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1. General Principles & Basic Science MCQs

Practice Set 379 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 7561

Topic: Biology, Genetics & Bone Healing

A 25-year-old man presents with progressive knee pain. Radiographs reveal an eccentric, entirely lytic epiphyseal lesion in the proximal tibia with no sclerotic border. A biopsy demonstrates sheets of mononuclear cells interspersed with numerous osteoclast-like giant cells. Due to the proximity to the joint, a specific monoclonal antibody is considered for systemic treatment prior to curettage to downstage the tumor. This medication exerts its effect by binding directly to which of the following?

. Receptor Activator of Nuclear factor Kappa-B (RANK)
. Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL)
. Osteoprotegerin (OPG)
. Vascular endothelial growth factor (VEGF)
. Macrophage colony-stimulating factor (M-CSF)

Correct Answer & Explanation

. Receptor Activator of Nuclear factor Kappa-B (RANK)

Explanation

The clinical and radiographic presentation of an eccentric, lytic epiphyseal lesion in a young adult is characteristic of a Giant Cell Tumor (GCT) of bone. Histologically, GCTs consist of neoplastic mononuclear stromal cells and reactive osteoclast-like giant cells. The neoplastic stromal cells overexpress Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). RANKL normally binds to the RANK receptor on the surface of osteoclast precursors, stimulating their differentiation and activation. Denosumab is a fully human monoclonal antibody that directly binds to and neutralizes RANKL, preventing it from binding to RANK. This halts the recruitment of osteoclast-like giant cells, leading to decreased bone resorption, tumor necrosis, and peripheral ossification of the lesion.

Question 7562

Topic: Biology, Genetics & Bone Healing

An 18-year-old man sustains a transverse midshaft femur fracture that is surgically stabilized with an intramedullary nail, leaving a minor gap at the fracture site. This mechanical environment promotes secondary (indirect) bone healing. Following the initial inflammatory phase and soft callus formation, endochondral ossification begins. During the hard callus phase, which of the following types of bone is initially deposited by osteoblasts?

. Lamellar bone
. Woven bone
. Haversian bone
. Osteonal bone
. Plexiform bone

Correct Answer & Explanation

. Lamellar bone

Explanation

Secondary (indirect) bone healing occurs under conditions of relative stability and involves fracture callus formation. The process progresses through hematoma/inflammation, soft callus (fibrocartilage), hard callus, and remodeling. During the hard callus phase, osteoblasts rapidly deposit woven bone via endochondral and intramembranous ossification. Woven bone is characterized by a disorganized, random arrangement of collagen fibers, which allows it to be formed quickly to bridge the fracture gap but makes it mechanically weak. Over months to years, basic multicellular units (osteoclasts and osteoblasts) remodel this woven bone into highly organized, mechanically robust lamellar bone (which includes Haversian/osteonal systems).

Question 7563

Topic: Biomechanics & Biomaterials

Ligaments and tendons are viscoelastic structures, meaning their mechanical behavior is dependent on the rate of loading and the time over which the load is applied. When an orthopedic surgeon tensions an ACL graft during reconstruction, the graft is pulled to a specific, constant length and held there. Over the course of several minutes, the tension (force) required to maintain that specific length gradually decreases. Which of the following biomechanical terms best describes this specific phenomenon?

. Creep
. Stress relaxation
. Hysteresis
. Fatigue failure
. Isotropy

Correct Answer & Explanation

. Creep

Explanation

Viscoelastic materials exhibit characteristic time-dependent properties. 'Stress relaxation' is defined as the steady decrease in stress (force) over time when a material is held at a constant strain (constant length). This is clinically relevant during ligament reconstruction or serial casting. 'Creep', conversely, is the gradual increase in strain (deformation or lengthening) over time when a material is subjected to a constant applied stress (constant load). 'Hysteresis' refers to the energy lost (usually as heat) during a loading and unloading cycle, represented by the area between the loading and unloading curves on a stress-strain graph. 'Isotropy' refers to materials that have the same mechanical properties in all directions, whereas ligaments are anisotropic.

