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Biology, Genetics & Bone Healing MCQs

Practice Set 160 of 212

This practice set contains high-yield board review questions covering key concepts in Biology, Genetics & Bone Healing. Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 3181

Topic: Biology, Genetics & Bone Healing

Teriparatide is administered for the treatment of severe osteoporosis in select patients. What is the precise mechanism of action by which this drug improves bone mineral density?

. Inhibition of osteoclast action via RANKL blockade
. Direct stimulation of osteoblast activity via PTH receptor agonism
. Inhibition of cathepsin K enzymatic activity
. Selective estrogen receptor modulation
. Binding to hydroxyapatite and inducing osteoclast apoptosis

Correct Answer & Explanation

. Direct stimulation of osteoblast activity via PTH receptor agonism

Explanation

Teriparatide is a recombinant fragment of parathyroid hormone (PTH 1-34). When administered intermittently, it possesses an anabolic effect by directly stimulating osteoblast activity and bone formation.

Question 3182

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory, what is the maximum tissue strain tolerated by lamellar bone formation during fracture healing?

. 2%
. 10%
. 15%
. 30%
. 100%

Correct Answer & Explanation

. 2%

Explanation

Perren's strain theory postulates that specific tissues tolerate specific maximum strain levels before failing. For lamellar bone to form, the strain must be less than 2%. This is the principle behind primary bone healing, which requires absolute stability. Woven bone can tolerate up to 10% strain, cartilage up to 30%, and granulation tissue can tolerate up to 100% strain.

Question 3183

Topic: Biology, Genetics & Bone Healing

A 52-year-old woman presents with diffuse bone pain and muscle weakness. Radiographs demonstrate bilateral symmetric radiolucent lines perpendicular to the cortex in her femoral necks. Laboratory evaluation reveals low serum calcium, low phosphorus, elevated alkaline phosphatase, and low 25-hydroxyvitamin D. What is the primary underlying defect in this condition?

. Failure of osteoclast-mediated bone resorption
. Defective mineralization of newly formed osteoid
. Abnormal cross-linking of type I collagen
. Gain-of-function mutation in the FGFR3 gene
. Excessive disorganized bone remodeling

Correct Answer & Explanation

. Defective mineralization of newly formed osteoid

Explanation

The clinical picture, lab profile, and radiographic findings (Looser zones/pseudofractures) are classic for osteomalacia. Osteomalacia is defined by defective mineralization of newly formed osteoid, most commonly due to severe vitamin D deficiency in adults.

Question 3184

Topic: Biology, Genetics & Bone Healing

Which of the following fixation constructs is most heavily reliant on cutting cone remodeling (primary bone healing) for the resolution of the fracture?

. Casting
. Intramedullary nailing
. Bridge plating
. Dynamic compression plating
. Circular external fixation

Correct Answer & Explanation

. Dynamic compression plating

Explanation

Primary (direct) bone healing occurs under conditions of absolute stability without callus formation and relies on cutting cones of osteoclasts followed by osteoblasts bridging the fracture gap. Dynamic compression plating provides the absolute stability required for primary bone healing. The other options provide relative stability and heal via secondary bone healing (callus formation).

Question 3185

Topic: Biology, Genetics & Bone Healing

Recombinant human bone morphogenetic protein-2 (rhBMP-2) is utilized in spine and trauma surgery for its osteoinductive properties. Once rhBMP-2 binds to its target cell surface receptor, which intracellular signaling pathway is primarily activated to upregulate osteoblast differentiation?

. Wnt/beta-catenin pathway
. Hedgehog (Ihh) pathway
. SMAD 1/5/8 pathway
. SMAD 2/3 pathway
. Notch signaling pathway

Correct Answer & Explanation

. SMAD 1/5/8 pathway

Explanation

BMPs belong to the TGF-beta superfamily. When osteoinductive BMPs (like BMP-2 and BMP-7) bind to their serine/threonine kinase receptors, they trigger the phosphorylation of receptor-regulated SMADs 1, 5, and 8. These complex with SMAD 4, enter the nucleus, and upregulate the transcription factor Runx2, driving osteoblast differentiation. SMAD 2/3 is associated with the TGF-beta/Activin pathway.

