A 65-year-old man with ankylosing spondylitis has neck pain after falling back over his lawnmower, striking his thoracic spine, and forcing his neck into extension. Examination reveals subtle weakness of the intrinsics and finger flexors at approxima

posterior laminectomy and spinal fusion.

A 25-year-old male polytrauma patient undergoes initial temporary external fixation for a femoral shaft fracture. He is converted to a femoral nail at 7 days. This management can be expected to result in

equal union and infection rates.

Which of the following is true regarding changes in the vascularity of the adult intervertebral disc with age? Review Topic

Neovascularization occurs from the nucleus pulposus

Bridge plating of the olecranon is MOST appropriate in which of the following clinical scenarios?

Fixation of an olecranon osteotomy used for distal humerus surgery in a 24-year-old male

Surgical restoration of sagittal balance of an adult spinal deformity will have which effect on outcome?

Improve quality of life and back pain

-Ten or more years after severe polytrauma, premenopausal women, compared to men

show higher rates of posttraumatic stress disorder and take more sick leave time.

-A high school distance runner reports a 3-week history of heel pain while running. Examination elicits no pain with dorsiflexion or with palpitation of the plantar fascia; however, pain is evident with palpitation over the muscular origin of the abd

Compression of the first branch of the lateral plantar nerve

Orthopedics Hyperguide Review MCQs | Dr Hutaif Ortho

Question 1

The ruffled border of osteoclasts represents the highly folded cell membrane where active bone resorption takes place. Which of the following is primarily responsible for the active acidification of the resorption lacuna at this site?

Explanation

Acidification of the resorption pit (Howship's lacuna) is essential for bone resorption. This is primarily achieved by the vacuolar-type H+ ATPase (V-ATPase) proton pump located heavily on the ruffled border. Carbonic anhydrase II generates the protons intracellularly, but the V-ATPase actively pumps them into the lacuna. Cathepsin K is a lysosomal enzyme that degrades the organic matrix (type I collagen) once the mineral is dissolved by the acidic environment.

Question 2

When analyzing the biomechanical properties of an orthopedic implant material using a standard stress-strain curve, the 'yield point' specifically represents:

Explanation

The yield point on a stress-strain curve defines the exact point at which a material transitions from elastic deformation (where it will return to its original shape when the load is removed) to plastic deformation (irreversible, permanent structural change). The ultimate tensile strength is the maximum stress on the curve. The slope of the elastic region is Young's modulus (stiffness). The total area under the curve represents toughness.

Question 3

Biofilm formation is a critical factor in the pathogenesis of prosthetic joint infections. Which of the following phases of biofilm development or bacterial state is targeted most effectively by the adjunctive use of Rifampin?

Explanation

Rifampin has a unique molecular ability to penetrate the exopolysaccharide matrix (glycocalyx) of established biofilms and kill sessile (dormant or slow-growing) staphylococcal organisms. It is highly effective against these biofilm-embedded bacteria, which is why it is utilized as an adjunctive treatment in prosthetic joint infections, provided the implant is retained and the bacterial burden has been surgically minimized. Standard IV antibiotics primarily target planktonic (free-floating) bacteria.

Question 4

A 16-year-old male presents with distal femur pain. Imaging reveals a destructive lesion, and biopsy confirms a high-grade osteosarcoma. A staging MRI indicates that the tumor has breached the posterior cortex and extends into the surrounding vastus intermedius muscle, without evidence of skip lesions or distant metastasis. According to the Enneking Musculoskeletal Tumor Staging System, what is the stage of this lesion?

Explanation

The Enneking staging system evaluates histological grade (G), local extent/compartmentalization (T), and presence of metastasis (M). G1 = low grade, G2 = high grade. T1 = intracompartmental, T2 = extracompartmental. M0 = no metastasis, M1 = metastasis. This patient has a high-grade (G2) lesion that is extracompartmental (T2, extending through the cortex into the muscle) with no metastasis (M0). Therefore, it is Stage IIB. Any lesion with metastasis (M1) is classified as Stage III.

Question 5

Nitrogen-containing bisphosphonates (e.g., Alendronate, Zoledronate) are commonly prescribed for osteoporosis and Paget's disease. What is their primary molecular mechanism of action?

Explanation

Nitrogen-containing bisphosphonates inhibit the enzyme farnesyl pyrophosphate (FPP) synthase within the mevalonate pathway inside osteoclasts. This prevents the essential prenylation (attachment of lipid tails) of small GTP-binding proteins (such as Ras, Rho, and Rac) that are critical for osteoclast ruffled border formation, function, and survival, ultimately leading to osteoclast apoptosis. Denosumab, a monoclonal antibody, works by inhibiting RANKL.

Question 6

According to Sunderland's classification of peripheral nerve injuries, a second-degree injury (Axonotmesis) is characterized by:

Explanation

Sunderland's classification: 1st degree (Neuropraxia) = local conduction block; 2nd degree (Axonotmesis) = axonal disruption with an intact endoneurium; 3rd degree = axonal and endoneurial disruption with an intact perineurium; 4th degree = disruption of everything inside the epineurium (perineurium torn, epineurium intact); 5th degree (Neurotmesis) = complete nerve transection. Second-degree injuries undergo Wallerian degeneration but have excellent recovery potential because the intact endoneurial tubes effectively guide regenerating axons.

Question 7

A patient with a modular primary total hip arthroplasty (THA) utilizing a titanium stem and a cobalt-chrome head presents with groin pain. Workup reveals an adverse local tissue reaction (ALTR) or pseudotumor. Which specific pathophysiologic mechanism is most responsible for this failure at the modular head-neck taper junction?

Explanation

Trunnionosis at the modular head-neck junction of a THA is classically driven by mechanically assisted crevice corrosion (MACC). The cyclical micromotion at the taper junction mechanically disrupts the protective oxide passivation layer (fretting). The restricted fluid exchange in this enclosed local geometry (crevice) allows for oxygen depletion and a localized drop in pH, creating an acidic, aggressive environment that accelerates the release of metal ions. This combination of mechanical wear and chemical corrosion is termed MACC.

Question 8

In the process of normal endochondral ossification at the physis, which specific cellular zone is primarily affected and functionally impaired in patients with achondroplasia?

