Question 1

A 45-year-old male presents with rapid, painless swelling and severe joint destruction of his right shoulder. Neurological examination reveals a loss of pain and temperature sensation in his upper extremities but preserved light touch and proprioception. Which of the following is the most likely underlying diagnosis?

Explanation

This patient has a Charcot arthropathy of the shoulder secondary to syringomyelia. The classic neurological finding is a dissociated sensory loss (loss of pain and temperature with preserved dorsal column function) in a cape-like distribution.

Question 2

A 12-year-old male presents with back pain and is found to have a 35-degree left-sided thoracic scoliotic curve. His neurological examination is normal. What is the most appropriate next step in evaluation?

Explanation

Atypical scoliosis features, such as a left-sided thoracic curve, pain, or rapid progression, warrant a total spine MRI. This is crucial to rule out an intraspinal anomaly like a syringomyelia, tethered cord, or tumor.

Question 3

A 13-year-old female presents with a 45-degree progressive left thoracic scoliosis. An MRI reveals an Arnold-Chiari I malformation and a large cervical syringomyelia. What is the most appropriate sequencing of treatment?

Explanation

In patients with scoliosis secondary to an Arnold-Chiari malformation and syringomyelia, the neurosurgical abnormality must be addressed first. Decompression of the posterior fossa often leads to stabilization or improvement of the scoliosis.

Question 4

Neuropathic arthropathy in the upper extremity associated with syringomyelia is primarily secondary to the destruction of which of the following spinal cord structures?

Explanation

Syringomyelia causes cystic enlargement of the central canal, which first compresses the crossing fibers of the lateral spinothalamic tract. This leads to the characteristic loss of pain and temperature sensation, predisposing the joint to Charcot arthropathy.

Question 5

A 15-year-old female presents with a painful, expansile, eccentric lytic lesion in the distal femoral metaphysis. Biopsy reveals blood-filled spaces lacking an endothelial lining. Which of the following genetic translocations is most specific to this primary lesion?

Explanation

Primary aneurysmal bone cysts (ABCs) are characterized by the t(16;17) translocation involving the CDH11 and USP6 genes. The USP6 upregulation drives the inflammatory and osteolytic nature of the tumor.

Question 6

Which of the following magnetic resonance imaging (MRI) findings is most characteristic of an aneurysmal bone cyst?

Explanation

Fluid-fluid levels on T2-weighted MRI are the hallmark of an aneurysmal bone cyst. This phenomenon occurs due to the separation of serum and red blood cells within the cystic spaces.

Question 7

An 8-year-old female is evaluated for multiple bony deformities, irregular café-au-lait spots, and early onset of menses. Which of the following gene mutations is responsible for her condition?

Explanation

This patient has McCune-Albright syndrome, a polyostotic form of fibrous dysplasia associated with endocrine abnormalities and coast of Maine café-au-lait spots. It is caused by an activating post-zygotic mutation in the GNAS1 gene, increasing cAMP levels.

Question 8

A 25-year-old with polyostotic fibrous dysplasia undergoes surgical correction of a shepherd's crook deformity. If bone grafting is necessary, which of the following materials is relatively contraindicated due to its tendency to be rapidly resorbed?

Explanation

Cancellous autograft is contraindicated in fibrous dysplasia because it is rapidly resorbed and replaced by dysplastic host tissue. Cortical allograft is preferred as it undergoes slow creeping substitution, providing prolonged structural support.

Question 9

When planning surgical fixation for an impending proximal femur fracture in a patient with a shepherd's crook deformity from fibrous dysplasia, which of the following implants provides the most reliable long-term outcome?

Explanation

Intramedullary devices, such as a cephalomedullary nail, are the implants of choice for fibrous dysplasia in the lower extremity. Plates and screws have an unacceptably high failure rate due to poor screw purchase in the dysplastic bone.

Question 10

Secondary aneurysmal bone cysts are most commonly found arising within which of the following pre-existing benign bone tumors?

Explanation

Secondary ABCs occur in up to 30% of cases and most commonly arise in association with a Giant Cell Tumor of bone. They can also be seen with chondroblastomas and osteoblastomas.

Question 11

A 14-year-old male presents with neck pain and a lytic lesion expanding the C4 lamina and pedicle. A needle biopsy confirms an aneurysmal bone cyst. What is the most appropriate next step in management prior to surgical resection?

Explanation

Spinal aneurysmal bone cysts are highly vascular and typically involve the posterior elements. Preoperative selective arterial embolization is recommended to significantly reduce intraoperative blood loss before excision.

Question 12

A 40-year-old female with known polyostotic fibrous dysplasia presents with a newly noticed, painless soft tissue mass in her thigh. Biopsy demonstrates a hypocellular lesion with stellate cells embedded in an abundant myxoid stroma. What is the diagnosis?

Explanation

Mazabraud syndrome is the rare association of fibrous dysplasia with single or multiple intramuscular myxomas. These soft tissue masses are typically benign and asymptomatic, though they can cause local mechanical symptoms.

Question 13

A patient with an expanding cervical syringomyelia will most likely present with which of the following classic early neurological deficits?

Explanation

The expanding syrinx compresses the decussating fibers of the spinothalamic tract in the anterior white commissure of the spinal cord. This typically causes a bilateral loss of pain and temperature sensation in a cape-like distribution over the arms and shoulders.

