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Biology, Genetics & Bone Healing MCQs

Practice Set 125 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 2481

Topic: Biology, Genetics & Bone Healing

A 65-year-old osteoporotic woman is prescribed alendronate. By which of the following intracellular mechanisms does this nitrogen-containing bisphosphonate inhibit osteoclast-mediated bone resorption?

. Inhibition of the Wnt/beta-catenin signaling pathway
. Binding to RANKL and preventing its interaction with RANK
. Inhibition of farnesyl pyrophosphate synthase in the mevalonate pathway
. Direct inhibition of cathepsin K in the resorption pit
. Activation of the calcium-sensing receptor on osteoclasts

Correct Answer & Explanation

. Inhibition of farnesyl pyrophosphate synthase in the mevalonate pathway

Explanation

Nitrogen-containing bisphosphonates like alendronate inhibit farnesyl pyrophosphate synthase. This disrupts the mevalonate pathway, preventing prenylation of small GTPases essential for osteoclast ruffled border formation and survival.

Question 2482

Topic: Biology, Genetics & Bone Healing

A 30-year-old female presents with knee pain. Radiographs show an eccentric, lytic epiphyseal lesion in the distal femur. Biopsy reveals multinucleated giant cells interspersed among mononuclear cells. Which cell population in this lesion harbors the primary neoplastic defect and what factor do they secrete?

. Multinucleated giant cells; secrete RANKL
. Mononuclear stromal cells; secrete RANKL
. Multinucleated giant cells; secrete Osteoprotegerin
. Mononuclear stromal cells; secrete Osteoprotegerin
. Osteoblasts; secrete Cathepsin K

Correct Answer & Explanation

. Mononuclear stromal cells; secrete RANKL

Explanation

In Giant Cell Tumor of bone, the spindle-shaped mononuclear stromal cells are the true neoplastic cells. They secrete high levels of RANKL, which recruits and activates the reactive multinucleated osteoclast-like giant cells.

Question 2483

Topic: Biology, Genetics & Bone Healing

Teriparatide is utilized in the management of severe osteoporosis. What is the fundamental mechanism by which intermittent, rather than continuous, administration of this parathyroid hormone analog results in an anabolic effect on bone?

. Downregulation of Wnt/beta-catenin signaling in osteoblasts
. Increased apoptosis of osteoclast precursors
. Promotion of osteoblast survival and inhibition of osteoblast apoptosis
. Direct stimulation of intestinal calcium absorption
. Irreversible binding to the RANK receptor

Correct Answer & Explanation

. Promotion of osteoblast survival and inhibition of osteoblast apoptosis

Explanation

Intermittent PTH exposure is highly anabolic because it stimulates osteoblast differentiation, inhibits osteoblast apoptosis, and promotes Wnt signaling. Conversely, continuous PTH exposure favors osteoclastogenesis via increased RANKL expression.

Question 2484

Topic: Biology, Genetics & Bone Healing

A surgeon plans to use a structural cortical bone allograft to reconstruct an intercalary defect. At approximately 6 months postoperatively, what is the primary reason the allograft experiences a significant transient reduction in biomechanical strength?

. Exhaustion of the native bone morphogenetic proteins
. Invasion of macrophages causing diffuse necrosis
. Osteoclastic resorption creating porosity prior to new bone formation
. Premature degradation of the collagen matrix by matrix metalloproteinases
. Lack of osteoconductive scaffolding in the central core

Correct Answer & Explanation

. Osteoclastic resorption creating porosity prior to new bone formation

Explanation

Cortical bone grafts incorporate via creeping substitution, initially beginning with osteoclastic resorption along Haversian canals. This creates significant porosity and a temporary nadir in mechanical strength around 6 months before new osteoblastic bone formation restores it.

Question 2485

Topic: Biology, Genetics & Bone Healing

A 68-year-old man presents with back pain, anemia, and hypercalcemia. Skeletal survey reveals multiple "punched-out" lytic lesions. The extensive bone destruction in this condition is primarily mediated by myeloma cells secreting which of the following factors?