Explanation

Achondroplasia is an autosomal dominant disorder caused by a gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. FGFR3 normally functions as a negative regulator of bone growth. Its constitutive activation leads to marked inhibition of chondrocyte proliferation. Therefore, the primary functional defect manifests in the proliferative zone of the physis, leading to decreased cartilage production and stunted endochondral bone growth (rhizomelic dwarfism).

Question 9

Which of the following Recombinant Human Bone Morphogenetic Proteins (rhBMPs) has been specifically FDA-approved for use in acute, open tibial shaft fractures stabilized with an intramedullary nail?

Explanation

Recombinant human BMP-2 (rhBMP-2) is FDA-approved for use in acute, open tibial shaft fractures stabilized with an intramedullary nail, as well as for single-level anterior lumbar interbody fusion (ALIF). rhBMP-7 (also known as Osteogenic Protein-1 or OP-1) was previously available under a Humanitarian Device Exemption for recalcitrant tibial nonunions but is not the approved BMP for acute open tibias. BMP-3 is notable for being an inhibitor of osteogenesis.

Question 10

Aspiration of a 3 cm central, radiolucent bone lesion in the proximal humerus of an 8-year-old child yields clear, serosanguinous fluid. Laboratory analysis of this specific aspirate fluid is most likely to demonstrate markedly elevated levels of:

Explanation

The clinical presentation (central lucent lesion in proximal humerus in a child) and aspiration findings describe a Unicameral Bone Cyst (UBC). Fluid analysis of a UBC classically reveals high levels of prostaglandins (particularly PGE2), interleukins, and other inflammatory mediators. These factors stimulate osteoclastic bone resorption and prevent bone healing, leading to cyst expansion. This pathophysiology forms the rationale behind treating UBCs with intralesional corticosteroid injections, which inhibit prostaglandin synthesis.

Question 11

A 55-year-old female undergoes a total thyroidectomy for carcinoma. Postoperatively, she develops severe muscle cramping and perioral numbness, and her intact PTH is notably undetectable. Which specific step in Vitamin D metabolism will be directly and severely impaired by this complication?

Explanation

Inadvertent removal of the parathyroid glands causes hypoparathyroidism and low Parathyroid Hormone (PTH). PTH is essential for upregulating the enzyme 1-alpha-hydroxylase in the proximal tubules of the kidney. This enzyme catalyzes the conversion of 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D (calcitriol). Without PTH, this critical final activation step is impaired, severely exacerbating her hypocalcemia.

Question 12

Articular cartilage is a highly specialized tissue designed to resist both compressive and shear forces. In which histologic zone of articular cartilage are the type II collagen fibers strictly oriented parallel to the joint surface to specifically resist shear forces?

Explanation

In the superficial (tangential) zone of articular cartilage, the chondrocytes are flattened, and the Type II collagen fibers are aligned parallel to the articular surface. This specific orientation provides the cartilage with its primary resistance to shear stress and acts as a gliding surface. In the deep (radial) zone, the thick collagen fibers are perpendicular to the surface to anchor the cartilage and resist compressive forces. The middle zone features a more random, oblique collagen network.

Question 13

Regarding the inherent mechanical properties of orthopedic implant materials and bone, which of the following materials possesses the lowest Young's modulus (thereby exhibiting the greatest relative elasticity/least stiffness)?

Explanation

Young's modulus is a measure of the stiffness of a solid material. A lower Young's modulus indicates less stiffness (greater elasticity). Cancellous bone has a significantly lower Young's modulus (approx. 0.1 - 2 GPa) compared to cortical bone (15 - 20 GPa). Among the common metals, Titanium alloy has the lowest modulus (~110 GPa), followed by Stainless steel (~200 GPa) and Cobalt-chromium (~210-250 GPa). Therefore, cancellous bone is the least stiff of the options provided.

Question 14

Which of the following best describes the normal physiologic direction of arterial blood flow within the diaphyseal cortex of a mature, uninjured long bone?

Explanation

In a normal, mature long bone, the diaphyseal cortex receives its primary blood supply from the high-pressure nutrient artery system within the medullary canal. The physiologic flow is centrifugal, moving from the higher-pressure medullary canal (endosteum) outward toward the lower-pressure periosteum. In the setting of trauma or disruption of the medullary supply (e.g., reamed intramedullary nailing), the flow can temporarily reverse, becoming centripetal as the intact periosteal blood supply compensates.

Question 15

A 68-year-old male presents with deep, aching pain in his right thigh and complains that his hats no longer fit. Radiographs of the femur show cortical thickening and coarse trabeculae. Baseline laboratory evaluation is most likely to show which of the following profiles for serum Calcium, Phosphorus, and Alkaline Phosphatase (ALP)?

Explanation

The clinical presentation (thigh pain, increased hat size) and radiographic appearance (cortical thickening, coarse trabeculae) are classic for Paget's disease of bone (osteitis deformans). In Paget's disease, there is massive, disorganized bone remodeling driven by overactive osteoclasts followed by chaotic osteoblast activity. Despite this intense turnover, serum calcium and phosphorus levels typically remain remarkably normal in untreated patients. The robust bone formation leads to an isolated, significantly elevated serum alkaline phosphatase (ALP).

Question 16

Highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) is widely used in total joint arthroplasty to minimize wear debris. However, which critical mechanical property is significantly decreased as a direct result of the high-dose irradiation process used to cross-link the polyethylene?

Explanation

Highly cross-linked polyethylene (HXLPE) is manufactured by subjecting UHMWPE to high-dose gamma or electron-beam irradiation, which severs molecular chains and induces dense cross-linking. While this process vastly improves adhesive and abrasive wear resistance, the trade-off is a measurable decrease in mechanical properties, specifically ultimate tensile strength, fatigue crack propagation resistance, and ductility. For this reason, HXLPE is used with caution in high-stress, non-conforming joints or in very thin acetabular liners where fatigue failure could occur.

Question 17

During the process of secondary fracture healing, the soft callus phase is primarily characterized by the formation of:

Explanation

Secondary fracture healing progresses sequentially through inflammation, soft callus formation, hard callus formation, and remodeling. The soft callus phase is primarily characterized by endochondral ossification, where chondrocytes produce a cartilaginous matrix that bridges the fracture site, providing initial semi-rigid stability. Woven bone formation subsequently characterizes the hard callus phase as it replaces the cartilage. Cutting cones are a feature of primary (direct) bone healing and late remodeling.