Question 14

What is considered the gold standard surgical treatment to minimize recurrence for an aggressive aneurysmal bone cyst of the proximal tibia in a 16-year-old?

Explanation

Extended curettage with a high-speed burr and the use of chemical or physical adjuvants (e.g., phenol, argon beam, cryotherapy) is the gold standard for ABCs. This aggressive local treatment significantly lowers the recurrence rate compared to simple curettage.

Question 15

Which of the following is the histological hallmark of fibrous dysplasia?

Explanation

The classic histological finding of fibrous dysplasia is irregular, C- or W-shaped trabeculae of woven bone ("Chinese characters") arising directly from a fibrous stroma. Crucially, these bone trabeculae lack functional osteoblastic rimming.

Question 16

Which medication class is currently considered the most effective first-line medical therapy for managing severe bone pain associated with polyostotic fibrous dysplasia?

Explanation

Intravenous bisphosphonates (such as pamidronate or zoledronic acid) are the mainstay of medical treatment for symptomatic fibrous dysplasia. They inhibit osteoclast activity, effectively reducing bone pain and potentially slowing lesion progression.

Question 17

The pathogenesis of syringomyelia in patients with a Chiari I malformation is best explained by which of the following mechanisms?

Explanation

Chiari I malformations cause cerebellar tonsillar ectopia, obstructing normal CSF flow at the foramen magnum. This altered hydrodynamic pulsatile pressure forces CSF into the central canal of the spinal cord, forming a syrinx.

Question 18

Under microscopic examination, the septations in a primary aneurysmal bone cyst typically contain which of the following cellular components?

Explanation

The septa of an ABC lack an endothelial lining and are instead composed of fibroblasts, myofibroblasts, and multinucleated giant cells. Reactive woven bone is also frequently seen within these fibrous septations.

Question 19

Which endocrinopathy is the most common and classic manifestation associated with McCune-Albright syndrome?

Explanation

Peripheral precocious puberty is the most common endocrine abnormality in McCune-Albright syndrome, particularly in females. It results from autonomous, gonadotropin-independent ovarian estrogen production driven by the GNAS mutation.

Question 20

Which of the following radiographic features best differentiates an aneurysmal bone cyst (ABC) from a unicameral bone cyst (UBC) in a long bone?

Explanation

Radiographically, an ABC is an eccentric, expansile, lytic lesion that may thin or "blow out" the cortex. In contrast, a UBC is typically centrally located within the metaphysis or diaphysis and rarely expands beyond the normal width of the physis.

Question 21

A 14-year-old female presents with a rapidly progressive left thoracic scoliosis. Neurological examination reveals a cape-like distribution of decreased pain and temperature sensation over her shoulders and arms, but intact proprioception and light touch. What is the precise anatomical location of the lesion causing her sensory deficit?

Explanation

The patient has syringomyelia, which typically expands centrally from the ependymal canal. It first compresses the crossing spinothalamic fibers in the anterior white commissure, leading to dissociated sensory loss (loss of pain/temperature with preserved proprioception/touch).

Question 22

A 12-year-old boy presents with knee pain and swelling. Radiographs show an eccentric, expansile lytic lesion in the distal femoral metaphysis. MRI demonstrates multiple fluid-fluid levels. Genetic analysis of the tissue is most likely to reveal which of the following translocations?

Explanation

The diagnosis is an aneurysmal bone cyst (ABC). Primary ABCs are neoplasms driven by a specific t(16;17) translocation that upregulates the USP6 gene.

Question 23

A 25-year-old female presents with a progressive bowing deformity of her proximal femur. Radiographs display a "ground-glass" appearance in the medullary canal with a "Shepherd's crook" deformity. A mutation in which of the following genes is responsible for this condition?

Explanation

The patient has fibrous dysplasia, caused by a post-zygotic, activating missense mutation in the GNAS gene. This leads to constitutive activation of the Gs-alpha protein and increased intracellular cAMP, disrupting normal osteoblast differentiation.

Question 24

Which of the following clinical features most strongly indicates the need for a total spine MRI to evaluate for syringomyelia or a neural axis abnormality in an adolescent presenting with scoliosis?

Explanation

A left thoracic curve is an atypical pattern in adolescent idiopathic scoliosis and carries a higher association with neural axis abnormalities like syringomyelia or Chiari malformation, warranting an MRI.

Question 25

Histological evaluation of a rapidly growing, expansile bone lesion in a 15-year-old boy reveals cavernous, blood-filled spaces separated by fibrous septae containing fibroblasts, multinucleated giant cells, and reactive woven bone. Which of the following is a definitive characteristic of the vascular spaces in this lesion?

Explanation

Aneurysmal bone cysts are characterized by blood-filled cystic spaces that are NOT true blood vessels. Therefore, they lack a true endothelial lining, differentiating them from vascular tumors like hemangiomas or angiosarcomas.

Question 26

A 9-year-old girl is diagnosed with polyostotic fibrous dysplasia. Physical examination reveals a large hyperpigmented macule on her back with irregular borders. Which of the following endocrine abnormalities is most commonly associated with her underlying syndrome?

Explanation

The patient has McCune-Albright syndrome (polyostotic fibrous dysplasia, "coast of Maine" café-au-lait spots, and endocrinopathies). Precocious puberty is the most common endocrine manifestation of this syndrome.