Question 3186

Topic: Biology, Genetics & Bone Healing

A 32-year-old female presents with knee pain. Radiographs reveal an eccentric, lytic, epiphyseal lesion of the distal femur without sclerotic margins. A biopsy confirms Giant Cell Tumor (GCT). If treated with Denosumab prior to surgical intervention, what is its primary mechanism of action?

. Inhibits RANKL, decreasing osteoclast-like giant cell activity
. Binds RANK receptor on stromal cells, inducing apoptosis
. Inhibits VEGF, decreasing tumor angiogenesis
. Cross-links DNA strands, preventing tumor cell replication
. Stimulates osteoprotegerin (OPG) production directly

Correct Answer & Explanation

. Inhibits RANKL, decreasing osteoclast-like giant cell activity

Explanation

Denosumab is a monoclonal antibody that binds to RANKL. In GCT, the neoplastic stromal cells overexpress RANKL, which recruits and activates the reactive, bone-resorbing multinucleated giant cells. By inhibiting RANKL, denosumab reduces the giant cell population, decreasing tumor-associated osteolysis and facilitating intralesional curettage.

Question 3187

Topic: Biology, Genetics & Bone Healing

During anterior cruciate ligament (ACL) reconstruction using a bone-patellar tendon-bone (BTB) autograft, the bone plugs heal to the host bone tunnels via which of the following primary biologic mechanisms?

. Creeping substitution
. Direct Sharpey fiber formation
. Endochondral ossification
. Membranous ossification
. Fibrocartilaginous zone formation

Correct Answer & Explanation

. Creeping substitution

Explanation

Bone-to-bone healing in a BTB graft occurs via creeping substitution, a process where osteoclasts resorb the graft bone and osteoblasts lay down new woven bone. This leads to earlier rigid fixation at approximately 6 weeks, compared to soft tissue grafts (e.g., hamstrings) which heal to the bone tunnel via a slower fibrovascular interface (Sharpey fibers) taking 8-12 weeks.

Question 3188

Topic: Biology, Genetics & Bone Healing

Following a diaphyseal femur fracture treated with a relatively stable but not absolutely rigid construct (e.g., intramedullary nail), secondary fracture healing occurs. Which phase of this healing process is fundamentally characterized by the conversion of woven bone to lamellar bone according to Wolff's law?

. Inflammatory phase
. Soft callus formation
. Hard callus formation
. Remodeling phase
. Hematoma organization

Correct Answer & Explanation

. Remodeling phase

Explanation

Secondary fracture healing consists of inflammation, soft callus (cartilage) formation, hard callus (woven bone) formation, and remodeling. The remodeling phase takes months to years and is driven by coupled osteoclast and osteoblast activity acting according to Wolff's law, where mechanically inferior woven bone is gradually replaced by mechanically robust lamellar bone along lines of stress.

Question 3189

Topic: Biology, Genetics & Bone Healing

A 55-year-old postmenopausal woman is evaluated for osteoporosis. She is started on denosumab. Which of the following best describes the primary mechanism of action of this medication?

. Direct inhibition of farnesyl pyrophosphate synthase
. Binding to RANK ligand (RANKL), preventing interaction with RANK
. Activation of the osteoblast Wnt signaling pathway
. Binding to parathyroid hormone receptors to stimulate bone formation
. Direct inhibition of cathepsin K

Correct Answer & Explanation

. Binding to RANK ligand (RANKL), preventing interaction with RANK

Explanation

Denosumab is a monoclonal antibody that targets RANKL, preventing it from binding to RANK on the surface of osteoclasts and their precursors. This inhibits osteoclastogenesis and decreases bone resorption. Bisphosphonates inhibit farnesyl pyrophosphate synthase. Romosozumab activates Wnt signaling.

Question 3190

Topic: Biology, Genetics & Bone Healing

A 72-year-old man presents with dull aching pain in his right thigh and an enlarging hat size. Radiographs of the femur show cortical thickening, coarse trabeculae, and a radiolucent V-shaped leading edge. Histologic evaluation of the affected bone would most likely demonstrate:

. A mosaic pattern of lamellar bone with prominent cement lines
. Sheets of uniform, small round blue cells
. A proliferation of woven bone lined by atypical osteoblasts
. Granulomas with caseous necrosis
. Extensive empty lacunae with necrotic marrow

Correct Answer & Explanation

. A mosaic pattern of lamellar bone with prominent cement lines

Explanation

The clinical presentation is classic for Paget's disease of bone. The V-shaped leading edge is the 'blade of grass' sign of the lytic phase. The hallmark histologic finding in the mixed phase of Paget's disease is a 'mosaic pattern' of lamellar bone with prominent, haphazard cement lines due to chaotic bone resorption and formation.

