularity (cupping or fraying) of the metaphyses, while the epiphyseal ossification centers remain relatively normal in size, shape, and time of appearance.

Question 11

A 4-year-old boy presents with short stature, a waddling gait, and progressive bowing of the lower extremities. Radiographs reveal flaring and irregularity of the metaphyses, particularly at the proximal femur, with coxa vara. The epiphyses are normal, and laboratory values for calcium, phosphorus, and alkaline phosphatase are within normal limits. Which of the following gene mutations is most likely responsible for this condition?

Explanation

Correct Answer: COL10A1

Schmid type metaphyseal chondrodysplasia is the most common of the metaphyseal chondrodysplasias. It is an autosomal dominant condition caused by mutations in the COL10A1 gene, which encodes type X collagen (found primarily in the hypertrophic zone of the physis). It presents with coxa vara, genu varum, and metaphyseal flaring. Normal laboratory values help differentiate it from rickets.

Question 12

A neonate is evaluated for severe short stature and prominent eyes. Radiographs demonstrate widely separated, irregular, and 'cupped' metaphyses. Laboratory studies reveal asymptomatic hypercalcemia and hypophosphatemia with normal parathyroid hormone (PTH) levels. What is the underlying pathophysiology of this skeletal dysplasia?

Explanation

Correct Answer: Activating mutation of the PTH/PTHrP receptor

Jansen type metaphyseal chondrodysplasia is a rare autosomal dominant disorder caused by an activating mutation in the PTH1R gene (PTH/PTHrP receptor). This leads to ligand-independent activation of the receptor, causing severe hypercalcemia, hypophosphatemia, and profound metaphyseal changes (cupping and irregularity) despite normal or low PTH levels.

Question 13

A 5-year-old girl with short-limbed dwarfism is noted to have fine, sparse, light-colored hair. Radiographs show metaphyseal flaring and irregularity. She has a history of recurrent severe respiratory infections. Which of the following is the most critical systemic manifestation to screen for in this patient?

Explanation

Correct Answer: Cellular immunodeficiency

Cartilage-hair hypoplasia (McKusick type metaphyseal chondrodysplasia) is an autosomal recessive disorder caused by mutations in the RMRP gene. It is characterized by short-limbed dwarfism, fine/sparse hair, and a significant cellular immunodeficiency (T-cell defect), making patients highly susceptible to severe, recurrent infections. They are also at increased risk for malignancies such as lymphoma.

Question 14

A 3-year-old boy presents with short stature and metaphyseal dysplasia. His medical history is significant for chronic diarrhea, failure to thrive, and recurrent bacterial infections. Complete blood count reveals absolute neutropenia. Which of the following is the most likely diagnosis?

Explanation

Correct Answer: Shwachman-Diamond syndrome

Shwachman-Diamond syndrome is an autosomal recessive disorder characterized by exocrine pancreatic insufficiency (causing diarrhea and malabsorption), bone marrow dysfunction (most commonly neutropenia), and skeletal abnormalities including metaphyseal dysplasia. It is caused by mutations in the SBDS gene.

Question 15

A 25-year-old patient is incidentally found to have striking widening of the metaphyses of the long bones, particularly the distal femur and proximal tibia, resembling an 'Erlenmeyer flask'. The patient is of normal height and has no significant clinical symptoms other than mild genu valgum. What is the most likely diagnosis?

Explanation

Correct Answer: Pyle disease

Pyle disease (familial metaphyseal dysplasia) is characterized by massive metaphyseal expansion (Erlenmeyer flask deformity) due to a defect in metaphyseal remodeling. Unlike craniometaphyseal dysplasia, cranial nerve palsies are rare, and patients typically have normal stature and are often asymptomatic, though they may have mild genu valgum or a slightly increased risk of fractures.

Question 16

A 6-year-old child presents with bowing of the legs and a waddling gait. Radiographs show widened physes and cupped, frayed metaphyses. Laboratory investigations reveal normal serum calcium, phosphorus, alkaline phosphatase, and Vitamin D levels. Which of the following is the most appropriate next step in management?

Explanation

Correct Answer: Genetic testing for COL10A1 mutation

The clinical and radiographic picture strongly resembles rickets, but the completely normal laboratory profile (normal Ca, PO4, ALP, Vit D) rules out nutritional and hypophosphatemic rickets. This presentation is classic for Schmid metaphyseal chondrodysplasia, which is caused by a COL10A1 mutation. Genetic testing confirms the diagnosis.

Question 17

A 45-year-old male with a known skeletal dysplasia characterized by rhizomelic shortening and a mutation in the FGFR3 gene presents with bilateral lower extremity neurogenic claudication. Radiographs of the spine show narrowing of the interpedicular distances from L1 to L5. What is the primary anatomical cause of his symptoms?

Explanation

Correct Answer: Congenital short pedicles

Achondroplasia is the most common rhizomelic dwarfism, caused by an FGFR3 mutation. A classic spinal manifestation is congenital spinal stenosis due to short, thickened pedicles and a progressive decrease in the interpedicular distance from the upper to the lower lumbar spine (the opposite of normal anatomy). This predisposes them to severe neurogenic claudication in adulthood.

Question 18

A 10-year-old boy is evaluated for short stature. He has mild disproportionate short limbs but lacks the characteristic facial features of achondroplasia (no frontal bossing or midface hypoplasia). Radiographs show mild narrowing of the lumbar interpedicular distances and mild metaphyseal flaring. Which of the following mutations is most likely responsible?

Explanation

Correct Answer: FGFR3

Hypochondroplasia is caused by mutations in the FGFR3 gene, similar to achondroplasia, but typically at a different locus (e.g., Asn540Lys instead of Gly380Arg). It presents as a milder form of achondroplasia without the classic craniofacial features, but still exhibits mild rhizomelia, metaphyseal flaring, and narrowing of the interpedicular distances.

Question 19

A 7-year-old child with Schmid metaphyseal chondrodysplasia presents with progressive bilateral coxa vara. The neck-shaft angle is 90 degrees bilaterally, and the Hilgenreiner-epiphyseal angle (HEA) is 65 degrees. What is the most appropriate surgical intervention?

Explanation

Correct Answer: Proximal femoral valgus osteotomies

In Schmid metaphyseal chondrodysplasia, progressive coxa vara is a common complication. Indications for surgical intervention include a neck-shaft angle less than 90-100 degrees or a Hilgenreiner-epiphyseal angle (HEA) greater than 60 degrees (which indicates high shear forces across the physis). Proximal femoral valgus osteotomy is the treatment of choice to correct the deformity and convert shear forces to compressive forces.

Question 20

A 2-year-old child presents with delayed walking, bowing of the long bones, and premature loss of deciduous teeth. Radiographs show irregular, radiolucent metaphyses resembling severe rickets. Laboratory tests reveal hypercalcemia and significantly decreased serum alkaline phosphatase. Which of the following is the underlying defect?