Question 27

A 45-year-old male presents with painless swelling, severe destruction, and dislocation of his right shoulder joint. Radiographs show a disorganized, fragmented joint with significant bony debris. He has a history of a cervical spinal cord injury resulting in a progressive fluid collection within the cord. This shoulder pathology is most directly mediated by the loss of which of the following?

Explanation

The patient has a Charcot joint (neuropathic arthropathy) of the shoulder due to syringomyelia. The loss of deep pain and proprioception removes protective joint reflexes, leading to repetitive microtrauma and severe joint destruction.

Question 28

A 14-year-old boy presents with back pain and mild lower extremity weakness. Imaging reveals an expansile, multiloculated lytic lesion in the posterior elements of the L3 vertebra with epidural extension. Biopsy confirms an aneurysmal bone cyst. What is the most appropriate next step in management to minimize surgical morbidity?

Explanation

ABCs in the spine and pelvis are highly vascular and prone to massive intraoperative hemorrhage. Preoperative selective arterial embolization is recommended to significantly reduce blood loss during subsequent intralesional curettage or excision.

Question 29

A 40-year-old female with known polyostotic fibrous dysplasia presents with a painless, deep-seated soft tissue mass in her right thigh. MRI shows a well-circumscribed intramuscular mass that is markedly hyperintense on T2-weighted images. What is the most likely diagnosis of this soft tissue lesion?

Explanation

The association of polyostotic fibrous dysplasia with intramuscular myxomas is known as Mazabraud syndrome. These myxomas are benign, appear bright on T2 MRI, and usually do not require resection unless symptomatic.

Question 30

An 8-year-old boy with a 25-degree scoliosis is diagnosed with a large cervical syringomyelia associated with a Chiari I malformation. He undergoes successful suboccipital decompression. What is the most likely natural history of his scoliosis following the neurosurgical intervention?

Explanation

In children with syringomyelia-associated scoliosis, neurosurgical decompression of the syrinx (via treating the Chiari malformation) often leads to stabilization or improvement of the scoliotic curve, particularly if the curve is less than 30 degrees at the time of surgery.

Question 31

Which of the following bone tumors is most frequently associated with the development of a secondary aneurysmal bone cyst (ABC)?

Explanation

Secondary ABCs occur in pre-existing bone lesions, with Giant Cell Tumor (GCT) of bone being the most common precursor. Other common precursors include chondroblastoma and osteoblastoma.

Question 32

A 30-year-old male with fibrous dysplasia requires surgical intervention for a progressive Shepherd's crook deformity of the proximal femur that has caused a severe limp and impending fracture. What is the biomechanically superior construct for stabilizing this deformity after corrective osteotomy?

Explanation

In fibrous dysplasia, the poor quality of woven bone leads to a high rate of hardware failure with plates and screws (which rely on cortical purchase). Load-sharing devices like intramedullary nails are the treatment of choice, usually preceded by a valgus-producing osteotomy.

Question 33

Histopathologic examination of a diaphyseal bone lesion shows irregular spicules of woven bone distributed within a bland fibrous stroma. A classic diagnostic feature of this pathology is the absence of which of the following?

Explanation

The classic histologic appearance of fibrous dysplasia is "Chinese character" trabeculae of woven bone emerging directly from a fibrous stroma without the normal osteoblastic rimming, distinguishing it from conditions like osteofibrous dysplasia.

Question 34

A patient with syringomyelia complains of loss of pain and temperature sensation. Which specific anatomical tract is compromised to cause these unilateral or bilateral sensory deficits prior to dorsal column involvement?

Explanation

The spinothalamic tract carries pain and temperature sensation. Fibers cross in the anterior white commissure, where they are typically first compressed by a centrally expanding syrinx, leading to the classic dissociated sensory loss.

Question 35

A 16-year-old female presents with an aggressively expanding, painful lytic lesion in her distal fibula with a very thin cortical shell. Biopsy confirms a solid variant of an aneurysmal bone cyst. Which molecular marker definitively distinguishes this primary solid ABC from a giant cell tumor of bone?

Explanation

Primary ABCs (including the solid variant, which lacks large cystic spaces) consistently show USP6 gene rearrangements. In contrast, Giant Cell Tumors of bone are characterized by H3F3A mutations.

Question 36

A 22-year-old female with polyostotic fibrous dysplasia presents with new-onset, deep, aching bone pain in her affected femur, without evidence of acute fracture or deformity progression. Which pharmacological intervention has been shown to be most effective in alleviating this specific type of bone pain?

Explanation

Intravenous bisphosphonates (like pamidronate or zoledronic acid) are the mainstay of medical treatment for symptomatic fibrous dysplasia, effectively reducing bone pain and potentially decreasing the rate of localized osteolysis.

Question 37

A 55-year-old patient with a history of fibrous dysplasia treated 20 years ago presents with rapid swelling, severe pain, and cortical destruction at the previous site. What historical treatment factor most significantly increased this patient's risk for malignant transformation?

Explanation

Malignant transformation in fibrous dysplasia is rare (<1%). However, a history of prior radiation therapy to the lesion significantly increases the risk of malignant degeneration, most commonly to osteosarcoma or fibrosarcoma.