. Macrophage inflammatory protein-1 alpha (MIP-1a) and DKK1
. Bone morphogenetic protein-2 (BMP-2)
. Transforming growth factor-beta (TGF-b)
. Insulin-like growth factor-1 (IGF-1)
. Fibroblast growth factor-23 (FGF-23)

Correct Answer & Explanation

. Macrophage inflammatory protein-1 alpha (MIP-1a) and DKK1

Explanation

Multiple myeloma causes lytic lesions via an uncoupling of bone remodeling. Myeloma cells secrete MIP-1alpha (stimulating osteoclasts) and DKK1 (inhibiting osteoblasts by antagonizing Wnt signaling).

Question 2486

Topic: Biology, Genetics & Bone Healing

A 5-year-old boy is diagnosed with X-linked hypophosphatemic rickets. Which of the following pathophysiologic sequences best explains his condition?

. PHEX mutation -> Decreased FGF23 -> Increased renal phosphate resorption
. PHEX mutation -> Increased FGF23 -> Decreased renal phosphate resorption
. FGF23 mutation -> Increased 1-alpha-hydroxylase activity -> Hypercalcemia
. Vitamin D receptor mutation -> End-organ resistance -> Hypocalcemia
. CASR mutation -> Resetting of the calcium set-point -> Hypercalciuria

Correct Answer & Explanation

. PHEX mutation -> Increased FGF23 -> Decreased renal phosphate resorption

Explanation

X-linked hypophosphatemic rickets is caused by a PHEX mutation leading to elevated FGF23 levels. High FGF23 causes profound renal phosphate wasting and inappropriately downregulates 1-alpha-hydroxylase, preventing calcitriol synthesis.

Question 2487

Topic: Biology, Genetics & Bone Healing

A patient with profound dietary restrictions presents with perifollicular hemorrhages, gingival bleeding, and poor wound healing. The underlying vitamin deficiency directly impairs which of the following steps in collagen synthesis?

. Cleavage of procollagen C- and N-terminals
. Hydroxylation of proline and lysine residues
. Glycosylation of hydroxylysine residues
. Extracellular cross-linking by lysyl oxidase
. Transcription of COL1A1 mRNA

Correct Answer & Explanation

. Hydroxylation of proline and lysine residues

Explanation

Scurvy is caused by Vitamin C deficiency. Vitamin C acts as a necessary cofactor for prolyl hydroxylase and lysyl hydroxylase, which are essential for stabilizing the collagen triple helix.

Question 2488

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory, what type of bone healing occurs in a fracture environment with absolute stability (strain less than 2%) and no gap?

. Endochondral ossification with abundant callus
. Intramembranous ossification with peripheral callus
. Primary bone healing via osteoclastic cutting cones
. Chondrogenesis followed by appositional bone growth
. Fibrous nonunion due to lack of micromotion

Correct Answer & Explanation

. Primary bone healing via osteoclastic cutting cones

Explanation

Under conditions of absolute stability (strain < 2%) and anatomic reduction, primary bone healing occurs. This process is mediated by osteoclastic cutting cones crossing the fracture site, followed by osteoblastic bone deposition, without the formation of an intermediate fracture callus.

Question 2489

Topic: Biology, Genetics & Bone Healing

A 6-year-old child presents with multiple unexplained fractures and severe anemia. Radiographs display symmetrically thickened, hyperdense bones ('marble bone') with an absence of normal medullary cavities. The pathogenesis of this condition (osteopetrosis) most commonly involves a defect in Carbonic Anhydrase II. This primarily disrupts which cellular process?

. Osteoblast matrix production
. Osteoclast ruffled border formation and acidification
. Osteocyte mechanotransduction
. Chondrocyte hypertrophy in the physis
. Extracellular Type I collagen cross-linking

Correct Answer & Explanation

. Osteoclast ruffled border formation and acidification

Explanation

Osteopetrosis is caused by defective osteoclast function, specifically the inability to acidify Howship's lacuna at the ruffled border. Mutations in Carbonic Anhydrase II (CAII) or the TCIRG1 gene prevent the necessary generation and transport of protons (H+) to dissolve bone mineral, leading to dense, brittle bones and obliterated marrow spaces.

Question 2490

Topic: Biology, Genetics & Bone Healing

Articular cartilage is a highly specialized tissue divided into four distinct zones. Which zone is characterized by having the highest concentration of water, the lowest concentration of proteoglycans, and collagen fibers oriented parallel to the articular surface?

