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Biology, Genetics & Bone Healing MCQs

Practice Set 82 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 1621

Topic: Biology, Genetics & Bone Healing

During embryonic development of the upper extremity, a genetic mutation results in dysfunction of the Zone of Polarizing Activity (ZPA). Which of the following developmental abnormalities is most likely to be observed in the developing fetus?

. Proximal-distal limb truncation (e.g., amelia)
. Anteroposterior axis defects (e.g., polydactyly)
. Dorsal-ventral axis defects (e.g., absent nails)
. Failure of normal joint cavitation
. Defective intramembranous ossification of the clavicle

Correct Answer & Explanation

. Proximal-distal limb truncation (e.g., amelia)

Explanation

The Zone of Polarizing Activity (ZPA) is a cluster of cells located at the posterior margin of the developing limb bud. It regulates the anteroposterior (radioulnar) patterning of the limb primarily via the expression of Sonic Hedgehog (SHH). Mutations affecting the ZPA or the SHH pathway typically result in radioulnar defects, such as preaxial or postaxial polydactyly, or mirror-image digits. Proximal-distal growth is regulated by the Apical Ectodermal Ridge (AER) via Fibroblast Growth Factors (FGFs), and dorsal-ventral patterning is regulated by the dorsal ectoderm via Wnt-7a.

Question 1622

Topic: Biology, Genetics & Bone Healing

A 55-year-old patient with end-stage renal disease presents with diffuse bone pain and an increased risk of fragility fractures. Laboratory tests reveal hypocalcemia, hyperphosphatemia, and significantly elevated parathyroid hormone (PTH) levels. Which of the following enzymes is primarily deficient in this patient, driving the pathogenesis of their metabolic bone disease?

. 25-hydroxylase
. 1-alpha-hydroxylase
. 24,25-hydroxylase
. Alkaline phosphatase
. Cathepsin K

Correct Answer & Explanation

. 25-hydroxylase

Explanation

In chronic kidney disease, the loss of functional renal parenchyma leads to a deficiency in 1-alpha-hydroxylase. This enzyme is responsible for converting 25-hydroxyvitamin D (calcidiol) into 1,25-dihydroxyvitamin D (calcitriol), the physiologically active form of vitamin D. The resulting deficiency of calcitriol impairs intestinal calcium absorption, leading to hypocalcemia and a compensatory, often severe, secondary hyperparathyroidism (renal osteodystrophy).

Question 1623

Topic: Biology, Genetics & Bone Healing

During a posterolateral lumbar fusion, a surgeon uses a commercially prepared demineralized bone matrix (DBM) to supplement local autograft. Based on the fundamental properties of bone grafting materials, which of the following biological properties does DBM inherently provide?

. Osteoconduction and osteogenesis
. Osteoinduction and osteogenesis
. Osteoconduction and osteoinduction
. Osteogenesis only
. Osteoinduction only

Correct Answer & Explanation

. Osteoconduction and osteogenesis

Explanation

Demineralized bone matrix (DBM) is an allograft product prepared by the acid extraction of the mineralized phase of bone, leaving behind the structural collagen matrix and noncollagenous proteins. The remaining matrix provides a physical scaffold for new bone growth (osteoconduction), while retained growth factors, primarily bone morphogenetic proteins (BMPs), stimulate host mesenchymal stem cells to differentiate into osteoblasts (osteoinduction). Because DBM is processed and sterilized, it contains no viable cells and thus possesses no intrinsic osteogenic potential.

Question 1624

Topic: Biology, Genetics & Bone Healing

A 35-year-old male undergoes open reduction and internal fixation of a diaphyseal femur fracture. The surgeon utilizes an absolute stability construct achieved via compression plating. Which of the following best describes the primary mode of bone healing expected and the corresponding mechanical strain environment at the fracture site?

. Endochondral ossification, strain < 2%
. Endochondral ossification, strain between 2% and 10%
. Intramembranous ossification, strain < 2%
. Primary (Haversian) bone healing, strain < 2%
. Primary (Haversian) bone healing, strain between 2% and 10%

Correct Answer & Explanation

. Endochondral ossification, strain < 2%

Explanation

Absolute stability constructs, such as compression plating, aim to achieve a strain environment of less than 2% across the fracture gap. Under these highly rigid conditions, secondary (callus) formation is suppressed, and the bone heals via primary (Haversian) bone healing. This process involves osteoclasts creating cutting cones across the fracture site, directly followed by osteoblasts laying down new lamellar bone. Strain environments between 2% and 10% (relative stability) promote secondary bone healing via endochondral ossification.