Question 38

A physical exam finding commonly associated with the presence of a syrinx in a pediatric patient with scoliosis, often prompting an MRI even before sensory changes are noticed, is which of the following?

Explanation

Asymmetric or absent abdominal reflexes are a subtle but classic upper motor neuron sign associated with intraspinal anomalies like syringomyelia. This finding strongly indicates the need for an MRI of the entire neural axis.

Question 39

When performing surgical treatment on a standard, accessible aneurysmal bone cyst in the proximal tibia of a 15-year-old, the current standard of care to minimize local recurrence involves intralesional curettage followed by which critical intraoperative step?

Explanation

The standard of care for an accessible ABC is extended intralesional curettage. Extending the margin using a high-speed burr (and often chemical or thermal adjuvants like phenol or argon beam) significantly reduces the recurrence rate compared to curettage alone.

Question 40

A radiologist describes a benign-appearing, expansile, diaphyseal radiolucent lesion with a "rind" of reactive sclerosis on the radiograph of a 30-year-old. This typical "rind" sign is most pathognomonic for which of the following conditions?

Explanation

Fibrous dysplasia lesions are typically diaphyseal or metadiaphyseal, exhibiting a "ground-glass" matrix and often surrounded by a thick, sclerotic rim of reactive bone known as the "rind" sign.

Question 41

A 12-year-old boy presents with a limp. Radiographs show a proximal femur lesion with a ground-glass appearance and a varus deformity. Which of the following genetic abnormalities is most likely responsible?

Explanation

Fibrous dysplasia is caused by a somatic, activating mutation in the GNAS gene encoding the alpha subunit of the Gs stimulatory protein. This leads to increased intracellular cAMP and impaired osteoblast differentiation.

Question 42

A 14-year-old girl presents with knee pain. MRI reveals an eccentric, expansile metaphyseal lesion in the proximal tibia with multiple fluid-fluid levels. Genetic testing of the tissue would most likely reveal which of the following?

Explanation

Primary aneurysmal bone cysts (ABCs) are neoplastic processes driven by rearrangements of the USP6 gene on chromosome 17. The classic MRI finding is multiple fluid-fluid levels representing blood breakdown products.

Question 43

A 45-year-old man presents with progressive, painless swelling and instability of his right shoulder. He has a history of severe thermal burns on his hands without recalling the pain. What is the most appropriate initial diagnostic step to identify the underlying cause?

Explanation

The patient's presentation of a Charcot (neuropathic) shoulder with loss of pain and temperature sensation is highly suspicious for syringomyelia. An MRI of the cervical spine is the gold standard to visualize the syrinx.

Question 44

An 8-year-old girl is diagnosed with polyostotic fibrous dysplasia. She has large cafe-au-lait spots with irregular borders on her trunk. Which of the following endocrine abnormalities is most commonly associated with her condition?

Explanation

McCune-Albright syndrome consists of polyostotic fibrous dysplasia, cafe-au-lait spots with coast of Maine borders, and endocrinopathies. The most common endocrine manifestation is precocious puberty.

Question 45

Which of the following surgical techniques is considered the gold standard for managing a symptomatic shepherd's crook deformity in a patient with polyostotic fibrous dysplasia?

Explanation

Cortical bone replacement via intramedullary nailing combined with a valgus osteotomy is the most reliable treatment for a shepherd's crook deformity. Plating has a high failure rate, and bone grafts tend to resorb.

Question 46

A 16-year-old boy is scheduled for surgical intervention of a left thoracic scoliotic curve measuring 45 degrees. Preoperative MRI reveals an Arnold-Chiari type I malformation and a large cervicothoracic syrinx. What is the most appropriate management strategy?

Explanation

In patients with scoliosis secondary to a syrinx and Chiari I malformation, neurosurgical decompression should be performed before deformity correction. This reduces the risk of neurologic injury and may stabilize the curve.

Question 47

A 15-year-old male presents with back pain. Imaging shows an expansile, lytic lesion involving the posterior elements of L3. Biopsy reveals blood-filled cystic spaces lacking endothelial lining. Which of the following adjuvant therapies is most appropriate if the lesion is surgically inaccessible?

Explanation

The histology describes an Aneurysmal Bone Cyst (ABC). In surgically inaccessible locations like the spine or sacrum, denosumab (a RANKL inhibitor) or selective arterial embolization can be used as effective treatments.

Question 48

Histopathological examination of a proximal femoral lesion demonstrates woven bone trabeculae in a fibrous stroma without osteoblastic rimming. What radiographic feature is most characteristic of this condition?

Explanation

The histology is pathognomonic for fibrous dysplasia, displaying Chinese character trabeculae. The classic radiographic finding is a well-circumscribed medullary lesion with a ground-glass appearance.

Question 49

A patient with polyostotic fibrous dysplasia develops multiple intramuscular masses in the right thigh adjacent to the bony lesions. Biopsy of the soft tissue masses reveals a hypocellular, myxoid stroma with spindle cells. What is the eponym for this clinical presentation?

Explanation

Mazabraud syndrome is the rare association of polyostotic fibrous dysplasia with intramuscular myxomas. These benign soft tissue masses are typically found in the same anatomic region as the bony lesions.