Question 18

In pediatric orthopedics, a Slipped Capital Femoral Epiphysis (SCFE) anatomically occurs predominantly through which specific zone of the proximal femoral physis?

Explanation

A Slipped Capital Femoral Epiphysis (SCFE), like most Salter-Harris type fractures, typically propagates through the hypertrophic zone of the physis. This zone is the weakest portion of the growth plate biomechanically because the chondrocytes are markedly swollen (hypertrophic) and the structural intercellular matrix is minimal, right before it gains structural integrity in the calcifying zone of provisional calcification.

Question 19

A 25-year-old male presents with persistent night pain in his left tibia that is profoundly relieved by ibuprofen. Imaging shows a 1 cm radiolucent nidus surrounded by dense reactive sclerosis in the diaphyseal cortex. If the nidus is excised, histologic examination is most likely to show:

Explanation

The clinical presentation (night pain dramatically relieved by NSAIDs) and radiographic appearance (lucent nidus < 2 cm with dense reactive sclerosis) are pathognomonic for Osteoid Osteoma. Histologically, the nidus consists of a highly vascularized network of interconnecting woven bone trabeculae lined by prominent, plump osteoblasts without malignant stroma. Cartilage cap = osteochondroma. Storiform fibroblastic stroma = Non-ossifying fibroma. Small round blue cells = Ewing sarcoma. Malignant osteoid = Osteosarcoma.

Question 20

Rivaroxaban is frequently prescribed for deep vein thrombosis (DVT) prophylaxis following lower extremity total joint arthroplasty. What is its specific mechanism of action in the coagulation cascade?

Explanation

Rivaroxaban (as well as Apixaban) is a Direct Oral Anticoagulant (DOAC) that specifically and directly inhibits Factor Xa in the coagulation cascade, preventing the conversion of prothrombin to thrombin. Dabigatran directly inhibits Thrombin (Factor IIa). Low Molecular Weight Heparins (like Enoxaparin) and Fondaparinux indirectly inhibit Factor Xa by binding to antithrombin III. Warfarin is a Vitamin K antagonist. Clopidogrel inhibits the ADP receptor on platelets.

Question 21

A 65-year-old man presents with groin pain 10 years after a total hip arthroplasty. Radiographs show eccentric wear of the polyethylene liner and large periacetabular radiolucencies. Which of the following cells is the primary mediator of the osteolysis seen in this patient?

Explanation

Macrophages are the primary mediators of particle-induced osteolysis. They phagocytose UHMWPE wear debris (optimal size 0.1-1.0 micrometers) and subsequently release inflammatory cytokines (such as TNF-alpha, IL-1, IL-6), which then stimulate osteoclasts to resorb bone.

Question 22

A new diagnostic test for diagnosing prosthetic joint infection has a sensitivity of 90% and a specificity of 95%. If the prevalence of the disease in the tested population increases from 2% to 10%, which of the following statistical parameters will also increase?

Explanation

Positive predictive value (PPV) is heavily dependent on disease prevalence. As the prevalence of a disease increases in a population, the PPV increases, while the Negative Predictive Value (NPV) decreases. Sensitivity and specificity are inherent characteristics of the test itself and do not change with population prevalence.

Question 23

A 70-year-old woman is prescribed rivaroxaban for venous thromboembolism prophylaxis following a total knee arthroplasty. Which of the following best describes the mechanism of action of this medication?

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that work by directly inhibiting Factor Xa. Warfarin inhibits Vitamin K epoxide reductase. Low Molecular Weight Heparin (LMWH) potentiates antithrombin III. Dabigatran is a direct thrombin inhibitor.

Question 24

A 12-year-old boy presents with multiple fractures and a history of anemia and cranial nerve palsies. Radiographs reveal generalized osteosclerosis and a 'bone-within-bone' appearance. Which of the following is the most likely cellular defect responsible for this condition?

Explanation

The patient has osteopetrosis, a metabolic bone disease characterized by osteoclast dysfunction leading to dense but brittle bones. A common genetic defect involves the TCIRG1 gene or carbonic anhydrase II, which results in the failure of osteoclasts to form a ruffled border and acidify the resorption pit, halting bone resorption.

Question 25

A 15-year-old boy presents with a 3-month history of a painful diaphyseal tibial lesion. The pain is worse at night and dramatically improves with NSAIDs. Radiographs show a small radiolucent nidus surrounded by reactive sclerosis. Which of the following inflammatory mediators is produced in high levels by this lesion?

Explanation

The clinical and radiographic presentation is classic for an osteoid osteoma. These benign bone-forming tumors possess a high expression of COX-2, leading to the production of extremely high levels of Prostaglandin E2 (PGE2). This pathophysiology explains the characteristic night pain that is exquisitely sensitive to NSAIDs.

Question 26

In the pathogenesis of periprosthetic joint infections, bacteria form a biofilm to evade host defenses and antibiotics. Which of the following terms describes the intercellular communication mechanism used by bacteria to coordinate gene expression and initiate biofilm formation?

Explanation

Quorum sensing is a process of cell-to-cell communication that allows bacteria to share information about cell density and adjust gene expression accordingly. This process is a critical early step for the transition from planktonic (free-floating) bacteria to the complex, highly organized sessile structure of a biofilm.

Question 27

Recombinant human bone morphogenetic protein-2 (rhBMP-2) is utilized in orthopedic and spine surgery to augment bone healing. BMP-2 initiates the osteoinductive signaling cascade by binding to serine-threonine kinase receptors. Which intracellular signaling molecules are directly phosphorylated by this activated receptor complex?

Explanation

BMPs bind to cell surface serine-threonine kinase receptors, which then directly phosphorylate the receptor-regulated SMADs (specifically SMAD 1, 5, and 8). These phosphorylated SMADs then complex with the common-mediator SMAD (SMAD 4) and translocate to the nucleus to regulate the transcription of target osteogenic genes.

Question 28

Which of the following modifications to a cortical bone screw design would most significantly increase its pullout strength?

Explanation

Screw pullout strength is proportional to the outer diameter, the length of thread engagement, and the shear strength of the bone. It is inversely proportional to the thread pitch (a decreased pitch means more threads per unit length, which increases pullout strength). Increasing the inner root diameter increases the screw's resistance to fatigue bending but does not directly increase pullout strength.

