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1. General Principles & Basic Science MCQs

Practice Set 423 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 8441

Topic: Biology, Genetics & Bone Healing

A 68-year-old man presents with dull, aching pain in his right thigh and an enlarging hat size. Radiographs of the femur demonstrate cortical thickening, increased bone density, and bowing. Laboratory studies show markedly elevated serum alkaline phosphatase, but normal calcium and phosphorus. The primary defect in this condition is characterized by which of the following?

. Excessive osteoblastic bone formation followed by secondary osteoclast overactivity
. Defective mineralization of osteoid leading to accumulation of unmineralized matrix
. Initial profound increase in osteoclastic bone resorption followed by disorganized osteoblastic bone formation
. A mutation in the FGFR3 gene causing abnormal enchondral ossification
. Parathyroid adenoma leading to autonomous parathyroid hormone secretion

Correct Answer & Explanation

. Initial profound increase in osteoclastic bone resorption followed by disorganized osteoblastic bone formation

Explanation

Paget's disease of bone is characterized by three phases: an initial purely osteoclastic (lytic) phase with profound and overactive bone resorption, a mixed phase with disorganized woven bone formation by osteoblasts attempting to compensate, and a final osteosclerotic (burnt-out) phase where bone is dense but mechanically weak. The primary cellular abnormality is excessive osteoclast activity.

Question 8442

Topic: 1. General Principles & Basic Science

A 20-year-old man complains of persistent left thigh pain that is worse at night and dramatically relieved by ibuprofen. Radiographs demonstrate cortical thickening in the diaphyseal femur with a small radiolucent nidus. What is the mechanism by which NSAIDs relieve the pain associated with this lesion?

. Inhibition of substance P release from the tumor cells
. Inhibition of excessive cyclooxygenase-2 (COX-2) enzyme and prostaglandin E2 (PGE2) production within the nidus
. Suppression of the marked perilesional inflammatory infiltrate and lymphocytes
. Direct inhibition of osteoclastic bone resorption around the nidus
. Downregulation of RANKL expression within the tumor stroma

Correct Answer & Explanation

. Inhibition of excessive cyclooxygenase-2 (COX-2) enzyme and prostaglandin E2 (PGE2) production within the nidus

Explanation

The nidus of an osteoid osteoma produces high levels of prostaglandins, specifically prostaglandin E2 (PGE2), due to the high expression of the cyclooxygenase-2 (COX-2) enzyme by the neoplastic osteoblasts. This leads to intense local vasodilation and nerve stimulation, causing pain that is classically worse at night and dramatically relieved by NSAIDs.

Question 8443

Topic: 1. General Principles & Basic Science
A 62-year-old man presents with back pain and fatigue. Laboratory investigations reveal anemia, hypercalcemia, and a monoclonal spike on serum protein electrophoresis. Plain radiographs of the spine are negative for obvious lytic lesions. What is the most sensitive imaging modality to detect skeletal involvement in this patient?
. Technetium-99m whole-body bone scan
. Whole-body low-dose CT (WBLDCT) or MRI
. Plain radiography skeletal survey
. Dual-energy X-ray absorptiometry (DEXA)
. Indium-111 leukocyte scan

Correct Answer & Explanation

. Whole-body low-dose CT (WBLDCT) or MRI

Explanation

Whole-body low-dose CT (WBLDCT) or whole-body MRI are the gold standards for detecting skeletal lesions in multiple myeloma because of their high sensitivity. A technetium-99m bone scan is generally not useful because myeloma lesions are purely lytic and often do not elicit an osteoblastic reaction, leading to false-negative results. Plain skeletal surveys have significantly lower sensitivity.