Question 50

A 30-year-old woman with a known history of syringomyelia presents to the clinic with a cape-like distribution of sensory loss over her shoulders. Which specific sensory modalities are typically lost FIRST in this condition?

Explanation

A syrinx typically expands centrally in the spinal cord, first compressing the crossing spinothalamic tracts at the anterior white commissure. This results in the classic dissociated sensory loss of pain and temperature.

Question 51

During a biopsy of a suspected aneurysmal bone cyst (ABC) in the distal femur, which of the following secondary lesions must the pathologist most carefully rule out due to its frequent harboring of secondary ABCs?

Explanation

Secondary ABCs can develop within several primary bone tumors, most notably giant cell tumor (GCT) of bone, chondroblastoma, and osteoblastoma. The pathologist must examine the tissue thoroughly to rule out an underlying GCT.

Question 52

A 25-year-old male with a history of monostotic fibrous dysplasia presents with a rapidly enlarging mass and severe pain in his humerus. He received radiation therapy for the lesion 10 years ago. Which of the following is the most likely diagnosis?

Explanation

Malignant transformation in fibrous dysplasia is rare (less than 1%) but is significantly associated with prior radiation therapy. The most common histology of the secondary malignancy is osteosarcoma.

Question 53

An 11-year-old boy undergoes extended intralesional curettage of an aneurysmal bone cyst in the distal tibia. Which of the following is most commonly used during this procedure to reduce the rate of local recurrence?

Explanation

Treatment of ABCs involves extended intralesional curettage, mechanical burring, and chemical or thermal adjuvants like phenol or liquid nitrogen. This multimodal approach effectively kills remaining cyst cells to prevent recurrence.

Question 54

What is the primary role of bisphosphonate therapy in the medical management of symptomatic polyostotic fibrous dysplasia?

Explanation

Bisphosphonates, such as pamidronate or zoledronic acid, are primarily indicated to manage severe bone pain in fibrous dysplasia. They do not alter disease progression, prevent fractures, or correct structural deformities.

Question 55

A 13-year-old boy presents with an atypical left thoracic scoliosis curve and absence of abdominal reflexes. MRI confirms a large syringomyelia. What pathomechanism is most commonly responsible for curve progression in this setting?

Explanation

Syringomyelia disrupts the anterior horn cells and descending tracts, leading to asymmetrical weakness and neuromuscular imbalance of the paraspinal muscles. This drives the rapid and atypical progression of the scoliotic curve.

Question 56

An 18-year-old woman is evaluated for a solitary, lytic, expansile lesion in the distal radius. Biopsy demonstrates cavernous blood-filled spaces. Karyotype shows a t(16;17) translocation. What protein is directly upregulated due to this genetic rearrangement?

Explanation

The t(16;17) translocation in primary aneurysmal bone cysts fuses the promoter region of CDH11 to the USP6 gene. This causes profound upregulation of USP6, a ubiquitin-specific protease driving the neoplasm.

Question 57

A patient with polyostotic fibrous dysplasia requires internal fixation for an impending subtrochanteric fracture. Which of the following biomechanical principles is critical for surgical management in this disease?

Explanation

Dysplastic bone has poor mechanical properties, leading to a high failure rate for cortical plates. Surgical management requires load-sharing intramedullary devices (e.g., IM nails) that span the entire length of the diseased bone.

Question 58

Which of the following clinical curve patterns is considered an absolute indication for obtaining a total spine MRI to rule out syringomyelia in a patient presenting with scoliosis?

Explanation

Atypical curve patterns, such as a left-sided thoracic curve, early onset, or rapid progression, are strong indications for an MRI. This imaging is crucial to rule out intraspinal abnormalities like a syrinx or tumor.

Question 59

A 21-year-old female presents with recurrent swelling of her proximal humerus 18 months after curettage of an Aneurysmal Bone Cyst. Which of the following surgical factors is most strongly associated with an increased risk of ABC recurrence?

Explanation

The most significant risk factor for the recurrence of an aneurysmal bone cyst is incomplete intralesional excision. Adding mechanical adjuvants like a high-speed burr significantly decreases the recurrence rate.

Question 60

A patient with suspected McCune-Albright syndrome exhibits a characteristic café-au-lait spot. How does the macroscopic appearance of this cutaneous lesion typically differ from those seen in Neurofibromatosis type 1 (NF1)?

Explanation

Café-au-lait spots in McCune-Albright syndrome typically have irregular, jagged borders commonly described as the coast of Maine. In contrast, the lesions in NF1 have smooth borders referred to as the coast of California.

Question 61

A 10-year-old girl presents with a rapidly progressive left-sided thoracic scoliosis. Her neurologic examination is completely normal. Which of the following is the most appropriate next step in evaluation?

Explanation

Left-sided thoracic curves, rapid progression, or abnormal neurologic findings are red flags for intraspinal anomalies like syringomyelia or Chiari malformation. These findings strongly warrant an MRI of the entire neuraxis prior to any intervention.

Question 62

A 45-year-old man presents with painless, massive swelling and instability of his right shoulder. Radiographs demonstrate severe glenohumeral destruction with osseous debris. Physical exam reveals a loss of pain and temperature sensation in both upper extremities. What is the most likely underlying diagnosis?