Question 29

A 22-year-old man sustains bilateral closed femur fractures in a motor vehicle accident. Forty-eight hours later, he develops confusion, tachypnea, and a petechial rash over his axilla and conjunctiva. Which of the following is the most appropriate initial management?

Explanation

The patient is presenting with classic Bergman's triad (hypoxia, neurologic compromise, petechial rash), indicative of Fat Embolism Syndrome (FES). The mainstay of treatment for FES is supportive care, primarily focusing on respiratory support (oxygenation/ventilation) and hemodynamic maintenance. Corticosteroids have not been shown to be effective in established FES, and systemic heparinization is contraindicated.

Question 30

According to Perren's strain theory, what is the maximum gap strain environment that will allow for primary (direct) bone healing without the formation of a visible fracture callus?

Explanation

Perren's strain theory states that the type of tissue that forms in a fracture gap depends on the strain environment. Primary (direct) bone healing via osteoclasts forming cutting cones followed by osteoblasts requires absolute stability with strain < 2%. Strain between 2-10% results in secondary bone healing (callus formation), while strain > 10% prevents bone formation and leads to nonunion.

Question 31

A neonate is diagnosed with achondroplasia. Both parents are of average stature. Which of the following best describes the pathogenesis of this skeletal dysplasia?

Explanation

Achondroplasia is the most common form of short-limb dwarfism and is caused by an autosomal dominant gain-of-function mutation in the FGFR3 gene. This overactive receptor inappropriately inhibits chondrocyte proliferation in the proliferative zone of the physis. Up to 80% of cases are new mutations, strongly associated with advanced paternal age.

Question 32

A 4-year-old boy presents with progressive bowing of his legs. Laboratory testing reveals normal serum calcium, markedly decreased serum phosphate, normal PTH, and elevated alkaline phosphatase. A genetic defect is identified on the X chromosome. Which of the following circulating factors is primarily responsible for the renal phosphate wasting in this patient?

Explanation

The patient has X-linked hypophosphatemic rickets, caused by a mutation in the PHEX gene. This leads to decreased degradation and subsequently elevated circulating levels of FGF-23. High FGF-23 inhibits sodium-phosphate cotransporters in the proximal renal tubule, causing severe phosphate wasting, and also downregulates 1-alpha-hydroxylase, preventing the synthesis of active vitamin D.

Question 33

A 14-year-old girl is evaluated for a large, destructive mass in the diaphysis of her femur. Biopsy reveals sheets of small, round, blue cells. Cytogenetic analysis is most likely to reveal which of the following translocations?

Explanation

The clinical presentation and histology ('small round blue cells') are characteristic of Ewing sarcoma. The vast majority of Ewing sarcomas (>90%) are associated with the t(11;22)(q24;q12) chromosomal translocation, which results in the EWS-FLI1 fusion protein. t(9;22) is seen in CML; t(12;16) in myxoid liposarcoma; t(X;18) in synovial sarcoma; t(2;13) in alveolar rhabdomyosarcoma.

Question 34

Tranexamic acid (TXA) has become a standard pharmacological agent to reduce perioperative blood loss in total joint arthroplasty. What is the precise mechanism of action of this drug?

Explanation

Tranexamic acid is an antifibrinolytic agent. It is a synthetic derivative of the amino acid lysine and reversibly binds to the lysine receptor sites on plasminogen. This competitive inhibition prevents plasminogen from converting to active plasmin, thereby preventing the degradation of existing fibrin clots.

Question 35

During a revision open reduction internal fixation of a distal femur fracture, a surgeon applies a titanium locking plate and secures it using stainless steel screws. What type of corrosion is most likely to occur at the interface between the plate and the screws?

Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., titanium and stainless steel) are placed in direct physical contact within a conductive fluid environment (like serum or interstitial fluid). The metal with the lower electrochemical potential (anodic) will corrode preferentially to protect the more noble metal (cathodic).

Question 36

Following a closed humerus fracture, a patient develops a radial nerve palsy. An EMG at 4 weeks shows fibrillation potentials, but the nerve remains anatomically continuous with disruption of axons and their myelin sheath, while the endoneurium, perineurium, and epineurium remain intact. This injury corresponds to which Sunderland grade?

Explanation

Sunderland Grade 2 (which corresponds to Seddon's axonotmesis) involves disruption of the axon and myelin sheath, but the surrounding connective tissue framework (endoneurium, perineurium, epineurium) is intact. Wallerian degeneration occurs distal to the injury, but the intact endoneurial tubes guide regenerating axons, leading to an excellent prognosis for recovery. Grade 1 is neurapraxia. Grades 3-5 involve progressive connective tissue disruption.

Question 37

When a massive structural cortical allograft is utilized in reconstructive surgery, the process of 'creeping substitution' dictates its incorporation. During the initial 6 to 12 months of this process, what happens to the mechanical strength of the allograft?

Explanation

Creeping substitution is the process by which a bone graft is simultaneously resorbed by osteoclasts and replaced by osteoblasts. In cortical bone grafts, osteoclastic resorption significantly outpaces osteoblastic bone formation initially. This leads to an increase in porosity and a significant decrease in mechanical strength (often weakening the graft by 50% or more) during the first 6 to 12 months, making it highly susceptible to fracture.

Question 38

Articular cartilage provides a nearly frictionless surface for joint articulation. Which zone of articular cartilage contains the highest concentration of water and collagen fibers oriented parallel to the joint surface, providing the highest resistance to tensile and shear forces?

Explanation

The superficial (tangential) zone of articular cartilage comprises 10-20% of the thickness. It has the highest water content, the highest concentration of type II collagen, and the lowest proteoglycan content. The collagen fibers are oriented parallel to the articular surface, allowing this zone to effectively resist the high tensile and shear forces experienced by the joint.

Question 39

During the normal human gait cycle, at what point does the maximum compressive force occur across the hip joint?

Explanation

During the single-limb support phase (mid-stance), the center of gravity shifts and the hip abductor muscles must fire forcefully to maintain a level pelvis. The combination of body weight and the powerful contraction force of the hip abductors results in the maximum joint reaction force (compressive force) across the hip, which can reach 2.5 to 3 times body weight.

