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1. General Principles & Basic Science MCQs

Practice Set 377 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 7521

Topic: Biology, Genetics & Bone Healing

A 70-year-old woman is prescribed denosumab for the treatment of severe postmenopausal osteoporosis. Which of the following best describes the exact molecular mechanism of action of this pharmacological agent?

. It directly binds to osteoprotegerin (OPG), preventing the inhibition of osteoclastogenesis
. It directly binds to RANK ligand (RANKL), preventing its interaction with the RANK receptor on osteoclasts
. It acts as a decoy receptor binding directly to the RANK receptor, blocking physiological ligand binding
. It selectively inhibits Cathepsin K inside the ruffled border of active osteoclasts
. It binds to Sclerostin, releasing the inhibition of the Wnt signaling pathway

Correct Answer & Explanation

. It directly binds to osteoprotegerin (OPG), preventing the inhibition of osteoclastogenesis

Explanation

Denosumab is a fully human monoclonal antibody designed to mimic the physiological effects of osteoprotegerin (OPG). It specifically binds to Receptor Activator of Nuclear Factor Kappa-B Ligand (RANKL). By neutralizing RANKL, it prevents RANKL from binding to the RANK receptor situated on the surface of osteoclasts and their precursors. This blockade dramatically reduces osteoclast differentiation, function, and survival, leading to decreased bone resorption. Romosozumab binds to Sclerostin, and Odanacatib (investigational) inhibits Cathepsin K.

Question 7522

Topic: Biology, Genetics & Bone Healing

Recombinant human bone morphogenetic protein-2 (rhBMP-2) is utilized in complex spine fusion surgery to induce osteoinduction. Bone morphogenetic proteins are potent growth factors belonging to the transforming growth factor-beta (TGF-β) superfamily. Which of the following best describes the principal intracellular signaling cascade directly activated following BMP binding to its specific cellular receptor?

. JAK-STAT pathway
. Wnt/β-catenin pathway
. Smad 1/5/8 pathway
. cAMP-Protein Kinase A pathway
. Notch-Hes pathway

Correct Answer & Explanation

. JAK-STAT pathway

Explanation

Bone Morphogenetic Proteins (BMPs) bind to cell surface type I and type II serine/threonine kinase receptors. Receptor binding induces cross-phosphorylation, which directly activates the intracellular canonical Smad signaling cascade. Specifically, for osteogenic BMPs like BMP-2, the activated receptors phosphorylate the receptor-regulated Smads (R-Smads): Smad 1, Smad 5, and Smad 8. These phosphorylated Smads then bind to the common-partner Smad (Co-Smad), Smad 4, and the entire complex translocates into the nucleus to act as transcription factors for osteogenic genes (like Runx2).

Question 7523

Topic: Biology, Genetics & Bone Healing

A 45-year-old patient requires a bone graft for a highly comminuted tibial nonunion. The surgeon chooses a graft that provides a structural physical scaffold for host osteogenic cells to migrate into, but the graft itself lacks living cells and does not contain intrinsic growth factors to stimulate stem cell differentiation. Which of the following best describes this graft's primary property and a classic example?

. Osteogenesis; autologous iliac crest bone graft
. Osteoinduction; demineralized bone matrix (DBM)
. Osteoconduction; cancellous allograft
. Osteoinduction; bone morphogenetic protein-2 (BMP-2)
. Osteoconduction; vascularized fibula autograft

Correct Answer & Explanation

. Osteogenesis; autologous iliac crest bone graft

Explanation

Osteoconduction refers to the provision of a passive structural scaffold (matrix) for host osteoprogenitor cells and blood vessels to migrate into and form new bone. Cancellous allografts and synthetic ceramics are primarily osteoconductive. Osteogenesis requires the presence of living, viable osteoprogenitor cells capable of forming new bone (e.g., fresh autograft). Osteoinduction is the stimulation of host mesenchymal stem cells to differentiate into osteoblasts, primarily mediated by growth factors such as bone morphogenetic proteins (BMPs); DBM and recombinant BMP-2 are osteoinductive.

Question 7524

Topic: 1. General Principles & Basic Science

Articular (hyaline) cartilage has a highly specialized extracellular matrix that relies heavily on its composition for biomechanical resilience. Which of the following best describes the structural and biochemical composition of normal adult articular cartilage?