Explanation

Correct Answer: Mutation in the ALPL gene encoding tissue-nonspecific alkaline phosphatase

Hypophosphatasia is a rare metabolic bone disease caused by mutations in the ALPL gene, leading to deficient tissue-nonspecific alkaline phosphatase (TNSALP). This causes defective bone mineralization (resembling rickets radiographically with metaphyseal irregularities) but is uniquely characterized by low alkaline phosphatase levels, hypercalcemia, and premature loss of deciduous teeth.

Question 21

An infant with achondroplasia is noted to have a flexible thoracolumbar kyphosis during a routine orthopedic evaluation. What is the most appropriate initial management for this spinal deformity?

Explanation

Correct Answer: Avoidance of unsupported sitting and use of a firm stroller

Thoracolumbar kyphosis is extremely common in infants with achondroplasia, primarily due to generalized hypotonia and a disproportionately large head. The deformity is typically flexible initially. The standard of care is to prevent the flexible deformity from becoming a rigid, structural wedging of the vertebrae. This is achieved by avoiding unsupported sitting and using firm backing in strollers and car seats. The kyphosis usually resolves spontaneously once the child develops adequate truncal tone and begins to walk. Surgical intervention or rigid bracing is reserved for severe, progressive, or rigid deformities that do not resolve with walking.

Question 22

Schmid metaphyseal chondrodysplasia is characterized by short stature, coxa vara, and a waddling gait. Which of the following gene mutations is responsible for the pathogenesis of this specific condition?

Explanation

Correct Answer: COL10A1

Schmid metaphyseal chondrodysplasia is an autosomal dominant disorder caused by mutations in the COL10A1 gene, which encodes type X collagen. Type X collagen is uniquely expressed by hypertrophic chondrocytes in the growth plate. A defect in this collagen leads to abnormal endochondral ossification, resulting in metaphyseal flaring, coxa vara, and short stature. FGFR3 mutations cause achondroplasia and hypochondroplasia. PTH1R mutations cause Jansen metaphyseal chondrodysplasia. RMRP mutations cause Cartilage-Hair Hypoplasia (McKusick type). COMP mutations are associated with pseudoachondroplasia and multiple epiphyseal dysplasia.

Question 23

A 3-year-old child presents with severe short stature, micrognathia, and prominent eyes. Radiographs demonstrate severe, irregular metaphyseal flaring with a 'ragged' appearance. Laboratory testing reveals hypercalcemia and hypophosphatemia. Which of the following is the most likely diagnosis?

Explanation

Correct Answer: Jansen metaphyseal chondrodysplasia

Jansen metaphyseal chondrodysplasia is a rare, severe autosomal dominant disorder caused by an activating mutation in the PTH1R gene (the receptor for parathyroid hormone and parathyroid hormone-related peptide). This ligand-independent activation leads to severe hypercalcemia, hypophosphatemia, and undetectable or very low PTH levels. Clinically, patients have severe short stature, characteristic facial features (micrognathia, prominent eyes), and profound metaphyseal irregularities. Schmid type does not have these severe lab abnormalities. Hypophosphatasia presents with low alkaline phosphatase, not hypercalcemia driven by PTH receptor overactivity.

Question 24

A patient diagnosed with Cartilage-Hair Hypoplasia (McKusick type metaphyseal chondrodysplasia) is at a significantly increased risk for which of the following systemic complications?

Explanation

Correct Answer: Cellular immunodeficiency

Cartilage-Hair Hypoplasia (McKusick type) is an autosomal recessive disorder caused by mutations in the RMRP gene. It is characterized by short-limbed dwarfism, fine and sparse hair, and a significant cellular immunodeficiency (T-cell defect), which makes these patients highly susceptible to severe, recurrent infections, particularly viral (like Varicella). They are also at an increased risk for Hirschsprung disease and certain malignancies (e.g., lymphomas). Exocrine pancreatic insufficiency is a hallmark of Shwachman-Diamond syndrome, another metaphyseal dysplasia.

Question 25

A 4-year-old boy presents with short stature and metaphyseal flaring on radiographs. His medical history is significant for recurrent bacterial infections and steatorrhea since infancy. A complete blood count reveals severe neutropenia. What is the most likely diagnosis?

Explanation

Correct Answer: Shwachman-Diamond syndrome

Shwachman-Diamond syndrome is a rare autosomal recessive disorder characterized by the classic triad of exocrine pancreatic insufficiency (leading to malabsorption and steatorrhea), bone marrow dysfunction (most commonly neutropenia, leading to recurrent infections), and skeletal abnormalities (predominantly metaphyseal chondrodysplasia). It is caused by mutations in the SBDS gene. Cartilage-hair hypoplasia features immunodeficiency but not pancreatic insufficiency. Schmid type is isolated to the skeleton without these systemic findings.

Question 26

In an infant with achondroplasia, which of the following clinical presentations should prompt immediate investigation for cervicomedullary compression at the foramen magnum?

Explanation

Correct Answer: Central sleep apnea and profound hypotonia

Foramen magnum stenosis is the most critical and potentially life-threatening neurological complication in infants with achondroplasia. The abnormal endochondral ossification affects the base of the skull, leading to a narrowed foramen magnum. Symptoms of cervicomedullary compression include central sleep apnea, profound hypotonia, hyperreflexia, clonus, and in severe cases, sudden infant death. Any infant with achondroplasia exhibiting these signs requires urgent MRI evaluation and potential neurosurgical decompression. Delayed walking, waddling gait, flexible kyphosis, and genu varum are expected, non-emergent findings in achondroplasia.

Question 27

A 6-year-old child with achondroplasia presents with progressive genu varum. What is the primary biomechanical and anatomical cause of this specific lower extremity deformity in this patient population?

Explanation

Correct Answer: Overgrowth of the fibula relative to the tibia

Genu varum is a very common deformity in patients with achondroplasia. The primary anatomical driver for this is the disproportionate growth between the tibia and the fibula. The fibula experiences relative overgrowth compared to the tibia, which acts as a tether, forcing the tibia into a varus bow. This is often accompanied by internal tibial torsion. While ligamentous laxity is present in achondroplasia, the structural bony disproportion is the primary cause of the varus deformity. Premature closure of the medial physis describes Blount disease, which is a different pathology.

Question 28

Which of the following clinical features most reliably distinguishes hypochondroplasia from achondroplasia during a physical examination?

Explanation

Correct Answer: Absence of frontal bossing and midface hypoplasia

Hypochondroplasia is clinically considered a milder variant of achondroplasia. Both are caused by mutations in the FGFR3 gene (though typically different specific mutations), both are autosomal dominant, and both present with disproportionate short stature. However, a key distinguishing clinical feature is the craniofacial appearance. Patients with achondroplasia classically have macrocephaly, frontal bossing, and midface hypoplasia. In contrast, patients with hypochondroplasia typically have a normal facial appearance and a normal or only slightly enlarged head circumference.

Question 29

An adult patient with achondroplasia presents with severe neurogenic claudication. Radiographs of the lumbar spine are obtained. Which of the following anatomical features is the primary congenital contributor to spinal stenosis in this condition?