Question 40

A 68-year-old woman is prescribed alendronate for the treatment of severe osteoporosis. By which of the following intracellular mechanisms does this drug primarily inhibit osteoclast-mediated bone resorption?

Explanation

Nitrogen-containing bisphosphonates (like alendronate, risedronate, zoledronic acid) act by inhibiting farnesyl pyrophosphate synthase, a key enzyme in the mevalonate pathway. This prevents the prenylation of small GTPase proteins (like Ras, Rho, Rac) that are essential for osteoclast function, ruffled border formation, and survival, ultimately leading to osteoclast apoptosis.

Question 41

Which type of corrosion is most likely to occur in the restricted, oxygen-depleted space between the head and neck taper of a modular total hip arthroplasty implant?

Explanation

Crevice corrosion occurs in restricted spaces (crevices) such as modular tapers or under screw heads, where oxygen depletion leads to a localized acidic environment that breaks down the implant's passivation layer.

Question 42

Recombinant human Bone Morphogenetic Protein-2 (rhBMP-2) is FDA-approved for use in acute, open tibial shaft fractures. Which intracellular signaling pathway is primarily activated by BMP-2 binding to its receptor?

Explanation

BMPs bind to serine/threonine kinase receptors, leading to the phosphorylation of receptor-regulated Smads (Smad 1, 5, and 8). These then form a complex with Smad 4 and translocate to the nucleus to regulate gene transcription.

Question 43

A patient with malignant infantile osteopetrosis is found to have defective osteoclasts. The loss of function of which of the following enzymes is a well-known genetic cause of the failure of osteoclasts to acidify the bone surface?

Explanation

Osteopetrosis is caused by impaired osteoclast function. A common mutation involves carbonic anhydrase II, which is necessary to generate protons (H+) for the proton pumps that acidify Howship's lacuna to dissolve bone mineral.

Question 44

On a stress-strain curve representing the mechanical properties of a bone plate, what does the area under the curve up to the yield point represent?

Explanation

Resilience is the capacity of a material to absorb energy when deformed elastically and then recover completely upon unloading. It corresponds to the area under the elastic portion of the stress-strain curve (up to the yield point).

Question 45

During a regional block, a patient inadvertently receives an intravascular injection of bupivacaine and subsequently develops refractory cardiac arrest. What is the recommended first-line antidote for this specific toxicity?

Explanation

Local anesthetic systemic toxicity (LAST), particularly from bupivacaine, causes severe cardiac depression and arrhythmias due to blockade of voltage-gated sodium channels. 20% Lipid emulsion therapy is the gold-standard treatment, acting as a 'lipid sink' to extract the highly lipophilic bupivacaine from myocardial tissue.

Question 46

Which of the following is the most sensitive whole-body imaging modality for detecting skeletal marrow involvement in a patient newly diagnosed with multiple myeloma?

Explanation

Whole-body MRI is highly sensitive for detecting diffuse marrow infiltration and focal lesions in multiple myeloma long before cortical destruction becomes evident on plain radiographs. Standard bone scans (Tc-99m MDP) are notoriously 'cold' in myeloma because the lesions are purely lytic with little to no reactive bone formation.

Question 47

Articular cartilage is composed of multiple zones. Which zone possesses the highest concentration of proteoglycans and the lowest water content?

Explanation

The deep (radial) zone of articular cartilage contains the largest diameter collagen fibrils oriented perpendicular to the joint surface, the highest concentration of proteoglycans, and the lowest water content.

Question 48

Rifampin is frequently utilized in the treatment of staphylococcal periprosthetic joint infections due to its ability to penetrate biofilms. What is its mechanism of action?

Explanation

Rifampin exerts its bactericidal effect by binding to the beta subunit of bacterial DNA-dependent RNA polymerase, thereby inhibiting RNA synthesis. It is highly active against stationary-phase bacteria within biofilms.

Question 49

A research group investigates the association between bisphosphonate use and atypical femur fractures by identifying a group of patients with atypical femur fractures and a group of matched patients without these fractures, then looking back through their records to compare their history of bisphosphonate use. What study design is this?

Explanation

Because the researchers identified patients based on their outcome (atypical femur fracture vs. no fracture) and then looked retrospectively at their exposure status (bisphosphonate use), this is a case-control study.

Question 50

During secondary fracture healing, endochondral ossification requires the coordinated differentiation of chondrocytes. Which transcription factor is the master regulator for chondrocyte hypertrophy and subsequent osteoblast differentiation?

Explanation

Runx2 (Cbfa1) is the master transcription factor necessary for chondrocyte hypertrophy and osteoblast differentiation. Sox9 is critical for early chondrogenesis but must be downregulated for chondrocytes to undergo hypertrophy.

Question 51

Duchenne muscular dystrophy is inherited in which of the following genetic patterns?

Explanation

Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder caused by a frameshift mutation in the dystrophin gene located on the X chromosome (Xp21).

Question 52

According to the Sunderland classification of peripheral nerve injuries, a third-degree nerve injury is characterized by disruption of the axon and which other structure(s)?

Explanation

A Sunderland III injury involves disruption of the axon and endoneurium, but the perineurium and epineurium remain intact. Recovery is unpredictable because internal scarring within the fascicle can obstruct axonal regeneration.

Question 53

Rivaroxaban is a widely used oral anticoagulant for venous thromboembolism prophylaxis after total joint arthroplasty. Which step in the coagulation cascade does it directly inhibit?

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants that work by competitively and reversibly inhibiting Factor Xa. Dabigatran is a direct thrombin (Factor IIa) inhibitor.

Question 54

When attempting to maximize the pull-out strength of a cortical bone screw, which of the following geometric modifications to the screw design will have the most significant positive effect?

Explanation

Pull-out strength of a screw is directly proportional to the outer (thread) diameter, the length of engagement in the bone, and the shear strength of the surrounding bone. Increasing the inner (core) diameter increases the screw's bending or fatigue strength but does not primarily increase pull-out strength.

Question 55

The incorporation of Vitamin E (alpha-tocopherol) into highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) components in total joint arthroplasty primarily serves to:

Explanation

Irradiation used to cross-link UHMWPE produces free radicals, which can lead to oxidation and embrittlement over time. Vitamin E is an antioxidant added to the polyethylene to scavenge these free radicals, preventing oxidation without the mechanical property compromises associated with post-irradiation remelting.