Question 1625

Topic: Biology, Genetics & Bone Healing

A 72-year-old woman is prescribed teriparatide for the treatment of severe osteoporosis complicated by multiple vertebral compression fractures. The primary cellular mechanism by which teriparatide increases total bone mass is by:

. Inhibiting the RANKL/RANK interaction, thereby preventing osteoclast activation
. Directly stimulating osteoblast differentiation and lifespan via intermittent PTH receptor activation
. Inhibiting the enzyme farnesyl pyrophosphate synthase in the mevalonate pathway
. Binding firmly to hydroxyapatite crystals and inducing osteoclast apoptosis
. Acting as a selective estrogen receptor modulator to decrease osteoclast-mediated bone resorption

Correct Answer & Explanation

. Inhibiting the RANKL/RANK interaction, thereby preventing osteoclast activation

Explanation

Teriparatide is a recombinant human parathyroid hormone (PTH 1-34) analog. When administered intermittently (e.g., as a daily subcutaneous injection), it has an anabolic effect on bone by stimulating osteoblast differentiation, activity, and lifespan, leading to a net increase in bone formation. Denosumab inhibits RANKL (Option A). Bisphosphonates inhibit farnesyl pyrophosphate synthase and promote osteoclast apoptosis (Options C and D). Raloxifene is a selective estrogen receptor modulator (Option E).

Question 1626

Topic: Biology, Genetics & Bone Healing

A surgeon elects to use a locking compression plate (LCP) for the fixation of a highly comminuted metaphyseal fracture in osteoporotic bone. Compared to a conventional non-locking plate, which of the following biomechanical principles represents the primary advantage of the locking plate construct in this scenario?

. It relies heavily on friction between the plate and the underlying cortex to maintain fracture reduction.
. It functions as a fixed-angle, single-beam construct that does not require compression against the bone.
. It requires strict bicortical screw purchase in every hole to achieve adequate angular stability.
. It relies entirely on the pull-out strength of individual screws rather than the overall construct stability.
. It induces primary (Haversian) bone healing by achieving absolute stability across the comminuted gap.

Correct Answer & Explanation

. It relies heavily on friction between the plate and the underlying cortex to maintain fracture reduction.

Explanation

Locking plates act as fixed-angle devices where the screw heads thread directly into the plate, creating a single-beam construct. This design provides stability independent of the friction between the plate and the bone, preserving periosteal blood supply. Because the screws are fixed to the plate, the construct relies on the aggregate pull-out strength rather than individual screw purchase, making it ideal for osteoporotic bone. Bridging a comminuted fracture with a locking plate typically provides relative stability, leading to secondary (callus) bone healing, not primary healing.

Question 1627

Topic: Biology, Genetics & Bone Healing

A 45-year-old man undergoes an open reduction and internal fixation of a diaphyseal radial fracture using dynamic compression plating. The goal is to achieve primary (direct) bone healing. Which of the following conditions is most critical for achieving this type of healing without callous formation?

. Interfragmentary strain between 2% and 10%
. Gap size of less than 1 mm and absolute stability
. Presence of intramembranous ossification mediators exclusively
. Micromotion at the fracture site to stimulate osteoblast activity
. Preservation of the fracture hematoma

Correct Answer & Explanation

. Interfragmentary strain between 2% and 10%

Explanation

Primary (direct) bone healing occurs without callus formation and requires absolute stability (interfragmentary strain < 2%) and direct cortical contact or a gap of less than 1 mm. This allows osteoclasts to create cutting cones across the fracture line, followed immediately by osteoblasts depositing lamellar bone. Interfragmentary strain between 2% and 10% or micromotion leads to secondary bone healing via endochondral ossification and callus formation.

Question 1628

Topic: Biology, Genetics & Bone Healing

Recombinant human bone morphogenetic proteins (rhBMPs) are commonly used in orthopedic surgery to promote osteoinduction. Which of the following bone morphogenetic proteins is FDA-approved as an adjunct for use in acute, open tibial shaft fractures treated with an intramedullary nail?