Explanation

Correct Answer: Progressive decrease in interpedicular distance from L1 to L5

In a normal spine, the interpedicular distance gradually increases from the upper lumbar spine (L1) down to the lower lumbar spine (L5). A hallmark radiographic finding in achondroplasia is the abnormal progressive decrease in the interpedicular distance from L1 to L5. This congenital narrowing of the spinal canal, combined with short, thickened pedicles and exaggerated lumbar lordosis, severely predisposes these patients to symptomatic lumbar spinal stenosis in adulthood, often requiring extensive laminectomies.

Question 30

A 5-year-old child with Schmid metaphyseal chondrodysplasia is evaluated for a waddling gait. Radiographs of the pelvis and lower extremities are most likely to demonstrate which of the following characteristic findings?

Explanation

Correct Answer: Coxa vara and genu varum with widened, irregular physes

Schmid metaphyseal chondrodysplasia is characterized radiographically by abnormalities confined primarily to the metaphyses, while the epiphyses remain normal (distinguishing it from epiphyseal dysplasias). The classic radiographic findings include coxa vara (decreased neck-shaft angle), genu varum, and widened, irregular, and cupped physes (metaphyseal flaring). The waddling gait is a direct clinical consequence of the coxa vara and resulting abductor mechanical disadvantage. Stippled epiphyses are seen in chondrodysplasia punctata, and dense sclerotic bands are seen in heavy metal poisoning or bisphosphonate use.

Question 31

A 3-year-old child presents with severe short stature, prominent eyes, and micrognathia. Laboratory investigations reveal marked hypercalcemia, hypophosphatemia, and undetectable intact parathyroid hormone (PTH) levels. Radiographs show severe metaphyseal irregularities and wide growth plates. Which of the following gene mutations is most likely responsible for this condition?

Explanation

Correct Answer: PTH1R

This clinical picture is classic for Jansen type metaphyseal chondrodysplasia. It is caused by an activating mutation in the PTH1R (parathyroid hormone 1 receptor) gene. This leads to ligand-independent activation of the receptor, resulting in severe hypercalcemia and hypophosphatemia despite suppressed or undetectable levels of PTH. Radiographically, it presents with severe metaphyseal changes. COL10A1 is associated with Schmid type, FGFR3 with achondroplasia/hypochondroplasia, RMRP with McKusick type (cartilage-hair hypoplasia), and COMP with pseudoachondroplasia.

Question 32

A 5-year-old boy presents with short stature, a waddling gait, and progressive bowing of the lower extremities. Radiographs reveal coxa vara, genu varum, and flaring of the metaphyses, but the spine and epiphyses are entirely normal. Genetic testing is most likely to reveal a mutation in the gene encoding for which of the following proteins?

Explanation

Correct Answer: Type X collagen

The patient's presentation of metaphyseal flaring, coxa vara, and genu varum with normal epiphyses and spine is characteristic of Schmid type metaphyseal chondrodysplasia. This condition is inherited in an autosomal dominant pattern and is caused by mutations in the COL10A1 gene, which encodes for Type X collagen. Type X collagen is primarily expressed by hypertrophic chondrocytes in the growth plate.

Question 33

A 2-year-old girl with disproportionate short stature and fine, sparse, light-colored hair is diagnosed with a metaphyseal chondrodysplasia. She has a history of recurrent severe respiratory infections. Which of the following gastrointestinal conditions is most strongly associated with her underlying skeletal dysplasia?

Explanation

Correct Answer: Hirschsprung disease

The patient has Cartilage-Hair Hypoplasia (McKusick type metaphyseal chondrodysplasia), which is characterized by short-limbed dwarfism, fine/sparse hair, and immunodeficiency (often T-cell or combined). It is caused by mutations in the RMRP gene. A known and significant association with this syndrome is Hirschsprung disease (aganglionic megacolon), as well as an increased risk of malignancies such as lymphomas.

Question 34

A 45-year-old male with achondroplasia presents with progressive neurogenic claudication and lower extremity weakness. Radiographs of the lumbar spine demonstrate narrowing of the interpedicular distances from L1 to L5. Which of the following anatomical abnormalities is the primary cause of his spinal stenosis?

Explanation

Correct Answer: Congenitally short pedicles

Spinal stenosis is a hallmark complication in adults with achondroplasia. The primary anatomical basis for this stenosis is congenitally short, thickened pedicles and a decreased interpedicular distance that typically worsens from L1 to L5 (the opposite of the normal spine). While degenerative changes like disc bulging and ligamentum flavum hypertrophy can exacerbate the stenosis later in life, the fundamental underlying cause is the abnormal pedicle morphology due to defective endochondral ossification.

Question 35

A 4-year-old boy presents with short stature, metaphyseal flaring, and a history of recurrent bacterial infections. Laboratory tests reveal chronic neutropenia. He also has a history of steatorrhea and failure to thrive. What is the most likely diagnosis?

Explanation

Correct Answer: Shwachman-Diamond syndrome

Shwachman-Diamond syndrome is an autosomal recessive disorder characterized by exocrine pancreatic insufficiency (causing steatorrhea and failure to thrive), bone marrow dysfunction (most commonly neutropenia, leading to recurrent infections), and skeletal abnormalities (metaphyseal dysostosis/dysplasia). It is typically caused by mutations in the SBDS gene.

Question 36

An infant with achondroplasia is noted to have central apnea, hyperreflexia, and delayed motor milestones. An MRI of the brain and cervical spine is ordered. What is the most likely finding responsible for these symptoms?

Explanation

Correct Answer: Cervicomedullary compression at the foramen magnum

Infants with achondroplasia are at high risk for foramen magnum stenosis due to abnormal endochondral ossification of the skull base. This can lead to cervicomedullary compression, presenting with central apnea, hyperreflexia, hypotonia, delayed motor milestones, and even sudden death. Prompt evaluation with MRI and potential neurosurgical decompression are critical.

Question 37

A 7-year-old child presents with disproportionate short stature. Radiographs show narrowing of the interpedicular distance in the lower lumbar spine and mild metaphyseal flaring. However, the child has a normal facial appearance without frontal bossing or midface hypoplasia. Which of the following is the most likely diagnosis?

Explanation

Correct Answer: Hypochondroplasia

Hypochondroplasia is caused by a different, typically milder, mutation in the FGFR3 gene compared to achondroplasia. Patients exhibit similar but less severe skeletal features, such as narrowing of the interpedicular distance and short limbs. Crucially, they lack the characteristic craniofacial features of achondroplasia, such as frontal bossing and midface hypoplasia. Pseudoachondroplasia involves both epiphyses and metaphyses and is caused by a COMP mutation.

Question 38

In a 10-year-old patient with achondroplasia and severe symptomatic genu varum, radiographic analysis typically shows varus deformity primarily associated with which of the following anatomical patterns?

