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1. General Principles & Basic Science MCQs

Practice Set 474 of 789

This practice set contains high-yield board review questions covering key concepts in 1. General Principles & Basic Science. Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 9461

Topic: 1. General Principles & Basic Science

An orthopedic surgeon is analyzing a long-leg radiograph to plan a deformity correction.

The mechanical axis line of the proximal segment and the mechanical axis line of the distal segment intersect at a specific point. What term strictly defines this intersection?

. Axis of Correction of Angulation (ACA)
. Center of Rotation of Angulation (CORA)
. Mechanical Axis Deviation (MAD) magnitude
. Osteotomy hinge point
. Joint Line Convergence Angle (JLCA)

Correct Answer & Explanation

. Center of Rotation of Angulation (CORA)

Explanation

The Center of Rotation of Angulation (CORA) is formally defined as the point of intersection between the mechanical or anatomic axes of the proximal and distal bone segments. The ACA is the surgical hinge point, which may or may not be placed at the CORA.

Question 9462

Topic: Biology, Genetics & Bone Healing

A 6-year-old child with NF1 presents with established anterolateral bowing and non-union of the tibia.

What is the primary histological finding at the site of the pseudoarthrosis that impairs healing?

. Avascular necrosis of the bone ends
. Thickened, fibrous hamartomatous periosteum
. Malignant transformation of osteoblasts
. Accumulation of woven bone without osteoclasts
. Infiltration of malignant peripheral nerve sheath tumor

Correct Answer & Explanation

. Thickened, fibrous hamartomatous periosteum

Explanation

In congenital anterolateral bowing of the tibia associated with NF1, the pseudoarthrosis site is enveloped by a thickened, fibrous, hamartomatous periosteum. This abnormal tissue acts as a mechanical and biological barrier, severely impairing local vascularization and bone union.

Question 9463

Topic: Biology, Genetics & Bone Healing

What is the primary function of the protein mutated in Neurofibromatosis type 1?

. Tyrosine kinase activation
. Wnt pathway inhibition
. Downregulation of the Ras pathway via GTPase activation
. RANKL inhibition
. Bone morphogenetic protein receptor signaling

Correct Answer & Explanation

. Downregulation of the Ras pathway via GTPase activation

Explanation

NF1 is caused by a mutation in the NF1 gene on chromosome 17q11.2, which encodes neurofibromin. Neurofibromin acts as a tumor suppressor by downregulating the Ras signaling pathway via its GTPase-activating protein (GAP) activity.

Question 9464

Topic: Biology, Genetics & Bone Healing

A 30-year-old female presents with an expansile, lytic lesion in the distal femur extending to the subchondral bone. Biopsy confirms a Giant Cell Tumor. What is the mechanism of action of Denosumab, a frequently used neoadjuvant therapy?

. Inhibits vascular endothelial growth factor (VEGF)
. Binds to RANKL, preventing osteoclast activation
. Direct cytotoxic effect on mononuclear neoplastic cells
. Tyrosine kinase inhibition
. Stimulates osteoblast bone formation

Correct Answer & Explanation

. Binds to RANKL, preventing osteoclast activation

Explanation

Denosumab is a monoclonal antibody that binds to RANKL, preventing it from interacting with the RANK receptor on osteoclasts. This inhibits osteoclast-mediated bone destruction, which is driven by the neoplastic mononuclear cells in GCT.

Question 9465

Topic: Biology, Genetics & Bone Healing

The gene mutated in Neurofibromatosis Type 1 (NF1) encodes for the protein neurofibromin. What is the normal cellular function of this protein?

. It acts as a GTPase-activating protein (GAP) that downregulates the Ras signaling pathway.
. It functions as a tyrosine kinase receptor promoting osteoblast differentiation.
. It acts as a tumor suppressor by directly binding to the p53 promoter region.
. It cross-links actin filaments to the cell membrane, stabilizing Schwann cells.
. It upregulates the Wnt/beta-catenin pathway to stimulate endochondral ossification.

Correct Answer & Explanation

. It acts as a GTPase-activating protein (GAP) that downregulates the Ras signaling pathway.

