ABOS Orthopedic Review: Dysplasias & Osteomalacia

Question 1

A 4-year-old boy presents with short stature, frontal bossing, and rhizomelic shortening. Radiographs show narrowing of the interpedicular distances in the lumbar spine. What is the underlying pathophysiology of this condition?

Explanation

Achondroplasia is caused by a gain-of-function mutation in the FGFR3 gene, which inhibits chondrocyte proliferation at the physis. This leads to rhizomelic dwarfism and spinal stenosis due to narrow interpedicular distances.

Question 2

A 5-year-old boy presents with progressively worsening varus deformity of both knees. Lab work shows low serum phosphate, normal serum calcium, and elevated alkaline phosphatase. Genetic testing reveals a PHEX gene mutation. Which of the following laboratory values is also expected?

Explanation

X-linked hypophosphatemic rickets (PHEX mutation) leads to an overproduction of FGF-23. Elevated FGF-23 causes renal phosphate wasting and inhibits 1-alpha-hydroxylase, preventing the activation of vitamin D.

Question 3

A 65-year-old patient presents with a chronically painful total knee arthroplasty done 3 years ago. CRP and ESR are mildly elevated. Aspiration yields 2,500 WBC/mcL with 65% PMNs. Which synovial fluid biomarker has the highest specificity for confirming a periprosthetic joint infection in this borderline scenario?

Explanation

Alpha-defensin is an antimicrobial peptide released by neutrophils that has highly specific and sensitive diagnostic value for periprosthetic joint infection (PJI), especially in borderline cases.

Question 4

A 4-year-old girl is diagnosed with oligoarticular Juvenile Idiopathic Arthritis (JIA) affecting her right knee. She is ANA positive and Rheumatoid Factor negative. What is the most critical screening protocol she requires to prevent a severe complication?

Explanation

Children with oligoarticular JIA who are ANA positive are at the highest risk for asymptomatic anterior uveitis. They require frequent slit-lamp examinations (typically every 3-4 months) to prevent permanent vision loss.

Question 5

A newborn presents with micromelic shortening of limbs, severe clubfeet, "hitchhiker" thumbs, and cystic swelling of the pinnae. Radiographs show first metacarpals that are oval-shaped. Which of the following is the defective mechanism in this disorder?

Explanation

Diastrophic dysplasia is caused by an autosomal recessive mutation in the SLC26A2 gene, resulting in a sulfate transport defect. This leads to undersulfated proteoglycans in cartilage matrix, presenting with cauliflower ears and hitchhiker thumbs.

Question 6

A 45-year-old male develops atraumatic rib fractures and profound muscle weakness. Laboratory testing reveals hypophosphatemia, hyperphosphaturia, and profoundly elevated levels of FGF-23. Whole-body functional imaging is most likely to identify which of the following?

Explanation

Tumor-induced osteomalacia is a paraneoplastic syndrome typically caused by a benign phosphaturic mesenchymal tumor secreting FGF-23. This causes renal phosphate wasting, leading to hypophosphatemia and osteomalacia.

Question 7

A 2-year-old boy presents with refusal to bear weight on the right leg and low-grade fever. Blood work shows a normal WBC count and mildly elevated CRP. Joint aspiration shows 45,000 WBCs. If the pathogen is a fastidious Gram-negative coccobacillus, what is the optimal method for isolation?

Explanation

Kingella kingae is a common cause of pediatric septic arthritis and osteomyelitis in children under 4. It is a fastidious organism that is best isolated by inoculating the synovial fluid directly into aerobic blood culture bottles.

Question 8

A 10-year-old boy presents with delayed eruption of secondary teeth and the ability to approximate his shoulders in the midline anteriorly. Radiographs demonstrate absent clavicles and delayed closure of cranial sutures. What bone formation process is primarily impaired in this condition?

Explanation

Cleidocranial dysplasia is caused by a mutation in the RUNX2 (CBFA1) gene, which primarily affects intramembranous ossification. This leads to hypoplastic or absent clavicles, open fontanelles, and dental anomalies.

Question 9

A 7-year-old girl with a history of daily spiking fevers, an evanescent salmon-pink rash, and polyarthritis now presents with sudden onset lethargy, mucosal bleeding, and hepatosplenomegaly. Labs reveal profound thrombocytopenia, elevated ferritin, and low fibrinogen. What is the most likely life-threatening complication?

Explanation

Macrophage activation syndrome (MAS) is a severe, potentially fatal complication of systemic JIA (Still's disease). It is characterized by pancytopenia, extremely high ferritin, liver dysfunction, and coagulopathy due to hyperactivation of macrophages.

Question 10

A 35-year-old immigrant presents with back pain, night sweats, and lower extremity weakness. MRI of the thoracic spine demonstrates destruction of the anterior vertebral bodies of T8 and T9 with a large paraspinal abscess, while the intervening disc space is relatively preserved. What is the most likely causative organism?

Explanation

Tuberculous spondylitis (Pott disease) typically involves the anterior aspects of two adjacent vertebral bodies and often spares the intervertebral disc until late in the disease process. A large, calcified paraspinal (cold) abscess is also highly characteristic.

Question 11

A 5-year-old child presents with disproportionate short-trunk dwarfism, a barrel chest, and coxa vara. Ophthalmologic exam reveals high myopia. Radiographs show flattened vertebral bodies and delayed ossification of the femoral heads. What is the underlying genetic defect?

Explanation

Spondyloepiphyseal dysplasia (SED) congenita is caused by a mutation in the COL2A1 gene, resulting in defective type II collagen. Clinical features include short-trunk dwarfism, coxa vara, and ocular issues like myopia and retinal detachment.

Question 12

A 55-year-old patient with end-stage renal disease presents with diffuse bone pain. Radiographs of the spine demonstrate alternating bands of sclerosis and lucency parallel to the vertebral endplates. What is the primary pathophysiologic driver of this specific radiographic finding?

Explanation

The "rugger jersey spine" is characteristic of renal osteodystrophy. It is caused by secondary hyperparathyroidism, where excess PTH stimulates osteoblastic activity leading to bands of osteosclerosis adjacent to the vertebral endplates.