Question 3191

Topic: Biology, Genetics & Bone Healing

Articular cartilage has a highly specialized extracellular matrix. Which of the following components is primarily responsible for the tissue's ability to resist compressive forces?

. Type I collagen
. Type II collagen
. Proteoglycans (e.g., aggrecan)
. Hyaluronic acid
. Chondronectin

Correct Answer & Explanation

. Proteoglycans (e.g., aggrecan)

Explanation

The ability of articular cartilage to withstand compressive forces is primarily provided by proteoglycans (predominantly aggrecan), which are highly negatively charged and attract water, creating a swelling pressure (Donnan osmotic effect). The Type II collagen network provides tensile strength.

Question 3192

Topic: Biology, Genetics & Bone Healing

A 68-year-old man is prescribed alendronate for the prevention of osteoporotic fragility fractures. At the cellular level, this medication ultimately inhibits bone resorption by which of the following mechanisms?

. Binding to estrogen receptors on osteoclasts
. Inhibiting the ruffled border proton pump (V-ATPase)
. Disrupting the mevalonate pathway, leading to osteoclast apoptosis
. Increasing the secretion of osteoprotegerin (OPG) from osteoblasts
. Directly inhibiting matrix metalloproteinases

Correct Answer & Explanation

. Disrupting the mevalonate pathway, leading to osteoclast apoptosis

Explanation

Nitrogen-containing bisphosphonates (like alendronate) inhibit farnesyl pyrophosphate synthase, a key enzyme in the mevalonate pathway. This prevents the prenylation of small GTP-binding proteins (like Ras, Rho, and Rab) essential for osteoclast function and survival, ultimately leading to osteoclast apoptosis and decreased bone resorption.

Question 3193

Topic: Biology, Genetics & Bone Healing

A 14-year-old boy presents with multiple fractures after minor trauma, recurrent dental infections, and mild hearing loss. Radiographs show diffusely dense, 'marble-like' bones with absent medullary canals. Which of the following cellular mechanisms is primarily defective in this condition?

. Osteoblast differentiation and maturation
. Osteocyte apoptosis in cortical bone
. Osteoclast ruffled border formation
. Type I collagen triple-helix synthesis
. Extracellular mineralization of osteoid

Correct Answer & Explanation

. Osteoclast ruffled border formation

Explanation

The clinical and radiographic presentation is classic for osteopetrosis (marble bone disease). Osteopetrosis is caused by defective osteoclast function, most commonly a failure to form the ruffled border, leading to impaired bone resorption. Common genetic defects include mutations in carbonic anhydrase II (CAII) and TCIRG1.

Question 3194

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of fracture healing, lamellar bone can only form when the interfragmentary strain at the fracture site is below what percentage?

. 100%
. 50%
. 10%
. 2%
. 0%

Correct Answer & Explanation

. 2%

Explanation

Perren's strain theory dictates that tissues cannot form in an environment where the strain exceeds their tolerance. Granulation tissue tolerates up to 100% strain, fibrocartilage tolerates up to 10%, and true lamellar bone formation requires a stable environment with an interfragmentary strain of less than 2%.

Question 3195

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy is evaluated for short stature and significant bowing of the lower extremities. Laboratory studies reveal normal serum calcium, profoundly decreased serum phosphate, normal PTH, and normal 25-hydroxyvitamin D. However, 1,25-dihydroxyvitamin D is inappropriately normal. Which of the following is the most likely genetic defect?

. Mutation in the Vitamin D receptor (VDR)
. Mutation in the PHEX gene
. Mutation in the COL1A1 gene
. Mutation in the FGFR3 gene
. Mutation in the CAII gene

Correct Answer & Explanation

. Mutation in the PHEX gene

Explanation

The clinical and laboratory profile (hypophosphatemia, normal calcium, normal PTH, and inappropriately normal 1,25-dihydroxyvitamin D despite low phosphate) is classic for X-linked hypophosphatemic rickets. This is caused by an inactivating mutation in the PHEX gene, leading to excessive levels of FGF23, which suppresses renal phosphate reabsorption and 1-alpha-hydroxylase activity.