. High cell density, Type I collagen, and dermatan sulfate
. Low cell density, Type II collagen, and aggrecan
. Avascular, Type I collagen, and hyaluronic acid
. Highly vascular, Type II collagen, and decorin
. Low cell density, Type III collagen, and chondroitin sulfate

Correct Answer & Explanation

. High cell density, Type I collagen, and dermatan sulfate

Explanation

Normal adult articular cartilage is avascular, aneural, and alymphatic. It has a very low cell density (chondrocytes make up <5% of tissue volume). The predominant collagen is Type II (comprising 90-95% of the collagen), which provides tensile strength. The primary proteoglycan is aggrecan, which interacts with hyaluronic acid to trap water, providing compressive stiffness. Type I collagen is the primary collagen found in fibrocartilage (e.g., meniscus) and bone.

Question 7525

Topic: 1. General Principles & Basic Science

Following a complete peripheral nerve transection (neurotmesis), Wallerian degeneration occurs in the distal stump to prepare the environment for potential axonal regeneration. Which of the following cells are primarily responsible for clearing myelin debris and forming bands to guide regenerating axons in the peripheral nervous system?

. Oligodendrocytes
. Astrocytes
. Schwann cells
. Microglia
. Fibroblasts

Correct Answer & Explanation

. Oligodendrocytes

Explanation

In the peripheral nervous system, Schwann cells play a crucial role in nerve regeneration. Following injury, Schwann cells undergo dedifferentiation and proliferation. Along with recruited macrophages, they phagocytose myelin debris and form longitudinal arrays known as bands of Büngner, which provide physical and molecular guidance for regenerating axonal sprouts. Oligodendrocytes and microglia serve related functions in the central nervous system, where regeneration is typically abortive.

Question 7526

Topic: Biology, Genetics & Bone Healing

A 70-year-old woman is treated with alendronate for postmenopausal osteoporosis. Bisphosphonates primarily reduce fracture risk by decreasing bone turnover. What is the precise molecular mechanism by which nitrogen-containing bisphosphonates inhibit osteoclast function?

. Direct inhibition of RANKL binding to RANK
. Inhibition of farnesyl pyrophosphate (FPP) synthase in the mevalonate pathway
. Direct competitive inhibition of cathepsin K
. Binding to and neutralizing circulating sclerostin
. Downregulation of osteoprotegerin (OPG) expression

Correct Answer & Explanation

. Direct inhibition of RANKL binding to RANK

Explanation

Nitrogen-containing bisphosphonates (e.g., alendronate, zoledronate, risedronate) inhibit bone resorption by inhibiting farnesyl pyrophosphate (FPP) synthase, a key enzyme in the mevalonate pathway. This prevents the prenylation of small regulatory GTPases (like Ras, Rho, Rac) essential for normal osteoclast function, cytoskeletal arrangement (ruffled border formation), and cell survival, ultimately inducing osteoclast apoptosis. Denosumab inhibits RANKL. Odanacatib inhibits cathepsin K. Romosozumab binds sclerostin.

Question 7527

Topic: Biology, Genetics & Bone Healing

A 35-year-old man presents with knee pain. Radiographs demonstrate an eccentric, lytic lesion extending into the epiphysis of the distal femur. Biopsy confirms a giant cell tumor of bone. Neoadjuvant treatment with denosumab is planned. What is the mechanism of action of this medication?

. Inhibition of the tyrosine kinase receptor
. Direct apoptosis of the multinucleated giant cells
. Binding to and neutralizing RANKL
. Competitive inhibition of the vascular endothelial growth factor (VEGF) receptor
. Cross-linking of DNA leading to cell cycle arrest

Correct Answer & Explanation

. Inhibition of the tyrosine kinase receptor

Explanation

Denosumab is a human monoclonal antibody that binds to Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL), thereby preventing it from activating RANK. In Giant Cell Tumor (GCT) of bone, the neoplastic mononuclear stromal cells express RANKL, which recruits and activates the osteoclast-like multinucleated giant cells that cause bone destruction. Denosumab inhibits this process.