Question 13

A 70-year-old man develops a chronic infection around his total hip arthroplasty, and cultures grow Staphylococcus epidermidis. Which phase of biofilm formation is characterized by the production of an extracellular polymeric substance (EPS) that creates a physical barrier against antibiotics?

Explanation

During the maturation phase of biofilm development, bacteria secrete an extracellular polymeric substance (glycocalyx) that encases the colony. This EPS matrix acts as a robust physical and chemical barrier against both host immune cells and systemic antibiotics.

Question 14

A 15-year-old boy with disproportionate short stature, frontal bossing, and rhizomelic shortening presents with severe leg pain and numbness after walking short distances. Genetic testing confirms a mutation in FGFR3. Which of the following is the most likely anatomic cause of his current symptoms?

Explanation

Achondroplasia is caused by an FGFR3 mutation leading to short pedicles and decreased interpedicular distance in the lumbar spine. This classically results in claudication and spinal stenosis in adolescence or adulthood.

Question 15

A 9-year-old child presents with a waddling gait, knee pain, and short stature. Radiographs reveal delayed, fragmented epiphyseal ossification centers and a "double-layered" patella on the lateral knee view. The most likely mutated gene encodes for which of the following proteins?

Explanation

Multiple Epiphyseal Dysplasia (MED) with a double-layered patella is classically associated with mutations in the Cartilage Oligomeric Matrix Protein (COMP) gene. It primarily affects the epiphyseal ossification centers, delaying their formation.

Question 16

A newborn is evaluated for severe micromelic dwarfism, bilateral rigid clubfeet, "hitchhiker" thumbs, and cystic swelling of the pinnae. The genetic mutation responsible for this condition directly affects which of the following cellular processes?

Explanation

Diastrophic dysplasia is caused by a mutation in the SLC26A2 gene, which encodes a sulfate transporter (DTDST). This results in undersulfated proteoglycans in the cartilage matrix, leading to the classic clinical triad of hitchhiker thumbs, cauliflower ears, and clubfeet.

Question 17

A 6-year-old boy with a disproportionately short trunk, coxa vara, and barrel chest is diagnosed with spondyloepiphyseal dysplasia congenita (SEDC). Before he undergoes surgical correction of his coxa vara, which of the following screening examinations is most critical?

Explanation

Spondyloepiphyseal dysplasia congenita (SEDC) is a type II collagenopathy associated with odontoid hypoplasia and atlantoaxial instability. Preoperative cervical spine screening is critical to prevent catastrophic neurologic injury during intubation.

Question 18

A 12-year-old girl presents with retained deciduous teeth and an ability to bring her shoulders together in the midline anteriorly. Radiographs demonstrate absent clavicles and delayed pubic symphysis ossification. The defective gene in this condition is an essential transcription factor for which of the following?

Explanation

Cleidocranial dysplasia results from a mutation in RUNX2 (CBFA1), a master transcription factor required for osteoblast differentiation. Patients classically present with absent clavicles, delayed cranial suture closure, and dental anomalies.

Question 19

A 4-year-old boy has progressive varus bowing of his legs. Lab testing shows low serum phosphate, normal calcium, normal PTH, normal vitamin D, and elevated FGF23 levels. What monoclonal antibody therapy directly targets the underlying pathophysiology of this condition?

Explanation

Burosumab is a monoclonal antibody that binds to and inhibits FGF23, directly addressing the underlying pathophysiology of X-linked hypophosphatemic rickets (XLH). This inhibition restores renal phosphate reabsorption and improves serum phosphate levels.

Question 20

A 14-month-old exclusively breastfed infant develops widening of the wrists and bowing of the distal femurs. Laboratory evaluation reveals significantly elevated parathyroid hormone (PTH) and alkaline phosphatase. Which of the following serum calcium and phosphate profiles is most consistent with this diagnosis?

Explanation

Nutritional Vitamin D deficiency rickets initially presents with low to normal serum calcium and persistently low phosphate due to secondary hyperparathyroidism. The elevated PTH promotes aggressive renal phosphate wasting.

Question 21

A 55-year-old male presents with severe diffuse bone pain, proximal muscle weakness, and multiple Looser zones on radiographs. Labs reveal marked hypophosphatemia, hyperphosphaturia, normal calcium, and elevated FGF23. If a causative neoplasm is identified, it is most likely to be which of the following?

Explanation

Tumor-induced osteomalacia (TIO) is a paraneoplastic syndrome typically caused by a benign phosphaturic mesenchymal tumor that secretes excessive FGF23. Complete resection of the tumor usually cures the osteomalacia.

Question 22

A 65-year-old female with a history of severe celiac disease complains of bilateral groin pain. Radiographs demonstrate transverse lucencies with sclerotic margins along the medial aspect of bilateral femoral necks. These lesions (Looser zones) represent which of the following?

Explanation

Looser zones (pseudofractures) are wide transverse lucencies that represent stress fractures repaired by inadequately mineralized osteoid. They classically occur on the compressive side of bones, such as the medial femoral neck, in patients with osteomalacia.

Question 23

A 2-year-old boy presents with a 3-day history of right knee swelling, limp, and low-grade fever. Synovial fluid aspiration shows a WBC count of 65,000/microL. Routine cultures at 48 hours are negative. Which of the following is the most likely causative organism and the best method for its detection?

Explanation

Kingella kingae is a leading cause of pediatric septic arthritis and osteomyelitis in children under 4 years old. Because it is fastidious and difficult to culture on standard media, PCR is the most sensitive and rapid diagnostic method.

Question 24

A 6-month-old infant is brought to the emergency department for irritability and refusal to move the left leg. Ultrasound reveals a left hip effusion. Which of the following features most strongly dictates the need for immediate surgical drainage of the hip joint?

Explanation

A purulent joint aspirate with a WBC count > 50,000/microL confirms septic arthritis, which is a surgical emergency. Immediate arthrotomy and irrigation of the infant hip are required to prevent irreversible cartilage destruction and avascular necrosis.

