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1. General Principles & Basic Science MCQs

Practice Set 231 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 4601

Topic: Biology, Genetics & Bone Healing

A 30-year-old female is diagnosed with a biopsy-proven giant cell tumor of the distal radius. Due to the extent of the lesion, she is treated with denosumab preoperatively. What is the precise mechanism of action of this pharmacological agent?

. It directly binds to and inhibits the RANK receptor on the surface of osteoclasts
. It binds to and inhibits the RANKL protein, preventing osteoclast formation and activation
. It induces targeted apoptosis of the neoplastic stromal cells through the p53 pathway
. It blocks the VEGF pathway to decrease tumor neoangiogenesis
. It acts as a synthetic analog of osteoprotegerin (OPG) produced by osteoblasts

Correct Answer & Explanation

. It binds to and inhibits the RANKL protein, preventing osteoclast formation and activation

Explanation

Denosumab is a fully human monoclonal antibody that binds to Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). Giant cell tumors of bone consist of neoplastic mononuclear stromal cells that overexpress RANKL, which in turn recruits and hyperactivates multinucleated giant cells (osteoclast-like cells) expressing the RANK receptor. By binding RANKL, denosumab prevents it from interacting with RANK on osteoclasts, severely halting bone resorption and leading to ossification of the tumor matrix.

Question 4602

Topic: 1. General Principles & Basic Science

According to the Seddon classification of nerve injuries, a peripheral nerve injury that involves complete disruption of the axon and myelin sheath, but preservation of the surrounding connective tissue framework (endoneurium, perineurium, and epineurium) is termed:

. Neuropraxia
. Axonotmesis
. Neurotmesis
. Wallerian degeneration
. Neuroma-in-continuity

Correct Answer & Explanation

. Axonotmesis

Explanation

In the Seddon classification: Neuropraxia is a local conduction block with intact axons (demyelination); recovery is fast and complete. Axonotmesis involves disruption of the axon and myelin sheath, resulting in distal Wallerian degeneration, but the connective tissue tubes (endoneurium, perineurium, epineurium) remain intact. This guides the regenerating axons, allowing for spontaneous recovery at ~1 mm/day. Neurotmesis is a complete transection of the nerve including the connective tissue framework, requiring surgical repair for any functional recovery.

Question 4603

Topic: Surgical Anatomy & Approaches

Which of the following surgical approaches for total hip arthroplasty (THA) uniquely utilizes a true internervous and intermuscular plane, dissecting superficially between the tensor fasciae latae and the sartorius, and deeply between the gluteus medius and the rectus femoris?

. Direct anterior approach (Smith-Petersen)
. Anterolateral approach (Watson-Jones)
. Direct lateral approach (Hardinge)
. Posterior approach (Moore/Southern)
. Posterolateral approach

Correct Answer & Explanation

. Direct anterior approach (Smith-Petersen)

Explanation

The direct anterior (Smith-Petersen) approach exploits a true internervous plane. Superficially, the plane is between the sartorius (femoral nerve) and the tensor fasciae latae (superior gluteal nerve). Deeply, the plane is between the rectus femoris (femoral nerve) and the gluteus medius/minimus (superior gluteal nerve). The anterolateral (Watson-Jones) plane is between the TFL and gluteus medius (both superior gluteal nerve, thus not internervous). The direct lateral approach splits the gluteus medius and vastus lateralis.

Question 4604

Topic: Biology, Genetics & Bone Healing

Which of the following transcription factors most strongly promotes the differentiation of mesenchymal stem cells into osteoblasts during fracture healing?

. Low oxygen tension
. High strain environment
. Core binding factor alpha-1 (Runx2)
. Sclerostin
. Tumor necrosis factor-alpha

Correct Answer & Explanation

. Core binding factor alpha-1 (Runx2)

Explanation

Core binding factor alpha-1 (Cbfa1), also known as Runx2, is the master transcription factor responsible for the differentiation of multipotent mesenchymal stem cells into the osteoblast lineage. Sclerostin is a Wnt antagonist that inhibits bone formation. High strain promotes fibrous tissue, and low oxygen promotes cartilage formation.

Question 4605

Topic: Biology, Genetics & Bone Healing
In the process of endochondral ossification during fracture healing, which type of collagen is most predominantly synthesized by chondrocytes in the hypertrophic zone just prior to calcification?
. Type I collagen
. Type II collagen
. Type III collagen
. Type IX collagen
. Type X collagen

Correct Answer & Explanation

. Type X collagen

Explanation

Hypertrophic chondrocytes uniquely synthesize Type X collagen during endochondral ossification. This collagen plays a critical role in the mineralization of the cartilaginous matrix, allowing subsequent vascular invasion and replacement by bone.