Question 7528

Topic: Biomechanics & Biomaterials

A new orthopedic polymer implant is being evaluated for use in load-bearing applications. When subjected to a constant load over a prolonged period of several months, the material demonstrates a progressive increase in strain and deformation. Which of the following viscoelastic properties best describes this phenomenon?

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

Correct Answer & Explanation

. Stress relaxation

Explanation

Creep is the progressive deformation of a material when it is subjected to a constant force or stress over time. Stress relaxation is the decrease in stress over time when a material is held at a constant deformation. Hysteresis refers to the energy lost (usually as heat) during the loading and unloading cycle of a viscoelastic material. Fatigue failure occurs from repetitive cyclical loading below the ultimate tensile strength. Anisotropy means a material's mechanical properties are dependent on the direction of loading.

Question 7529

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with knee pain. Radiographs reveal an eccentric, lytic epiphyseal lesion in the proximal tibia extending to the subchondral bone. Biopsy demonstrates numerous multinucleated giant cells in a background of mononuclear stromal cells. If medical therapy is considered to downstage this unresectable tumor, what is the mechanism of action of the most appropriate targeted agent?

. Inhibition of tyrosine kinase activity
. Monoclonal antibody against RANK ligand
. Inhibition of osteoclast proton pumps
. Direct cytotoxicity to mononuclear stromal cells
. Selective estrogen receptor modulation

Correct Answer & Explanation

. Inhibition of tyrosine kinase activity

Explanation

The lesion described is a giant cell tumor (GCT) of bone. Denosumab is frequently used for unresectable or metastatic GCTs, or to downstage tumors before surgery. Denosumab is a fully human monoclonal antibody that binds to RANKL, preventing it from activating RANK on the surface of osteoclasts and their precursors, thereby inhibiting osteoclast-mediated bone destruction. In GCT, the neoplastic mononuclear stromal cells express RANKL, which recruits the non-neoplastic multinucleated giant cells (osteoclasts) that cause bone lysis.

Question 7530

Topic: Biology, Genetics & Bone Healing

A 45-year-old man undergoes open reduction and internal fixation of a transverse radial shaft fracture using absolute stability techniques with a dynamic compression plate. Six weeks postoperatively, radiographs show a well-reduced fracture with no visible callus formation. Which of the following is the predominant mechanism of bone healing occurring in this scenario?

. Endochondral ossification
. Intramembranous ossification
. Primary bone healing via cutting cones
. Gap healing with initial fibrous tissue formation
. Secondary bone healing

Correct Answer & Explanation

. Endochondral ossification

Explanation

When absolute stability is achieved (e.g., with compression plating) and the fracture gap is minimal, primary bone healing occurs. This process bypasses callus formation. Instead, osteoclasts at the head of cutting cones cross the fracture site, followed closely by osteoblasts that lay down new lamellar bone (Haversian remodeling). Endochondral ossification is the hallmark of secondary bone healing, which involves a cartilaginous intermediate and callus formation, typical of non-rigid fixation (e.g., intramedullary nailing or cast immobilization).

Question 7531

Topic: 1. General Principles & Basic Science

You are designing a new cortical screw to maximize pullout strength in osteoporotic bone. According to the formula for screw pullout strength, which of the following modifications to the thread geometry will result in the greatest proportional increase in pullout strength, assuming bone failure is the limiting factor?

. Increasing the inner root diameter
. Decreasing the thread pitch
. Increasing the outer diameter
. Increasing the length of the screw shank
. Decreasing the thread depth

Correct Answer & Explanation

. Increasing the inner root diameter

Explanation

Screw pullout strength can be approximated by the formula: Pullout strength = S × L × π × D_major × TSF (where S is the shear strength of bone, L is the length of thread engagement, D_major is the outer thread diameter, and TSF is the thread shape factor). The outer diameter has the most direct linear relationship with pullout strength. Increasing the outer diameter significantly increases the volume of bone interposed between the threads. Decreasing pitch increases pullout strength but has limits before threads strip bone. Increasing the inner root diameter increases bending strength but decreases thread depth, potentially reducing pullout strength.

Question 7532

Topic: 1. General Principles & Basic Science

Articular cartilage is a highly specialized, avascular tissue composed of specific structural zones. In which zone of articular cartilage are the type II collagen fibers predominantly oriented perpendicular to the joint surface to provide maximum resistance to compressive forces?

