Menu

1. General Principles & Basic Science MCQs

Practice Set 735 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 14681

Topic: 1. General Principles & Basic Science

According to Paley's osteotomy rules, if the osteotomy is executed at a level separate from the CORA, and the correction hinge is placed directly at the osteotomy site, what is the biomechanical outcome upon correction of the angulation?

. The mechanical axes will perfectly align without translation
. The bone will undergo significant shortening requiring segmental transport
. A secondary translation will occur, causing the mechanical axes to remain misaligned
. Pure translation occurs without altering the initial angular deformity
. The joint orientation angles will reverse their alignment completely

Correct Answer & Explanation

. A secondary translation will occur, causing the mechanical axes to remain misaligned

Explanation

Paley's Rule 3 indicates that if both the osteotomy and the hinge are placed away from the CORA, correcting the angulation will simultaneously induce a translational deformity. This results in parallel but misaligned mechanical axes.

Question 14682

Topic: 1. General Principles & Basic Science

When planning a deformity correction using a circular fixator, the surgeon places the hinge at the center of rotation of angulation (CORA) but performs the osteotomy at a different level. According to Paley's osteotomy rules, what is the expected outcome at the osteotomy site?

. Angulation without translation
. Angulation with translation
. Pure translation
. Compression only
. Distraction only

Correct Answer & Explanation

. Angulation with translation

Explanation

According to Osteotomy Rule 2, when the osteotomy is separate from the CORA but the hinge is placed at the CORA, the mechanical axes will realign properly. However, this configuration inevitably causes translation at the osteotomy site.

Question 14683

Topic: 1. General Principles & Basic Science

Review the lower extremity deformity shown.

When analyzing a diaphyseal deformity with two distinct centers of rotation of angulation (CORAs), which of the following mechanical axis lines must be established to determine the intervening segment's true axis?

. Proximal and distal mechanical axes only
. Proximal mechanical axis, distal mechanical axis, and a line connecting the two CORAs
. Proximal mechanical axis, distal mechanical axis, and a line parallel to the joint line
. Anatomic axis of the proximal segment only
. Mechanical axis of the entire lower extremity

Correct Answer & Explanation

. Proximal mechanical axis, distal mechanical axis, and a line connecting the two CORAs

Explanation

In multi-apical deformities, the axis of the middle segment is defined by a line connecting the two CORAs. These CORAs are found at the intersections of this intervening line with the respective proximal and distal mechanical or anatomic axes.

Question 14684

Topic: 1. General Principles & Basic Science

When applying a hexapod circular external fixator for complex lower extremity deformity correction, accurately inputting mounting parameters is essential. Which of the following defines the "axial translation" mounting parameter?

. The AP distance from the center of the ring to the reference bone axis
. The mediolateral distance from the center of the ring to the reference bone axis
. The proximal-distal distance from the reference ring to the origin/CORA
. The angular difference between the ring and the mechanical axis
. The distance between the proximal and distal rings

Correct Answer & Explanation

. The proximal-distal distance from the reference ring to the origin/CORA

Explanation

In hexapod frame software, axial translation refers to the vertical (proximal-distal) distance from the reference ring's center to the deformity's origin (CORA). Anteroposterior and lateral translations denote the orthogonal offsets.

Question 14685

Topic: Biology, Genetics & Bone Healing

A 32-year-old female presents with a purely lytic, eccentric lesion in the distal femur extending to the subchondral bone. Biopsy confirms giant cell tumor of bone. Neoadjuvant therapy with denosumab is considered. What is the mechanism of action of this drug?

. Monoclonal antibody against RANK ligand (RANKL)
. Inhibits osteoclast ruffled border proton pumps
. Binds to VEGF to inhibit angiogenesis
. Tyrosine kinase inhibitor of c-KIT
. Direct cytotoxic effect on multinucleated giant cells

Correct Answer & Explanation

. Monoclonal antibody against RANK ligand (RANKL)

Explanation

Denosumab is a human monoclonal antibody that binds to RANKL, preventing it from activating RANK on the surface of osteoclasts and their precursors. In giant cell tumors, it halts the tumor-induced osteolysis mediated by the reactive giant cells.