Question 7564

Topic: Biomechanics & Biomaterials

The sterilization and processing methods of ultra-high-molecular-weight polyethylene (UHMWPE) heavily influence its long-term wear properties in total joint arthroplasty. Historically, UHMWPE components were sterilized using gamma irradiation in the presence of air. This practice was largely abandoned because it reliably led to which of the following detrimental outcomes?

. Insufficient initial sterilization against spore-forming bacteria
. Immediate catastrophic decrease in ultimate tensile strength
. Oxidative degradation leading to embrittlement and subsurface delamination
. Excessive cross-linking resulting in extreme stiffness
. Thermal melting and loss of component modularity

Correct Answer & Explanation

. Insufficient initial sterilization against spore-forming bacteria

Explanation

Historically, gamma irradiation in air (oxygen) was the standard sterilization method for UHMWPE. The high-energy gamma rays break polymer chains and generate free radicals. While this causes some beneficial cross-linking, the free radicals react rapidly with oxygen to form hydroperoxides and ketones, a process called oxidative degradation. This oxidation continues progressively over time, even while the implant sits on the shelf, leading to a profound decrease in molecular weight, severe embrittlement, and ultimately disastrous subsurface delamination and rapid wear in vivo. Modern polyethylene is irradiated in inert environments (vacuum or argon) and often thermally treated (remelted or annealed) or infused with Vitamin E to extinguish free radicals and prevent this oxidative degradation.

Question 7565

Topic: Infection, Pharmacology & VTE

Orthopedic implant-associated infections are notoriously difficult to eradicate. Pathogenic bacteria, such as Staphylococcus aureus, form biofilms on the surface of foreign materials. Once a mature biofilm is established, the bacteria become highly resistant to both systemic antibiotics and host immune clearance. Which of the following components is primarily responsible for conferring this structural integrity and profound antibiotic tolerance to the mature biofilm?

. The rigid peptidoglycan cell wall
. Teichoic acid and lipoteichoic acid
. The extracellular polymeric substance (EPS)
. Surface-bound Protein A
. Intracellular fibronectin-binding proteins

Correct Answer & Explanation

. The rigid peptidoglycan cell wall

Explanation

A critical feature of orthopedic device-related infections is the ability of bacteria like S. aureus and S. epidermidis to form a mature biofilm. The defining characteristic of a mature biofilm is the production of an extracellular polymeric substance (EPS), also known as the glycocalyx. The EPS is a complex, self-produced matrix consisting of polysaccharides (such as PIA/PNAG), structural proteins, and extracellular DNA. This thick matrix encases the bacteria, providing a formidable physical and chemical barrier that prevents the penetration of host immune cells (e.g., macrophages, neutrophils) and drastically reduces the diffusion and efficacy of antimicrobial agents. While peptidoglycan, teichoic acid, and Protein A are important virulence factors for S. aureus, it is the EPS matrix that specifically defines the mature biofilm phenotype and its characteristic recalcitrance to medical therapy.

Question 7566

Topic: Biology, Genetics & Bone Healing

A 45-year-old man undergoes a posterolateral lumbar fusion. Recombinant human bone morphogenetic protein-2 (rhBMP-2) is utilized. Which of the following best describes the intracellular signaling mechanism immediately following BMP binding to its receptor?

. Activation of the Wnt/beta-catenin pathway
. Phosphorylation of Smad 1, 5, and 8
. Inhibition of osteoprotegerin (OPG)
. Activation of adenylyl cyclase and cAMP production
. Cleavage of the Notch intracellular domain

Correct Answer & Explanation

. Activation of the Wnt/beta-catenin pathway

Explanation

BMPs are members of the TGF-beta superfamily. They bind to cell surface serine/threonine kinase receptors. Upon activation, these receptors phosphorylate intracellular Smad proteins, specifically Smad 1, 5, and 8. These phosphorylated Smads then form a complex with Smad 4, which translocates to the nucleus to regulate the transcription of osteogenic genes, including Runx2. The Wnt pathway involves beta-catenin, while OPG is a decoy receptor for RANKL.