Question 25

A 7-year-old girl from Connecticut presents with a massive right knee effusion. Surprisingly, she bears weight with a minimal limp and has only mild discomfort with range of motion. Joint aspiration reveals 40,000 WBCs/microL with 80% neutrophils. What is the most appropriate next step in management?

Explanation

Lyme arthritis commonly presents with a massive knee effusion, but patients paradoxically maintain weight-bearing and have minimal pain compared to those with bacterial septic arthritis. Diagnosis requires two-tiered serologic testing, and it is primarily managed medically rather than with urgent surgical washout.

Question 26

A 45-year-old immigrant presents with chronic back pain, night sweats, and weight loss. MRI of the thoracic spine demonstrates destruction of the T8 and T9 vertebral bodies with a large paraspinal abscess, but relative preservation of the T8-T9 intervertebral disc. Which of the following is the most likely diagnosis?

Explanation

Tuberculous spondylitis (Pott's disease) classically demonstrates anterior vertebral body destruction with relative preservation of the intervertebral disc spaces until late in the disease. Pyogenic infections, in contrast, typically destroy the disc early.

Question 27

A 4-year-old boy presents with severe bowing of the lower extremities. Labs show normal calcium, low phosphorus, and elevated alkaline phosphatase. Genetic testing reveals a PHEX gene mutation. Which of the following is the primary mechanism of this disease?

Explanation

PHEX mutations lead to decreased degradation of FGF-23 in X-linked hypophosphatemic rickets. Elevated FGF-23 causes increased renal phosphate excretion and inhibits 1-alpha-hydroxylase, leading to severe hypophosphatemia and rickets.

Question 28

A 45-year-old male with achondroplasia presents with neurogenic claudication. Which of the following best describes the underlying pathophysiology of his skeletal dysplasia?

Explanation

Achondroplasia is caused by a gain-of-function mutation in the FGFR3 gene (G380R). This mutation excessively inhibits chondrocyte proliferation in the proliferative zone of the physis, leading to rhizomelic dwarfism and narrowed spinal pedicles.

Question 29

A 10-year-old child presents with bilateral hip pain, a waddling gait, and normal height. Radiographs show small, irregular, and fragmented epiphyses with a "double-layer" appearance of the patella on the lateral knee radiograph. Which gene is most commonly mutated in this condition?

Explanation

Multiple Epiphyseal Dysplasia (MED) is most commonly caused by mutations in the COMP (Cartilage Oligomeric Matrix Protein) gene. The "double-layer" patella is a pathognomonic radiographic finding for this condition.

Question 30

A newborn is evaluated for severe short-limbed dwarfism. Examination reveals bilateral clubfeet, "hitchhiker" thumbs, and prominent swelling of the auricles. Which of the following is the underlying genetic defect?

Explanation

Diastrophic dysplasia is an autosomal recessive condition caused by mutations in the SLC26A2 sulfate transporter gene. Classic findings include hitchhiker thumbs, cauliflower ears, and severe rigid clubfeet.

Question 31

A 3-year-old girl is diagnosed with oligoarticular Juvenile Idiopathic Arthritis (JIA) affecting her right knee. She has a positive ANA and negative Rheumatoid Factor. What is the most critical screening protocol she must undergo?

Explanation

Young girls with ANA-positive oligoarticular JIA are at the highest risk for asymptomatic anterior uveitis. They require frequent slit-lamp examinations (every 3-4 months) by an ophthalmologist to prevent permanent vision loss.

Question 32

A 50-year-old male presents with diffuse bone pain and recurrent metatarsal fractures. Labs show severe hypophosphatemia, normal calcium, and elevated FGF-23 levels. A whole-body DOTATATE PET/CT reveals a small soft-tissue mass in his thigh. What is the expected outcome following complete marginal excision of the mass?

Explanation

Tumor-induced osteomalacia is a paraneoplastic syndrome caused by an FGF-23 secreting phosphaturic mesenchymal tumor. Complete surgical resection of the tumor typically cures the condition, leading to rapid normalization of phosphate and FGF-23.

Question 33

A 14-month-old child presents with a limp, mild fever, and a swollen knee. Aspiration yields purulent fluid. Gram stain is negative, but PCR confirms the presence of an organism. Given the patient's age and clinical presentation, which organism is increasingly recognized as a leading cause?

Explanation

Kingella kingae is a fastidious Gram-negative organism that is now recognized as a leading cause of osteoarticular infections in children aged 6 months to 4 years. It is best detected via PCR or by inoculating synovial fluid directly into blood culture vials.

Question 34

A 7-year-old boy presents for orthopedic evaluation. He is remarkably able to bring his shoulders together anteriorly in the midline. Radiographs reveal absent clavicles and delayed closure of the cranial sutures. A mutation in which of the following transcription factors is responsible?

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by mutations in the RUNX2 (CBFA1) gene, an essential transcription factor for osteoblast differentiation. It is characterized by absent or hypoplastic clavicles and delayed cranial suture closure.

Question 35

A 5-year-old child with short-trunk dwarfism and severe myopia presents with a waddling gait. Radiographs show flattened vertebral bodies and delayed ossification of the capital femoral epiphyses. Prior to undergoing general anesthesia, what imaging study is most critical?

Explanation

Spondyloepiphyseal Dysplasia (SED) Congenita is a type II collagenopathy (COL2A1) frequently associated with odontoid hypoplasia and atlantoaxial instability. Flexion-extension cervical spine radiographs are essential before general anesthesia to prevent catastrophic spinal cord injury.

Question 36

A 55-year-old female with end-stage renal disease presents with diffuse bone pain. Radiographs of her spine demonstrate sclerotic bands at the superior and inferior endplates of the vertebral bodies. Which of the following lab profiles is most consistent with her condition?

Explanation

Renal osteodystrophy typically causes secondary hyperparathyroidism due to phosphate retention and renal failure to produce 1,25-OH vitamin D. This results in hypocalcemia, hyperphosphatemia, and elevated PTH, causing the classic "rugger jersey" spine appearance.

