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1. General Principles & Basic Science MCQs

Practice Set 376 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 7501

Topic: Biomechanics & Biomaterials

A 65-year-old woman is scheduled to undergo a total hip arthroplasty. The surgeon plans to use a highly cross-linked polyethylene (HXLPE) liner. Which of the following describes the primary tribological and mechanical trade-off associated with highly cross-linked polyethylene compared to conventional ultra-high-molecular-weight polyethylene (UHMWPE)?

. Decreased volumetric wear but decreased fracture toughness
. Increased volumetric wear but increased ultimate tensile strength
. Decreased generation of wear debris but increased fatigue crack propagation resistance
. Increased osteolysis risk but improved conformability
. Elimination of free radicals resulting in increased yield strength

Correct Answer & Explanation

. Decreased volumetric wear but decreased fracture toughness

Explanation

Highly cross-linked polyethylene (HXLPE) was developed to reduce volumetric wear and the subsequent risk of particle-induced osteolysis, which is its primary clinical advantage. However, the cross-linking process (typically via gamma or electron beam irradiation) followed by thermal treatment (melting or annealing) alters the material properties. The primary trade-off is a reduction in mechanical properties, specifically decreased fracture toughness, reduced fatigue resistance, and decreased ultimate tensile strength compared to conventional UHMWPE.

Question 7502

Topic: Surgical Anatomy & Approaches

During a direct anterior approach to the hip for a total hip arthroplasty, the surgeon develops the internervous plane between the sartorius and the tensor fasciae latae. Which of the following nerves is at greatest risk of iatrogenic injury during the superficial dissection of this approach?

. Femoral nerve
. Lateral femoral cutaneous nerve
. Superior gluteal nerve
. Obturator nerve
. Ilioinguinal nerve

Correct Answer & Explanation

. Femoral nerve

Explanation

The direct anterior approach to the hip utilizes the internervous plane between the sartorius (innervated by the femoral nerve) and the tensor fasciae latae (innervated by the superior gluteal nerve). The lateral femoral cutaneous nerve (LFCN) typically crosses the sartorius muscle distally and laterally from the anterior superior iliac spine (ASIS). It is at high risk of stretch or transection injury during the superficial dissection, which can lead to meralgia paresthetica (numbness, dysesthesia, or pain in the anterolateral thigh).

Question 7503

Topic: Biology, Genetics & Bone Healing

A 62-year-old man presents with progressive anterolateral bowing of his left tibia. He reports his hat size has increased over the last few years. Laboratory evaluation reveals a markedly elevated serum alkaline phosphatase, but normal serum calcium, phosphorus, and parathyroid hormone levels. What is the fundamental cellular abnormality initiating this disease process?

. Defective mineralization of osteoid by osteoblasts
. Increased resorptive activity of large, hypernucleated osteoclasts
. A mutation in the COL1A1 gene leading to abnormal type I collagen
. Paraneoplastic secretion of parathyroid hormone-related peptide (PTHrP)
. Monoclonal proliferation of plasma cells

Correct Answer & Explanation

. Defective mineralization of osteoid by osteoblasts

Explanation

The patient's clinical presentation (tibial bowing, increasing head size) and laboratory profile (isolated elevation of alkaline phosphatase) are classic for Paget's disease of bone (osteitis deformans). The primary cellular abnormality initiating Paget's disease is intense, focal osteoclastic overactivity. The osteoclasts are markedly enlarged and hypernucleated (sometimes containing up to 100 nuclei). This is followed by a compensatory but disorganized osteoblastic response, leading to structurally weak, woven bone.

Question 7504

Topic: 1. General Principles & Basic Science

In the ultrastructure of normal adult articular cartilage, which zone is characterized by the highest concentration of proteoglycans, the lowest water content, and collagen fibrils arranged perpendicular to the articular surface?