Question 7567

Topic: Biology, Genetics & Bone Healing

A 65-year-old woman with severe osteoporosis requires spinal instrumentation. To maximize pedicle screw pull-out strength in osteoporotic bone, which of the following alterations in screw design is most effective?

. Decreasing the outer diameter of the screw
. Decreasing the thread pitch (fewer threads per inch)
. Increasing the inner diameter of the screw core
. Increasing the outer diameter of the screw
. Utilizing a screw with a constant inner diameter rather than conical

Correct Answer & Explanation

. Decreasing the outer diameter of the screw

Explanation

The pull-out strength of a pedicle screw is most significantly influenced by the outer diameter of the screw, the length of bone engagement, and the bone mineral density. Increasing the outer diameter increases the volume of bone caught between the threads, thereby maximizing pull-out strength. Increasing the inner diameter (core) decreases thread depth, which reduces pull-out strength but increases the fatigue strength of the screw. Decreasing thread pitch (fewer threads) decreases engagement and reduces pull-out resistance.

Question 7568

Topic: Biomechanics & Biomaterials

A 32-year-old man sustained a femur fracture treated with a stainless steel plate and screws 2 years ago. He presents with a nonunion and broken hardware. During revision surgery, the surgeon considers using titanium screws through the retained intact stainless steel plate. Which of the following is the most likely consequence of mixing these two metals?

. Fretting corrosion leading to failure of the titanium screws
. Galvanic corrosion causing accelerated degradation of the stainless steel plate
. Galvanic corrosion causing accelerated degradation of the titanium screws
. Crevice corrosion isolated to the screw holes
. No significant corrosion due to the formation of a stable passivation layer on both metals

Correct Answer & Explanation

. Fretting corrosion leading to failure of the titanium screws

Explanation

Mixing dissimilar metals in a conductive environment (body fluid) leads to galvanic corrosion. The less noble (more anodic) metal undergoes accelerated corrosion, while the more noble (cathodic) metal is protected. Titanium is more noble than stainless steel. Therefore, if titanium screws are used with a stainless steel plate, the stainless steel plate will act as the anode and undergo accelerated galvanic corrosion.

Question 7569

Topic: 1. General Principles & Basic Science

A 28-year-old patient suffers a nerve injury characterized by loss of axonal continuity and disruption of the endoneurial tubes, while the perineurium and epineurium remain completely intact. According to Sunderland's classification, which grade does this represent, and what is its Seddon classification equivalent?

. Sunderland Grade I; Neurapraxia
. Sunderland Grade II; Axonotmesis
. Sunderland Grade III; Axonotmesis
. Sunderland Grade IV; Neuroma-in-continuity
. Sunderland Grade V; Neurotmesis

Correct Answer & Explanation

. Sunderland Grade I; Neurapraxia

Explanation

Sunderland Grade III nerve injuries involve disruption of the axon and endoneurium, with an intact perineurium and epineurium. Intrafascicular scarring can occur, which often impedes spontaneous regeneration and may require surgical intervention. It falls under Seddon's classification of axonotmesis. Grade I is neurapraxia. Grade II involves an intact endoneurium. Grade IV involves disruption of the perineurium (intact epineurium). Grade V is a complete transection (neurotmesis).

Question 7570

Topic: Biology, Genetics & Bone Healing

A 70-year-old man presents with aseptic loosening of his cementless total hip arthroplasty 15 years after the index procedure. Radiographs show extensive periprosthetic osteolysis. The primary biological mechanism involves particulate wear debris stimulating macrophages to release cytokines. Which of the following is the final common pathway leading to bone resorption in this condition?