Question 8444

Topic: Biology, Genetics & Bone Healing

A 35-year-old woman presents with persistent right knee pain. Radiographs reveal an eccentric, purely lytic lesion in the distal femoral epiphysis extending into the metaphysis and reaching the subchondral bone. A core needle biopsy demonstrates numerous multinucleated giant cells uniformly dispersed among round-to-oval mononuclear stromal cells. Due to the high risk of joint collapse with surgical curettage, targeted medical therapy is considered. The most appropriate pharmacological agent for this lesion primarily targets which of the following molecular pathways?

. Vascular endothelial growth factor (VEGF) inhibition
. Receptor activator of nuclear factor kappa-B ligand (RANKL) inhibition
. Macrophage colony-stimulating factor (M-CSF) inhibition
. Tyrosine kinase inhibition
. Platelet-derived growth factor (PDGF) inhibition

Correct Answer & Explanation

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

Explanation

The clinical presentation and histologic description are characteristic of a giant cell tumor (GCT) of bone. GCTs typically present as eccentric, lytic epiphyseal/metaphyseal lesions in young adults after skeletal maturity. The true neoplastic cells in GCT are the mononuclear stromal cells, which express high levels of RANKL. RANKL stimulates the recruitment, differentiation, and activation of non-neoplastic osteoclast-like multinucleated giant cells, leading to extensive local bone resorption. Denosumab is a fully human monoclonal antibody that binds to and inhibits RANKL, effectively halting osteoclast-mediated bone destruction. It is indicated for GCTs that are recurrent or when surgical resection is deemed to have unacceptably high morbidity.

Question 8445

Topic: Biology, Genetics & Bone Healing

A 65-year-old man presents with severe back pain, fatigue, and generalized weakness. Laboratory tests reveal a normocytic anemia, hypercalcemia, and an elevated serum creatinine. Radiographs of the spine and skull demonstrate multiple, well-circumscribed, 'punched-out' lytic lesions without reactive sclerosis. The extensive bone destruction seen in this condition is primarily mediated by which of the following cellular mechanisms?

. Direct degradation of the bone matrix by neoplastic cells
. Systemic secretion of parathyroid hormone-related peptide (PTHrP)
. Upregulation of RANKL and downregulation of osteoprotegerin (OPG) within the marrow microenvironment
. Increased endogenous production of fibroblast growth factor 23 (FGF23)
. Defective mineralization of the newly formed osteoid

Correct Answer & Explanation

. Upregulation of RANKL and downregulation of osteoprotegerin (OPG) within the marrow microenvironment

Explanation

The clinical picture of back pain, anemia, hypercalcemia, renal insufficiency, and 'punched-out' lytic lesions is characteristic of multiple myeloma. The osteolytic bone disease in multiple myeloma is heavily driven by an uncoupling of the normal bone remodeling process. Neoplastic plasma cells and adjacent bone marrow stromal cells overexpress RANKL while simultaneously downregulating osteoprotegerin (OPG), an endogenous decoy receptor for RANKL. This imbalance leads to massive osteoclastogenesis and excessive bone resorption. Furthermore, myeloma cells secrete factors that inhibit osteoblast differentiation (e.g., DKK1), further worsening bone loss. Direct tumor degradation of bone is not the primary mechanism. Unlike solid tumor metastases (e.g., breast or lung cancer), multiple myeloma rarely relies on PTHrP to induce bone resorption.

Question 8446

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with worsening knee pain over several months. Radiographs demonstrate an eccentric, expansile, lytic epiphyseal lesion in the distal femur. A biopsy confirms a giant cell tumor of bone. She is treated with a targeted monoclonal antibody prior to surgical curettage to downstage the tumor. What is the precise mechanism of action of the medication most likely administered to this patient?