Question 14686

Topic: Biology, Genetics & Bone Healing

A 32-year-old male presents with knee pain. Radiographs reveal an eccentric, lytic epiphyseal lesion in the distal femur. Biopsy reveals multinucleated giant cells and mononuclear stromal cells.

Which cell population harbors the neoplastic mutation, and what marker do they overexpress?

. Multinucleated giant cells expressing RANKL
. Mononuclear stromal cells expressing RANK
. Mononuclear stromal cells expressing RANKL
. Multinucleated giant cells expressing RANK
. Macrophages expressing CD68

Correct Answer & Explanation

. Mononuclear stromal cells expressing RANKL

Explanation

In Giant Cell Tumor of bone, the mononuclear spindle-like stromal cells are the true neoplastic cells. They express RANKL, which recruits and stimulates normal osteoclast precursors to form the reactive multinucleated giant cells.

Question 14687

Topic: Biology, Genetics & Bone Healing

Which of the following skeletal dysplasias is characterized by a mutation in the RUNX2 (CBFA1) gene, delayed closure of cranial sutures, and supernumerary teeth?

. Osteogenesis Imperfecta
. Cleidocranial dysplasia
. Diastrophic dysplasia
. Achondroplasia
. Multiple epiphyseal dysplasia

Correct Answer & Explanation

. Cleidocranial dysplasia

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by a RUNX2 mutation, leading to defective intramembranous ossification. Hallmarks include absent or hypoplastic clavicles, delayed cranial suture closure, and supernumerary teeth.

Question 14688

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with knee pain. Radiographs show an eccentric, lytic epiphyseal-metaphyseal lesion of the distal femur. Biopsy confirms a Giant Cell Tumor. If she is treated with denosumab preoperatively, which histological change is most expected?

. Increased mitotic figures in mononuclear cells
. Depletion of multinucleated osteoclast-like giant cells
. Extensive cartilage matrix production
. Transformation into a high-grade sarcoma
. Appearance of atypical spindle cells

Correct Answer & Explanation

. Depletion of multinucleated osteoclast-like giant cells

Explanation

Denosumab is a monoclonal antibody against RANKL. It prevents the RANK-RANKL interaction, leading to the rapid depletion of the reactive multinucleated giant cells and promotion of woven bone formation, while the underlying neoplastic mononuclear stromal cells persist.

Question 14689

Topic: Biology, Genetics & Bone Healing

In a Giant Cell Tumor of bone, which specific cell population harbors the pathognomonic H3F3A mutation and is considered the true neoplastic component?

. The multinucleated osteoclast-like giant cells
. The mononuclear spindle-shaped stromal cells
. The reactive histiocytes
. The recruited osteoblasts
. The perivascular pericytes

Correct Answer & Explanation

. The mononuclear spindle-shaped stromal cells

Explanation

The mononuclear spindle-shaped stromal cells are the true neoplastic cells in a Giant Cell Tumor of bone and typically harbor a mutation in the H3F3A gene. These cells secrete RANKL, which subsequently recruits and activates the non-neoplastic, reactive multinucleated giant cells.

Question 14690

Topic: Biology, Genetics & Bone Healing

In the management of a massive, unresectable recurrent giant cell tumor of the sacrum, which of the following targeted medical therapies is the most appropriate primary treatment?

. Imatinib
. Denosumab
. Zoledronic acid
. Rituximab
. Methotrexate

Correct Answer & Explanation

. Denosumab

Explanation

Denosumab is a monoclonal antibody against RANKL, which is overexpressed by the neoplastic mononuclear stromal cells in GCT. It effectively inhibits the recruitment and function of reactive osteoclast-like giant cells, making it the treatment of choice for unresectable cases.