Explanation

Syringomyelia commonly causes neuropathic (Charcot) arthropathy in the upper extremities, particularly the shoulder. This occurs due to central cord cavity expansion disrupting the spinothalamic tracts, eliminating protective pain and temperature sensation.

Question 63

A central spinal cord syrinx typically causes early neurologic deficits by initially compressing which specific neural structure?

Explanation

A syrinx centrally expands and initially damages the crossing fibers of the spinothalamic tract in the anterior white commissure. This results in the classic bilateral "cape-like" loss of pain and temperature sensation in the upper extremities.

Question 64

A 12-year-old patient presents with a 45-degree right thoracic scoliosis curve and an MRI revealing a large cervical syrinx associated with a Chiari I malformation. What is the most appropriate sequence of management?

Explanation

Neurosurgical decompression of the syrinx/Chiari malformation should always precede scoliosis surgery to reduce the high risk of intraoperative neurologic injury. Furthermore, the scoliosis curve may stabilize or even improve following neurosurgical decompression.

Question 65

A 14-year-old boy undergoes biopsy of an expansile, eccentric lytic lesion in the distal femur. Histology shows blood-filled spaces lined by fibrous septa containing multinucleated giant cells. Which genetic abnormality is pathognomonic for the primary form of this lesion?

Explanation

Primary Aneurysmal Bone Cysts (ABCs) are true neoplasms characterized by USP6 gene translocations, most commonly t(16;17)(q22;p13). This distinguishes primary ABCs from secondary ABCs, which lack this genetic marker.

Question 66

Which of the following MRI findings is highly characteristic, though not entirely specific, for an aneurysmal bone cyst?

Explanation

Fluid-fluid levels on MRI represent separated blood components of different densities. While highly characteristic of ABCs, they are not pathognomonic and can also be seen in telangiectatic osteosarcoma and giant cell tumors.

Question 67

Secondary aneurysmal bone cysts can arise within pre-existing osseous lesions. Which of the following benign tumors is most commonly associated with the development of a secondary ABC?

Explanation

Approximately 30% of ABCs are secondary. Giant cell tumor of bone, chondroblastoma, and osteoblastoma are the most common benign lesions associated with the formation of a secondary ABC.

Question 68

A 10-year-old girl presents with a primary ABC of the proximal humerus causing severe cortical thinning, but no fracture. What is the most widely accepted initial surgical management?

Explanation

The standard of care for an active primary ABC is extended intralesional curettage using a high-speed burr, often combined with local adjuvants (like phenol or argon beam) followed by bone grafting or cementation to reduce recurrence.

Question 69

A 16-year-old girl presents with back pain and L3 radiculopathy. Imaging shows an expansile lytic lesion in the L3 posterior elements with fluid-fluid levels. To minimize intraoperative complications, which preoperative intervention is highly recommended?

Explanation

Spinal ABCs commonly involve the posterior elements and can be extremely vascular. Preoperative selective arterial embolization is strongly recommended to significantly decrease the risk of massive intraoperative hemorrhage.

Question 70

Fibrous dysplasia is driven by a somatic mutation in the GNAS gene. This mutation directly leads to the constitutive activation of which of the following cellular pathways?

Explanation

The GNAS mutation results in a constitutively active Gs-alpha protein. This leads to continuous activation of adenylate cyclase and an overproduction of intracellular cAMP, which disrupts normal osteoblast differentiation.

Question 71

A 7-year-old girl presents with a limp, irregular café-au-lait spots on her trunk, and a history of early vaginal bleeding. Pelvic radiographs show a lytic ground-glass lesion in the proximal femur. Which endocrine abnormality is most characteristic of this syndrome?

Explanation

McCune-Albright syndrome is characterized by the classic triad of polyostotic fibrous dysplasia, café-au-lait macules with "coast of Maine" irregular borders, and hyperfunctioning endocrinopathies, most commonly precocious puberty.

Question 72

An adult female with known polyostotic fibrous dysplasia presents with a painless, slow-growing soft tissue mass in her thigh. A biopsy of the mass reveals a hypocellular lesion with abundant myxoid stroma. What is the diagnosis?

Explanation

Mazabraud syndrome is a rare condition characterized by the association of polyostotic fibrous dysplasia and single or multiple intramuscular myxomas. These soft tissue tumors often occur in the same anatomic region as the severely affected bone.

Question 73

A 25-year-old man with fibrous dysplasia presents with severe varus deformity of the proximal femur (shepherd's crook) and an impending fracture. Following a corrective valgus osteotomy, which is the most appropriate fixation method?

Explanation

Intramedullary nailing is the preferred fixation method for shepherd's crook deformities in fibrous dysplasia. Plates and screws have a high failure rate due to poor screw purchase in dysplastic bone, and cortical bone grafts are rapidly resorbed by the disease process.

Question 74

Which of the following medical therapies is considered the most effective for the management of severe, recalcitrant bone pain associated with polyostotic fibrous dysplasia?

Explanation

Intravenous bisphosphonates (e.g., pamidronate or zoledronic acid) are the mainstay of medical treatment for symptomatic fibrous dysplasia. They effectively reduce bone pain, decrease bone turnover, and may slow the progression of lytic lesions.

Question 75

If left completely untreated, a progressively expanding cervical syringomyelia is most likely to eventually result in which of the following late neurologic complications?