Question 56

Within a cortical bone osteon, adjacent osteocytes communicate with each other and share nutrients via gap junctions located within microscopic channels. What are these channels called?

Explanation

Canaliculi are narrow, microscopic channels that radiate from the lacunae housing osteocytes. The dendritic processes of osteocytes extend through the canaliculi to contact adjacent cells via gap junctions, facilitating communication and nutrient exchange.

Question 57

A 4-year-old child presents with rhizomelic short stature, frontal bossing, and midface hypoplasia. Radiographs show a narrowing of the interpedicular distances from L1 to L5. This condition is most commonly caused by a mutation in which gene?

Explanation

The clinical presentation is classic for achondroplasia. Achondroplasia is caused by an activating (gain-of-function) mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene, leading to inhibition of chondrocyte proliferation in the growth plate.

Question 58

Paget's disease of bone occurs in three distinct phases. Which of the following best describes the primary cellular abnormality during the initial phase?

Explanation

Paget's disease typically begins with an intense osteolytic phase characterized by a profound increase in the number, size, and activity of osteoclasts. These osteoclasts are abnormally large and have numerous nuclei. This is followed by a mixed phase and finally a sclerotic phase.

Question 59

A 55-year-old male presents with acute severe pain in the first metatarsophalangeal joint. Aspiration yields negatively birefringent, needle-shaped crystals. For long-term management to prevent recurrent attacks, which of the following drugs acts by inhibiting xanthine oxidase?

Explanation

Allopurinol is a xanthine oxidase inhibitor used for the chronic management of gout to decrease uric acid production. Colchicine inhibits microtubule assembly, probenecid is a uricosuric agent, and indomethacin is an NSAID used for acute attacks.

Question 60

In a patient with X-linked hypophosphatemic rickets (XLH), which of the following laboratory profiles is typically observed?

Explanation

X-linked hypophosphatemic rickets is caused by a mutation in the PHEX gene, leading to elevated FGF23 and subsequent renal phosphate wasting. Typical lab values show low serum phosphate, normal serum calcium, and a normal or only mildly elevated PTH level. This contrasts with nutritional rickets, which features low calcium and significantly elevated PTH.

Question 61

Which of the following best describes the primary mechanism of action of osteoprotegerin (OPG) in bone metabolism?

Explanation

Osteoprotegerin (OPG) acts as a soluble decoy receptor that binds to RANKL. By binding RANKL, it prevents it from interacting with the RANK receptor on osteoclast precursors, thereby inhibiting osteoclast differentiation and bone resorption.

Question 62

According to Perren's strain theory, what is the maximum amount of interfragmentary strain that can be tolerated to achieve primary (direct) bone healing without a cartilaginous intermediate?

Explanation

Primary bone healing requires absolute stability, which corresponds to an interfragmentary strain of less than 2%. Under these conditions, cutting cones cross the fracture site directly followed by osteoblast-mediated lamellar bone deposition.

Question 63

Which of the following biochemical changes is characteristic of articular cartilage in the early stages of osteoarthritis, distinguishing it from the normal aging process?

Explanation

In early osteoarthritis, the initial degradation of the collagen matrix allows the hydrophilic proteoglycans to swell, leading to an increased water content in the cartilage. In contrast, normal aging cartilage undergoes dehydration and exhibits decreased water content.

Question 64

When comparing the torsional rigidity of a solid intramedullary nail to a hollow nail of the same material, the polar moment of inertia is primarily determined by which dimensional parameter?

Explanation

Torsional rigidity is governed by the polar moment of inertia. For a solid cylinder, this value is directly proportional to the radius to the fourth power (r^4), demonstrating how small increases in nail diameter drastically increase torsional stiffness.

Question 65

The combination of a 316L stainless steel screw with a titanium plate in an orthopedic fracture construct significantly increases the risk of which type of corrosion?

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in contact within a conductive fluid environment. The less noble metal acts as an anode and undergoes accelerated corrosion.

Question 66

A 45-year-old weightlifter experiences an acute distal biceps tendon rupture. This specific injury biomechanically occurs most frequently during which type of muscle contraction?

Explanation

Eccentric contractions occur when a muscle generates tension while actively lengthening, yielding the highest peak force of any contraction type. This massive force generation renders the muscle-tendon unit most susceptible to mechanical failure and rupture.

Question 67

A patient sustains a closed midshaft humerus fracture with an associated radial nerve palsy. According to Sunderland's classification, a 3rd-degree nerve injury involves the disruption of the axon and myelin sheath along with which of the following connective tissue layers?

Explanation

A Sunderland 3rd-degree injury involves damage to the axon, myelin, and the endoneurium, while the perineurium remains intact. Because the endoneurial tubes are disrupted, axonal regeneration may be misdirected, leading to incomplete or aberrant functional recovery.

Question 68

What is the primary pharmacological mechanism by which low-molecular-weight heparin (LMWH) provides deep vein thrombosis prophylaxis in postoperative orthopedic patients?

Explanation

LMWH binds to and induces a conformational change in antithrombin III. Unlike unfractionated heparin, its smaller molecular size causes it to predominantly accelerate the inhibition of Factor Xa rather than thrombin (Factor IIa).

Question 69

A 15-year-old male presents with a permeative, moth-eaten lytic lesion in the femoral diaphysis with an "onion-skin" periosteal reaction. Biopsy reveals small round blue cells. Which chromosomal translocation is highly specific for this pathology?

Explanation

The clinical presentation and histology are classic for Ewing sarcoma. The vast majority of these tumors contain the t(11;22)(q24;q12) translocation, which creates the oncogenic EWS-FLI1 fusion protein.

Question 70

A 65-year-old male undergoes a total hip arthroplasty for severe secondary osteoarthritis. Preoperative radiographs reveal extensive cortical thickening and a "cotton wool" appearance of the hemipelvis. Histological examination of this bone would most likely demonstrate:

Explanation

The radiographic findings suggest Paget's disease of bone. The pathognomonic histologic feature is a mosaic pattern of lamellar bone characterized by prominent, haphazardly arranged cement lines due to erratic, coupled bone resorption and formation.

Question 71

Methicillin-resistant Staphylococcus aureus (MRSA) poses a severe challenge in periprosthetic joint infections. Its specific resistance to beta-lactam antibiotics is primarily mediated by which of the following mechanisms?