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

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

Articular cartilage is divided into several zones. The deep (radial) zone is characterized by the highest proteoglycan content, the lowest water content, and large-diameter collagen fibrils (primarily Type II) that are arranged strictly perpendicular to the articular surface. This specific structural arrangement provides the deep zone with the highest resistance to compressive loads. The superficial zone has the highest water and collagen content, with fibrils arranged parallel to the joint surface to resist shear and tensile stresses.

Question 7505

Topic: Infection, Pharmacology & VTE

A 68-year-old man is prescribed rivaroxaban for venous thromboembolism prophylaxis following an elective total knee arthroplasty. What is the precise mechanism of action of this pharmacological agent?

. Indirect inhibition of Factor Xa via antithrombin III binding
. Direct, reversible inhibition of free and clot-bound Factor Xa
. Direct inhibition of thrombin (Factor IIa)
. Irreversible inhibition of the adenosine diphosphate (ADP) P2Y12 receptor
. Competitive antagonism of vitamin K epoxide reductase

Correct Answer & Explanation

. Indirect inhibition of Factor Xa via antithrombin III binding

Explanation

Rivaroxaban is an oral anticoagulant that functions as a direct, reversible, and highly selective inhibitor of Factor Xa (both free and clot-bound forms). By inhibiting Factor Xa, it interrupts the intrinsic and extrinsic pathways of the blood coagulation cascade, inhibiting both thrombin generation and thrombus development. Indirect inhibition of Factor Xa via antithrombin III is the mechanism of fondaparinux and low-molecular-weight heparins. Direct inhibition of thrombin is the mechanism of dabigatran.

Question 7506

Topic: Biology, Genetics & Bone Healing

Which of the following biological descriptions accurately represents primary (strain-free) bone healing?

. It involves endochondral ossification leading to robust callus formation.
. It requires absolute stability and proceeds via cutting cones crossing the fracture site.
. It is typically achieved through intramedullary nailing of diaphyseal fractures.
. It relies heavily on the early proliferation of a hematoma into a soft cartilaginous callus.
. It is primarily regulated by the release of transforming growth factor-beta (TGF-beta) to induce chondrogenesis.

Correct Answer & Explanation

. It involves endochondral ossification leading to robust callus formation.

Explanation

Primary bone healing (also known as direct bone healing) occurs only when there is absolute fracture stability (strain < 2%) and intimate cortical contact, typically achieved via rigid internal fixation (e.g., compression plating). It bypasses the formation of a cartilaginous intermediate and fracture callus. Instead, it proceeds via direct remodeling of lamellar bone by osteoclastic 'cutting cones' that cross the fracture line, followed immediately by osteoblasts laying down new bone. Secondary bone healing involves callus formation and endochondral ossification, which occurs with relative stability (e.g., intramedullary nailing or casting).

Question 7507

Topic: Biology, Genetics & Bone Healing

A 35-year-old woman presents with knee pain. Radiographs reveal an eccentric, lytic lesion in the distal femur extending to the subchondral bone without a sclerotic rim. Biopsy confirms a giant cell tumor of bone. She is treated with denosumab preoperatively. What is the mechanism of action of denosumab in this clinical setting?

. Direct cytotoxic effect on multinucleated giant cells
. Inhibition of vascular endothelial growth factor (VEGF)
. Binding to RANKL, preventing activation of the RANK receptor on osteoclast precursors
. Inhibition of matrix metalloproteinases (MMPs)
. Binding to the RANK receptor, preventing interaction with RANKL

Correct Answer & Explanation

. Direct cytotoxic effect on multinucleated giant cells

Explanation

Denosumab is a human monoclonal antibody that binds to the Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). In Giant Cell Tumor of bone (GCT), the neoplastic stromal cells express RANKL, which recruits and activates the reactive multinucleated giant cells (osteoclast-like cells) expressing RANK. By binding RANKL, denosumab prevents the interaction with the RANK receptor, thereby decreasing bone resorption and causing tumor necrosis and ossification.