Question 14691

Topic: Biology, Genetics & Bone Healing

Which of the following structural defects is primarily responsible for the clinical manifestations seen in patients with Osteogenesis Imperfecta?

. Defect in type II collagen synthesis
. Defect in type I collagen folding or synthesis
. Impaired osteoclast carbonic anhydrase II function
. Mutation in the RUNX2 gene
. Mutation in the Fibroblast Growth Factor Receptor 3 gene

Correct Answer & Explanation

. Defect in type I collagen folding or synthesis

Explanation

Osteogenesis imperfecta is most commonly caused by mutations in the COL1A1 or COL1A2 genes. This leads to quantitative or qualitative defects in Type I collagen, which is the major structural protein in bone, sclera, and dentin.

Question 14692

Topic: Biology, Genetics & Bone Healing

A 28-year-old female presents with knee pain. Radiographs show an eccentric, lytic epiphyseal lesion extending to the subchondral bone. Biopsy confirms a tumor characterized by mononuclear stromal cells and multinucleated giant cells. Which of the following best describes the mechanism of the targeted medical therapy for this condition?

. Inhibition of RANKL
. Inhibition of osteoprotegerin
. Tyrosine kinase inhibition
. Inhibition of VEGF
. Bisphosphonate-mediated apoptosis

Correct Answer & Explanation

. Inhibition of RANKL

Explanation

Giant cell tumor of bone is driven by RANKL-expressing neoplastic stromal cells that recruit osteoclast-like giant cells. Denosumab, a monoclonal antibody that inhibits RANKL, is the targeted medical therapy used for advanced or unresectable cases.

Question 14693

Topic: Biology, Genetics & Bone Healing

A 14-year-old boy has multiple painless bony protuberances around his knees and shoulders. Genetic testing reveals a mutation in the EXT1 gene. This mutation primarily affects which of the following cellular processes?

. Heparan sulfate synthesis
. Fibroblast growth factor receptor signaling
. Type I collagen formation
. Runx2 transcription
. Wnt/beta-catenin signaling

Correct Answer & Explanation

. Heparan sulfate synthesis

Explanation

Multiple Hereditary Exostoses (MHE) is caused by mutations in the EXT1 or EXT2 genes. These genes encode glycosyltransferases responsible for heparan sulfate synthesis, disrupting chondrocyte regulation at the physis.

Question 14694

Topic: Biology, Genetics & Bone Healing

A 4-year-old girl with a history of multiple low-energy fractures, blue sclerae, and dentinogenesis imperfecta is started on pamidronate. What is the primary mechanism of action of this medication in treating her condition?

. Stimulates osteoblast proliferation
. Inhibits osteoclast-mediated bone resorption
. Increases type I collagen cross-linking
. Enhances calcium absorption in the gut
. Promotes chondrocyte hypertrophy

Correct Answer & Explanation

. Inhibits osteoclast-mediated bone resorption

Explanation

Osteogenesis Imperfecta is treated medically with bisphosphonates like pamidronate. Bisphosphonates function by inhibiting osteoclast-mediated bone resorption, thereby increasing bone mineral density and reducing fracture risk.

Question 14695

Topic: Biology, Genetics & Bone Healing

A 15-year-old boy presents with the ability to appose his shoulders at the midline. He has delayed eruption of permanent teeth and an unusually wide face. Radiographs show absent clavicles. This condition is primarily due to a defect in which of the following?

. Endochondral ossification
. Intramembranous ossification
. Type 1 collagen synthesis
. Osteoclast function
. Lysosomal storage

Correct Answer & Explanation

. Intramembranous ossification

Explanation

Cleidocranial dysplasia is caused by a mutation in RUNX2 (CBFA1), an essential transcription factor for osteoblast differentiation. It primarily affects bones formed by intramembranous ossification, such as the clavicles and cranial vault.