Question 4606

Topic: Biomechanics & Biomaterials

Galvanic corrosion between orthopedic implants occurs when two dissimilar metals are in contact within an electrolytic environment. Which of the following combinations of metals is most likely to result in significant galvanic corrosion in vivo and is generally contraindicated?

. Titanium and Titanium alloy
. Stainless steel and Cobalt-Chromium
. Stainless steel and Titanium
. Cobalt-Chromium and Titanium
. Zirconium and Titanium

Correct Answer & Explanation

. Stainless steel and Cobalt-Chromium

Explanation

Stainless steel and titanium have a large electrochemical potential difference. When placed in contact (e.g., a stainless steel plate with a titanium screw), the stainless steel acts as the anode and corrodes rapidly. Cobalt-chromium and titanium are routinely used together (e.g., THA stems and heads) with an acceptable passivation layer preventing severe corrosion.

Question 4607

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) play a crucial role in osteoinduction and are utilized in various spine and trauma applications.

Which of the following intracellular signaling molecules is directly phosphorylated following the binding of BMP-2 or BMP-7 to its serine/threonine kinase membrane receptor?

. Beta-catenin
. Smad 1/5/8
. Smad 2/3
. Smad 4
. Runx2

Correct Answer & Explanation

. Smad 1/5/8

Explanation

BMPs exert their effects by binding to specific serine/threonine kinase receptors on the cell surface. This binding leads to the direct phosphorylation of the receptor-regulated Smads (R-Smads), specifically Smad 1, 5, and 8. These then form a complex with the common-mediator Smad 4, which translocates to the nucleus to upregulate osteogenic genes like Runx2. TGF-beta, in contrast, signals via Smad 2 and 3.

Question 4608

Topic: Biomechanics & Biomaterials

Articular cartilage exhibits complex viscoelastic properties under loading. When a constant deformation (strain) is rapidly applied and maintained on articular cartilage, the internal peak stress gradually decreases over time until it reaches an equilibrium. Which of the following terms best describes this specific biomechanical phenomenon?

. Creep
. Stress relaxation
. Hysteresis
. Anisotropy
. Fatigue failure

Correct Answer & Explanation

. Stress relaxation

Explanation

Stress relaxation is a hallmark of viscoelastic materials, defined as the decrease in internal stress over time when subjected to a constant strain (deformation). Creep, conversely, is the progressive increase in strain (deformation) over time when subjected to a constant stress (load). Hysteresis refers to the energy lost (usually as heat) during the loading and unloading cycle of a viscoelastic material.

Question 4609

Topic: Infection, Pharmacology & VTE

A 55-year-old male with poorly controlled diabetes mellitus presents with a swollen, erythematous, and painless left foot.

Radiographs show fragmentation and subluxation of the midfoot. To differentiate between acute Charcot arthropathy and superimposed osteomyelitis, an MRI is obtained. Which of the following MRI findings is highly specific for osteomyelitis in this setting?

. Diffuse bone marrow edema
. Significant joint effusion
. Subchondral cysts
. Presence of a sinus tract or the 'ghost sign'
. Periarticular soft tissue edema

Correct Answer & Explanation

. Presence of a sinus tract or the 'ghost sign'

Explanation

Differentiating Charcot arthropathy from osteomyelitis on MRI can be challenging as both present with marrow edema. However, finding a sinus tract extending from a skin ulcer to the bone, or the 'ghost sign' (disappearance of a bone on T1-weighted images that reappears on T2 or post-contrast images), is highly specific for osteomyelitis.

Question 4610

Topic: 1. General Principles & Basic Science

Articular cartilage relies on a highly specialized architectural organization to resist complex mechanical loads.

In the deep (radial) zone of articular cartilage, which statement accurately describes the orientation of Type II collagen fibers and the concentration of proteoglycans?

. Collagen fibers are parallel to the joint surface; proteoglycan concentration is lowest.
. Collagen fibers are parallel to the joint surface; proteoglycan concentration is highest.
. Collagen fibers are perpendicular to the joint surface; proteoglycan concentration is lowest.
. Collagen fibers are perpendicular to the joint surface; proteoglycan concentration is highest.
. Collagen fibers are randomly oriented; proteoglycan concentration is intermediate.

Correct Answer & Explanation

. Collagen fibers are perpendicular to the joint surface; proteoglycan concentration is highest.

Explanation

Articular cartilage is divided into zones. In the superficial zone, collagen fibers are parallel to the surface to resist shear, water content is highest, and proteoglycan content is lowest. In the deep (radial) zone, the collagen fibers are oriented perpendicularly to the subchondral bone to resist compressive forces, and the proteoglycan concentration is at its highest.