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

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

The superficial (tangential) zone comprises 10-20% of articular cartilage thickness. It has the highest water content (up to 80%), the lowest proteoglycan content, and densely packed collagen type II fibers aligned parallel to the joint surface to resist shear and tensile stresses.

Question 2491

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of fracture healing, which type of tissue can tolerate the highest amount of interfragmentary strain before failing, thereby acting as the necessary initial bridging tissue in secondary bone healing?

. Lamellar bone
. Woven bone
. Cartilage
. Granulation tissue
. Fibrous connective tissue

Correct Answer & Explanation

. Granulation tissue

Explanation

Granulation tissue can tolerate up to 100% strain before failing. Fibrous tissue tolerates ~17%, cartilage ~2-10%, and lamellar bone only 2%. Therefore, in the high-strain environment immediately following a fracture, only granulation tissue can form. As the callus builds, stiffness increases, strain decreases, and subsequent tissue types can form.

Question 2492

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with knee pain. Radiographs reveal an eccentric, lytic epiphyseal lesion extending into the subchondral bone of the distal femur. A biopsy confirms a giant cell tumor of bone. If medical therapy is indicated to shrink the tumor prior to intralesional curettage, which of the following mechanisms best describes the action of the most appropriate drug?

. Inhibition of the mTOR pathway
. Direct inhibition of osteoclast proton pumps
. Binding to the RANK ligand (RANKL) to prevent osteoclast activation
. Inhibition of vascular endothelial growth factor (VEGF)
. Competitive binding to estrogen receptors

Correct Answer & Explanation

. Binding to the RANK ligand (RANKL) to prevent osteoclast activation

Explanation

Denosumab is a monoclonal antibody used to treat giant cell tumors of bone by binding to RANKL, preventing the activation of RANK on osteoclasts and osteoclast-like giant cells. This halts bone resorption and allows the stroma to form woven bone, facilitating easier surgical curettage.

Question 2493

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) initiate osteoinductive signaling pathways. Upon BMP binding to its surface receptor, which intracellular signaling molecules are directly phosphorylated to translocate to the nucleus?

. Wnt/beta-catenin
. Smad 1, 5, and 8
. Receptor activator of NF-kB (RANK)
. Osteoprotegerin
. Mitogen-activated protein kinases (MAPK)

Correct Answer & Explanation

. Smad 1, 5, and 8

Explanation

BMPs bind to serine-threonine kinase receptors, which then phosphorylate receptor-regulated Smads (Smad 1, 5, and 8). These form a complex with co-Smad 4 and translocate to the nucleus to regulate target gene transcription for osteoblast differentiation.

Question 2494

Topic: Biology, Genetics & Bone Healing

Denosumab is increasingly used as a neoadjuvant treatment for large, marginally resectable Giant Cell Tumors (GCT) of bone. What is the specific mechanism of action of this medication?

. Inhibition of matrix metalloproteinases
. Direct induction of osteoblast apoptosis
. Monoclonal antibody binding to RANKL
. Inhibition of the Wnt/beta-catenin pathway
. Tyrosine kinase inhibition

Correct Answer & Explanation

. Monoclonal antibody binding to RANKL

Explanation

Denosumab is a fully human monoclonal antibody that binds directly to Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). This prevents RANKL from binding to RANK on osteoclasts and giant cells, halting osteoclast-mediated bone destruction.

Question 2495

Topic: Biology, Genetics & Bone Healing

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

. < 2%
. 2-10%
. 10-30%
. 30-100%
. > 100%

Correct Answer & Explanation

. < 2%

Explanation

Perren's strain theory dictates that specific tissues tolerate different maximum strains before rupturing and failing to bridge a fracture gap. Lamellar bone is highly rigid and tolerates <2% strain. Woven bone tolerates up to 10%, cartilage up to 30%, and granulation tissue can tolerate up to 100% strain.

Question 2496

Topic: Biology, Genetics & Bone Healing

A 55-year-old woman with a history of gastric bypass surgery presents with bilateral groin pain. Radiographs reveal bilateral Looser zones in the femoral necks. Laboratory evaluation for her underlying metabolic bone disease will most likely show which of the following profiles?