. BMP-2
. BMP-3
. BMP-4
. BMP-7
. BMP-9

Correct Answer & Explanation

. BMP-2

Explanation

Bone morphogenetic proteins (BMPs) are members of the TGF-beta superfamily that promote osteoinduction by stimulating the differentiation of mesenchymal stem cells into osteoblasts. rhBMP-2 is FDA-approved for use in acute, open tibial shaft fractures treated with an intramedullary nail, as well as for anterior lumbar interbody fusion (ALIF). BMP-7 (OP-1) was previously approved under a humanitarian device exemption for recalcitrant long-bone nonunions but has had limited application. BMP-3 acts as an antagonist to osteogenesis.

Question 1629

Topic: Biology, Genetics & Bone Healing

A 3-year-old boy presents with a femur fracture following minimal trauma. This is his fourth long bone fracture. Clinical examination reveals blue sclerae and dentinogenesis imperfecta. Genetic testing confirms a mutation in the COL1A1 gene. He is started on intravenous pamidronate. What is the primary mechanism of action of this pharmacological therapy?

. Stimulation of osteoblast proliferation and differentiation
. Inhibition of osteoclast-mediated bone resorption
. Supplementation of endogenous type I collagen synthesis
. Activation of the calcium-sensing receptor in the parathyroid gland
. Inhibition of sclerostin to promote uninhibited bone formation

Correct Answer & Explanation

. Inhibition of osteoclast-mediated bone resorption

Explanation

The patient has Osteogenesis Imperfecta (OI), typically caused by an autosomal dominant mutation in type I collagen genes (COL1A1 or COL1A2). Intravenous bisphosphonates, such as pamidronate, are the standard of care for moderate to severe OI to reduce fracture frequency and improve bone density. Bisphosphonates function primarily by attaching to hydroxyapatite binding sites on bony surfaces, leading to the inhibition of osteoclast-mediated bone resorption. They do not repair the underlying genetic defect in collagen synthesis.

Question 1630

Topic: Biology, Genetics & Bone Healing

A 3-year-old girl with recurrent long bone fractures, blue sclerae, and dentinogenesis imperfecta is diagnosed with osteogenesis imperfecta. She is started on a medical therapy that aims to increase bone mineral density and reduce the fracture rate. What is the primary mechanism of action of this medication class?

. Inhibition of osteoclast-mediated bone resorption
. Stimulation of osteoblast proliferation
. Enhanced calcium absorption in the gastrointestinal tract
. Cross-linking of type I collagen
. Inhibition of parathyroid hormone secretion

Correct Answer & Explanation

. Inhibition of osteoclast-mediated bone resorption

Explanation

The patient has osteogenesis imperfecta and is being treated with bisphosphonates (e.g., pamidronate or zoledronic acid). Bisphosphonates inhibit osteoclast-mediated bone resorption by inducing osteoclast apoptosis, which leads to increased bone mineral density, a reduced rate of fractures, and improvement in vertebral body shape in children with OI.

Question 1631

Topic: Biology, Genetics & Bone Healing

A 3-year-old child with a known diagnosis of Osteogenesis Imperfecta Type III is admitted to the hospital for elective placement of telescopic intramedullary rods in bilateral femurs. To decrease fracture burden and improve bone mineral density, the patient receives cyclical intravenous pamidronate therapy. What is the primary cellular mechanism of action of this pharmacological treatment?

. Stimulation of osteoblast-mediated bone formation
. Inhibition of osteoclast-mediated bone resorption
. Direct cross-linking of type 1 collagen fibrils in the extracellular matrix
. Inhibition of parathyroid hormone (PTH) secretion
. Enhancement of intestinal calcium and phosphate absorption

Correct Answer & Explanation

. Inhibition of osteoclast-mediated bone resorption

Explanation

Pamidronate is a nitrogen-containing bisphosphonate. The primary mechanism of action of all bisphosphonates is the inhibition of osteoclast-mediated bone resorption. Specifically, nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate synthase in the mevalonate pathway, leading to osteoclast apoptosis and decreased bone turnover. They do not primarily stimulate osteoblasts, alter collagen cross-linking (the primary structural defect in OI), or significantly affect PTH secretion or direct intestinal absorption.

Question 1632

Topic: Biology, Genetics & Bone Healing

A 5-year-old boy with a history of multiple low-energy fractures, blue sclerae, and dentinogenesis imperfecta is being evaluated for surgical intervention of a severely bowed femur. He receives intravenous bisphosphonate therapy every 3 months. What is the primary mechanism of action of bisphosphonates in this condition?