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

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

Articular cartilage is structurally divided into four distinct zones. In the superficial (tangential) zone, collagen fibers are oriented parallel to the joint surface to resist shear forces. In the middle (transitional) zone, collagen fibers are organized obliquely. In the deep (radial) zone, the collagen fibers are oriented perpendicular to the joint surface, anchoring the cartilage to the underlying subchondral bone and providing high resistance to compressive forces. The calcified zone anchors cartilage to bone, separated from the deep zone by the tidemark.

Question 7533

Topic: Biology, Genetics & Bone Healing

A 65-year-old man presents with severe back pain and fatigue. Radiographs demonstrate multiple punched-out lytic lesions in his skull and vertebral bodies. Laboratory studies show hypercalcemia, an elevated serum creatinine, and an abnormal M-spike on serum protein electrophoresis. A bone marrow biopsy reveals sheets of plasma cells. Which of the following is the most appropriate initial medical management directly targeting his skeletal disease?

. High-dose systemic methotrexate
. Intravenous bisphosphonates
. Subcutaneous teriparatide
. Recombinant BMP-2 injections
. Broad-spectrum antibiotics

Correct Answer & Explanation

. High-dose systemic methotrexate

Explanation

The patient has multiple myeloma, characterized by the CRAB criteria (HyperCalcemia, Renal failure, Anemia, Bone lesions). The bony destruction in multiple myeloma is heavily mediated by increased osteoclast activity stimulated by RANKL and MIP-1 alpha produced by myeloma cells. Intravenous bisphosphonates (like zoledronic acid) are the standard of care to reduce skeletal-related events (SREs), treat hypercalcemia, and directly inhibit osteoclast-mediated bone resorption. Teriparatide is contraindicated in patients with an active bone malignancy.

Question 7534

Topic: 1. General Principles & Basic Science

In healthy normal articular cartilage, the highest concentration of water is found in which of the following histological zones?

. Superficial zone
. Middle (transitional) zone
. Deep zone
. Calcified zone
. Subchondral bone plate

Correct Answer & Explanation

. Superficial zone

Explanation

Normal articular cartilage is highly hydrated. The water content is highest at the superficial zone (approximately 80%) and decreases linearly as one moves toward the deep zone (approximately 65%). In osteoarthritis, the overall water content of the cartilage paradoxically increases, which is accompanied by a decrease in proteoglycan content and disruption of the collagen network.

Question 7535

Topic: Biology, Genetics & Bone Healing

Osteoclasts resorb bone by secreting hydrogen ions and proteolytic enzymes into the sealed Howship's lacuna. Which enzyme plays a critical role in the generation of the intracellular hydrogen ions necessary for this process?

. Cathepsin K
. Carbonic anhydrase II
. Matrix metalloproteinase-9
. Tartrate-resistant acid phosphatase
. Alkaline phosphatase

Correct Answer & Explanation

. Cathepsin K

Explanation

Osteoclasts produce hydrogen ions intracellularly through the action of carbonic anhydrase II, which catalyzes the hydration of carbon dioxide to form carbonic acid, which then dissociates into protons (H+) and bicarbonate. The protons are actively pumped into the resorption pit via a V-type H+-ATPase. Cathepsin K is a protease responsible for degrading the organic bone matrix (collagen) once it is demineralized. Mutations in carbonic anhydrase II cause osteopetrosis with renal tubular acidosis.

Question 7536

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with a painful mass in the distal radius. Radiographs reveal an eccentrically located, purely lytic lesion in the epiphysis extending to the subchondral bone. A biopsy demonstrates multinucleated giant cells in a stroma of mononuclear cells. Treatment with denosumab is considered. What is the specific cellular or molecular target of denosumab in the treatment of this condition?

. The multinucleated giant cells
. The RANK receptor on the osteoclast precursors
. The RANK ligand produced by the neoplastic mononuclear stromal cells
. Osteoprotegerin
. Vascular endothelial growth factor (VEGF)

Correct Answer & Explanation

. The multinucleated giant cells

Explanation

Giant cell tumor of bone (GCTB) is characterized by neoplastic mononuclear stromal cells that express high levels of RANK Ligand (RANKL). This overproduction of RANKL recruits and activates normal osteoclast precursors to form the characteristic multinucleated giant cells, leading to aggressive bone resorption. Denosumab is a monoclonal antibody that specifically binds to RANKL, preventing it from interacting with the RANK receptor on osteoclasts and their precursors, thereby halting bone destruction.