Explanation

Neurofibromin functions as a GTPase-activating protein (GAP) that normally inhibits the Ras signaling pathway. Loss of neurofibromin leads to constitutively active Ras, driving uncontrolled cell proliferation and tumor formation in NF1 patients.

Question 9466

Topic: Biology, Genetics & Bone Healing

A 15-year-old boy presents with knee pain. Radiographs reveal an eccentric, lytic lesion in the distal femoral epiphysis extending to the subchondral bone. Biopsy confirms a Giant Cell Tumor (GCT). Which cellular component is the true neoplastic cell, and which signaling molecule does it express to drive osteolysis?

. Multinucleated giant cells; expressing RANKL.
. Multinucleated giant cells; expressing Osteoprotegerin (OPG).
. Mononuclear stromal cells; expressing RANKL.
. Mononuclear stromal cells; expressing RANK.
. Osteoblasts; expressing Sclerostin.

Correct Answer & Explanation

. Mononuclear stromal cells; expressing RANKL.

Explanation

In GCT of bone, the spindle-shaped mononuclear stromal cells are the true neoplastic elements. They overexpress RANKL, which recruits and activates normal host macrophages to fuse into the osteolytic multinucleated giant cells.

Question 9467

Topic: 1. General Principles & Basic Science
A 45-year-old patient presents with six large café-au-lait spots, axillary freckling, and Lisch nodules. He states his mother had similar skin findings and a leg deformity. What is the inheritance pattern and the chromosome involved in this disorder?
. Autosomal recessive, Chromosome 17
. Autosomal dominant, Chromosome 17
. Autosomal dominant, Chromosome 22
. X-linked recessive, Chromosome 17
. Autosomal recessive, Chromosome 22

Correct Answer & Explanation

. Autosomal dominant, Chromosome 17

Explanation

Neurofibromatosis Type 1 (NF1) is an autosomal dominant genetic disorder caused by a mutation in the NF1 gene located on Chromosome 17. NF2 involves Chromosome 22.

Question 9468

Topic: Biology, Genetics & Bone Healing

A 5-year-old boy is diagnosed with Neurofibromatosis type 1 (NF-1). Which of the following best describes the molecular pathophysiology of his condition?

. Decreased inhibition of the Ras-MAPK pathway
. Gain-of-function mutation in the FGFR3 gene
. Defect in type 1 collagen synthesis
. Overexpression of the RANKL protein
. Mutation in the RUNX2 transcription factor

Correct Answer & Explanation

. Decreased inhibition of the Ras-MAPK pathway

Explanation

NF-1 is caused by a mutation in the NF1 gene on chromosome 17, which encodes the tumor suppressor neurofibromin. Neurofibromin normally downregulates the Ras-MAPK pathway; its loss leads to uncontrolled cell proliferation and tumor formation.

Question 9469

Topic: Biology, Genetics & Bone Healing

A 28-year-old woman presents with knee pain. Radiographs show an eccentric, expansile, lytic lesion in the distal femur extending to the subchondral bone without a sclerotic margin. Biopsy shows multinucleated giant cells in a background of mononuclear stromal cells. If systemic medical therapy is indicated, which molecular pathway is the primary target?

. Vascular endothelial growth factor (VEGF) pathway
. RANK ligand (RANKL) pathway
. Fibroblast growth factor receptor 3 (FGFR3) pathway
. Wnt/beta-catenin signaling pathway
. PD-1 immune checkpoint pathway

Correct Answer & Explanation

. RANK ligand (RANKL) pathway

Explanation

Giant cell tumors of bone are driven by neoplastic mononuclear stromal cells that overexpress RANKL, which aggressively recruits and activates osteoclast-like multinucleated giant cells. Denosumab, a monoclonal antibody against RANKL, blocks this pathway and is used for advanced or inoperable cases.

Question 9470

Topic: Biology, Genetics & Bone Healing

A 28-year-old female presents with knee pain. Radiographs show an eccentric, lytic epiphyseal lesion in the proximal tibia. Biopsy confirms Giant Cell Tumor (GCT). If medical management is considered prior to curettage to downstage the tumor, what is the mechanism of action of the preferred agent?