Question 37

A 7-year-old boy with a history of systemic Juvenile Idiopathic Arthritis (JIA) presents to the emergency department with high continuous fever, hepatosplenomegaly, purpura, and abrupt onset of severe lethargy. Labs reveal falling ESR, extreme hyperferritinemia, and thrombocytopenia. What is the most likely life-threatening complication occurring?

Explanation

Macrophage Activation Syndrome (MAS) is a severe, life-threatening complication of systemic JIA. It is characterized by sudden fever, hepatosplenomegaly, cytopenias, a paradoxically falling ESR, and severely elevated ferritin, requiring immediate high-dose corticosteroids.

Question 38

A 24-month-old toddler presents with a waddling gait and bowing of the legs. Radiographs show widening and cupping of the distal femoral physes, as well as radiolucent bands perpendicular to the cortex in the medial femoral neck. Which of the following is the most likely diagnosis for the radiolucent bands?

Explanation

Looser zones, or pseudofractures, are wide transverse radiolucencies traversing part of the bone, representing accumulations of unmineralized osteoid. They are a classic radiographic hallmark of osteomalacia and severe rickets.

Question 39

A 9-year-old boy is admitted with severe, multifocal MRSA osteomyelitis and an associated deep vein thrombosis (DVT) of the affected extremity. The severity of this infection and the development of DVT are most strongly associated with which bacterial virulence factor?

Explanation

Panton-Valentine leukocidin (PVL) is a cytotoxin produced by community-acquired MRSA strains that destroys leukocytes and causes severe tissue necrosis. PVL-positive strains are strongly associated with multifocal disease, aggressive osteomyelitis, and adjacent deep vein thrombosis.

Question 40

A 3-year-old child with frequent fractures, blue sclerae, and dentinogenesis imperfecta is started on intravenous bisphosphonate therapy (pamidronate). What is the primary cellular mechanism of action of this medication in treating this disease?

Explanation

Bisphosphonates are analogues of inorganic pyrophosphate that bind to hydroxyapatite and are ingested by osteoclasts. They inhibit osteoclastic bone resorption by disrupting the mevalonate pathway and inducing osteoclast apoptosis, thereby increasing bone mineral density in Osteogenesis Imperfecta.

Question 41

A 6-month-old male with achondroplasia presents with witnessed episodes of central apnea, poor head control, and hyperreflexia in the lower extremities. What is the most appropriate next step in management?

Explanation

Central apnea and hyperreflexia in an infant with achondroplasia are major red flags for cervicomedullary compression. MRI is the diagnostic modality of choice to evaluate foramen magnum stenosis before proceeding to surgical decompression.

Question 42

A 5-year-old boy sustains a low-energy femur fracture. On examination, he has blue sclerae and generalized osteopenia on radiographs. A genetic defect in which of the following is most likely responsible for his condition?

Explanation

Osteogenesis imperfecta is primarily caused by autosomal dominant mutations in the COL1A1 or COL1A2 genes. This leads to quantitative or qualitative defects in type I collagen, the major structural protein in bone.

Question 43

A 10-year-old boy presents with progressive knee pain and short stature. Radiographs demonstrate delayed ossification of the capital femoral epiphyses and irregular secondary ossification centers. A lateral radiograph of the knee reveals a pathognomonic finding. What is this finding?

Explanation

Multiple epiphyseal dysplasia (MED) is characterized by delayed ossification of multiple epiphyses. A double-layered patella on a lateral knee radiograph is considered a pathognomonic finding for this dysplasia.

Question 44

A 4-year-old child with short-trunk dwarfism, severe myopia, and coxa vara is scheduled for a surgical procedure requiring general anesthesia. To prevent a catastrophic neurologic complication during intubation, which abnormality must be ruled out?

Explanation

Spondyloepiphyseal dysplasia congenita affects type II collagen and causes short-trunk dwarfism. Patients frequently have odontoid hypoplasia leading to atlantoaxial instability, strictly requiring cervical flexion-extension films before anesthesia.

Question 45

A 3-year-old child presents with progressive varus bowing of the lower extremities. Laboratory studies reveal a normal serum calcium, significantly decreased serum phosphate, normal PTH, and elevated alkaline phosphatase. What is the primary pathophysiologic mechanism of this disease?

Explanation

X-linked hypophosphatemic rickets is caused by a PHEX gene mutation that leads to excess FGF23 production. FGF23 decreases renal tubular reabsorption of phosphate, resulting in profound hypophosphatemia and secondary rickets.

Question 46

A neonate is noted to have a large anterior fontanelle, delayed closure of cranial sutures, and unusual mobility of the shoulders allowing them to be opposed anteriorly. A mutation in which of the following transcription factors is responsible for this condition?

Explanation

Cleidocranial dysplasia is caused by an autosomal dominant mutation in the RUNX2 (CBFA1) gene, which regulates osteoblast differentiation. This leads to defective intramembranous ossification, characterized by absent/hypoplastic clavicles and delayed fontanelle closure.

Question 47

A newborn is evaluated in the NICU and noted to have very short limbs, rigid equinovarus foot deformities, 'hitchhiker' thumbs, and cystic swelling of the external ear. The genetic mutation causing this dysplasia affects which of the following?

Explanation

Diastrophic dysplasia is an autosomal recessive disorder caused by a mutation in the SLC26A2 sulfate transporter gene (DTDST). Clinical hallmarks include cauliflower ears, hitchhiker thumbs, and severe, rigid clubfeet.

Question 48

A 2-year-old child recently immigrated to the US presents with irritability, delayed walking, and widening of the wrists. Radiographs show cupping and fraying of the metaphyseal-physeal junctions. Which of the following laboratory profiles confirms the diagnosis of nutritional rickets?

Explanation

Nutritional rickets results from Vitamin D deficiency, leading to decreased calcium absorption. This triggers secondary hyperparathyroidism, which increases serum calcium toward normal but causes profound renal phosphate wasting.

Question 49

A 45-year-old male with end-stage renal disease on hemodialysis presents with diffuse bone pain. Radiographs demonstrate 'rugger jersey' spine and osteopenia. What is the primary driving mechanism for his metabolic bone disease?