Question 7508

Topic: 1. General Principles & Basic Science

A 19-year-old male presents with right shin pain that is worse at night and dramatically improves with ibuprofen. Radiographs reveal cortical thickening of the anterior tibial diaphysis. CT scan shows a 7-mm radiolucent nidus surrounded by dense reactive sclerosis. What is the primary biochemical mechanism responsible for the classic pain pattern in this condition?

. Localized production of high levels of prostaglandins by the nidus
. Mechanical expansion of the periosteum by the growing tumor
. Secretion of nerve growth factor causing localized hyperalgesia
. High levels of local RANKL expression inducing osteoclast-mediated bone pain
. Ischemic pain from tumor-induced thrombosis of local intramedullary vessels

Correct Answer & Explanation

. Localized production of high levels of prostaglandins by the nidus

Explanation

The classic presentation of an osteoid osteoma is night pain that is exquisitely sensitive to NSAIDs or aspirin. This is due to the very high concentration of prostaglandins (specifically PGE2) produced by the osteoblasts within the tumor nidus. Prostaglandins sensitize local nerve endings, causing pain. NSAIDs inhibit cyclooxygenase, decreasing prostaglandin production and dramatically relieving the pain.

Question 7509

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with persistent knee pain. Radiographs demonstrate an eccentric, lytic epiphyseal lesion in the distal femur without sclerotic margins. A core needle biopsy reveals mononuclear cells intermixed with multinucleated giant cells. The patient is prescribed denosumab to downstage the tumor prior to surgery. What is the mechanism of action of this medication in the context of this tumor?

. It binds to the RANK receptor on the multinucleated giant cells, directly inhibiting their activity
. It binds to RANKL secreted by the neoplastic mononuclear stromal cells, preventing osteoclast differentiation
. It inhibits vascular endothelial growth factor (VEGF), reducing tumor angiogenesis
. It promotes apoptosis of the neoplastic multinucleated giant cells via p53 activation
. It directly inhibits osteoclast proton pumps, thereby reducing bone resorption

Correct Answer & Explanation

. It binds to the RANK receptor on the multinucleated giant cells, directly inhibiting their activity

Explanation

In giant cell tumor of bone (GCTB), the neoplastic cells are the mononuclear stromal cells, not the giant cells. These stromal cells highly express RANKL. The RANKL stimulates the recruitment and differentiation of non-neoplastic multinucleated giant cells (osteoclast-like cells) which cause extensive bone destruction. Denosumab is a monoclonal antibody that binds to RANKL, preventing it from interacting with the RANK receptor on the multinucleated giant cells, thereby inhibiting their differentiation and osteolytic activity.

Question 7510

Topic: Biology, Genetics & Bone Healing

A 65-year-old man presents with progressive bowing of his right tibia and dull, aching leg pain. Blood tests reveal an isolated, significantly elevated alkaline phosphatase level with normal calcium and phosphorus. Radiographs show cortical thickening, trabecular coarsening, and anterior bowing of the tibia. A biopsy of the bone would most likely demonstrate which of the following histologic features?

. Haphazardly arranged woven bone surrounded by bland fibrous tissue
. A mosaic pattern of lamellar bone with prominent cement lines
. Woven bone trabeculae lined by prominent osteoblasts with multinucleated giant cells in a hemorrhagic stroma
. Islands of cartilage with endochondral ossification extending from the physis
. Spindle cells arranged in a herringbone pattern

Correct Answer & Explanation

. Haphazardly arranged woven bone surrounded by bland fibrous tissue

Explanation

The clinical presentation is classic for Paget's disease of bone (osteitis deformans). The classic histologic finding in the mixed and sclerotic phases of Paget's disease is a 'mosaic' or 'jigsaw puzzle' pattern of lamellar bone with irregular, prominent cement lines. This pattern is the result of erratic and decoupled osteoclast and osteoblast activity. Option A describes fibrous dysplasia. Option C describes an aneurysmal bone cyst or hyperparathyroidism (brown tumor). Option E describes fibrosarcoma.