. Inhibition of the tyrosine kinase receptor to reduce angiogenesis
. Binding to the RANK ligand, preventing the activation of RANK on osteoclast-like giant cells
. Cross-linking of DNA leading to apoptosis of the neoplastic stromal cells
. Inhibition of matrix metalloproteinases, reducing tumor invasiveness
. Direct binding to the osteoclast proton pump, halting bone resorption

Correct Answer & Explanation

. Binding to the RANK ligand, preventing the activation of RANK on osteoclast-like giant cells

Explanation

The patient is being treated with denosumab, a human monoclonal antibody that specifically binds to the Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). In giant cell tumors of bone, the neoplastic mononuclear stromal cells express RANKL, which recruits and activates the reactive, multinucleated giant cells (osteoclast-like cells) expressing RANK. By binding RANKL, denosumab prevents RANK activation, thereby potently inhibiting bone resorption, facilitating tumor ossification, and decreasing both the size and vascularity of the tumor prior to surgery.

Question 8447

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with worsening knee pain. Radiographs reveal an eccentric, purely lytic lesion in the distal femur extending to the subchondral bone without a sclerotic margin. Biopsy confirms a giant cell tumor of bone. In cases where the tumor is deemed unresectable, which of the following targeted therapies acts by binding to RANKL to inhibit osteoclast-like giant cells?

. Zoledronic acid
. Imatinib
. Denosumab
. Mifamurtide
. Doxorubicin

Correct Answer & Explanation

. Denosumab

Explanation

Denosumab is a fully human monoclonal antibody that binds to RANKL, preventing the activation of the RANK receptor on the surface of osteoclasts and the multinucleated giant cells characteristic of giant cell tumor (GCT) of bone. It is FDA-approved for adults and skeletally mature adolescents with GCT of bone that is unresectable or where surgical resection is likely to result in severe morbidity.

Question 8448

Topic: Biology, Genetics & Bone Healing

Nitrogen-containing bisphosphonates (e.g., alendronate, zoledronic acid) are frequently used in the management of metastatic bone disease and osteoporosis. They primarily inhibit osteoclastic bone resorption through which of the following intracellular mechanisms?

. Inhibition of farnesyl pyrophosphate synthase
. Direct inhibition of the RANK receptor
. Inhibition of cathepsin K
. Activation of osteoprotegerin (OPG)
. Binding to the Wnt/beta-catenin pathway

Correct Answer & Explanation

. Inhibition of farnesyl pyrophosphate synthase

Explanation

Nitrogen-containing bisphosphonates accumulate in osteoclasts and inhibit farnesyl pyrophosphate synthase, a key enzyme in the mevalonate pathway. This inhibition prevents the prenylation of small GTPase proteins (like Ras, Rho, and Rab) that are essential for osteoclast function, ruffled border formation, and cell survival, ultimately leading to osteoclast apoptosis.

Question 8449

Topic: Biology, Genetics & Bone Healing

During the process of secondary bone healing (enchondral ossification), which of the following local microenvironmental factors is primarily responsible for directing the differentiation of mesenchymal stem cells into chondrocytes during the formation of the soft callus?

. Absolute rigid mechanical fixation
. High oxygen tension at the fracture site
. Low oxygen tension combined with moderate mechanical strain
. Suppression of Transforming Growth Factor-beta (TGF-beta)
. Overexpression of Sclerostin

Correct Answer & Explanation

. Low oxygen tension combined with moderate mechanical strain

Explanation

Secondary bone healing (healing with callus formation) requires relative stability (moderate mechanical strain) and occurs in an environment that initially has lower oxygen tension (hypoxia) due to the disruption of local blood supply. The combination of hypoxia and micro-motion promotes the differentiation of mesenchymal stem cells into chondrocytes, leading to the formation of a cartilaginous soft callus that is later calcified.

Question 8450

Topic: 1. General Principles & Basic Science

In the evaluation of a peripheral nerve injury, electromyography (EMG) is often utilized to assess the functional status of the motor unit. Fibrillation potentials and positive sharp waves observed on needle EMG of a resting muscle typically indicate which of the following?