Question 4611

Topic: Biology, Genetics & Bone Healing

A 30-year-old woman presents with worsening knee pain. Radiographs reveal an eccentric, lytic, expansile lesion located in the epiphysis of the proximal tibia, extending to the subchondral bone. There is no sclerotic margin. Histology demonstrates mononuclear cells and numerous uniformly distributed osteoclast-like multinucleated giant cells. Which of the following is the most effective targeted medical therapy for advanced, unresectable cases of this tumor?

. Imatinib
. Denosumab
. Methotrexate
. Doxorubicin
. Rituximab

Correct Answer & Explanation

. Denosumab

Explanation

The clinical and radiographic presentation is classic for a Giant Cell Tumor of Bone (GCTB). The neoplastic mononuclear cells in GCTB express high levels of RANKL, which recruits and activates the osteoclast-like giant cells, leading to massive bone destruction. Denosumab, a monoclonal antibody against RANKL, is highly effective in treating GCTB and is used for unresectable or recurrent lesions, or as a neoadjuvant therapy.

Question 4612

Topic: Biology, Genetics & Bone Healing

Demineralized bone matrix (DBM) is commonly used as a bone graft extender in spinal fusion surgery. Which of the following correctly describes the biologic properties of DBM?

. Osteogenic, osteoinductive, and osteoconductive
. Osteoinductive and osteoconductive only
. Osteoconductive only
. Osteogenic and osteoconductive only
. Osteoinductive only

Correct Answer & Explanation

. Osteoinductive and osteoconductive only

Explanation

Demineralized bone matrix (DBM) is processed allograft bone that has had the inorganic mineral component removed, leaving the organic collagen matrix and growth factors (such as bone morphogenetic proteins, BMPs). It is osteoinductive (due to the presence of BMPs) and osteoconductive (providing a biologic scaffold). It is not osteogenic because the processing destroys living cells (osteoblasts or osteoprogenitor cells).

Question 4613

Topic: 1. General Principles & Basic Science

In normal articular cartilage, which structural zone contains the largest diameter collagen fibrils oriented perpendicular to the joint surface and provides the highest resistance to compressive forces?

. Superficial tangential zone
. Middle (transitional) zone
. Deep (radial) zone
. Calcified zone
. Tidemark

Correct Answer & Explanation

. Deep (radial) zone

Explanation

The deep (radial) zone of articular cartilage contains the largest diameter collagen fibrils that are oriented perpendicular to the joint surface. This zone also has the highest concentration of proteoglycans and the lowest water content, providing the primary resistance to compressive loads. The superficial zone has collagen parallel to the joint, which primarily resists shear forces.

Question 4614

Topic: Biology, Genetics & Bone Healing

Denosumab is widely utilized as a neoadjuvant or definitive therapy for unresectable Giant Cell Tumor of Bone. By what mechanism does denosumab arrest the progression of this lesion?

. It induces apoptosis of the neoplastic mononuclear stromal cells directly via p53 activation
. It cross-links DNA via alkylation, preventing mitosis in highly proliferative cells
. It competitively inhibits vascular endothelial growth factor (VEGF), leading to tumor ischemia
. It stimulates osteoblastic activity by agonizing the Wnt/beta-catenin pathway
. It acts as a monoclonal antibody against RANKL, preventing osteoclast-like giant cell formation

Correct Answer & Explanation

. It acts as a monoclonal antibody against RANKL, preventing osteoclast-like giant cell formation

Explanation

Giant Cell Tumor of Bone consists of neoplastic mononuclear stromal cells that overexpress RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand). This overproduction recruits and activates reactive multinucleated osteoclast-like giant cells, which cause massive localized bone destruction. Denosumab is a fully human monoclonal antibody that binds directly to RANKL, inhibiting the formation, function, and survival of these giant cells, thereby arresting osteolysis and allowing the bone to re-ossify.

Question 4615

Topic: Surgical Anatomy & Approaches

During an ilioinguinal approach to the acetabulum, massive bleeding occurs while dissecting over the superior pubic ramus near the symphysis. The injured vessel, commonly referred to as the 'corona mortis,' typically represents an anastomosis between the obturator artery and which of the following?

. Internal pudendal artery
. Superior gluteal artery
. Inferior epigastric or external iliac artery
. Femoral artery
. Deep circumflex iliac artery

Correct Answer & Explanation

. Inferior epigastric or external iliac artery

Explanation

The 'corona mortis' (crown of death) is an anatomical variant describing an anastomosis between the obturator system and the external iliac or inferior epigastric system. It is located on the posterior aspect of the superior pubic ramus. Injury to this vessel during pelvic or acetabular surgery (like the ilioinguinal or Stoppa approaches) can lead to severe, difficult-to-control hemorrhage.