. Increased production of Osteoprotegerin (OPG)
. Macrophage-mediated direct phagocytosis of bone
. Increased expression of RANKL binding to RANK on osteoclast precursors
. Decreased expression of Macrophage Colony-Stimulating Factor (M-CSF)
. Activation of the Wnt/beta-catenin pathway in osteoblasts

Correct Answer & Explanation

. Increased production of Osteoprotegerin (OPG)

Explanation

In periprosthetic osteolysis, wear debris (most notably polyethylene) is phagocytosed by macrophages. These activated macrophages release pro-inflammatory cytokines such as TNF-alpha, IL-1, and IL-6. These cytokines stimulate osteoblasts and other local cells to increase the expression of Receptor Activator of Nuclear Factor Kappa-B Ligand (RANKL). RANKL binds to the RANK receptor on osteoclast precursors, stimulating their differentiation and activation into mature, bone-resorbing osteoclasts.

Question 7571

Topic: Biomechanics & Biomaterials

During a mechanical test of a human anterior cruciate ligament (ACL) graft, a constant tensile load is applied and maintained over an extended period. It is observed that the graft gradually elongates over time under this constant load. Which viscoelastic property is being demonstrated?

. Stress relaxation
. Creep
. Hysteresis
. Fatigue failure
. Strain rate dependency

Correct Answer & Explanation

. Stress relaxation

Explanation

Creep is a fundamental viscoelastic property characterized by the progressive deformation (elongation) of a material when subjected to a constant load (stress) over a prolonged period. In contrast, stress relaxation occurs when a material is held at a constant length (strain), resulting in a gradual decrease in stress. Hysteresis is the energy lost (usually as heat) between the loading and unloading phases of a cycle.

Question 7572

Topic: Biology, Genetics & Bone Healing

A 35-year-old patient with a displaced transverse midshaft tibial fracture is treated with an intramedullary nail. The construct allows for a moderate amount of interfragmentary motion. According to Perren's strain theory, what type of tissue will primarily form in the fracture gap if the interfragmentary strain is between 2% and 10%?

. Woven bone
. Lamellar bone
. Cartilage
. Granulation tissue
. Fibrous tissue

Correct Answer & Explanation

. Woven bone

Explanation

Perren's strain theory states that the type of tissue that can form in a fracture gap depends on the local mechanical strain. Tissues cannot form if the strain exceeds their tolerance. Granulation tissue can tolerate up to 100% strain. Fibrous tissue can tolerate up to 17% strain. Cartilage tolerates strains between 2% and 10%. Bone formation requires very low strain environments; lamellar bone requires strain less than 2%. With 2% to 10% strain, a cartilaginous callus (endochondral ossification) will form.

Question 7573

Topic: Infection, Pharmacology & VTE

A 62-year-old woman is scheduled for an elective total knee arthroplasty. She has a history of atrial fibrillation and is currently taking apixaban. When considering her perioperative deep vein thrombosis (DVT) prophylaxis and bleeding risk, the mechanism of action of apixaban must be accounted for. Apixaban exerts its anticoagulant effect by directly inhibiting which of the following?

. Thrombin (Factor IIa)
. Factor Xa
. Vitamin K epoxide reductase
. Antithrombin III
. Platelet ADP receptor (P2Y12)

Correct Answer & Explanation

. Thrombin (Factor IIa)

Explanation

Apixaban and rivaroxaban are direct oral anticoagulants (DOACs) that function as direct, reversible inhibitors of Factor Xa. Dabigatran is a direct thrombin (Factor IIa) inhibitor. Warfarin inhibits vitamin K epoxide reductase, affecting factors II, VII, IX, and X. Heparins function by potentiating Antithrombin III. Clopidogrel and ticagrelor are platelet P2Y12 ADP receptor inhibitors.

Question 7574

Topic: Biology, Genetics & Bone Healing

A 45-year-old man undergoes a posterolateral lumbar fusion using a structural cortical allograft. What is the primary mechanism by which this structural allograft will incorporate over the next 12 months?