Explanation

In renal osteodystrophy, failing kidneys cannot excrete phosphate or adequately synthesize 1,25-dihydroxyvitamin D. The resulting hyperphosphatemia and hypocalcemia drive severe secondary hyperparathyroidism, leading to high-turnover bone disease.

Question 50

A 2-year-old child presents with a limp, fever, and a swollen knee. Aspiration yields purulent synovial fluid with a WBC of 65,000. Standard aerobic and anaerobic cultures on solid media are negative at 48 hours. What technique would have best optimized the isolation of the most likely responsible organism?

Explanation

Kingella kingae is a highly fastidious organism and the most common cause of septic arthritis in children under 4 years old. Isolation is significantly enhanced by inoculating synovial fluid directly into aerobic BACTEC blood culture vials.

Question 51

Staphylococcus epidermidis frequently causes chronic prosthetic joint infections due to its ability to form a robust biofilm. Which specific structural component produced by this bacteria is primarily responsible for adherence and biofilm architecture?

Explanation

S. epidermidis secures itself to orthopaedic implants via a biofilm composed of an exopolysaccharide matrix. The critical structural component of this glycocalyx is polysaccharide intercellular adhesin (PIA).

Question 52

A 55-year-old male with diabetes presents with a rapidly progressive, extremely painful, swollen, and erythematous lower extremity. You calculate a Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC) score. Which of the following laboratory parameters is NOT included in this score?

Explanation

The LRINEC score helps differentiate necrotizing fasciitis from severe cellulitis. Its variables include CRP, WBC, hemoglobin, sodium, creatinine, and glucose; serum potassium is not part of the score.

Question 53

A 65-year-old female presents with a painful total knee arthroplasty 2 years post-op. Synovial fluid aspiration reveals a WBC count of 3,500 cells/uL with 75% neutrophils. Which of the following biomarkers in the synovial fluid has the highest specificity for confirming a periprosthetic joint infection?

Explanation

Alpha-defensin is an antimicrobial peptide released by neutrophils in response to pathogens. It is a highly sensitive and specific synovial fluid biomarker currently used to confirm periprosthetic joint infection.

Question 54

A 4-year-old female is recently diagnosed with oligoarticular Juvenile Idiopathic Arthritis. Her laboratory workup is positive for Antinuclear Antibody (ANA). Which of the following represents the most critical screening protocol for this patient?

Explanation

Young females with ANA-positive oligoarticular JIA carry the highest risk of developing asymptomatic anterior uveitis. Routine slit-lamp examinations every 3 to 4 months are critical to prevent permanent visual impairment or blindness.

Question 55

A 6-year-old boy presents with daily spiking fevers, generalized lymphadenopathy, a transient salmon-colored macular rash, and polyarthritis. Which potentially fatal hematologic complication is most uniquely associated with this specific subtype of Juvenile Idiopathic Arthritis?

Explanation

Systemic JIA (Still's disease) uniquely predisposes patients to Macrophage Activation Syndrome (MAS). MAS is a severe, life-threatening complication characterized by an overwhelming inflammatory cascade and multi-organ failure.

Question 56

A 26-year-old female with a long-standing history of polyarticular Juvenile Idiopathic Arthritis is scheduled for a bilateral total hip arthroplasty. During preoperative templating and surgical planning, which anatomic deformity on the femoral side should the surgeon anticipate?

Explanation

Patients with severe JIA commonly develop hypoplastic bone anatomy due to early physeal closure and lack of normal weight-bearing. The proximal femur characteristically exhibits excessive anteversion, coxa valga, and an extremely narrow intramedullary canal.

Question 57

Which of the following radiographic findings is typically the earliest and most characteristic manifestation of Juvenile Idiopathic Arthritis in the cervical spine?

Explanation

The hallmark of JIA in the cervical spine is early ankylosis of the apophyseal (facet) joints, classically beginning at the C2-C3 level. This progressive fusion alters cervical biomechanics and frequently leads to compensatory subaxial subluxation.

Question 58

A 4-year-old boy presents with rhizomelic short stature, frontal bossing, and midface hypoplasia. Radiographs reveal narrowing of the interpedicular distances in the lumbar spine. Which of the following genes is mutated in this condition?

Explanation

Achondroplasia is caused by an activating mutation in the FGFR3 gene, which inhibits chondrocyte proliferation in the proliferative zone of the physis. It presents with rhizomelic dwarfism and narrowing of the lumbar interpedicular distance, increasing the risk of spinal stenosis.

Question 59

A 7-year-old child presents with a short trunk, coxa vara, and normal length hands and feet. Radiographs reveal flattened vertebral bodies. What is the most critical anatomical area to evaluate to prevent catastrophic neurologic injury in this patient?

Explanation

Spondyloepiphyseal dysplasia congenita (caused by a COL2A1 mutation) is highly associated with odontoid hypoplasia, leading to atlantoaxial instability. Flexion-extension cervical spine radiographs are mandatory before any surgical procedure.

Question 60

A 5-year-old girl with severe bowing of the lower extremities is diagnosed with X-linked hypophosphatemic rickets (XLH). Which laboratory profile is most specific to the pathophysiology of her disease?

Explanation

XLH is caused by a PHEX gene mutation that leads to excess production of FGF23. Elevated FGF23 causes profound renal phosphate wasting, resulting in hypophosphatemia with normal serum calcium and normal PTH levels.

Question 61

Which of the following patients with Juvenile Idiopathic Arthritis (JIA) requires the most frequent slit-lamp examinations due to having the highest risk for developing silent anterior uveitis?

Explanation

The highest risk for silent, asymptomatic anterior uveitis in JIA occurs in young, female patients with oligoarticular onset who are antinuclear antibody (ANA) positive. Frequent routine screening is mandatory to prevent blindness.

Question 62

A 68-year-old man presents with a painful total knee arthroplasty 3 years postoperatively. Synovial fluid analysis shows a WBC of 4,500 cells/mcL with 85% neutrophils. Which synovial fluid biomarker, if elevated, is most highly specific for diagnosing a periprosthetic joint infection?

Explanation

Alpha-defensin is an antimicrobial peptide released by neutrophils in response to pathogens. It is highly sensitive and specific for diagnosing periprosthetic joint infection (PJI) and is included in the updated MSIS diagnostic criteria.