Explanation

As a cervical syrinx expands outward from the central canal, it progressively compresses the laterally situated corticospinal tracts and anterior horn cells. This ultimately leads to long-tract signs including spastic paraparesis and sphincter dysfunction.

Question 76

Telangiectatic osteosarcoma can closely mimic an aneurysmal bone cyst on standard MRI. Which histologic feature best differentiates telangiectatic osteosarcoma from a primary ABC?

Explanation

While both lesions show blood-filled cystic spaces separated by septa, telangiectatic osteosarcoma exhibits marked cytologic atypia, pleomorphism, and atypical mitoses within the septal stromal cells. The fibroblasts in an ABC are inherently benign.

Question 77

Although rare (less than 1%), spontaneous malignant transformation can occur in lesions of fibrous dysplasia. When transformation occurs, which of the following is the most common histologic subtype?

Explanation

Malignant transformation of fibrous dysplasia is rare but is most commonly identified as osteosarcoma, followed by fibrosarcoma or chondrosarcoma. A history of prior radiation therapy to the lesion significantly increases this risk.

Question 78

In cases of surgically inaccessible or highly morbid aneurysmal bone cysts (e.g., within the sacrum), which targeted systemic therapy has shown significant efficacy in shrinking the lesion and promoting ossification?

Explanation

Denosumab, a monoclonal antibody against RANKL, has shown excellent efficacy as an off-label treatment for surgically inaccessible ABCs. It inhibits the osteoclast-like giant cells within the lesion, promoting rapid ossification and stabilization.

Question 79

On a standard plain radiograph, fibrous dysplasia typically presents with which of the following classic morphologic appearances?

Explanation

Fibrous dysplasia typically appears as a radiolucent, expansile medullary lesion with a hazy, "ground-glass" matrix. This radiographic appearance directly reflects the histologic replacement of normal cancellous bone by abnormal fibrous tissue and irregular woven bone trabeculae.

Question 80

A 15-year-old male with an atypical left-sided thoracic scoliosis undergoes an MRI revealing a holocord syrinx. He has no gross neurologic deficits. Why is immediate surgical correction of the scoliosis strictly contraindicated prior to neurosurgical evaluation?

Explanation

Performing spinal deformity correction in the presence of an untreated syrinx carries a massive risk of acute neurologic injury or paraplegia. The syrinx limits the cord's compliance and space to adapt to the mechanical shifts of deformity correction.

Question 81

A 12-year-old girl presents with a rapidly progressive left thoracic scoliotic curve. Physical exam reveals absent superficial abdominal reflexes. Which of the following is the most appropriate next step in evaluation?

Explanation

A left-sided thoracic curve and absent abdominal reflexes are atypical findings for adolescent idiopathic scoliosis and highly suggestive of a neural axis abnormality, such as syringomyelia. A total spine MRI is the gold standard for evaluating neuroaxial anomalies prior to any intervention.

Question 82

A 15-year-old boy presents with a pathologic fracture of the proximal femur with a characteristic "shepherd's crook" deformity. Biopsy reveals woven bone trabeculae lacking osteoblastic rimming within a fibrous stroma. What is the underlying genetic mechanism of this condition?

Explanation

The clinical and histologic description is classic for fibrous dysplasia. This condition is caused by an activating missense mutation in the GNAS gene, which encodes the alpha subunit of the Gs G-protein, leading to elevated intracellular cAMP.

Question 83

A 14-year-old boy has an expansile, eccentric, radiolucent lesion in the proximal tibial metaphysis. MRI demonstrates multiple fluid-fluid levels. Cytogenetic analysis is most likely to reveal an abnormality involving which of the following?

Explanation

The presence of a metaphyseal expansile lesion with MRI fluid-fluid levels strongly suggests an aneurysmal bone cyst (ABC). Primary ABCs are characterized by translocations involving the USP6 gene on chromosome 17, most commonly t(16;17).

Question 84

An 11-year-old boy is diagnosed with a large syringomyelia and an associated Chiari I malformation, along with a 50-degree thoracic scoliotic curve. What is the recommended treatment sequence?

Explanation

In patients with a large syrinx and a surgically significant scoliosis curve, neurosurgical decompression of the syrinx/Chiari malformation should be performed before spinal deformity correction. This reduces the risk of neurologic injury during spinal correction and may result in curve stabilization or improvement.

Question 85

A 9-year-old girl is found to have polyostotic fibrous dysplasia, café-au-lait spots with irregular borders, and precocious puberty. Which of the following is true regarding surgical management of impending fractures in this patient?

Explanation

Patients with McCune-Albright syndrome have polyostotic fibrous dysplasia, which produces structurally weak bone. Fixation requires load-sharing intramedullary devices spanning the entire lesion; plates/screws and isolated autografts have unacceptably high failure rates due to bone resorption.

Question 86

A 17-year-old female presents with neck pain. Imaging reveals an expansile, multiloculated lytic lesion in the posterior elements of the C4 vertebra. Biopsy demonstrates blood-filled spaces lined by fibrous septa with multinucleated giant cells. Which of the following treatment options is most appropriate to minimize recurrence?

Explanation

The diagnosis is an aneurysmal bone cyst (ABC), which classically occurs in the posterior elements of the spine. Extended curettage using a high-speed burr along with local adjuvants (e.g., argon beam, phenol, or cryotherapy) significantly reduces the recurrence rate compared to simple curettage.