Question 14696

Topic: Biology, Genetics & Bone Healing

A 6-year-old boy presents with recurrent fractures, anemia, and hepatosplenomegaly. Radiographs reveal a 'bone-within-a-bone' appearance and generalized osteosclerosis. What is the most definitive curative treatment for the underlying systemic disease?

. High-dose bisphosphonates
. Hematopoietic stem cell transplantation
. Parathyroid hormone analogs
. Intravenous enzyme replacement therapy
. Denosumab therapy

Correct Answer & Explanation

. Hematopoietic stem cell transplantation

Explanation

Infantile malignant osteopetrosis is caused by defective osteoclast function. Because osteoclasts are derived from the hematopoietic monocyte-macrophage lineage, hematopoietic stem cell transplantation (HSCT) is the only curative treatment.

Question 14697

Topic: Biology, Genetics & Bone Healing

An infant presents with failure to thrive, hepatosplenomegaly, and severe anemia. Radiographs show diffusely dense bones with a "bone-within-bone" appearance in the metaphyses. A mutation impairing which of the following cellular mechanisms is most likely responsible?

. Osteoblast differentiation via RUNX2
. Collagen cross-linking in the extracellular matrix
. Osteoclast acidification of the resorption pit
. Chondrocyte proliferation in the resting zone
. Fibroblast growth factor receptor signaling

Correct Answer & Explanation

. Osteoclast acidification of the resorption pit

Explanation

This is malignant infantile osteopetrosis, characterized by defective osteoclast function. Mutations often involve TCIRG1 or Carbonic Anhydrase II, preventing the osteoclast from acidifying the resorption pit and dissolving hydroxyapatite.

Question 14698

Topic: Biology, Genetics & Bone Healing

Denosumab is increasingly used in the neoadjuvant management of unresectable or complex giant cell tumors of bone.

What is the specific target of this monoclonal antibody?

. RANK receptor on osteoclasts
. Osteoprotegerin (OPG)
. RANKL produced by neoplastic stromal cells
. Vascular endothelial growth factor (VEGF)
. Macrophage colony-stimulating factor (M-CSF)

Correct Answer & Explanation

. RANKL produced by neoplastic stromal cells

Explanation

Denosumab is a monoclonal antibody that binds to RANK Ligand (RANKL), which is overexpressed by the neoplastic mononuclear stromal cells in giant cell tumors. This prevents RANKL from binding to the RANK receptor, thereby inhibiting osteoclast giant cell formation.

Question 14699

Topic: Biology, Genetics & Bone Healing

A 15-year-old patient is evaluated for delayed dental eruption, open cranial sutures, and abnormal shoulder mobility, allowing them to approximate their shoulders anteriorly. This autosomal dominant condition is caused by a mutation in the RUNX2 (CBFA1) gene. This gene normally controls the differentiation of which cell type?

. Chondrocytes
. Osteoclasts
. Osteoblasts
. Fibroblasts
. Hematopoietic stem cells

Correct Answer & Explanation

. Osteoblasts

Explanation

The patient has cleidocranial dysplasia. The RUNX2 (CBFA1) gene is a master transcription factor essential for osteoblast differentiation and subsequent intramembranous ossification.

Question 14700

Topic: Biology, Genetics & Bone Healing

A 65-year-old male presents with increasing hat size, asymmetric hearing loss, and bowing of his right tibia. Laboratory testing reveals markedly elevated alkaline phosphatase with normal serum calcium and phosphorus. Which phase of this specific disease process is characterized by intense, unregulated osteoclastic resorption?

. Sclerotic phase
. Mixed phase
. Lytic phase
. Latent phase
. Malignant phase

Correct Answer & Explanation

. Lytic phase

Explanation

Paget's disease of bone begins with an initial lytic phase, characterized by aggressive, unregulated osteoclastic bone resorption. This is followed by a mixed phase and finally a sclerotic phase where weak, woven bone is haphazardly laid down.

Test Yourself

Switch to an interactive, timed exam simulation to truly master this topic.

On this Page