Explanation

Correct Answer: Proximal and distal tibial varus with relative fibular overgrowth

Genu varum is a very common orthopedic manifestation in achondroplasia. It is typically driven by relative overgrowth of the fibula compared to the tibia, leading to varus bowing at both the proximal and distal tibia. Surgical correction often requires tibial osteotomies combined with fibular shortening or epiphysiodesis to address the length discrepancy between the two bones.

Question 39

A 4-year-old child presents with bowing of the legs and a waddling gait. Radiographs show widening and cupping of the metaphyses, strongly resembling rickets. However, laboratory investigations reveal normal serum calcium, phosphorus, alkaline phosphatase, and vitamin D levels. What is the most likely diagnosis?

Explanation

Correct Answer: Schmid metaphyseal chondrodysplasia

Schmid metaphyseal chondrodysplasia is often misdiagnosed as rickets due to the similar radiographic appearance of widened, cupped, and irregular metaphyses (especially in the lower extremities causing bowing). However, unlike nutritional or hypophosphatemic rickets, patients with Schmid type have completely normal serum calcium, phosphorus, and alkaline phosphatase levels.

Question 40

A couple, both of whom have achondroplasia, are expecting a child. They seek genetic counseling regarding the risk to their offspring. What is the probability that their child will be born with homozygous achondroplasia, a condition that is typically lethal in the perinatal period?

Explanation

Correct Answer: 25%

Achondroplasia is an autosomal dominant disorder. When both parents are heterozygous for the FGFR3 mutation (which is the case for viable adults with achondroplasia), there is a 25% chance of the offspring inheriting two mutant alleles (homozygous achondroplasia). This condition causes severe rib cage deformity and pulmonary hypoplasia, and is almost universally lethal shortly after birth. There is a 50% chance of heterozygous achondroplasia and a 25% chance of average stature.

Question 41

A 4-year-old boy is diagnosed with the most common form of short-limbed dwarfism, characterized by rhizomelic shortening and frontal bossing. Which of the following best describes the pathophysiology of his condition?

Explanation

Correct Answer: Gain-of-function mutation in the FGFR3 gene leading to decreased chondrocyte proliferation

Achondroplasia is the most common skeletal dysplasia and is characterized by predominantly metaphyseal involvement with rhizomelic short stature. It is caused by a gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. Normally, FGFR3 acts as a negative regulator of bone growth. The mutation causes the receptor to be constitutively active, leading to profound inhibition of chondrocyte proliferation and hypertrophy in the proliferative zone of the physis.

Question 42

In a patient with achondroplasia, which of the following radiographic findings is most characteristic of the lumbar spine?

Explanation

Correct Answer: Progressive narrowing of the interpedicular distance from L1 to L5

A hallmark radiographic feature of achondroplasia is the progressive narrowing of the interpedicular distance from the upper to the lower lumbar spine (L1 to L5). In a normal spine, this distance widens. This narrowing, combined with short pedicles and thickened ligamentum flavum, strongly predisposes these patients to severe spinal stenosis later in life. Anterior beaking is characteristic of mucopolysaccharidoses (like Hurler or Morquio syndrome), and coronal clefts are seen in chondrodysplasia punctata.

Question 43

A 5-year-old child presents with short stature, a waddling gait, coxa vara, and genu varum. Radiographs show flaring and irregularity of the metaphyses, but the epiphyses and spine are completely normal. Genetic testing reveals a mutation in the COL10A1 gene. What is the most likely diagnosis?

Explanation

Correct Answer: Schmid metaphyseal chondrodysplasia

Schmid metaphyseal chondrodysplasia is an autosomal dominant condition caused by mutations in the COL10A1 gene, which encodes type X collagen (found in the hypertrophic zone of the physis). It presents with short stature, coxa vara, and genu varum. Radiographically, it affects the metaphyses (flaring, cupping) while sparing the epiphyses and the spine. Jansen type is caused by a PTH1R mutation, and Cartilage-hair hypoplasia (McKusick type) is associated with the RMRP gene.

Question 44

An infant presents with severe short stature, prominent eyes, and a highly arched palate. Radiographs demonstrate severe metaphyseal irregularities described as a 'chopped wood' appearance. Laboratory studies are most likely to show which of the following abnormalities?

Explanation

Correct Answer: Hypercalcemia and hypophosphatemia

The clinical and radiographic presentation ('chopped wood' metaphyses) is classic for Jansen metaphyseal chondrodysplasia. This rare autosomal dominant disorder is caused by an activating mutation in the PTH/PTHrP receptor (PTH1R). This ligand-independent activation leads to biochemical findings identical to primary hyperparathyroidism: severe hypercalcemia and hypophosphatemia, but with undetectable or very low levels of actual circulating PTH.

Question 45

A 3-year-old Amish child presents with disproportionate short stature, fine sparse hair, and flaring of the rib costochondral junctions. Radiographs show metaphyseal widening and irregularity. This patient is at the highest risk for which of the following systemic complications?

Explanation

Correct Answer: Cellular immunodeficiency

The patient has Cartilage-Hair Hypoplasia (McKusick type metaphyseal chondrodysplasia), an autosomal recessive disorder common in the Amish and Finnish populations, caused by mutations in the RMRP gene. It is characterized by short-limbed dwarfism, fine sparse hair, and metaphyseal dysplasia. A critical systemic association is cellular immunodeficiency (T-cell defect), making these patients highly susceptible to severe infections, particularly varicella. They also have an increased risk of malignancies like lymphomas.

Question 46

A 7-year-old boy with achondroplasia presents with progressive, symptomatic genu varum. Radiographs confirm severe bowing centered at the proximal tibia with relative fibular overgrowth. What is the most appropriate surgical management?

Explanation

Correct Answer: Proximal tibial and fibular valgus-producing osteotomies

Genu varum is common in achondroplasia and is typically driven by relative overgrowth of the fibula compared to the tibia, leading to a varus thrust and lateral ligamentous laxity. When symptomatic and severe, surgical correction is indicated. The standard of care is valgus-producing osteotomies of the proximal tibia and fibula. Guided growth on the lateral side would worsen the varus. While medial guided growth is sometimes attempted, the fibular overgrowth often necessitates concurrent fibular management or definitive osteotomy.

Question 47

Parents of a newborn diagnosed with achondroplasia are counseled regarding potential complications. Which of the following represents the most significant cause of mortality in the first year of life for patients with this condition?

Explanation

Correct Answer: Cervicomedullary compression due to foramen magnum stenosis

The most critical and potentially lethal complication in infants with achondroplasia is stenosis of the foramen magnum. This can lead to cervicomedullary compression, resulting in central apnea, profound hypotonia, quadriparesis, and sudden death. All infants with achondroplasia require careful neurological evaluation and often neuroimaging (MRI) or polysomnography to screen for this condition. If severe compression is identified, suboccipital decompression is urgently indicated.

Question 48

Which of the following clinical or radiographic features best differentiates hypochondroplasia from achondroplasia?