Question 7511

Topic: Biology, Genetics & Bone Healing

A 68-year-old man presents with lower back pain and fatigue. Radiographs of the lumbar spine and pelvis reveal multiple 'punched-out' lytic lesions without reactive sclerosis. Laboratory studies show mild hypercalcemia and a normocytic anemia. Which of the following diagnostic tests would be most appropriate to confirm the underlying diagnosis and evaluate the extent of skeletal involvement?

. Technetium-99m bone scan and prostate-specific antigen (PSA)
. Serum protein electrophoresis (SPEP) and a whole-body low-dose CT scan
. MRI of the spine and a chest CT scan
. Bone marrow biopsy and a dual-energy x-ray absorptiometry (DEXA) scan
. Positron emission tomography (PET) scan and carcinoembryonic antigen (CEA)

Correct Answer & Explanation

. Technetium-99m bone scan and prostate-specific antigen (PSA)

Explanation

The clinical picture of age, punched-out lytic lesions, hypercalcemia, and anemia strongly suggests multiple myeloma. Serum protein electrophoresis (SPEP) and urine protein electrophoresis (UPEP) are essential to detect monoclonal paraproteins. To evaluate skeletal involvement, a whole-body low-dose CT scan (or whole-body MRI/PET-CT) is indicated. Technetium-99m bone scans depend on osteoblastic activity; since myeloma lesions are purely osteolytic and suppress osteoblasts, bone scans are classically 'cold' or falsely negative in myeloma.

Question 7512

Topic: 1. General Principles & Basic Science

A 45-year-old man undergoes a complex revision total hip arthroplasty utilizing a massive structural cortical allograft. Which of the following best describes the expected incorporation process of this allograft compared to a cancellous autograft?

. Initial osteoblastic bone apposition followed by osteoclastic resorption
. Initial osteoclastic resorption followed by osteoblastic bone apposition
. Incorporation purely by osteoinduction without a creeping substitution phase
. Complete replacement of the allograft with viable host bone within 6 months
. Enhanced neovascularization leading to superior mechanical strength at 1 year compared to cancellous graft

Correct Answer & Explanation

. Initial osteoblastic bone apposition followed by osteoclastic resorption

Explanation

Cortical bone grafts incorporate via a process of initial osteoclastic resorption of the Haversian canals, followed by osteoblastic bone apposition (creeping substitution). Cancellous bone, in contrast, undergoes initial osteoblastic apposition on the existing trabeculae followed by subsequent remodeling. Cortical grafts remain an admixture of necrotic graft and viable host bone for years and are initially mechanically weakened during the resorptive phase.

Question 7513

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with a lytic, expansile lesion in the distal femur extending to the subchondral bone. Biopsy confirms a giant cell tumor of bone. She is treated preoperatively with denosumab to consolidate the lesion. What is the specific mechanism of action of this medication?

. Direct inhibition of the ruffled border of osteoclasts
. Monoclonal antibody binding to RANK ligand (RANKL), preventing osteoclast activation
. Binding to RANK receptor on the osteoclast precursor, inducing apoptosis
. Inhibition of farnesyl pyrophosphate synthase, disrupting osteoclast function
. Activation of osteoprotegerin (OPG), competitively inhibiting RANKL

Correct Answer & Explanation

. Direct inhibition of the ruffled border of osteoclasts

Explanation

Denosumab is a fully human monoclonal antibody that binds directly to RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand). By binding RANKL, it prevents RANKL from interacting with the RANK receptor on osteoclasts and their precursors, thereby inhibiting osteoclast formation, function, and survival. Bisphosphonates inhibit farnesyl pyrophosphate synthase.

Question 7514

Topic: 1. General Principles & Basic Science

Which zone of normal articular cartilage contains the largest diameter collagen fibrils oriented strictly perpendicular to the articular surface, providing the primary resistance to compressive forces?