. Normal voluntary muscle activation
. Active muscle denervation
. Early muscle reinnervation
. An upper motor neuron lesion
. Myasthenia gravis

Correct Answer & Explanation

. Active muscle denervation

Explanation

On electromyography (EMG), fibrillation potentials and positive sharp waves are classic signs of active muscle denervation. They arise from the spontaneous, unstable depolarizations of individual muscle fibers that have lost their nerve supply. These findings typically become evident 2 to 3 weeks following an acute peripheral nerve injury. In contrast, reinnervation is characterized by the appearance of polyphasic motor unit action potentials (MUAPs).

Question 8451

Topic: Biology, Genetics & Bone Healing



A 32-year-old woman presents with worsening right knee pain. Radiographs show an eccentric, lytic epiphyseal lesion in the proximal tibia extending directly to the subchondral bone without a sclerotic rim. Biopsy reveals mononuclear cells interspersed with multinucleated giant cells. Because the lesion is large and close to the joint surface, she is prescribed a targeted biologic agent to downstage the tumor prior to surgical curettage. What is the primary mechanism of action of this medication?

. Inhibition of vascular endothelial growth factor (VEGF)
. Monoclonal antibody against CD20
. Binding and neutralization of RANK-ligand (RANKL)
. Tyrosine kinase inhibition
. Activation of Wnt/beta-catenin signaling

Correct Answer & Explanation

. Binding and neutralization of RANK-ligand (RANKL)

Explanation

The clinical and radiographic presentation is classic for a giant cell tumor (GCT) of bone. Denosumab is a monoclonal antibody often used to treat large or unresectable GCTs. It works by binding to RANK-ligand (RANKL), which is secreted by the neoplastic mononuclear stromal cells. This prevents RANKL from binding to the RANK receptor on osteoclast precursors, thereby inhibiting their recruitment, differentiation, and bone-resorbing activity.

Question 8452

Topic: Biology, Genetics & Bone Healing
A 9-year-old girl is brought to the clinic due to a limb-length discrepancy and a noticeable limp. Radiographs demonstrate a 'shepherd's crook' deformity of her proximal femur with a well-defined medullary lesion showing a 'ground-glass' appearance. On examination, she has large café-au-lait macules with irregular, 'coast of Maine' borders. She also has a history of early onset menarche at age 7. The underlying cellular pathophysiology of her musculoskeletal condition involves a mutation resulting directly in:
. Overactivation of adenylate cyclase and increased intracellular cAMP
. Inactivation of the retinoblastoma (Rb) tumor suppressor gene
. Defective assembly of collagen type I triple helices
. Increased extracellular RANKL expression by osteoblasts
. Aberrant fibroblast growth factor receptor 3 (FGFR3) signaling

Correct Answer & Explanation

. Overactivation of adenylate cyclase and increased intracellular cAMP

Explanation

This patient has McCune-Albright syndrome, a triad of polyostotic fibrous dysplasia, café-au-lait spots (with irregular borders), and precocious puberty (or other endocrinopathies). It is caused by an activating post-zygotic somatic mutation in the GNAS gene, which encodes the alpha subunit of the stimulatory G protein (Gs-alpha). This mutation leads to constitutive activation of adenylate cyclase, resulting in high intracellular levels of cyclic AMP (cAMP). This aberrant signaling impairs the differentiation of bone-marrow stromal cells into mature osteoblasts, leading to the formation of structurally weak fibrous tissue and woven bone.

Question 8453

Topic: Biology, Genetics & Bone Healing

A 45-year-old male undergoes a posterolateral lumbar fusion using recombinant human bone morphogenetic protein-2 (rhBMP-2). Once rhBMP-2 binds to its target cell surface receptor, which of the following intracellular signaling molecules is primarily responsible for translocating to the nucleus to upregulate osteogenic gene expression?

. Beta-catenin
. SMAD 1/5/8
. RANKL
. NF-kappa B
. JAK/STAT

Correct Answer & Explanation

. SMAD 1/5/8

Explanation

BMPs act via serine/threonine kinase receptors that, upon activation, phosphorylate intracellular SMAD proteins (specifically SMAD 1, 5, and 8). These receptor-regulated SMADs form a complex with the co-SMAD (SMAD 4), translocate to the nucleus, and regulate the transcription of osteogenic genes such as Runx2. Beta-catenin is the primary intracellular mediator for the Wnt signaling pathway, while NF-kappa B is involved in RANK/RANKL signaling in osteoclasts.