Explanation

Correct Answer: Absence of frontal bossing and midface hypoplasia

Hypochondroplasia is a milder skeletal dysplasia that is also caused by mutations in the FGFR3 gene (though typically different specific mutations than achondroplasia). While patients with hypochondroplasia have short stature and may have narrowing of the lumbar interpedicular distances, they characteristically lack the distinct craniofacial features of achondroplasia, such as frontal bossing and midface hypoplasia. Their facial appearance is typically normal.

Question 49

A 9-month-old infant with achondroplasia is noted to have a flexible thoracolumbar kyphosis. Neurological examination is normal. What is the most appropriate initial management?

Explanation

Correct Answer: Avoidance of unsupported sitting and use of a firm mattress

Thoracolumbar kyphosis is present in over 90% of infants with achondroplasia. It is primarily postural, exacerbated by hypotonia and a large head size. The vast majority of these curves are flexible and will resolve spontaneously once the child begins to walk and develops lumbar lordosis. The standard initial management is to avoid unsupported sitting (which exacerbates the kyphosis) and to carry the child in a supportive device. Bracing is reserved for curves that persist after walking age or become rigid.

Question 50

A 4-year-old boy presents with short stature, metaphyseal dysplasia, neutropenia, and a history of steatorrhea. Which of the following is the most likely diagnosis?

Explanation

Correct Answer: Shwachman-Diamond syndrome

Shwachman-Diamond syndrome is a rare autosomal recessive disorder characterized by exocrine pancreatic insufficiency (causing steatorrhea), bone marrow dysfunction (most commonly neutropenia), and skeletal abnormalities, specifically metaphyseal chondrodysplasia. The combination of metaphyseal dysplasia with gastrointestinal and hematological abnormalities is classic for this syndrome.

Question 51

A 4-year-old child presents with bowed legs, coxa vara, and a waddling gait. Radiographs show widened, irregular metaphyses but completely normal epiphyses. Genetic testing confirms a mutation in the COL10A1 gene. Which zone of the physis is primarily targeted by this genetic mutation?

Explanation

Schmid metaphyseal chondrodysplasia is caused by a mutation in COL10A1, which encodes type X collagen. Type X collagen is uniquely expressed by chondrocytes in the hypertrophic zone of the physis.

Question 52

A 3-year-old boy presents with disproportionate short stature, fine sparse hair, and a history of recurrent severe pulmonary infections. Radiographs demonstrate metaphyseal widening and irregularity. Which of the following systemic manifestations is most closely associated with this condition?

Explanation

Cartilage-hair hypoplasia (McKusick type) is an autosomal recessive metaphyseal dysplasia caused by RMRP mutations. It is strongly associated with severe cellular (T-cell) immunodeficiency and an increased risk of malignancy.

Question 53

A 2-year-old is evaluated for short stature and prominent metaphyseal flaring. The patient also has a history of steatorrhea and failure to thrive. Complete blood count shows severe neutropenia. Which of the following genes is most likely mutated in this patient?

Explanation

Shwachman-Diamond syndrome is caused by a mutation in the SBDS gene. It presents with metaphyseal chondrodysplasia, exocrine pancreatic insufficiency (causing steatorrhea), and bone marrow dysfunction leading to neutropenia.

Question 54

An infant with severe micromelic dwarfism and prominent eyes is found to have widespread irregular, cupped metaphyses on radiographs. Laboratory studies reveal profound hypercalcemia and hypophosphatemia, yet parathyroid hormone levels are undetectable. The defective receptor in this condition physiologically binds to which of the following ligands?

Explanation

Jansen type metaphyseal chondrodysplasia is caused by an activating mutation in the PTHR1 gene. This receptor normally binds both PTH and PTHrP, leading to ligand-independent activation and subsequent severe hypercalcemia.

Question 55

A 12-year-old presents for evaluation of genu valgum. Radiographs reveal massive cortical thinning and an "Erlenmeyer flask" deformity of the distal femur and proximal tibia metaphyses. The patient's stature is normal, and cranial nerves are completely intact. What is the most likely diagnosis?

Explanation

Pyle disease (familial metaphyseal dysplasia) is characterized by striking metaphyseal widening (Erlenmeyer flask deformity) with cortical thinning. Unlike craniometaphyseal dysplasia, Pyle disease typically spares the skull and cranial nerves.

Question 56

A 10-year-old child presents with progressive facial distortion, a wide nasal bridge, and bilateral conductive hearing loss. Radiographs show widened metaphyses of the long bones. A mutation in the ANKH gene is identified. What is the primary pathomechanism of this mutation?

Explanation

Craniometaphyseal dysplasia is caused by mutations in the ANKH gene, which regulates the transmembrane transport of inorganic pyrophosphate. This leads to altered osteoclast formation and defective bone remodeling.

Question 57

An Amish child is diagnosed with cartilage-hair hypoplasia after presenting with short-limb dwarfism and an unusually severe varicella infection. The mutated gene responsible for this condition (RMRP) is unique because it encodes for which of the following?

Explanation

Cartilage-hair hypoplasia is uniquely caused by mutations in the RMRP gene. Unlike most skeletal dysplasias, RMRP encodes an untranslated, non-coding RNA component of the mitochondrial RNA-processing endoribonuclease.

Question 58

Which of the following combined lower extremity deformities is the most characteristic clinical hallmark of Schmid metaphyseal chondrodysplasia?

Explanation

Patients with Schmid metaphyseal chondrodysplasia classically present with significant coxa vara and progressive genu varum. This leads to the characteristic waddling gait often observed when they begin walking.

Question 59

A 4-year-old girl is evaluated for a waddling gait and disproportionate short stature. Unlike patients with achondroplasia, her facial features and head circumference are entirely normal. Radiographs show delayed epiphyseal ossification and irregular, widened metaphyses. Which gene is most likely implicated?

Explanation

Pseudoachondroplasia mimics the dwarfism of achondroplasia but spares the craniofacial skeleton. It is an epimetaphyseal dysplasia caused by a mutation in the COMP (Cartilage Oligomeric Matrix Protein) gene.

Question 60

A 5-year-old boy with metaphyseal chondrodysplasia has recurrent pneumonia. A sweat chloride test is normal, but stool elastase is abnormally low. Radiographs show symmetrical metaphyseal widening primarily affecting the hips and knees. He is at highest risk for developing which of the following complications?

Explanation

This patient has Shwachman-Diamond syndrome, characterized by pancreatic insufficiency and metaphyseal dysplasia. These patients have bone marrow dysfunction and carry a high risk of malignant transformation to myelodysplastic syndrome or acute myeloid leukemia.

Question 61

A male infant is noted to have severe short stature and bowing of the femurs. Radiographs at birth show marked metaphyseal irregularity and splaying. By age 3, these radiographic abnormalities spontaneously resolve, though the patient remains short. Which gene is most likely responsible for this self-limiting condition?

Explanation

Metaphyseal anadysplasia is characterized by severe metaphyseal changes in early infancy that spontaneously regress. It is most commonly associated with mutations in the matrix metalloproteinase genes, particularly MMP13 or MMP9.