Question 4616

Topic: Biomechanics & Biomaterials

In total hip arthroplasty (THA), the adoption of highly cross-linked polyethylene (HXLPE) has significantly reduced wear rates compared to conventional polyethylene. However, the high-dose irradiation process alters the mechanical properties of the material. Which mechanical property is most significantly decreased as a direct result of irradiation for cross-linking?

. Modulus of elasticity
. Wear resistance
. Fatigue crack propagation resistance
. Coefficient of friction
. Oxidation resistance prior to remelting

Correct Answer & Explanation

. Fatigue crack propagation resistance

Explanation

While high-dose irradiation creates cross-links that greatly improve wear resistance, it also makes the polyethylene more brittle. This results in a decrease in ductility, ultimate tensile strength, and most notably, fatigue crack propagation resistance (fracture toughness). This is why highly cross-linked polyethylene is generally avoided in unicompartmental knees or very thin tibial inserts where high stress concentrations might lead to component fracture.

Question 4617

Topic: Biomechanics & Biomaterials

Titanium alloy (Ti-6Al-4V) is frequently utilized in orthopedic implants, including fracture plates and intramedullary nails. Compared to stainless steel (316L) and cobalt-chrome (Co-Cr) alloys, which of the following is a primary biomechanical advantage of titanium?

. Higher modulus of elasticity leading to better load sharing
. Lower modulus of elasticity resulting in decreased stress shielding
. Superior resistance to galvanic corrosion when mixed with other metals
. Higher density providing increased implant weight and stability
. Greater resistance to wear and scratching in articulating surfaces

Correct Answer & Explanation

. Lower modulus of elasticity resulting in decreased stress shielding

Explanation

Titanium has a significantly lower modulus of elasticity (closer to that of cortical bone) compared to stainless steel and cobalt-chrome. This allows for more load sharing with the host bone, thereby decreasing the phenomenon of stress shielding (which can lead to bone resorption around the implant). Titanium is famously poor for wear resistance on articulating surfaces.

Question 4618

Topic: Biology, Genetics & Bone Healing

A 28-year-old male presents with knee pain. Radiographs reveal an eccentric, purely lytic epiphyseal lesion extending to the subchondral bone of the proximal tibia. Biopsy histology is shown.

Multinucleated giant cells in a background of mononuclear stromal cells are seen. Which systemic medication is specifically designed to target the pathophysiology of this lesion?

. Methotrexate
. Doxorubicin
. Imatinib
. Denosumab
. Bisphosphonates

Correct Answer & Explanation

. Denosumab

Explanation

The patient has a Giant Cell Tumor (GCT) of bone. The neoplastic mononuclear stromal cells in GCT express high levels of RANKL, which stimulates the recruitment and differentiation of non-neoplastic multinucleated giant cells that cause the osteolysis. Denosumab is a monoclonal antibody that inhibits RANKL, effectively halting this process and promoting ossification of the lytic defect. It is utilized for unresectable tumors or to facilitate joint salvage.

Question 4619

Topic: 1. General Principles & Basic Science

Normal articular cartilage is divided into distinct zones. In which zone is the concentration of water and collagen the highest, with chondrocytes characteristically flattened and arranged parallel to the joint surface?

. Superficial (tangential) zone
. Middle (transitional) zone
. Deep (radial) zone
. Tidemark
. Calcified cartilage zone

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

The superficial (tangential) zone of articular cartilage contains the highest concentration of water (approximately 80%) and the highest concentration of collagen (primarily Type II). The collagen fibrils and flattened chondrocytes are aligned parallel to the articular surface to resist shear forces.

Question 4620

Topic: Biology, Genetics & Bone Healing

A 32-year-old female presents with progressive knee pain. Imaging reveals an eccentric, lytic epiphyseal-metaphyseal lesion of the distal femur. A biopsy demonstrates multinucleated giant cells in a background of mononuclear stromal cells.

If neoadjuvant medical therapy is utilized prior to curettage, the mechanism of action of the most appropriate drug is:

. Direct cytotoxicity targeting multinucleated giant cells
. Inhibition of osteoclast proton pumps
. Inhibition of RANK ligand (RANKL)
. Inhibition of Vascular Endothelial Growth Factor (VEGF)
. Downregulation of parathyroid hormone receptors

Correct Answer & Explanation

. Inhibition of RANK ligand (RANKL)

Explanation

The diagnosis is Giant Cell Tumor (GCT) of bone. Denosumab is a monoclonal antibody often used in the medical management of GCT. Its mechanism of action is the inhibition of RANK ligand (RANKL). In GCT, the neoplastic cells are the mononuclear stromal cells, which express RANKL. This recruits and activates the non-neoplastic, multinucleated osteoclast-like giant cells that cause the massive bone resorption.

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