. Stimulation of osteoblast proliferation
. Inhibition of osteoclast-mediated bone resorption
. Direct enhancement of type I collagen synthesis
. Increased intestinal calcium absorption
. Promotion of chondrocyte hypertrophy

Correct Answer & Explanation

. Inhibition of osteoclast-mediated bone resorption

Explanation

Osteogenesis imperfecta is most commonly caused by a genetic defect in type I collagen synthesis. Bisphosphonates are a mainstay of medical management to increase bone mineral density and reduce fracture rates. They function primarily by binding to hydroxyapatite crystals in bone and inhibiting osteoclast-mediated bone resorption, which leads to a relative increase in bone mass, despite the collagen matrix remaining abnormal.

Question 1633

Topic: Biology, Genetics & Bone Healing

A 2-year-old child with a known diagnosis of neurofibromatosis type 1 (NF1) presents with anterolateral bowing of the tibia that has progressed to a complete fracture. Radiographs confirm congenital pseudarthrosis of the tibia (CPT). Which of the following biological adjuncts is most strongly supported in the surgical management of this condition to improve bone union rates?

. Systemic bisphosphonates
. Recombinant human bone morphogenetic protein-2 (rhBMP-2)
. Teriparatide (PTH 1-34)
. Non-steroidal anti-inflammatory drugs (NSAIDs)
. Vitamin C supplementation

Correct Answer & Explanation

. Recombinant human bone morphogenetic protein-2 (rhBMP-2)

Explanation

Congenital pseudarthrosis of the tibia (CPT) is extremely challenging to treat, with high rates of nonunion and refracture. Surgical management typically involves resection of the pseudarthrosis, rigid stabilization (often combining intramedullary fixation with an Ilizarov external fixator), and extensive bone grafting. The use of recombinant human bone morphogenetic protein-2 (rhBMP-2) as an adjunct has been shown in multiple studies to significantly increase union rates and decrease the time to union in children with CPT.

Question 1634

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy presents with a diaphyseal femur fracture after a minor fall. He has a history of multiple fractures, blue sclerae, and dentinogenesis imperfecta. This condition is most commonly caused by a mutation in genes coding for which of the following proteins?

. Fibroblast growth factor receptor 3 (FGFR3)
. Cartilage oligomeric matrix protein (COMP)
. Type I collagen
. Type II collagen
. Runx2

Correct Answer & Explanation

. Type I collagen

Explanation

Osteogenesis imperfecta is most commonly an autosomal dominant disorder caused by mutations in the COL1A1 or COL1A2 genes, which encode Type I collagen. This defect leads to brittle bones, blue sclerae, and associated dental anomalies.

Question 1635

Topic: Biology, Genetics & Bone Healing

A 6-year-old boy with blue sclerae, dentinogenesis imperfecta, and a history of multiple low-energy fractures is being treated with intravenous pamidronate. What is the primary cellular mechanism of action of this medication in managing his condition?

. Stimulation of osteoblast differentiation and bone formation
. Inhibition of osteoclast-mediated bone resorption
. Direct cross-linking of defective type 1 collagen fibers
. Increased intestinal calcium absorption via vitamin D pathways
. Stimulation of physeal chondrocyte proliferation

Correct Answer & Explanation

. Inhibition of osteoclast-mediated bone resorption

Explanation

Pamidronate is a bisphosphonate used as the gold-standard medical therapy for osteogenesis imperfecta. It functions by inhibiting osteoclast-mediated bone resorption, thereby increasing bone mineral density and reducing fracture frequency.

Question 1636

Topic: Biology, Genetics & Bone Healing

Periprosthetic osteolysis is the leading cause of late aseptic loosening following total joint arthroplasty, primarily driven by a biologic response to particulate wear debris. Following the phagocytosis of wear particles by macrophages, which of the following receptor-ligand interactions represents the terminal obligate pathway for the differentiation and activation of osteoclasts?

. IL-1 binding to the IL-1 receptor (IL-1R)
. TNF-alpha binding to the TNF receptor 1 (TNFR1)
. RANKL binding to RANK
. Osteoprotegerin (OPG) binding to RANKL
. BMP-2 binding to the BMP receptor (BMPR)

Correct Answer & Explanation

. RANKL binding to RANK

Explanation

The biologic cascade of periprosthetic osteolysis begins when macrophages phagocytose particulate wear debris (most commonly polyethylene). The activated macrophages release pro-inflammatory cytokines such as TNF-alpha, IL-1, and IL-6. These cytokines stimulate osteoblasts and other cells to upregulate Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). The terminal and obligate step for osteoclastogenesis and subsequent bone resorption is the binding of RANKL to its receptor, RANK, which is located on the surface of osteoclast precursors. Osteoprotegerin (OPG) acts as a decoy receptor that binds to RANKL, inhibiting this process.