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

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

Articular cartilage is divided into specific structural zones. The superficial (tangential) zone has densely packed, fine collagen fibrils oriented parallel to the joint surface to resist shear forces. The deep (radial) zone has the largest diameter collagen fibrils oriented strictly perpendicular to the articular surface, crossing into the tidemark. This structural arrangement allows the deep zone to provide the highest resistance to compressive loads.

Question 7515

Topic: Biology, Genetics & Bone Healing

A 68-year-old woman taking oral alendronate for 10 years presents with a 3-month history of dull right thigh pain. Radiographs reveal focal lateral cortical thickening (beaking) and a transverse radiolucent line in the subtrochanteric femur. What is the fundamental alteration in bone physiology responsible for this condition?

. Excessive osteoclastic bone resorption outstripping osteoblastic formation
. Severely suppressed bone turnover leading to the accumulation of physiologic microdamage
. Vitamin D deficiency causing impaired mineralization of osteoid
. A mutation in the type I collagen gene causing structural weakness
. Hyperparathyroidism inducing generalized osteitis fibrosa cystica

Correct Answer & Explanation

. Excessive osteoclastic bone resorption outstripping osteoblastic formation

Explanation

Long-term use of bisphosphonates suppresses normal bone turnover. By inhibiting osteoclastic resorption, targeted remodeling of bone is arrested, which prevents the repair of physiologic microcracks. Over time, this microdamage accumulates, altering the biomechanical properties of the femur and leading to an increased risk of atypical femur fractures (AFFs). These typically begin as tension-sided cortical stress reactions.

Question 7516

Topic: Biology, Genetics & Bone Healing

A researcher is developing a novel therapeutic agent for osteoporosis that directly inhibits the physiological processes at the ruffled border of osteoclasts. Which of the following best describes the primary mechanism by which osteoclasts create the acidic environment necessary for bone resorption at this ruffled border?

. Carbonic anhydrase II generation of hydrogen ions and subsequent active transport via a vacuolar H+-ATPase pump
. Active transport of hydrochloric acid by a local Na+/K+ ATPase pump in exchange for extracellular calcium
. Direct action of TRAP (Tartrate-resistant acid phosphatase) lowering the local pH within the sealed zone
. Binding of RANK-Ligand to the OPG receptor, causing an intracellular surge of lysozymes
. Secretion of Cathepsin K and matrix metalloproteinases that primarily degrade hydroxyapatite crystals

Correct Answer & Explanation

. Carbonic anhydrase II generation of hydrogen ions and subsequent active transport via a vacuolar H+-ATPase pump

Explanation

Osteoclasts resorb bone by tightly adhering to the bone surface (creating a sealed zone) and forming a ruffled border. Within the osteoclast, carbonic anhydrase II catalyzes the conversion of H2O and CO2 into hydrogen ions (H+) and bicarbonate (HCO3-). The hydrogen ions are actively pumped across the ruffled border into the resorption pit by a vacuolar H+-ATPase, creating a highly acidic environment (pH ~4) that dissolves inorganic hydroxyapatite. Once the mineral is dissolved, enzymes like Cathepsin K degrade the remaining organic collagen matrix.

Question 7517

Topic: Biomechanics & Biomaterials

A 65-year-old man who underwent a primary total hip arthroplasty using a titanium stem and a cobalt-chromium modular head 5 years ago presents with new-onset groin pain. Laboratory tests show elevated serum cobalt levels, while chromium levels are mildly elevated. MRI reveals a solid-cystic pseudotumor around the hip joint. The primary mechanism responsible for the release of these metal ions at the modular head-neck junction is best described as:

. Galvanic corrosion
. Mechanically assisted crevice corrosion
. Pitting corrosion
. Third-body wear
. Adhesive wear

Correct Answer & Explanation

. Galvanic corrosion

Explanation

The release of metal ions at the modular head-neck junction (trunnion) of a total hip arthroplasty is primarily due to mechanically assisted crevice corrosion (MACC), also known as fretting corrosion. Micromotion at the modular junction continuously disrupts the protective passivation layer of the metals, leading to subsequent corrosive attack and the release of metal debris and ions (particularly cobalt). While galvanic corrosion can technically occur when two dissimilar metals are in contact, mechanically assisted crevice corrosion is the overwhelmingly dominant mode of failure driving trunnionosis in modern modular implants.