Question 3196

Topic: Biology, Genetics & Bone Healing

In mature cortical bone, the Haversian systems (osteons) run parallel to the long axis of the bone. Which of the following structures is responsible for connecting adjacent Haversian canals to facilitate vascular communication between osteons?

. Canaliculi
. Volkmann canals
. Howship lacunae
. Sharpey's fibers
. Cement lines

Correct Answer & Explanation

. Volkmann canals

Explanation

Volkmann canals are transverse or oblique channels in cortical bone that contain blood vessels and connect adjacent longitudinally running Haversian canals to each other, as well as to the periosteal and endosteal blood supplies.

Question 3197

Topic: Biology, Genetics & Bone Healing

A 68-year-old male presents with deep, aching bone pain in his right femur and an enlarging hat size. Laboratory tests show a significantly elevated serum alkaline phosphatase but normal calcium and phosphorus. A biopsy of the femur is most likely to demonstrate which of the following pathognomonic histological features?

. Irregular areas of unmineralized osteoid with an absence of calcified cartilage
. A mosaic pattern of lamellar bone with prominent, irregular cement lines
. Sheets of plasma cells with 'clock-face' nuclei
. Uniform sheets of small round blue cells with scant cytoplasm
. Non-caseating granulomas with multinucleated giant cells

Correct Answer & Explanation

. A mosaic pattern of lamellar bone with prominent, irregular cement lines

Explanation

The clinical scenario is classic for Paget's disease of bone (osteitis deformans). The pathognomonic histological finding in the mixed and late/sclerotic phases is a mosaic, 'jigsaw puzzle' pattern of lamellar bone with prominent, haphazard cement lines, reflecting the chaotic, rapid cycles of bone resorption and formation.

Question 3198

Topic: Biology, Genetics & Bone Healing
A 35-year-old man undergoes closed reduction and cast application for a closed tibial shaft fracture. Secondary bone healing is anticipated. During the cartilaginous callus phase, which collagen type is predominantly synthesized before being replaced by woven bone?
. Type I collagen
. Type II collagen
. Type III collagen
. Type IV collagen
. Type X collagen

Correct Answer & Explanation

. Type II collagen

Explanation

Secondary bone healing involves a soft cartilaginous callus phase characterized by the production of Type II collagen by chondrocytes. As the cartilage undergoes endochondral ossification, hypertrophic chondrocytes produce Type X collagen, followed by osteoblasts producing Type I collagen in the hard callus.

Question 3199

Topic: Biology, Genetics & Bone Healing

A 68-year-old man presents with increasing right thigh pain and bowing of his femur. Radiographs show cortical thickening and trabecular coarsening. Serum alkaline phosphatase is markedly elevated, but calcium and phosphorus are normal. If a bone biopsy were performed, what characteristic histological finding would be expected?

. Woven bone with empty lacunae
. Mosaic pattern of lamellar bone with prominent cement lines
. Sheets of plasma cells with clock-face nuclei
. Non-mineralized osteoid seams surrounding trabeculae
. Multinucleated giant cells in a background of mononuclear stromal cells

Correct Answer & Explanation

. Mosaic pattern of lamellar bone with prominent cement lines

Explanation

Paget's disease of bone is characterized by increased, disorganized bone remodeling. Histologically, it presents as a mosaic pattern of lamellar bone with prominent, haphazardly arranged cement lines due to irregular periods of osteoclastic resorption and osteoblastic formation.

Question 3200

Topic: Biology, Genetics & Bone Healing

A 72-year-old woman with a history of a vertebral fragility fracture is prescribed alendronate. At the cellular level, this medication inhibits bone resorption primarily by inhibiting which of the following enzymes?

. Cathepsin K
. Carbonic anhydrase II
. Farnesyl pyrophosphate synthase
. Tartrate-resistant acid phosphatase
. Matrix metalloproteinase-9

Correct Answer & Explanation

. Farnesyl pyrophosphate synthase

Explanation

Nitrogen-containing bisphosphonates, such as alendronate, inhibit farnesyl pyrophosphate synthase within the mevalonate pathway. This disruption prevents the prenylation of small GTPase proteins (like Ras and Rho), ultimately leading to osteoclast apoptosis and decreased bone resorption.

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