. Inhibits osteoclast activity via direct bisphosphonate binding
. Monoclonal antibody that binds and inhibits RANKL
. Tyrosine kinase inhibitor targeting VEGF
. Selective estrogen receptor modulator reducing osteoclastogenesis
. Cross-links DNA to inhibit tumor replication directly

Correct Answer & Explanation

. Monoclonal antibody that binds and inhibits RANKL

Explanation

Denosumab is a monoclonal antibody that binds to RANKL, inhibiting osteoclast differentiation and function. It is frequently used in challenging GCT cases to ossify the tumor periphery and facilitate a more complete curettage.

Question 9471

Topic: Biology, Genetics & Bone Healing

A 32-year-old female presents with a destructive, expansile, purely lytic lesion in the distal femur epiphysis. Biopsy confirms a Giant Cell Tumor (GCT) of bone. She is treated preoperatively with denosumab. What is the primary mechanism of action of this medication in the context of GCT?

. Directly induces apoptosis of the neoplastic mononuclear stromal cells
. Inhibits RANK ligand (RANKL), preventing the recruitment and activation of reactive osteoclast-like giant cells
. Blocks vascular endothelial growth factor (VEGF) to decrease tumor vascularity
. Inhibits the mTOR pathway, leading to tumor necrosis
. Binds to the extracellular matrix to inhibit tumor invasion

Correct Answer & Explanation

. Inhibits RANK ligand (RANKL), preventing the recruitment and activation of reactive osteoclast-like giant cells

Explanation

Denosumab is a monoclonal antibody that binds to RANKL, which is overexpressed by the neoplastic mononuclear stromal cells in GCT. This prevents RANKL from binding to RANK on osteoclast precursors, thereby halting the formation of bone-resorbing giant cells.

Question 9472

Topic: Biology, Genetics & Bone Healing

Regarding Glucocorticoid-Induced Osteoporosis (GIO) in orthopedic patients undergoing chronic steroid therapy, which mechanism primarily drives the rapid early bone loss?

. Increased intestinal calcium absorption
. Decreased renal calcium excretion
. Increased osteoblast apoptosis and reduced osteoblastogenesis
. Overproduction of parathyroid hormone (PTH)
. Increased calcitonin secretion

Correct Answer & Explanation

. Increased osteoblast apoptosis and reduced osteoblastogenesis

Explanation

GIO primarily results from direct inhibitory effects on bone formation. Glucocorticoids increase the apoptosis of mature osteoblasts and osteocytes while simultaneously decreasing new osteoblastogenesis.

Question 9473

Topic: Biology, Genetics & Bone Healing
A 5-year-old boy presents with six café-au-lait spots and axillary freckling. He is diagnosed with Neurofibromatosis Type 1 (NF1). The mutated gene in this condition normally functions to downregulate which of the following signaling pathways?
. Wnt/beta-catenin
. Hedgehog
. Ras-MAPK
. PI3K/AKT
. TGF-beta

Correct Answer & Explanation

. Ras-MAPK

Explanation

NF1 is caused by an autosomal dominant mutation in the NF1 gene on chromosome 17. This gene encodes neurofibromin, a GTPase-activating protein that normally downregulates the Ras-MAPK pathway to prevent uncontrolled cellular proliferation.

Question 9474

Topic: Biology, Genetics & Bone Healing

A 32-year-old female presents with a large, lytic epiphyseal lesion of the distal femur. Biopsy confirms Giant Cell Tumor of bone (GCT). If medical therapy with denosumab is considered prior to surgery, which cellular interaction is primarily targeted?

. Inhibition of RANKL produced by the multinucleated giant cells
. Inhibition of RANKL produced by the neoplastic mononuclear stromal cells
. Direct apoptosis of the neoplastic mononuclear stromal cells
. Activation of osteoprotegerin (OPG) by the multinucleated giant cells
. Inhibition of RANK receptor on the neoplastic mononuclear stromal cells

Correct Answer & Explanation

. Inhibition of RANKL produced by the neoplastic mononuclear stromal cells

Explanation

In GCT, the true neoplastic cells are the mononuclear stromal cells, which overexpress RANKL. Denosumab binds to this RANKL, preventing it from activating the RANK receptor on the reactive, bone-resorbing multinucleated giant cells.