Question 8454

Topic: Biomechanics & Biomaterials

A 65-year-old woman presents with groin pain 5 years after a total hip arthroplasty utilizing a cobalt-chromium modular head on a titanium alloy stem. Aspiration yields fluid with elevated cobalt levels but no bacterial growth. What is the predominant mechanism of corrosion at the modular head-neck junction?

. Mechanically assisted crevice corrosion
. Pure galvanic corrosion
. Pitting corrosion
. Intergranular corrosion
. Stress corrosion cracking

Correct Answer & Explanation

. Mechanically assisted crevice corrosion

Explanation

Trunnionosis at the modular head-neck junction is primarily driven by mechanically assisted crevice corrosion (MACC), also known as fretting corrosion. Micro-motion at the interface disrupts the passive oxide layer (fretting), and the confined space of the modular junction leads to oxygen depletion and a drop in pH (crevice corrosion), accelerating material loss. While mixing dissimilar metals theoretically risks pure galvanic corrosion, the mechanical disruption of the passivation layer in a confined crevice is the primary driver in modern modular orthopedic implants.

Question 8455

Topic: Biology, Genetics & Bone Healing

An infant presents with multiple fractures and is diagnosed with malignant infantile osteopetrosis. Radiographs reveal diffuse osteosclerosis and an 'erlenmeyer flask' deformity of the distal femurs. The pathogenesis of this specific severe autosomal recessive disorder is most commonly related to a mutation affecting which of the following?

. Type I collagen synthesis
. Osteoclast ruffled border proton pump (TCIRG1)
. Fibroblast growth factor receptor 3 (FGFR3)
. Runx2 transcription factor
. Osteoblast RANKL production

Correct Answer & Explanation

. Osteoclast ruffled border proton pump (TCIRG1)

Explanation

Osteopetrosis is characterized by defective osteoclast function leading to impaired bone resorption and overly dense but brittle bone. The most common mutation in malignant infantile (autosomal recessive) osteopetrosis affects the TCIRG1 gene, which encodes the a3 subunit of the vacuolar H+-ATPase pump essential for creating the acidic environment at the osteoclast ruffled border required for bone resorption. Type I collagen defects cause osteogenesis imperfecta. FGFR3 mutations cause achondroplasia. Runx2 mutations cause cleidocranial dysplasia.

Question 8456

Topic: Biomechanics & Biomaterials

Ligaments and tendons exhibit viscoelastic properties. During a continuous traction experiment on a cadaveric ACL, a constant load is applied over a prolonged period. The observation that the ligament continues to slowly elongate over time under this steady load is termed:

. Stress relaxation
. Creep
. Hysteresis
. Fatigue failure
. Isotropic strain

Correct Answer & Explanation

. Creep

Explanation

Creep is the progressive deformation (elongation) of a viscoelastic material when subjected to a constant load over time. Stress relaxation, conversely, is the decrease in internal stress over time when a material is held at a constant strain (constant length). Hysteresis represents the energy lost (as heat) during the loading and unloading cycles of a viscoelastic material.

Question 8457

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy is evaluated for severe bowing of the lower extremities. Laboratory results demonstrate low serum phosphorus, normal serum calcium, normal parathyroid hormone (PTH), and elevated alkaline phosphatase. Genetic testing reveals a mutation in the PHEX gene. Which of the following best describes the underlying pathophysiology?