Explanation

MRSA resistance is directly conferred by the mecA gene, which encodes for an altered penicillin-binding protein, PBP2a. This mutant protein has an extremely low affinity for beta-lactam antibiotics, allowing cell wall cross-linking to continue even in their presence.

Question 72

Which type of collagen forms the predominant structural tensile framework in the annulus fibrosus of the intervertebral disc?

Explanation

The annulus fibrosus consists predominantly of Type I collagen, which provides the necessary tensile strength to contain the inner structures. Conversely, the central nucleus pulposus is rich in Type II collagen and water-binding proteoglycans.

Question 73

Bone morphogenetic proteins (BMPs) play a crucial osteoinductive role in fracture healing. Which intracellular signaling molecules are phosphorylated by BMP receptors to translocate to the nucleus and regulate osteogenic gene expression?

Explanation

BMPs are part of the TGF-beta superfamily and signal through serine/threonine kinase receptors. Upon activation, these receptors phosphorylate receptor-regulated Smad proteins (such as Smad1/5/8), which then bind Smad4 and translocate to the nucleus.

Question 74

In orthopedic biomechanics, the pullout strength of a standard cortical bone screw is most significantly increased by maximizing which of the following design parameters?

Explanation

The pullout strength of a screw is most strongly related to the outer (thread) diameter because it dictates the volume of bone engaged between the threads. Increasing the inner core diameter increases bending strength but does not primarily maximize pullout resistance.

Question 75

According to the inverse square law of radiation physics, if an orthopedic surgeon steps back from 1 meter to 2 meters away from the C-arm during intraoperative fluoroscopy, the radiation exposure is reduced to what fraction of the original dose?

Explanation

The inverse square law dictates that radiation intensity is inversely proportional to the square of the distance from the source. Doubling the distance (from 1m to 2m) reduces the exposure by a factor of 2 squared, leaving the surgeon exposed to only one-fourth (25%) of the original dose.

Question 76

When administering local anesthetics for an intra-articular block, the clinical duration of action of the agent is primarily determined by its:

Explanation

The duration of action of a local anesthetic is primarily determined by its affinity for plasma and tissue proteins. Agents with high protein binding, such as bupivacaine, remain attached to the sodium channel receptor site longer, providing prolonged clinical anesthesia.

Question 77

During the incorporation of a massive cortical bone allograft, the process of "creeping substitution" is accurately defined by which initial sequence of events?

Explanation

Cortical allografts incorporate via creeping substitution, wherein host osteoclasts first resorb the necrotic graft matrix to create cutting cones. This resorptive phase temporarily weakens the graft before host osteoblasts trail behind to deposit new, living bone.

Question 78

In the structural organization of normal articular cartilage, the deep (basal) zone is characterized by which specific biomechanical arrangement of its collagen fibers?

Explanation

In the deep zone of articular cartilage, Type II collagen fibers are arranged perpendicularly to the articular surface. This vertical orientation allows them to firmly anchor the cartilage across the tidemark into the underlying subchondral bone, effectively resisting compressive forces.

Question 79

During the normal human gait cycle, the tibialis anterior muscle exhibits its peak electromyographic activity immediately following heel strike. What type of contraction is it performing, and what is its primary biomechanical function during this phase?

Explanation

Immediately following heel strike (initial contact), the ankle moves toward foot flat. The tibialis anterior contracts eccentrically (lengthening under tension) to control the rate of plantarflexion, slowly lowering the forefoot to the ground and preventing an audible "foot slap."

Question 80

Tranexamic acid (TXA) is now routinely utilized in major orthopedic procedures like arthroplasty to minimize perioperative blood loss. Which of the following accurately describes its molecular mechanism of action?

Explanation

Tranexamic acid is a synthetic lysine analog that reversibly binds to the lysine-binding sites on plasminogen. This competitively inhibits the activation of plasminogen to plasmin, thereby preventing the degradation of established fibrin clots (a potent antifibrinolytic effect).

Question 81

Which of the following best describes the primary mechanism by which a cancellous bone autograft incorporates into the host site, as compared to a cortical bone autograft?

Explanation

Cancellous bone incorporates by initial osteoblastic apposition directly onto the existing trabecular framework (osteoconduction). In contrast, cortical bone must undergo initial osteoclastic resorption (creeping substitution) before new bone can be formed.

Question 82

During the biomechanical testing of an articular cartilage explant, a constant strain is applied and maintained over time. The gradual decrease in the force required to maintain this constant strain is biomechanically defined as:

Explanation

Stress relaxation is the time-dependent decrease in stress (force) under a constant state of strain. Creep, conversely, is the progressive deformation (strain) of a material under a constant stress (load).

Question 83

During an extended deltopectoral approach to the shoulder, the cephalic vein is identified. To minimize bleeding and preserve its venous drainage, the cephalic vein should ideally be retracted in which direction, and what is its anatomic rationale?

Explanation

The cephalic vein lies in the deltopectoral groove and should be retracted laterally with the deltoid during the approach. This is because it receives the majority of its delicate venous tributaries from the deltoid muscle.

Question 84

A 14-year-old boy presents with severe thigh pain. Radiographs show a destructive, permeative diaphyseal lesion of the femur with an 'onion-skin' periosteal reaction. Biopsy reveals small round blue cells. Which of the following chromosomal translocations is most pathognomonic for this tumor?

Explanation

This presentation is characteristic of Ewing sarcoma, a small round blue cell tumor. The t(11;22) translocation creates the EWS-FLI1 fusion protein, which is diagnostic for Ewing sarcoma.

Question 85

An obese 12-year-old boy presents with a stable slipped capital femoral epiphysis (SCFE) of the left hip. Under which of the following conditions is prophylactic in situ pinning of the contralateral, asymptomatic right hip most strongly indicated?

Explanation

Prophylactic pinning of the contralateral hip in SCFE is highly recommended for patients with underlying endocrine disorders (such as hypothyroidism) or renal osteodystrophy. These conditions carry a significantly higher risk for bilateral involvement.

Question 86

A surgeon revises a nonunion treated previously with a stainless steel plate by placing a new titanium plate over the existing retained stainless steel screws. Which type of corrosion is most likely to rapidly degrade the implant interface?