Question 7537

Topic: Biology, Genetics & Bone Healing

A 34-year-old woman presents with knee pain. Radiographs reveal an eccentric, purely lytic lesion in the distal femur extending to the subchondral bone without a sclerotic rim. Biopsy confirms a giant cell tumor of bone. Due to extensive joint involvement and a high risk of morbidity with immediate curettage, neoadjuvant pharmacological therapy is planned to consolidate the lesion prior to joint-salvage surgery. Which of the following describes the mechanism of action of the most appropriate medical therapy?

. Inhibition of vascular endothelial growth factor (VEGF) receptors
. Binding to and competitive inhibition of the RANK ligand (RANKL)
. Inhibition of the mammalian target of rapamycin (mTOR) pathway
. Cross-linking of DNA double strands to prevent cellular replication
. Selective modulation of estrogen receptors (SERM) on osteoclasts

Correct Answer & Explanation

. Inhibition of vascular endothelial growth factor (VEGF) receptors

Explanation

The diagnosis is a giant cell tumor (GCT) of bone, which typically presents in young adults as an eccentric, lytic epiphyseal/metaphyseal lesion. The neoplastic stromal cells in GCT express high levels of RANKL, which recruits and activates the multinucleated giant cells (osteoclast-like cells) responsible for the aggressive osteolysis. Denosumab is a human monoclonal antibody that specifically binds to and inhibits RANKL, thereby halting osteoclastogenesis and reducing osteolysis. It is highly effective as a neoadjuvant therapy in advanced GCTs to induce a thick peripheral rim of bone, facilitating joint-salvage procedures such as extended curettage.

Question 7538

Topic: Biology, Genetics & Bone Healing

A 45-year-old patient undergoes a posterolateral spinal fusion. Demineralized bone matrix (DBM) is used as an adjunct to local autograft. DBM primarily relies on which of the following mechanisms to promote bone formation?

. Osteogenesis
. Osteoinduction
. Osteoconduction
. Osteointegration
. Creeping substitution

Correct Answer & Explanation

. Osteogenesis

Explanation

Demineralized bone matrix (DBM) is primarily an osteoinductive and osteoconductive material. The decalcification process exposes bone morphogenetic proteins (BMPs) and other growth factors, which induce local mesenchymal stem cells to differentiate into osteoblasts—a process known as osteoinduction. It lacks live cells, so it is not osteogenic.

Question 7539

Topic: 1. General Principles & Basic Science

The extracellular matrix of normal articular cartilage is designed to withstand significant mechanical loads. The compressive stiffness of articular cartilage is primarily provided by which of the following components?

. Type II collagen
. Hyaluronic acid
. Lubricin
. Aggrecan
. Type I collagen

Correct Answer & Explanation

. Type II collagen

Explanation

Proteoglycans, predominantly aggrecan, are highly negatively charged due to their glycosaminoglycan (GAG) side chains (e.g., chondroitin sulfate and keratin sulfate). This creates a high osmotic pressure that draws water into the tissue, providing the cartilage with its ability to resist compressive forces. Type II collagen forms a fibrillar network that primarily provides tensile strength.

Question 7540

Topic: Biology, Genetics & Bone Healing

In the process of normal bone remodeling, osteoblast lineage cells regulate the activity of osteoclasts. Which of the following molecules acts as a soluble decoy receptor to inhibit osteoclast differentiation and activation?

. Osteoprotegerin (OPG)
. Sclerostin
. Macrophage colony-stimulating factor (M-CSF)
. Calcitonin
. Osteocalcin

Correct Answer & Explanation

. Osteoprotegerin (OPG)

Explanation

Osteoprotegerin (OPG) is secreted by osteoblasts and binds to RANKL, preventing it from interacting with RANK on the surface of osteoclast precursors. This inhibits osteoclastogenesis and reduces bone resorption. PTH and other factors can stimulate bone resorption by decreasing OPG and increasing RANKL expression.

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