Question 87

A 45-year-old woman with known polyostotic fibrous dysplasia presents with new, painless soft tissue masses in her thigh musculature. MRI confirms well-circumscribed intramuscular myxomas. What is this specific clinical association called?

Explanation

Mazabraud syndrome is a rare disorder characterized by the association of polyostotic fibrous dysplasia with single or multiple intramuscular myxomas. These myxomas are benign but are frequently located in the same anatomic region as the severely affected bones.

Question 88

A patient with syringomyelia presents with characteristic neurologic deficits in the upper extremities. Which of the following physical exam findings is most classically associated with a central cord syrinx?

Explanation

A syrinx typically expands from the central canal, first damaging the crossing spinothalamic fibers in the anterior white commissure. This classically produces a "cape-like" distribution of bilateral loss of pain and temperature sensation in the upper extremities and neck, while preserving light touch and proprioception.

Question 89

In the treatment of a recurrent aneurysmal bone cyst (ABC) of the pelvis that is deemed surgically unresectable due to excessive morbidity, which of the following medical therapies is most appropriate?

Explanation

Denosumab, a RANKL inhibitor, has proven effective in the medical management of surgically challenging or unresectable ABCs by inhibiting osteoclast-mediated bone destruction and inducing lesional ossification. Selective arterial embolization is also a common nonsurgical alternative.

Question 90

A biopsy of a metaphyseal bone lesion shows irregular, curvilinear trabeculae of woven bone lacking osteoblastic rimming, set within a moderately cellular fibrous stroma. Radiographs show a "ground-glass" appearance. What is the most likely diagnosis?

Explanation

The histologic description of "Chinese character" or "alphabet soup" trabeculae of woven bone without osteoblastic rimming in a fibrous stroma is the pathognomonic feature of fibrous dysplasia. This matches the classic radiographic "ground-glass" matrix.

Question 91

A 16-year-old male is undergoing extended curettage of an aneurysmal bone cyst in the distal femur. The surgeon decides to use an adjuvant to reduce the risk of recurrence. Which of the following is considered a chemical adjuvant in this setting?

Explanation

Phenol acts as a chemical cauterizing agent (adjuvant) used to induce necrosis of any microscopic tumor cells remaining after curettage. Liquid nitrogen (cryotherapy) and argon beam are physical thermal adjuvants, while a high-speed burr is mechanical.

Question 92

An orthopedic surgeon is evaluating a 20-year-old patient with an atypical scoliotic curve. An MRI of the spine reveals a fluid-filled cavity within the spinal cord extending from C5 to T8. Which associated finding is most likely to be seen on a sagittal MRI of the head and neck?

Explanation

Syringomyelia is frequently associated with Chiari I malformation, which is characterized by the downward displacement (ectopia) of the cerebellar tonsils through the foramen magnum. This obstruction of CSF flow is thought to drive syrinx formation.

Question 93

Which of the following bone grafting materials is most prone to failure due to complete resorption when used for filling a defect after curettage of a fibrous dysplasia lesion?

Explanation

Cancellous autograft is universally recognized to have a high failure rate in fibrous dysplasia because the host's dysplastic bone forming process rapidly resorbs the autograft and replaces it with weak, dysplastic woven bone. Structural allografts or synthetic substitutes are preferred if grafting is necessary.

Question 94

An aneurysmal bone cyst (ABC) can occasionally arise as a secondary lesion within a pre-existing primary bone tumor. Which of the following benign bone tumors is most frequently associated with secondary ABC formation?

Explanation

Secondary ABCs account for up to 30% of all ABCs and most commonly arise within giant cell tumors of bone, chondroblastomas, and osteoblastomas. A biopsy must carefully evaluate for an underlying primary neoplastic process to ensure proper diagnosis and treatment.

Question 95

A 10-year-old child presents with severe bone pain secondary to polyostotic fibrous dysplasia. Nonsteroidal anti-inflammatory drugs have provided minimal relief. What is the most appropriate first-line pharmacologic therapy for managing this patient's bone pain?

Explanation

Intravenous bisphosphonates (e.g., pamidronate or zoledronic acid) are the first-line pharmacologic treatment for severe, refractory bone pain in patients with polyostotic fibrous dysplasia. They work by inhibiting osteoclast activity, reducing bone turnover, and improving symptoms.

Question 96

An 18-year-old male with an untreated, symptomatic Chiari I malformation and a large cervicothoracic syrinx presents for scoliosis correction due to a progressive 65-degree curve. If spinal fusion is performed prior to neurosurgical decompression, what is the most significant risk?

Explanation

Performing scoliosis correction on a patient with an untreated syrinx carries an unacceptably high risk of neurologic injury (e.g., paraplegia) during curve correction due to altered spinal cord compliance. Neurosurgical decompression must precede deformity correction to mitigate this risk.

Orthopaedic Surgery Board Exam Review: ABOS Part I & AAOS OITE Prep Questions | Part 22210

Syringomyelia, Aneurysmal Bone Cyst, Fibrous Dysplasia: ABOS Board Review | Part 18

Key Takeaway

Syringomyelia, Aneurysmal Bone Cyst, Fibrous Dysplasia: ABOS Board Review | Part 18

Chapter Index