Question 62

A 35-year-old male with achondroplasia presents with neurogenic claudication, progressive leg weakness, and hyperreflexia in the lower extremities. Which anatomical feature of his skeletal dysplasia most directly contributes to this presentation?

Explanation

Achondroplasia is characterized by narrowing of the interpedicular distances from the upper to lower lumbar spine. This anatomical defect severely predisposes adults to central spinal canal stenosis and neurogenic claudication.

Question 63

A newborn presents with disproportionate short stature, postaxial polydactyly, and hypoplastic nails. An echocardiogram reveals an atrial septal defect. Radiographs show progressive distal shortening of the limbs (acromesomelia). What is the most likely diagnosis?

Explanation

Ellis-van Creveld syndrome (chondroectodermal dysplasia) is characterized by short limbs, postaxial polydactyly, ectodermal defects (nail/tooth dysplasia), and congenital heart disease, most commonly a single atrium or ASD.

Question 64

An infant presents with a remarkably narrow, bell-shaped thorax, short limbs, and polydactyly. Pelvic radiographs display a trident appearance of the acetabulum with spurs. The infant requires mechanical ventilation. Mutations affecting which cellular structure are most commonly responsible for this phenotype?

Explanation

Asphyxiating thoracic dystrophy (Jeune syndrome) presents with a narrow, rigid thorax leading to severe respiratory distress. It belongs to the family of short-rib polydactyly syndromes, which are uniformly caused by defects in the primary cilia (ciliopathies).

Question 65

A 6-year-old is evaluated for short-trunk short stature, scoliosis, and generalized joint laxity. Radiographs demonstrate platyspondyly, delayed epiphyseal ossification, and irregular, flared metaphyses. A mutation in the TRPV4 gene is identified. This gene encodes for an ion channel primarily permeable to which of the following?

Explanation

Spondyloepimetaphyseal dysplasia (SEMD), Maroteaux type, is caused by mutations in the TRPV4 gene. TRPV4 functions as a calcium-permeable, non-selective cation channel essential for normal chondrocyte function.

Question 66

A 6-month-old infant with achondroplasia is noted during a routine exam to have brisk deep tendon reflexes, clonus, and reports of central sleep apnea. Which of the following is the most appropriate next step in management?

Explanation

Infants with achondroplasia are at high risk for foramen magnum stenosis, which can cause cervicomedullary compression. Signs of upper motor neuron findings or central apnea mandate an urgent MRI of the cranio-cervical junction to assess for cord compression.

Question 67

An 8-year-old child with genetically confirmed Schmid metaphyseal chondrodysplasia has progressive severe coxa vara with a neck-shaft angle of 90 degrees. He is experiencing hip pain and a worsening Trendelenburg gait. What is the most appropriate surgical management?

Explanation

Severe progressive coxa vara in metaphyseal chondrodysplasia (neck-shaft angle <100-110 degrees, or large Hilgenreiner-epiphyseal angle) is a prime indication for a valgus-producing proximal femoral osteotomy to restore normal hip biomechanics and prevent premature osteoarthritis.

Question 68

In achondroplasia, the underlying genetic defect is a point mutation in the FGFR3 gene. This mutation results in a gain-of-function that directly causes which of the following abnormalities at the growth plate?

Explanation

FGFR3 is a negative regulator of bone growth. The gain-of-function mutation in achondroplasia causes constitutive activation of the receptor, which abnormally inhibits both the proliferation and terminal hypertrophy of chondrocytes in the physis.

Question 69

A 3-year-old girl is diagnosed with nutritional rickets due to bowed legs and cupped, widened metaphyses on radiographs. However, her condition does not improve despite 6 months of high-dose vitamin D therapy. Labs show perfectly normal calcium, phosphorus, alkaline phosphatase, and 25-OH vitamin D levels. What is the most likely diagnosis?

Explanation

Schmid metaphyseal chondrodysplasia closely mimics the clinical and radiographic features of rickets. However, unlike all forms of true rickets, patients with Schmid dysplasia have completely normal serum calcium, phosphorus, and alkaline phosphatase levels.

Question 70

A 7-year-old boy is evaluated for disproportionate short stature. He lacks the frontal bossing and midface hypoplasia typical of classic achondroplasia. Radiographs show mild narrowing of the lumbar interpedicular distances and squared, shortened long bone diaphyses with slightly flared metaphyses. Which genetic abnormality is most likely present?

Explanation

Hypochondroplasia shares similarities with achondroplasia but is clinically milder, typically lacking the pronounced craniofacial features. It is caused by different, less severe mutations in the exact same gene (FGFR3), most commonly the N540K substitution.

Question 71

A 6-year-old boy presents with short stature, severe genu varum, and a waddling gait. Laboratory studies, including serum calcium, phosphorus, alkaline phosphatase, and vitamin D, are entirely normal. Radiographs reveal widened physes and irregular, flared metaphyses. A mutation in the COL10A1 gene is confirmed. Which physeal zone is primarily affected by this genetic mutation?

Explanation

Schmid metaphyseal chondrodysplasia is caused by a COL10A1 mutation, which specifically affects type X collagen. Type X collagen is exclusively expressed by chondrocytes in the hypertrophic zone of the physis.

Question 72

A 3-year-old Amish child presents with disproportionate short stature, sparse light-colored hair, and a history of recurrent severe respiratory infections. Radiographs demonstrate flaring and irregularity of the metaphyses. This patient is at the highest risk for developing which of the following complications?

Explanation

This clinical picture describes Cartilage-Hair Hypoplasia (McKusick type), caused by an RMRP gene mutation. Patients have impaired T-cell immunity, predisposing them to recurrent infections and malignancies such as non-Hodgkin lymphoma.

Question 73

A neonate is diagnosed with Jansen metaphyseal chondrodysplasia. Laboratory studies reveal severe hypercalcemia and hypophosphatemia. The genetic mutation associated with this disorder leads to which of the following mechanisms?

Explanation

Jansen metaphyseal chondrodysplasia is caused by an activating mutation in the PTH1R gene. This leads to ligand-independent, constitutive activation of the PTH/PTHrP receptor, biochemically mimicking hyperparathyroidism despite low endogenous PTH levels.

Question 74

A 4-year-old child presents with failure to thrive, steatorrhea, and short stature. Radiographs show symmetrical metaphyseal dysplasia primarily affecting the hips and knees. A complete blood count reveals absolute neutropenia. What is the most likely diagnosis?

Explanation

Shwachman-Diamond syndrome presents with metaphyseal dysostosis, exocrine pancreatic insufficiency (steatorrhea), and bone marrow dysfunction (neutropenia). It is an autosomal recessive disorder caused by mutations in the SBDS gene.

Question 75

A 17-year-old boy with achondroplasia complains of progressive bilateral leg weakness, numbness, and neurogenic claudication after walking short distances. He has a normal upper extremity neurological examination. Which of the following is the most likely cause of his symptoms?