Question 9475

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with a lytic epiphyseal lesion in her proximal tibia. Biopsy confirms Giant Cell Tumor (GCT) of bone. Because the tumor is deemed unresectable, medical therapy is initiated. The prescribed medication targets which of the following?

. RANKL
. Osteoprotegerin
. VEGF
. Tyrosine kinase
. PD-1

Correct Answer & Explanation

. RANKL

Explanation

Denosumab is a monoclonal antibody that targets RANKL, which is overexpressed by the neoplastic mononuclear stromal cells in GCT of bone. This prevents RANK activation on osteoclasts, dramatically reducing bone resorption and tumor progression.

Question 9476

Topic: Infection, Pharmacology & VTE

A 12-year-old boy with systemic juvenile idiopathic arthritis (JIA) presents to the emergency department with acute high fever, hepatosplenomegaly, and altered mental status. Laboratory results show a sudden drop in ESR, thrombocytopenia, and hypofibrinogenemia. What is the most likely diagnosis?

. Septic arthritis
. Osteomyelitis
. Macrophage activation syndrome
. Reactive arthritis
. Acute lymphoblastic leukemia

Correct Answer & Explanation

. Macrophage activation syndrome

Explanation

Macrophage activation syndrome (MAS) is a life-threatening complication of systemic JIA characterized by a paradoxical drop in ESR, coagulopathy, and cytopenias. It requires immediate medical intervention.

Question 9477

Topic: 1. General Principles & Basic Science

Early-onset juvenile idiopathic arthritis commonly involves the temporomandibular joint (TMJ). Which of the following facial deformities is most characteristically seen as a consequence of this involvement?

. Prognathism
. Maxillary hyperplasia
. Micrognathia
. Hemifacial microsomia
. Cleft palate

Correct Answer & Explanation

. Micrognathia

Explanation

TMJ arthritis in JIA can cause premature closure of the mandibular condyle growth plate. This leads to restricted mandibular growth, resulting in micrognathia (the classic 'bird-face' deformity).

Question 9478

Topic: Biology, Genetics & Bone Healing

A 30-year-old female presents with a large, lytic, expansile lesion in the proximal tibia extending to the subchondral bone. Biopsy demonstrates mononuclear cells interspersed with osteoclast-like multinucleated cells. Which targeted pharmacological agent can be used to downstage this tumor prior to surgery?

. Imatinib
. Bisphosphonates
. Rituximab
. Denosumab
. Methotrexate

Correct Answer & Explanation

. Denosumab

Explanation

The diagnosis is Giant Cell Tumor (GCT) of bone. Denosumab, a monoclonal antibody against RANKL, inhibits the recruitment and action of the osteoclast-like giant cells, causing tumor calcification and facilitating surgical curettage.

Question 9479

Topic: Physiology & Rehabilitation

A patient with syringomyelia presents with the classic 'cape-like' bilateral loss of pain and temperature sensation over the shoulders and upper extremities.

This specific neurological deficit is caused by the expanding syrinx compressing which structure within the spinal cord?

. Dorsal columns
. Lateral corticospinal tract
. Anterior horn cells
. Anterior white commissure
. Spinocerebellar tract

Correct Answer & Explanation

. Anterior white commissure

Explanation

A central cord syrinx initially compresses the crossing spinothalamic fibers in the anterior white commissure. This disruption causes the classic dissociated sensory loss (loss of pain and temperature, with preserved fine touch and proprioception).

Question 9480

Topic: Biology, Genetics & Bone Healing

A 4-year-old girl is diagnosed with ANA-positive oligoarticular Juvenile Idiopathic Arthritis (JIA). What is the most critical routine screening required for this patient?

. Slit-lamp examination every 3-4 months
. Echocardiogram yearly
. Pulmonary function tests every 6 months
. Renal ultrasound yearly
. DEXA scan at diagnosis and every 2 years

Correct Answer & Explanation

. Slit-lamp examination every 3-4 months

Explanation

ANA-positive oligoarticular JIA carries the highest risk for asymptomatic uveitis/iridocyclitis. Routine slit-lamp examination every 3-4 months is critical to prevent permanent visual impairment or blindness.

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