Question 63

A newborn presents with short-limbed dwarfism, hitchhiker thumbs, bilateral severe clubfeet, and cystic swelling of the external ears. What is the genetic mechanism of this disorder?

Explanation

Diastrophic dysplasia is caused by a mutation in the SLC26A2 gene (DTDST), which leads to a defect in a sulfate transporter essential for cartilage proteoglycan sulfation. The presence of 'cauliflower ear' is pathognomonic.

Question 64

A 14-month-old presents with acute knee swelling and refusal to bear weight. Aspiration yields cloudy fluid. Routine synovial cultures on blood agar are negative at 48 hours. What is the optimal method to isolate the most likely organism in this specific age group?

Explanation

Kingella kingae is a fastidious Gram-negative organism and a leading cause of septic arthritis in children under 4 years of age. It is best isolated by inoculating synovial fluid directly into BACTEC blood culture vials or via PCR testing.

Question 65

A 55-year-old man presents with diffuse bone pain and progressive muscle weakness. Labs show profoundly low serum phosphate, normal calcium, normal PTH, and elevated alkaline phosphatase. A whole-body MRI reveals a small soft tissue mass in his thigh. Excision of this mass will reverse his symptoms by decreasing levels of:

Explanation

Oncogenic osteomalacia (tumor-induced osteomalacia) is typically caused by a phosphaturic mesenchymal tumor that secretes excessive FGF23. Elevated FGF23 causes renal phosphate wasting; excision of the tumor is curative.

Question 66

A 14-year-old female with long-standing polyarticular JIA presents for preoperative clearance for a total hip arthroplasty. Which of the following cervical spine abnormalities is most characteristic of JIA and poses a significant intubation risk?

Explanation

JIA commonly affects the cervical spine, leading to early fusion (ankylosis) of the apophyseal joints, most frequently at C2-C3. This restricts neck extension and complicates airway management during general anesthesia.

Question 67

A 10-year-old boy presents with bilateral knee and hip pain. Radiographs show delayed, irregular ossification of the capital femoral epiphyses and a double-layered patella on the lateral view. His spine is radiographically normal. Which of the following genes is most commonly mutated in this condition?

Explanation

Multiple Epiphyseal Dysplasia (MED) is characterized by abnormal epiphyses with a normal spine, and a double-layered patella is a classic radiographic finding. It is most commonly caused by an autosomal dominant mutation in the COMP gene.

Question 68

According to the Cierny-Mader classification of adult osteomyelitis, what specifically defines a Type B host?

Explanation

In the Cierny-Mader classification, host status is divided into Type A (normal), Type B (compromised locally or systemically by factors like diabetes or smoking), and Type C (treatment is worse than the disease).

Question 69

A 1-year-old exclusively breastfed infant presents with bowing of the distal radius and widening of the physes. Which laboratory profile is most consistent with the early symptomatic stages of nutritional rickets?

Explanation

Vitamin D deficiency rickets initially presents with decreased enteral calcium absorption, leading to hypocalcemia. This triggers secondary hyperparathyroidism (elevated PTH), which normalizes calcium slightly but causes renal phosphate wasting (hypophosphatemia) and elevates alkaline phosphatase.

Question 70

A 12-year-old girl presents with excessive shoulder mobility, allowing her to touch her shoulders together anteriorly. Radiographs reveal aplastic clavicles. What other clinical finding is most characteristic of this genetic condition?

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by a RUNX2 (CBFA1) mutation. It is classically associated with absent or hypoplastic clavicles, delayed skull suture closure, and supernumerary or delayed eruption of teeth.

Question 71

A 7-year-old with systemic JIA develops a sudden high unremitting fever, hepatosplenomegaly, and bleeding gums. Labs show a rapidly falling ESR, severe pancytopenia, and a ferritin level of 12,000 ng/mL. What is the most appropriate initial management for this complication?

Explanation

Macrophage Activation Syndrome (MAS) is a severe, life-threatening complication of systemic JIA characterized by pancytopenia, paradoxical falling ESR (fibrinogen consumption), and markedly elevated ferritin. Immediate treatment with high-dose corticosteroids and immunomodulators is required.

Question 72

A 9-year-old boy with a history of recurrent skin boils presents with femur osteomyelitis and a concurrent deep vein thrombosis in the adjacent femoral vein. Which virulence factor is most responsible for this specific hypervirulent presentation?

Explanation

Community-acquired MRSA osteomyelitis often presents with severe disease, including adjacent DVT and multifocal abscesses. This hypervirulence is strongly associated with the Panton-Valentine leukocidin (PVL) cytotoxin, which causes extensive tissue necrosis and leukocyte destruction.

Question 73

A 6-month-old infant presents with bowing of the extremities and failure to thrive. Radiographs show severe osteopenia and widened, irregular physes resembling rickets. Labs reveal hypercalcemia and an exceptionally low serum alkaline phosphatase. What is the primary underlying defect?

Explanation

Hypophosphatasia mimics rickets clinically but is caused by an ALPL gene mutation leading to deficient tissue-nonspecific alkaline phosphatase (TNSALP). This prevents the cleavage of inorganic pyrophosphate, a potent inhibitor of bone mineralization.

Question 74

A 10-year-old boy presents with a femur fracture after a minor fall. Radiographs demonstrate a 'bone-within-a-bone' appearance and an absence of medullary canals. He also has a history of cranial nerve palsies. The pathogenesis involves failure of which cellular mechanism?

Explanation

Osteopetrosis (Albers-Schonberg disease) results from defective osteoclast function, commonly due to an inability to form a ruffled border or acidify the resorption pit (e.g., Carbonic Anhydrase II mutations). This leads to dense, brittle bones and cranial nerve entrapment from narrowed foramina.

Question 75

A 22-year-old sexually active female presents with a 3-day history of migratory polyarthralgia, now localized as a swollen right wrist. She has sparse, painless pustular skin lesions on her distal extremities. What is the most common finding on synovial fluid culture from the wrist?