. Decreased production of 1,25-dihydroxyvitamin D in the kidney due to 1-alpha-hydroxylase mutation
. Increased FGF-23 leading to renal phosphate wasting
. Defective calcium-sensing receptors in the parathyroid gland
. Nutritional vitamin D deficiency leading to secondary hyperparathyroidism
. Autoimmune destruction of osteoclasts causing impaired remodeling

Correct Answer & Explanation

. Increased FGF-23 leading to renal phosphate wasting

Explanation

The patient has X-linked hypophosphatemic (XLH) rickets, the most common heritable form of rickets. It is caused by a mutation in the PHEX gene. This leads to an overproduction or reduced degradation of Fibroblast Growth Factor 23 (FGF-23). High levels of circulating FGF-23 inhibit sodium-phosphate cotransporters (NaPi-IIa and NaPi-IIc) in the proximal renal tubule, causing severe renal phosphate wasting. FGF-23 also inhibits 1-alpha-hydroxylase, preventing the compensatory rise in 1,25-dihydroxyvitamin D.

Question 8458

Topic: 1. General Principles & Basic Science

Articular cartilage relies on its unique extracellular matrix to provide a nearly frictionless surface and distribute joint loads. Which of the following combinations best describes the composition and structural arrangement of articular cartilage in the superficial zone compared to the deep zone?

. Higher water content, collagen fibers parallel to the joint surface
. Lower water content, collagen fibers perpendicular to the joint surface
. Higher proteoglycan content, collagen fibers parallel to the joint surface
. Lower water content, random collagen fiber orientation
. Higher water content, collagen fibers perpendicular to the joint surface

Correct Answer & Explanation

. Higher water content, collagen fibers parallel to the joint surface

Explanation

Articular cartilage is divided into distinct zones. The superficial (tangential) zone has the highest water content (approx 80%) and lowest proteoglycan concentration. Its Type II collagen fibers are oriented parallel to the joint surface to resist shear forces. In contrast, the deep zone has the lowest water content (approx 65%), the highest proteoglycan concentration, and collagen fibers oriented perpendicular to the joint surface to resist compressive loads.

Question 8459

Topic: Infection, Pharmacology & VTE
A 70-year-old patient is scheduled for a total knee arthroplasty. The patient has a history of atrial fibrillation and takes rivaroxaban daily. What is the precise mechanism of action of this anticoagulant?
. Direct inhibition of thrombin (Factor IIa)
. Direct inhibition of Factor Xa
. Vitamin K antagonism via epoxide reductase inhibition
. Activation of antithrombin III
. Inhibition of ADP-induced platelet aggregation

Correct Answer & Explanation

. Direct inhibition of Factor Xa

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that exert their effect by directly binding to and inhibiting Factor Xa, blocking the conversion of prothrombin to thrombin. Dabigatran is a direct thrombin (Factor IIa) inhibitor. Warfarin is a vitamin K antagonist. Heparin and low molecular weight heparins (like enoxaparin) act by enhancing the activity of antithrombin III.

Question 8460

Topic: Infection, Pharmacology & VTE

Following total joint arthroplasty, periprosthetic joint infections are notoriously difficult to eradicate due to bacterial biofilm formation. Which of the following represents the critical first step in the formation of a biofilm on an orthopedic implant?

. Quorum sensing and colony dispersion
. Secretion of extracellular polymeric substance (EPS)
. Reversible bacterial adherence to a host conditioning film
. Maturation of water channels within the colony
. Irreversible adherence via species-specific adhesins

Correct Answer & Explanation

. Reversible bacterial adherence to a host conditioning film

Explanation

The formation of a biofilm on an implant occurs in a stepwise fashion: 1) Within minutes of implantation, a 'conditioning film' of host proteins (fibronectin, fibrinogen) coats the implant; 2) Planktonic bacteria undergo reversible adherence to this film via non-specific physical forces; 3) Adherence becomes irreversible via specific bacterial adhesins (e.g., MSCRAMMs); 4) Bacteria proliferate and secrete an extracellular polymeric substance (EPS) to form the mature biofilm matrix; 5) The biofilm matures and later uses quorum sensing to coordinate dispersion.

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