. High calcium, low phosphorus, normal alkaline phosphatase
. Low calcium, low phosphorus, high alkaline phosphatase
. Normal calcium, normal phosphorus, high alkaline phosphatase
. Low calcium, high phosphorus, low alkaline phosphatase
. Normal calcium, normal phosphorus, normal alkaline phosphatase

Correct Answer & Explanation

. Low calcium, low phosphorus, high alkaline phosphatase

Explanation

The patient has osteomalacia secondary to vitamin D malabsorption post-gastric bypass (Looser zones/pseudofractures are pathognomonic). Malabsorption leads to low calcium, triggering secondary hyperparathyroidism. Elevated PTH increases renal phosphate excretion (lowering phosphorus) while attempting to normalize calcium. High alkaline phosphatase reflects increased osteoblast activity attempting to mineralize excess osteoid.

Question 2497

Topic: Biology, Genetics & Bone Healing

A 65-year-old male presents with deep bone pain, increasing hat size, and bowing of the tibiae. Laboratory evaluation shows a markedly elevated alkaline phosphatase with normal serum calcium and phosphorus. Which cellular defect drives the primary initial phase of this disease process?

. Defective mineralization of osteoid by osteoblasts due to vitamin D deficiency.
. Increased, disorganized osteoclast activity often linked to SQSTM1 mutations.
. Malignant proliferation of plasma cells stimulating osteoclast-activating factors.
. Overproduction of parathyroid hormone leading to cortical bone resorption.
. Deficient Type I collagen synthesis leading to brittle woven bone.

Correct Answer & Explanation

. Increased, disorganized osteoclast activity often linked to SQSTM1 mutations.

Explanation

The patient has Paget's disease of bone. The primary defect in Paget's disease is intense, localized, and disorganized osteoclastic bone resorption (the lytic phase), often associated with mutations in the SQSTM1 gene (encoding p62). This is followed by a compensatory but disorganized osteoblastic response (blastic/sclerotic phase).

Question 2498

Topic: Biology, Genetics & Bone Healing

Demineralized bone matrix (DBM) is commonly used as a bone graft substitute in orthopedic surgery. Which of the following best describes its biological properties?

. Osteogenic and osteoinductive
. Osteoconductive and osteogenic
. Osteoinductive and osteoconductive
. Osteoconductive only
. Osteogenic only

Correct Answer & Explanation

. Osteoinductive and osteoconductive

Explanation

Demineralized bone matrix (DBM) contains bone morphogenetic proteins (BMPs) exposed during the demineralization process, providing osteoinductive properties. Its residual collagenous matrix acts as a scaffold, providing osteoconductive properties. Because it lacks live cells, it is not osteogenic.

Question 2499

Topic: Biology, Genetics & Bone Healing

A 2-year-old child presents with progressive bowing of the lower extremities. Laboratory findings show normal serum calcium, markedly decreased serum phosphate, and elevated alkaline phosphatase. Genetic testing reveals a mutation in the PHEX gene. What is the most appropriate medical treatment?

. High-dose Vitamin D3 (Cholecalciferol)
. Oral calcium and Vitamin D supplementation
. Oral phosphate and calcitriol (1,25-dihydroxyvitamin D)
. Intravenous Bisphosphonates
. Subcutaneous Calcitonin

Correct Answer & Explanation

. Oral phosphate and calcitriol (1,25-dihydroxyvitamin D)

Explanation

The clinical and laboratory presentation is consistent with X-linked hypophosphatemic rickets (Vitamin D-resistant rickets). It is caused by a PHEX mutation leading to excess FGF23, which causes renal phosphate wasting. Standard treatment consists of oral phosphate supplementation and active vitamin D (calcitriol).

Question 2500

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of bone healing, what is the maximum strain environment that allows for the formation of lamellar bone across a fracture gap?

. Less than 2%
. 2% to 10%
. 10% to 30%
. 30% to 50%
. Greater than 50%

Correct Answer & Explanation

. Less than 2%

Explanation

Perren's strain theory posits that specific tissues tolerate different amounts of mechanical strain before failure. Granulation tissue tolerates up to 100% strain, fibrous tissue and cartilage up to 10-20%, woven bone up to 10%, but lamellar bone can only form in a rigid, low-strain environment of less than 2%.

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