Explanation

Disseminated gonococcal infection classically presents with migratory polyarthritis, tenosynovitis, and dermatitis. Synovial fluid cultures are frequently negative (representing a sterile reactive phase), but mucosal cultures or PCR often confirm the diagnosis.

Question 76

A 14-year-old girl with a history of precocious puberty and large cafe-au-lait spots presents with a pathologic fracture of her proximal femur. Radiographs show a ground-glass lytic lesion with a shepherd's crook deformity. What is the underlying genetic mutation?

Explanation

McCune-Albright syndrome consists of polyostotic fibrous dysplasia, cafe-au-lait spots (with irregular 'coast of Maine' borders), and endocrine abnormalities. It is caused by a post-zygotic somatic activating mutation in the GNAS1 gene.

Question 77

A 6-year-old child presents with a disproportionately short trunk, barrel chest, and coxa vara. Radiographs reveal delayed ossification of the pubic bones, platyspondyly, and a hypoplastic odontoid. What is the underlying genetic mutation associated with this condition?

Explanation

This presentation is classic for spondyloepiphyseal dysplasia (SED) congenita, which is caused by a defect in type II collagen due to a COL2A1 gene mutation. Type II collagen is crucial for proper hyaline cartilage formation, affecting the spine and epiphyses.

Question 78

A 45-year-old presents with progressive diffuse bone pain and muscle weakness. Labs show profound hypophosphatemia, hyperphosphaturia, normal calcium, and low 1,25-dihydroxyvitamin D. Parathyroid hormone levels are normal. What is the most likely underlying mechanism?

Explanation

The clinical picture describes tumor-induced osteomalacia, typically caused by a phosphaturic mesenchymal tumor. These tumors secrete FGF-23, which promotes renal phosphate wasting and inhibits 1-alpha-hydroxylase, leading to low active Vitamin D levels.

Question 79

A 14-month-old child presents with a limp, a temperature of 38.2C, and refusal to bear weight on the right leg. Routine blood cultures are negative, but synovial fluid PCR detects the causative organism. Which of the following best describes the most likely organism responsible for this presentation?

Explanation

Kingella kingae is a Gram-negative diplobacillus and is the most common cause of pediatric septic arthritis in children under 4 years old. It is fastidious and often requires specific BACTEC blood culture vials or PCR for detection.

Question 80

A 16-year-old female with long-standing polyarticular juvenile idiopathic arthritis is scheduled for a bilateral total hip arthroplasty. Before proceeding with intubation and surgery, which of the following is the most critical radiographic screening to perform?

Explanation

Patients with polyarticular or systemic JIA frequently develop cervical spine involvement, including atlantoaxial instability and subaxial subluxation. Flexion-extension cervical spine radiographs are mandatory before any procedure requiring intubation to prevent catastrophic neurologic injury.

Question 81

A newborn presents with short-limbed dwarfism, bilateral rigid clubfeet, hitchhiker thumbs, and cystic swelling of the external ear. Which of the following genes is mutated in this condition?

Explanation

This describes diastrophic dysplasia, an autosomal recessive condition characterized by hitchhiker thumbs, cauliflower ears, and severe clubfeet. It is caused by a mutation in the SLC26A2 gene (also known as DTDST), which encodes a sulfate transporter.

Question 82

A 68-year-old male is 3 weeks post-operative from a primary total hip arthroplasty. He presents with 3 days of acute hip pain, focal erythema, and a draining sinus tract. Radiographs show a well-fixed implant without loosening. What is the most appropriate surgical management?

Explanation

In cases of acute post-operative periprosthetic joint infection (less than 4 weeks from index surgery) with well-fixed components and intact soft tissues, DAIR with modular component exchange is the standard of care. Two-stage exchange is reserved for chronic infections or loose components.

Question 83

A 2-year-old boy presents with bowing of the lower extremities and delayed walking. Laboratory studies demonstrate hypercalcemia, normal phosphorus, and markedly decreased serum alkaline phosphatase. Urine phosphoethanolamine is elevated. What is the primary biochemical defect?

Explanation

This clinical profile is pathognomonic for hypophosphatasia, an inborn error of metabolism caused by a mutation in the TNSALP gene. It leads to impaired skeletal mineralization resembling rickets, but uniquely presents with profoundly low alkaline phosphatase.

Question 84

A 10-year-old girl is evaluated for delayed dental eruption and abnormal shoulder mobility. Examination shows she can bring her anterior shoulders together in the midline. Which of the following transcription factors is deficient in this condition?

Explanation

The patient has cleidocranial dysplasia, characterized by absent or hypoplastic clavicles, delayed cranial suture closure, and dental abnormalities. It is caused by a mutation in the RUNX2 (CBFA1) gene, an essential transcription factor for osteoblast differentiation.

Question 85

A 3-year-old girl is diagnosed with oligoarticular juvenile idiopathic arthritis involving her right knee. She is ANA positive and Rheumatoid Factor negative. Which of the following is the most important regular screening test for this patient?

Explanation

Patients with ANA-positive oligoarticular JIA are at the highest risk for developing asymptomatic chronic anterior uveitis. Regular slit-lamp examinations by an ophthalmologist are critical to prevent permanent vision loss.

Question 86

A 9-year-old boy from Connecticut presents with a massive, painless effusion of the right knee. He is afebrile and has no history of trauma. Aspiration reveals 45,000 WBC/mcL with 80% PMNs. What is the most appropriate initial diagnostic test for the suspected etiology?

Explanation

A massive, relatively painless knee effusion in an afebrile child from an endemic area is highly suspicious for Lyme arthritis. The appropriate initial diagnostic step is two-tier serologic testing starting with an ELISA for Borrelia burgdorferi, followed by a Western blot if positive.

Question 87

A 5-year-old child presents with short-limbed dwarfism, a waddling gait, and joint laxity. The patient facial features and head circumference are completely normal. Radiographs show small, irregular epiphyses and metaphyseal flaring. What is the mutated gene?

Explanation

Pseudoachondroplasia presents with short-limbed dwarfism but uniquely spares the craniofacial skeleton (normal face and head). It is caused by mutations in the Cartilage Oligomeric Matrix Protein (COMP) gene.