. Osteogenesis followed by osteoinduction
. Osteoconduction followed by creeping substitution
. Direct intramembranous ossification
. Endochondral ossification
. Osteoinduction followed by osteogenesis

Correct Answer & Explanation

. Osteogenesis followed by osteoinduction

Explanation

Structural allografts incorporate primarily through osteoconduction, acting as a scaffold for host bone growth. This is followed by creeping substitution, a process where host osteoclasts gradually resorb the graft and osteoblasts lay down new woven and lamellar bone. Allografts lack living osteogenic cells and have minimal osteoinductive potential compared to autografts, as the processing required to prevent rejection diminishes osteoinductive proteins.

Question 7575

Topic: 1. General Principles & Basic Science

In normal articular cartilage, the organization of collagen fibers and chondrocytes varies significantly by zone. Which of the following accurately describes the deep (basal) zone of articular cartilage?

. Highest concentration of water and lowest concentration of proteoglycans
. Collagen fibers are oriented parallel to the articular surface
. Chondrocytes are flattened and metabolically inactive
. Collagen fibers are oriented perpendicular to the articular surface
. Highest concentration of collagen and lowest concentration of proteoglycans

Correct Answer & Explanation

. Highest concentration of water and lowest concentration of proteoglycans

Explanation

Articular cartilage is divided into distinct zones. In the superficial (tangential) zone, collagen fibers are oriented parallel to the joint surface to resist shear stress, and the water content is highest. In the deep (basal) zone, collagen fibers are oriented perpendicular to the joint surface, crossing the tidemark to firmly anchor the cartilage to the underlying subchondral bone. Proteoglycan concentration is highest in the deep zone, while water concentration is lowest.

Question 7576

Topic: Biology, Genetics & Bone Healing

A 35-year-old woman presents with severe knee pain. Radiographs reveal an eccentric, lytic, epiphyseal-metaphyseal lesion in the distal femur extending to the subchondral bone. Biopsy shows mononuclear cells and multinucleated giant cells. Which of the following describes the mechanism of action of the most appropriate targeted medical therapy for a recurrent or unresectable form of this disease?

. Inhibition of vascular endothelial growth factor (VEGF)
. Binding to the RANK ligand (RANKL) to prevent osteoclast activation
. Inhibition of tyrosine kinase receptors
. Binding to bisphosphonates to inhibit osteoblast apoptosis
. Direct alkylation of DNA in rapidly dividing cells

Correct Answer & Explanation

. Inhibition of vascular endothelial growth factor (VEGF)

Explanation

The patient has a giant cell tumor (GCT) of bone. Denosumab is a targeted monoclonal antibody used for unresectable or recurrent GCTs. The neoplastic cells in GCT are the mononuclear stromal cells, which express high levels of RANKL. Denosumab binds directly to RANKL, preventing it from binding to the RANK receptor on the reactive multinucleated giant cells (osteoclast precursors). This inhibits giant cell formation, function, and survival, leading to tumor suppression.

Question 7577

Topic: Biomechanics & Biomaterials

A researcher is evaluating different calcium phosphate bone void fillers for use in metaphyseal defects. Which of the following formulations is characterized by being formed via an exothermic reaction, possesses the highest compressive strength, and undergoes the slowest rate of in vivo resorption?

. Calcium sulfate
. Tricalcium phosphate
. Calcium phosphate cement (hydroxyapatite)
. Demineralized bone matrix
. Cancellous allograft chips

Correct Answer & Explanation

. Calcium sulfate

Explanation

Calcium phosphate cements harden in an exothermic reaction to form a poorly crystalline hydroxyapatite. They provide high compressive strength (similar to cancellous bone) but possess very low tensile strength. Due to their dense, crystalline structure and lack of macroporosity, they resorb very slowly in vivo, often remaining visible on radiographs for years. In contrast, calcium sulfate and tricalcium phosphate resorb much more rapidly.