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in physical contact within a conducting fluid medium (like body fluids). This creates an electrochemical cell, leading to accelerated corrosion of the less noble metal.

Question 87

A 45-year-old farmer sustains a Gustilo-Anderson Type IIIA open tibia fracture heavily contaminated with soil and manure. According to current evidence-based trauma guidelines, which of the following antibiotic regimens is most appropriate?

Explanation

For severe open fractures with farm or soil contamination (Type III), high-dose penicillin is added to the standard regimen of a first-generation cephalosporin and an aminoglycoside. This specific addition provides coverage against Clostridium species to prevent gas gangrene.

Question 88

The compressive strength and stiffness of normal articular cartilage are primarily provided by the interaction between interstitial fluid and which of the following extracellular matrix components?

Explanation

Aggrecan, a large proteoglycan, is highly negatively charged and draws water into the cartilage matrix via Donnan osmotic pressure. This swelling pressure is constrained by the Type II collagen network, providing cartilage with its vital compressive strength.

Question 89

A 10-year-old boy sustains a transverse subtrochanteric fracture. Radiographs reveal generalized, symmetric increased bone density with an absent medullary canal (Erlenmeyer flask deformity). The underlying pathophysiology of this condition is a defect in which of the following?

Explanation

This clinical picture describes osteopetrosis, a genetic disorder characterized by excessively dense, brittle bones. It is caused by defective osteoclast function, preventing normal bone resorption and remodeling.

Question 90

A 45-year-old man presents with an acutely swollen, erythematous knee. Joint aspiration yields opaque synovial fluid. Which of the following synovial fluid analysis profiles is most diagnostic for acute bacterial septic arthritis?

Explanation

Septic arthritis typically presents with a synovial fluid WBC count > 50,000 cells/mcL, > 90% polymorphonuclear leukocytes (PMNs), and decreased glucose compared to serum. While definitive diagnosis requires culture, these parameters dictate immediate empiric treatment.

Question 91

Tranexamic acid (TXA) is frequently used in major orthopedic arthroplasty and trauma to reduce perioperative blood loss. Which of the following best describes its primary mechanism of action?

Explanation

TXA is a synthetic analog of the amino acid lysine. It competitively and reversibly binds to the lysine receptor sites on plasminogen, preventing its activation to plasmin and thereby inhibiting fibrinolysis.

Question 92

Recombinant human bone morphogenetic protein-2 (rhBMP-2) enhances bone healing by inducing the differentiation of mesenchymal stem cells into osteoblasts. This osteoinductive effect is mediated by activating which of the following intracellular signaling pathways?

Explanation

BMPs exert their effects by binding to serine/threonine kinase transmembrane receptors. This binding phosphorylates and activates the intracellular Smad 1/5/8 pathway, which translocates to the nucleus to regulate osteogenic gene transcription.

Question 93

During internal fixation of a dense cortical bone fracture, the pullout strength of the orthopedic screw is determined by several design parameters. Which of the following geometric modifications most significantly increases the screw's pullout strength?

Explanation

Pullout strength of a bone screw is directly proportional to the outer (thread) diameter, the length of thread engagement, and the shear strength of the bone. Increasing the outer diameter maximizes the volume of bone captured between threads.

Question 94

A 32-year-old male sustains a closed midshaft tibia fracture and subsequently complains of severe pain requiring increasing doses of narcotics. Which of the following clinical findings is considered the earliest and most reliable indicator of acute compartment syndrome?

Explanation

Pain out of proportion to the injury and pain with passive stretching of the muscles within the affected compartment are the earliest and most sensitive clinical signs of acute compartment syndrome. Pulselessness and paralysis are late, ominous signs of irreversible damage.

Question 95

In an adult patient presenting with neurogenic claudication secondary to an L4-L5 degenerative spondylolisthesis, which neural element is most commonly compressed, producing radicular symptoms?

Explanation

Degenerative spondylolisthesis most commonly occurs at the L4-L5 level with an intact pars interarticularis. This condition leads to central canal and lateral recess stenosis, which compresses the traversing L5 nerve root rather than the exiting L4 nerve root.

Question 96

A 19-year-old male presents with severe nocturnal thigh pain that is rapidly and completely relieved by ibuprofen. Radiographs demonstrate diaphyseal cortical thickening with a 1 cm radiolucent nidus. The dramatic pain relief is due to NSAID inhibition of which specific molecule secreted by the nidus?

Explanation

This classic presentation describes an osteoid osteoma. The nidus secretes abnormally high levels of Prostaglandin E2 (PGE2), which mediates the severe nocturnal pain. NSAIDs provide dramatic relief by inhibiting cyclooxygenase, halting PGE2 synthesis.

Question 97

Following a total hip arthroplasty, a patient is prescribed rivaroxaban for routine venous thromboembolism prophylaxis. Which of the following best describes the precise molecular target of this medication?

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that work by directly and reversibly inhibiting Factor Xa. This interrupts the intrinsic and extrinsic pathways of the blood coagulation cascade.

Question 98

A 7-year-old child sustains a Salter-Harris Type II fracture of the distal radius. The fracture line propagating transversely through the physis typically traverses which of the following histologic zones of the growth plate?

Explanation

Fractures through the physis typically occur in the zone of hypertrophy. This zone is mechanically the weakest because of a high cell-to-matrix ratio, large chondrocyte volume, and a relative lack of collagen.

Question 99

During the process of secondary fracture healing, the initial cartilaginous soft callus is gradually replaced by a hard woven bone callus. Which of the following cell types is primarily responsible for the production of the initial woven bone via endochondral ossification?

Explanation

During the hard callus phase of secondary fracture healing, osteoblasts lay down woven bone to replace the cartilaginous framework. This process of endochondral ossification turns the unstable soft callus into a rigid hard callus.

Question 100

Intermittent, low-dose administration of parathyroid hormone (PTH) analogs, such as teriparatide, is utilized for severe osteoporosis. While continuous elevated PTH causes net bone resorption, intermittent administration primarily stimulates bone formation by directly activating receptors on which cell type?

Explanation

PTH receptors are located directly on the surface of osteoblasts (and osteocytes), but not on osteoclasts. Intermittent administration of PTH promotes osteoblast survival and stimulates them to form new bone, whereas continuous administration upregulates RANKL to activate osteoclasts.

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