Explanation

In older children and adults with achondroplasia, neurogenic claudication is typically due to lumbar spinal stenosis caused by congenitally short pedicles and decreased interpedicular distance. Foramen magnum stenosis usually presents in infancy with central apnea or high cervical myelopathy.

Question 76

A 14-year-old boy is evaluated for mild genu valgum. Radiographs reveal massive widening of the metaphyses of the long bones, classically described as an "Erlenmeyer flask" deformity, with marked cortical thinning. His skull radiographs are entirely normal. What is the most likely diagnosis?

Explanation

Pyle disease (familial metaphyseal dysplasia) is characterized by striking Erlenmeyer flask deformities of the long bones but normal cranial bones. This normal skull finding distinguishes it from craniometaphyseal dysplasia, which involves significant hyperostosis of the skull.

Question 77

Achondroplasia is caused by a gain-of-function mutation in the FGFR3 gene. This mutation primarily inhibits chondrocyte proliferation and differentiation. Which zone of the physis is directly suppressed in this condition?

Explanation

FGFR3 is a negative regulator of bone growth. Its constitutive activation in achondroplasia suppresses chondrocyte replication, primarily affecting the proliferative zone of the physis.

Question 78

A 5-year-old girl is referred for bilateral bowed legs and a waddling gait. Radiographs show widened, irregular metaphyses and coxa vara. Serum calcium, phosphorus, alkaline phosphatase, and 25(OH) vitamin D levels are strictly within normal limits. Based on the most likely diagnosis, how should her condition be initially managed?

Explanation

The normal laboratory values rule out rickets, pointing to Schmid metaphyseal chondrodysplasia. Treatment is mostly supportive, with corrective valgus osteotomies reserved for severe, progressive coxa vara or marked lower extremity malalignment.

Question 79

A 10-year-old boy presents with progressive facial paralysis and hearing loss. Radiographs show mild metaphyseal flaring of the long bones and severe hyperostosis of the cranial vault and skull base. A mutation in the ANKH gene is identified. The primary mechanism of this disease involves defective:

Explanation

Craniometaphyseal dysplasia is caused by ANKH gene mutations, leading to defective intracellular pyrophosphate transport. This results in severe cranial hyperostosis, which frequently compresses cranial nerves, causing facial palsy and deafness.

Question 80

A 6-year-old boy presents with a waddling gait and bowing of the lower extremities. Laboratory studies reveal normal calcium, phosphorus, and alkaline phosphatase. Radiographs demonstrate coxa vara and widened, irregular metaphyses, but the epiphyses are completely normal. Genetic testing reveals a mutation in COL10A1. Which specific zone of the physis is primarily disrupted by this pathogenesis?

Explanation

Schmid metaphyseal chondrodysplasia is caused by a mutation in the COL10A1 gene, leading to defective type X collagen. Type X collagen is exclusively synthesized by chondrocytes in the hypertrophic zone of the physis.

Question 81

A 2-year-old child of Amish descent is evaluated for short-limbed dwarfism. Physical examination notes fine, sparse hair and redundant skin. The child has a history of recurrent severe respiratory infections. Radiographs reveal metaphyseal flaring and irregularity. A mutation in which of the following is most likely responsible for this syndrome?

Explanation

Cartilage-hair hypoplasia (McKusick type) is an autosomal recessive disorder caused by mutations in the RMRP gene. It is characterized by metaphyseal dysplasia, sparse hair, and profound T-cell immunodeficiency leading to recurrent infections.

Question 82

A 4-year-old girl is referred to pediatric orthopedics for short stature and symmetrical metaphyseal irregularities on radiographs. Her medical history is significant for chronic steatorrhea and failure to thrive. Which of the following hematologic abnormalities is most commonly associated with her underlying diagnosis?

Explanation

This patient has Shwachman-Diamond syndrome, characterized by metaphyseal chondrodysplasia, exocrine pancreatic insufficiency, and bone marrow dysfunction. Cyclic neutropenia is a classic hematologic hallmark, predisposing these patients to recurrent infections.

Question 83

A 35-year-old male with achondroplasia presents with progressively worsening bilateral leg pain and weakness brought on by walking and relieved by sitting. Neurological examination reveals hyperreflexia in the lower extremities. Imaging confirms severe lumbar spinal stenosis. Which anatomic abnormality is the primary driver of central canal stenosis in this patient?

Explanation

In achondroplasia, premature closure of the neurocentral synchondroses results in congenitally shortened pedicles and a narrowed interpedicular distance. This developmental anomaly strictly limits the spinal canal diameter, making minor degenerative changes highly symptomatic.

Question 84

A 15-year-old asymptomatic female is evaluated for mild genu valgum. Radiographs exhibit striking, symmetrical widening of the distal femoral and proximal tibial metaphyses, classically described as an "Erlenmeyer flask" deformity, with significant cortical thinning. Her calvarium and cranial nerves are clinically and radiographically normal. What is the most likely diagnosis?

Explanation

Pyle disease (familial metaphyseal dysplasia) is characterized by massive metaphyseal flaring (Erlenmeyer flask deformity) and cortical thinning. It is typically asymptomatic and distinctively lacks the severe cranial sclerosis and cranial nerve impingement seen in craniometaphyseal dysplasia.

Question 85

A 7-year-old boy is diagnosed with pseudoachondroplasia after presenting with short-limbed dwarfism, a waddling gait, and joint laxity. His facial features are entirely normal. Radiographs show small, irregular epiphyses and widened metaphyses. The pathophysiology of this disease involves the intracellular accumulation of an abnormal protein within the rough endoplasmic reticulum. Which gene is mutated?

Explanation

Pseudoachondroplasia is caused by mutations in the COMP (Cartilage Oligomeric Matrix Protein) gene. The mutation leads to the accumulation of misfolded COMP in the rough endoplasmic reticulum of chondrocytes, causing premature cell death.

Question 86

A 10-year-old child evaluated for short stature is suspected of having a milder variant of an FGFR3 mutation. Genetic testing confirms the diagnosis of hypochondroplasia. Which of the following clinical features is typically ABSENT in this patient compared to a patient with classic achondroplasia?

Explanation

Hypochondroplasia is a milder skeletal dysplasia caused by FGFR3 mutations. Unlike classic achondroplasia, patients with hypochondroplasia typically lack the prominent craniofacial features such as frontal bossing and midface hypoplasia.

Question 87

An infant is born with a remarkably long, narrow trunk and severely shortened limbs. A prominent tail-like appendage is noted over the sacrum. Radiographs demonstrate marked platyspondyly and severe metaphyseal flaring, giving the long bones a dumbbell shape. As this child matures, what is the most critical and progressive orthopedic complication that requires close monitoring?

Explanation

The clinical picture describes metatropic dysplasia (TRPV4 mutation), named for the "changing" proportions as the child grows. While they are born with a long trunk and short limbs, they universally develop severe, rapidly progressive, and often treatment-resistant kyphoscoliosis, leading to a short trunk later in life.