Question 7518

Topic: Biology, Genetics & Bone Healing

A 28-year-old woman presents with knee pain. Radiographs show an eccentric, lytic lesion in the distal femoral epiphysis extending precisely to the subchondral bone. Biopsy confirms a giant cell tumor of bone. Which of the following molecular targets is specifically responsible for the aggressive osteolysis in this tumor and is targeted in its medical management?

. Receptor activator of nuclear factor kappa-B ligand (RANKL)
. Vascular endothelial growth factor (VEGF)
. Platelet-derived growth factor (PDGF)
. Fibroblast growth factor receptor (FGFR)
. Mammalian target of rapamycin (mTOR)

Correct Answer & Explanation

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

Explanation

Giant cell tumor of bone (GCT) is primarily driven by neoplastic mononuclear stromal cells that overexpress RANKL. This overexpression recruits and stimulates normal osteoclast precursors to fuse into reactive, multinucleated giant cells, leading to aggressive and rapid bone resorption. Denosumab, a monoclonal antibody that binds to and neutralizes RANKL, is utilized in the medical management of advanced, unresectable, or recurrent GCTs to inhibit bone destruction and induce ossification of the tumor matrix.

Question 7519

Topic: Biology, Genetics & Bone Healing

Denosumab has revolutionized the non-surgical management of advanced giant cell tumors of bone. What is the precise cellular mechanism of action of this targeted medication?

. It directly inhibits osteoclast ruffled border formation by binding to alphav-beta3 integrins.
. It binds to and neutralizes RANK ligand, preventing its interaction with the RANK receptor on osteoclast precursors.
. It directly induces apoptosis of mature osteoclasts by irreversibly inhibiting farnesyl pyrophosphate synthase.
. It acts as a competitive antagonist at the parathyroid hormone (PTH) receptor on osteoblasts.
. It inhibits matrix metalloproteinases (MMPs) secreted by neoplastic stromal cells.

Correct Answer & Explanation

. It directly inhibits osteoclast ruffled border formation by binding to alphav-beta3 integrins.

Explanation

Denosumab is a fully human monoclonal antibody that specifically targets and binds to RANK Ligand (RANKL). By binding to RANKL, denosumab prevents the ligand from interacting with its designated receptor, RANK, which is located on the surface of osteoclasts and osteoclast precursors. This blockade effectively halts osteoclast formation, function, and survival, thereby rapidly decreasing tumor-induced bone resorption. In contrast, bisphosphonates work by inhibiting farnesyl pyrophosphate synthase, leading to osteoclast apoptosis.

Question 7520

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with knee pain. Radiographs reveal an eccentric, lytic epiphyseal lesion extending to the subchondral bone of the distal femur. Biopsy reveals multinucleated giant cells in a background of mononuclear stromal cells. She is prescribed a medication that binds to a specific ligand to prevent osteoclast activation. What is the target of this medication?

. Osteoprotegerin (OPG)
. Macrophage colony-stimulating factor (M-CSF)
. Vascular endothelial growth factor (VEGF)
. RANK ligand (RANKL)
. RANK receptor

Correct Answer & Explanation

. Osteoprotegerin (OPG)

Explanation

The clinical and radiographic presentation is classic for a Giant Cell Tumor (GCT) of bone. The neoplastic mononuclear stromal cells of GCT express RANKL, which recruits and activates the reactive multinucleated osteoclast-like giant cells that express the RANK receptor. Denosumab is a fully human monoclonal antibody that directly targets and binds to RANK ligand (RANKL), thereby preventing osteoclast activation and subsequent bone destruction. Osteoprotegerin (OPG) is the body's natural decoy receptor for RANKL.

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