Question 88

A 55-year-old patient with end-stage renal disease presents with severe bone pain and proximal muscle weakness. Radiographs of the spine demonstrate a rugger jersey appearance. Lab results show hypocalcemia, hyperphosphatemia, and markedly elevated PTH. Which of the following pathophysiologic mechanisms best explains these findings?

Explanation

Renal osteodystrophy is driven by a failing kidney's inability to excrete phosphate and its lack of 1-alpha-hydroxylase activity. This results in low active Vitamin D (calcitriol), causing hypocalcemia and severe secondary hyperparathyroidism, leading to bone resorption and osteitis fibrosa cystica.

Question 89

A 42-year-old aquarium worker presents with a chronic, indolent, violaceous nodule on his dominant hand that has progressed to tenosynovitis over several months. Routine bacterial cultures are negative. What is the most likely causative organism?

Explanation

Mycobacterium marinum is an atypical mycobacterium commonly found in saltwater and freshwater aquariums. It causes chronic, indolent granulomatous infections (fish tank granuloma) that can progress to deep tenosynovitis, requiring acid-fast cultures grown at lower temperatures.

Question 90

A 7-year-old with systemic juvenile idiopathic arthritis acutely develops a high continuous fever, hepatosplenomegaly, and a bleeding diathesis. Laboratory tests show new-onset cytopenias, markedly elevated ferritin, and a sudden paradoxical drop in the erythrocyte sedimentation rate (ESR). What is the most likely diagnosis?

Explanation

Macrophage activation syndrome (MAS) is a severe, potentially fatal complication of systemic JIA. It is characterized by hyperferritinemia, cytopenias, coagulopathy, and a paradoxical drop in ESR due to hypofibrinogenemia caused by hepatic dysfunction.

Question 91

A 12-year-old boy presents with bilateral hip and knee pain after physical activity. Radiographs reveal irregular, flattened epiphyses in the hips and knees, with a double-layer appearance of the patella on the lateral view. His height and spine radiographs are normal. What is the most likely diagnosis?

Explanation

Multiple epiphyseal dysplasia (MED) affects the epiphyses of the appendicular skeleton, causing early-onset osteoarthritis and a classic double-layer patella on lateral radiographs. The spine is generally spared, differentiating it from spondyloepiphyseal dysplasia.

Question 92

A 3-year-old girl presents with severe bowing of the legs, growth retardation, and total alopecia. Lab testing reveals hypocalcemia, hypophosphatemia, and markedly elevated levels of 1,25-dihydroxyvitamin D. What is the underlying pathophysiology?

Explanation

Vitamin D dependent rickets Type II is characterized by end-organ resistance to active Vitamin D due to a mutation in the Vitamin D receptor. This leads to markedly elevated 1,25-dihydroxyvitamin D levels and is uniquely associated with total alopecia.

Question 93

A 60-year-old diabetic male presents with severe leg pain out of proportion to exam findings, swelling, and bullae. You suspect necrotizing fasciitis and calculate a Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC) score. Which of the following serum laboratory parameters is utilized in this scoring system?

Explanation

The LRINEC score utilizes six laboratory variables to risk-stratify for necrotizing fasciitis: CRP, total WBC count, hemoglobin, serum sodium, serum creatinine, and serum glucose. Hyponatremia (sodium less than 135 mmol/L) adds points to the score.

Question 94

A newborn presents with short-limbed dwarfism, abducted "hitchhiker" thumbs, clubfeet, and cystic swelling of the pinnae that later develops into a "cauliflower ear" deformity. Radiographs demonstrate shortening of the tubular bones and a characteristic first metacarpal anomaly. Which of the following gene mutations is responsible for this condition?

Explanation

This clinical presentation is classic for diastrophic dysplasia, an autosomal recessive condition. It is caused by a mutation in the SLC26A2 (DTDST) gene, which impairs the sulfate transporter essential for normal cartilage matrix formation.

Question 95

A 4-year-old girl presents with a swollen, painless right knee and an antalgic gait of 3 months duration. She has no fever, rash, or other joint involvement. Laboratory testing reveals a positive ANA and negative Rheumatoid Factor. Based on the most likely diagnosis, which of the following screening protocols is most critical for this patient?

Explanation

The patient has early-onset, ANA-positive oligoarticular Juvenile Idiopathic Arthritis (JIA). These patients are at the highest risk for developing asymptomatic anterior uveitis, necessitating frequent slit-lamp examinations every 3-4 months to prevent blindness.

Question 96

A 45-year-old male presents with severe diffuse bone pain, proximal muscle weakness, and multiple metatarsal stress fractures. Laboratory results show severe hypophosphatemia, normal serum calcium, normal PTH, and markedly elevated FGF-23 levels. What is the most appropriate next step in the workup of this patient?

Explanation

This patient's presentation of adult-onset hypophosphatemia with elevated FGF-23 strongly suggests tumor-induced osteomalacia (TIO). A DOTATATE PET/CT or Octreoscan is the most appropriate step to locate the benign phosphaturic mesenchymal tumor responsible for the condition.

Question 97

An 18-month-old boy presents with a 2-day history of right knee swelling and refusal to bear weight. He is afebrile with a WBC count of 11,000/mcL, ESR of 35 mm/hr, and CRP of 2.5 mg/dL. Knee aspiration yields purulent fluid, but standard culture plates show no growth at 72 hours. A PCR test of the synovial fluid returns positive. What is the most likely pathogen and appropriate antibiotic therapy?

Explanation

Kingella kingae is a fastidious Gram-negative organism and the most common cause of septic arthritis in children 6 to 36 months old. It often fails to grow on standard solid media but is reliably detected via PCR, and is generally highly susceptible to beta-lactams like cefazolin.

Skeletal Dysplasias of the Spine - Arab Board MCQ Prep

Master ABOS Orthopedic Board Review: Dysplasias, Osteomalacia, Infections, JIA | Part 7

Key Takeaway

Master ABOS Orthopedic Board Review: Dysplasias, Osteomalacia, Infections, JIA | Part 7

Chapter Index