Question 1637

Topic: Biology, Genetics & Bone Healing

Romosozumab is a monoclonal antibody utilized in the treatment of severe osteoporosis. What is the precise molecular target and subsequent downstream effect of this medication?

. Binds to RANKL, inhibiting osteoclast maturation
. Binds to Sclerostin, allowing Wnt to bind to LRP5/6 and upregulating bone formation
. Acts as an agonist at the parathyroid hormone receptor, stimulating osteoblast activity
. Inhibits Cathepsin K, preventing the degradation of type I collagen
. Binds directly to osteoprotegerin (OPG), preventing RANKL-RANK interaction

Correct Answer & Explanation

. Binds to Sclerostin, allowing Wnt to bind to LRP5/6 and upregulating bone formation

Explanation

Romosozumab is a humanized monoclonal antibody that targets and inhibits sclerostin. Sclerostin is naturally secreted by osteocytes and normally inhibits the Wnt/beta-catenin pathway by binding to LRP5/6 receptors on osteoblasts. By inhibiting sclerostin, Romosozumab disinhibits this pathway, leading to a dual effect: substantially increased osteoblast activity (anabolic) and decreased osteoclast activity (anti-resorptive), thereby increasing bone mineral density.

Question 1638

Topic: Biology, Genetics & Bone Healing

Romosozumab is a monoclonal antibody utilized in the treatment of severe osteoporosis to decrease the risk of fractures. What is its exact mechanism of action at the molecular level?

. Binds and neutralizes receptor activator of nuclear factor kappa-B ligand (RANKL)
. Inhibits Cathepsin K, preventing the degradation of bone matrix
. Binds to and inhibits sclerostin, unblocking the Wnt/beta-catenin signaling pathway
. Directly stimulates the parathyroid hormone (PTH) receptor
. Inhibits farnesyl pyrophosphate synthase within osteoclasts

Correct Answer & Explanation

. Binds to and inhibits sclerostin, unblocking the Wnt/beta-catenin signaling pathway

Explanation

Romosozumab is a monoclonal antibody that targets and inhibits sclerostin. Sclerostin is a glycoprotein secreted by osteocytes that normally downregulates bone formation by inhibiting the Wnt/beta-catenin signaling pathway in osteoblasts. By inhibiting sclerostin, romosozumab exerts a dual effect: it significantly promotes bone formation and also mildly decreases bone resorption, making it a highly effective anabolic agent for severe osteoporosis.

Question 1639

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) play a crucial role in fracture healing by inducing the differentiation of mesenchymal stem cells into osteoblasts. BMPs primarily exert their intracellular effects through which of the following signaling pathways?

. Wnt/beta-catenin pathway
. JAK/STAT pathway
. Smad pathway
. Notch signaling pathway
. MAP kinase pathway

Correct Answer & Explanation

. Smad pathway

Explanation

BMPs are members of the Transforming Growth Factor-beta (TGF-beta) superfamily. Upon binding to their specific serine/threonine kinase cell surface receptors, they trigger the phosphorylation of intracellular receptor-regulated Smad proteins (specifically Smad1, 5, and 8). These activated Smads form a complex with the common-mediator Smad4, which then translocates into the nucleus to regulate the transcription of osteogenic target genes, notably Runx2.

Question 1640

Topic: Biology, Genetics & Bone Healing

Which of the following best describes the mechanism of action of teriparatide in the treatment of osteoporosis?

. Binds to the RANK receptor on osteoclasts to inhibit their activation
. Inhibits the enzyme farnesyl pyrophosphate synthase in the mevalonate pathway
. Acts as a selective estrogen receptor modulator (SERM) with agonistic effects on bone
. Continuously stimulates PTH receptors leading to decreased osteoblast apoptosis
. Intermittently stimulates PTH receptors leading to preferential activation of osteoblasts over osteoclasts

Correct Answer & Explanation

. Intermittently stimulates PTH receptors leading to preferential activation of osteoblasts over osteoclasts

Explanation

Teriparatide is an anabolic agent and a recombinant human parathyroid hormone (PTH 1-34) analog. While continuous endogenous PTH elevation leads to bone resorption via osteoclast activation, intermittent (daily subcutaneous) administration of teriparatide preferentially stimulates osteoblastic bone formation, increasing bone mineral density and reducing fracture risk.

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