Question 88

A 3-year-old boy presents with profound short stature, micrognathia, and hypercalcemia. Radiographs display wide, irregular, and markedly cupped metaphyses. Laboratory evaluation reveals hypophosphatemia and undetectable levels of endogenous parathyroid hormone (PTH). What is the exact pathophysiologic mechanism for his hypercalcemia?

Explanation

Jansen metaphyseal chondrodysplasia is caused by an activating mutation in the PTH1R gene (the PTH/PTHrP receptor). This constitutive activation leads to a pseudo-hyperparathyroid state with hypercalcemia, hypophosphatemia, and appropriately suppressed endogenous PTH.

Question 89

An 8-year-old boy with a known history of Schmid type metaphyseal chondrodysplasia presents with worsening waddling gait and bilateral hip pain. Radiographs confirm severe, progressive coxa vara with a neck-shaft angle of 85 degrees and a vertical physeal angle. What is the most appropriate definitive management?

Explanation

In Schmid metaphyseal chondrodysplasia, severe and progressive coxa vara (neck-shaft angle <100 degrees or vertical Hilgenreiner-epiphyseal angle) alters hip biomechanics and risks physeal slipping. Proximal femoral valgus osteotomy is the standard of care to correct the mechanical axis and alleviate symptoms.

Question 90

A 2-year-old boy from an Amish community presents with disproportionate short-limbed dwarfism, sparse, light-colored hair, and a history of recurrent severe viral infections. Radiographs reveal flaring and irregularity of the metaphyses. Which of the following genes is most likely mutated in this patient?

Explanation

This patient has Cartilage-Hair Hypoplasia (McKusick type), an autosomal recessive metaphyseal chondrodysplasia. It is caused by mutations in the RMRP gene and is uniquely associated with cellular immunodeficiency and sparse hair.

Question 91

A 4-year-old girl is diagnosed with a metaphyseal chondrodysplasia. She also suffers from chronic diarrhea, steatorrhea, and recurrent bacterial infections. Complete blood count shows absolute neutropenia. A mutation affecting which of the following cellular processes is most likely responsible?

Explanation

Shwachman-Diamond syndrome presents with metaphyseal chondrodysplasia, exocrine pancreatic insufficiency, and neutropenia. It is caused by mutations in the SBDS gene, which is essential for normal ribosome maturation.

Question 92

In a patient with Schmid type metaphyseal chondrodysplasia, genetic testing reveals a mutation in the COL10A1 gene. The defective protein produced by this gene is primarily synthesized in which specific region of the growing physis?

Explanation

Schmid metaphyseal chondrodysplasia is caused by mutations in the COL10A1 gene, which encodes type X collagen. Type X collagen is exclusively synthesized by chondrocytes in the hypertrophic zone of the physis.

Question 93

An infant presents with profound hypercalcemia, hypophosphatemia, and markedly irregular, cupped metaphyses on radiographs. Serum parathyroid hormone (PTH) levels are undetectable. What is the primary pathophysiologic mechanism of this condition?

Explanation

Jansen metaphyseal chondrodysplasia is caused by an activating (gain-of-function) mutation in the PTH1R gene. This leads to ligand-independent signaling of the PTH/PTHrP receptor, causing hypercalcemia and severe metaphyseal dysplasia despite suppressed PTH.

Question 94

A 7-year-old boy with known Schmid metaphyseal chondrodysplasia presents with a worsening waddling gait. Pelvic radiographs demonstrate bilateral coxa vara with a Hilgenreiner-epiphyseal (HE) angle of 75 degrees. What is the most appropriate management?

Explanation

In Schmid metaphyseal chondrodysplasia, progressive coxa vara with a Hilgenreiner-epiphyseal (HE) angle greater than 60 degrees carries a high risk of pseudarthrosis and progressive deformity. Valgus-producing proximal femoral osteotomies are indicated to correct the mechanical axis and shear forces.

Question 95

A 6-year-old boy presents with short stature. Examination shows mild rhizomelic limb shortening, but a normal facial appearance without frontal bossing or midface hypoplasia. Radiographs show narrowing of the interpedicular distances in the lower lumbar spine and mild metaphyseal flaring. Which genetic mutation is most likely?

Explanation

The patient's presentation is consistent with hypochondroplasia, a milder variant of achondroplasia. It is most commonly caused by the N540K mutation in the FGFR3 gene, and patients typically lack the severe craniofacial features seen in classic achondroplasia.

Question 96

A rare, autosomal recessive form of metaphyseal chondrodysplasia presents with severe genu varum and metaphyseal irregularities indistinguishable from the Schmid type on plain radiographs. Which of the following describes this specific condition?

Explanation

Spahr type metaphyseal chondrodysplasia is an autosomal recessive disorder that clinically and radiographically mimics the autosomal dominant Schmid type. It typically presents with significant bowing of the lower extremities (genu varum).

Question 97

A 5-month-old infant with achondroplasia is brought to the clinic due to episodic apneic spells, brisk deep tendon reflexes, and sustained ankle clonus. What is the most critical next step in management?

Explanation

Infants with achondroplasia are at high risk for foramen magnum stenosis, leading to cervicomedullary compression. Signs of myelopathy (hyperreflexia, clonus) or central apnea require urgent MRI of the craniocervical junction to evaluate the need for surgical decompression.

Question 98

A neonate is born with extremely short limbs, postaxial polydactyly, and a remarkably narrow, bell-shaped thorax. Radiographs show irregular metaphyses and handlebar-shaped clavicles. What is the most common cause of early mortality in this syndrome?

Explanation

Asphyxiating thoracic dysplasia (Jeune syndrome) is characterized by a narrow thorax, short limbs, and metaphyseal irregularities. The severely restricted rib cage causes pulmonary hypoplasia, making respiratory failure the leading cause of early mortality.

Question 99

A 9-year-old boy is evaluated for mild short stature. Historical radiographs from age 2 showed striking metaphyseal irregularities and fraying. Current radiographs, however, show that these metaphyseal changes have nearly completely resolved. Mutations in which gene are most classically associated with this spontaneously resolving dysplasia?

Explanation

Metaphyseal anadysplasia is a rare skeletal dysplasia defined by severe metaphyseal changes in early childhood that spontaneously regress or resolve with age. It is typically caused by mutations in the MMP13 or MMP9 genes.

Question 100

A 38-year-old male with classic achondroplasia presents with neurogenic claudication, complaining of bilateral leg pain and heaviness that worsens with walking and improves when sitting. What is the primary anatomical basis for his spinal stenosis?

Explanation

Adults with achondroplasia frequently develop symptomatic lumbar spinal stenosis. The stenosis is driven by underlying osseous anatomy, specifically congenitally shortened pedicles and decreased interpedicular distances, compounded by a thickened ligamentum flavum.

Pediatric Orthopaedics: Comprehensive MCQ Question Bank & Exam Prep

Metaphyseal Skeletal Dysplasias MCQs - Orthopedic Board

Metaphyseal Skeletal Dysplasias MCQs - Orthopedic Board

Chapter Index