Question 7578

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with progressive knee pain and swelling. Radiographs display an eccentric, expansile, purely lytic lesion in the distal femoral epiphysis extending to the subchondral bone plate without a sclerotic margin. Histologic examination demonstrates a proliferation of neoplastic mononuclear stromal cells mixed with numerous multinucleated giant cells. For surgically unsalvageable or recurrent disease, targeted systemic therapy would appropriately focus on inhibiting which of the following molecular targets?

. Receptor activator of nuclear factor kappa-B ligand (RANKL)
. Platelet-derived growth factor receptor (PDGFR)
. Epidermal growth factor receptor (EGFR)
. Vascular endothelial growth factor (VEGF)
. Tyrosine kinase (TK)

Correct Answer & Explanation

. Receptor activator of nuclear factor kappa-B ligand (RANKL)

Explanation

The diagnosis is giant cell tumor of bone (GCTB), a locally aggressive benign bone tumor typically affecting the epiphyses of long bones in young adults. The pathogenesis relies on the neoplastic mononuclear stromal cells expressing high levels of RANKL, which recruits and activates the non-neoplastic, bone-resorbing multinucleated giant cells. Denosumab, a human monoclonal antibody that specifically binds to and inhibits RANKL, is an effective targeted therapy utilized for GCTB that is unresectable, recurrent, or would require highly morbid surgery.

Question 7579

Topic: 1. General Principles & Basic Science

Articular cartilage is divided into several distinct zones to optimize its biomechanical properties. Which of the following best describes the organization of collagen fibers and chondrocytes in the superficial (tangential) zone?

. Collagen fibers are oriented perpendicular to the joint surface, and chondrocytes are arranged in columns.
. Collagen fibers are oriented parallel to the joint surface, and chondrocytes are flattened.
. Collagen fibers are randomly oriented, and chondrocytes are spherical.
. This zone contains the tidemark and highest concentration of proteoglycans.
. Collagen fibers are thickest, and it has the highest compressive modulus.

Correct Answer & Explanation

. Collagen fibers are oriented perpendicular to the joint surface, and chondrocytes are arranged in columns.

Explanation

The superficial (tangential) zone of articular cartilage comprises approximately 10% to 20% of articular cartilage thickness. It has collagen fibers (primarily type II) oriented parallel to the joint surface to effectively resist shear forces. The chondrocytes in this layer are flattened. In contrast, the deep zone has collagen fibers oriented perpendicular to the joint surface to resist compressive loads, and its chondrocytes are arranged in vertical columns. Proteoglycan concentration is highest in the deep zone, not the superficial zone.

Question 7580

Topic: Biology, Genetics & Bone Healing

Denosumab is a targeted biological therapy frequently used in the treatment of osteoporosis and giant cell tumor of bone. Which of the following best describes its specific mechanism of action?

. It binds to the RANK receptor on osteoclasts, directly stimulating apoptosis.
. It binds to RANKL, preventing the interaction between RANKL and RANK on osteoclast precursors.
. It acts as a decoy receptor similar to osteoprotegerin (OPG), binding directly to the osteoclast ruffled border.
. It inhibits farnesyl pyrophosphate synthase within the mevalonate pathway.
. It stimulates Wnt/beta-catenin signaling in osteoblasts to increase bone formation.

Correct Answer & Explanation

. It binds to the RANK receptor on osteoclasts, directly stimulating apoptosis.

Explanation

Denosumab is a fully human monoclonal antibody that specifically binds to receptor activator of nuclear factor kappa-B ligand (RANKL). By binding to RANKL, it prevents RANKL from activating its receptor, RANK, which is located on the surface of osteoclasts and their precursors. This inhibition prevents osteoclast formation, function, and survival, leading to decreased bone resorption. Bisphosphonates act by inhibiting farnesyl pyrophosphate synthase.

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