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Biology, Genetics & Bone Healing MCQs

Practice Set 40 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 781

Topic: Biology, Genetics & Bone Healing

A 65-year-old woman with osteoporosis presents with acute onset of severe back pain after bending over. Plain radiographs reveal a vertebral compression fracture at L1. Neurological examination is unremarkable. Which of the following is the most appropriate initial management?

. Immediate surgical stabilization with instrumentation.
. Vertebroplasty or kyphoplasty.
. Brace immobilization, analgesia, and osteoporosis management.
. Bed rest for 2 weeks followed by physical therapy.
. MRI to rule out spinal cord compression.

Correct Answer & Explanation

. Brace immobilization, analgesia, and osteoporosis management.

Explanation

For an osteoporotic vertebral compression fracture without neurological deficit, the initial management is typically non-operative. This includes pain management with analgesia, bracing (e.g., thoracolumbosacral orthosis - TLSO) for comfort and stability, and aggressive management of osteoporosis to prevent future fractures. Vertebroplasty or kyphoplasty can be considered if pain remains intractable despite conservative measures, but are not always first-line. Surgical stabilization is reserved for unstable fractures or those with neurological compromise. Bed rest for prolonged periods is generally discouraged due to deconditioning. An MRI is important if there's suspicion of neurological involvement or malignancy, but if the neurological exam is unremarkable, it might not be immediately emergent after initial radiographs confirm the fracture.

Question 782

Topic: Biology, Genetics & Bone Healing

A 65-year-old male presents with a non-union of his tibial shaft fracture after 9 months. Investigations reveal normal calcium and PTH levels. Which of the following cellular mechanisms is most critically impaired in cases of delayed union or non-union of a long bone fracture?

. Osteoclast apoptosis
. Endochondral ossification
. Intramembranous ossification
. Fibroblast proliferation
. Chondrocyte hypertrophy

Correct Answer & Explanation

. Endochondral ossification

Explanation

Long bone fracture healing primarily occurs via endochondral ossification, where a cartilaginous callus forms and is subsequently replaced by woven bone, then remodeled into lamellar bone. Intramembranous ossification is more characteristic of flat bone healing or direct bone formation without a cartilage intermediate, but its impairment is less likely to be theprimarycritical issue in long bone non-union than a failure of the endochondral pathway. Osteoclast apoptosis is important for remodeling but not the initial bone formation. Fibroblast proliferation contributes to the initial hematoma and fibrous tissue, but effective healing requires subsequent cartilage and bone formation. Chondrocyte hypertrophy is a key step in endochondral ossification, and failure of this process would certainly contribute, but endochondral ossification encompasses the entire pathway including cartilage formation, hypertrophy, mineralization, and bone replacement.

Question 783

Topic: Biology, Genetics & Bone Healing

Parathyroid hormone (PTH) plays a crucial role in calcium homeostasis. At the skeletal level, prolonged elevation of PTH primarily stimulates which of the following?

. Osteoblast differentiation
. Bone matrix mineralization
. Osteocyte apoptosis
. Osteoclastic bone resorption
. Collagen synthesis by osteoblasts

Correct Answer & Explanation

. Osteoclastic bone resorption

Explanation

Parathyroid hormone (PTH) has a complex effect on bone. Intermittent, low-dose administration of PTH (e.g., teriparatide) is anabolic, stimulating osteoblast activity and bone formation. However, sustained or chronically elevated levels of PTH, as seen in conditions like primary hyperparathyroidism, primarily promote osteoclastic bone resorption. PTH acts indirectly on osteoclasts by binding to receptors on osteoblasts, which then produce RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand) and inhibit osteoprotegerin (OPG). This shifts the balance towards increased osteoclast differentiation, activation, and survival, leading to enhanced bone resorption and calcium release from bone.

Question 784

Topic: Biology, Genetics & Bone Healing

Mesenchymal Stem Cells (MSCs) are often investigated for orthopedic regenerative applications due0 to their multipotent differentiation capacity. Which of the following is a key cell surface marker commonly used to identify human MSCs?

. CD34
. CD45
. CD90
. CD14
. CD19

Correct Answer & Explanation

. CD90

Explanation

Mesenchymal Stem Cells (MSCs) are characterized by their adherence to plastic, multipotent differentiation capacity (into osteoblasts, chondrocytes, adipocytes), and a specific immunophenotype. The International Society for Cellular Therapy (ISCT) defines minimal criteria for MSCs, which include expressing CD73, CD90, and CD105, and lacking the expression of hematopoietic markers such as CD34, CD45, CD14, CD11b, CD79a, or CD19, and HLA-DR. Therefore, CD90 is a commonly used positive marker to identify human MSCs. CD34 and CD45 are markers for hematopoietic stem cells and leukocytes, respectively.

Question 785

Topic: Biology, Genetics & Bone Healing

Which of the following cells is directly responsible for sensing mechanical stress and initiating the bone remodeling cascade in response to altered loading?

. Osteoblasts
. Osteoclasts
. Osteocytes
. Chondrocytes
. Fibroblasts

Correct Answer & Explanation

. Osteocytes

Explanation

Osteocytes, which are mature osteoblasts embedded within the mineralized bone matrix, are considered the primary mechanosensors of bone. They form an intricate network through their dendritic processes, which connect via gap junctions within canaliculi. When mechanical stresses are applied to bone, the fluid flow through these canaliculi is altered, stimulating osteocytes. In response, osteocytes release signaling molecules (e.g., sclerostin, RANKL) that can influence the activity and recruitment of osteoblasts and osteoclasts, thereby initiating the bone remodeling process to adapt bone structure to mechanical demands.

Question 786

Topic: Biology, Genetics & Bone Healing
The natural healing process of a ruptured Achilles tendon often results in tissue with inferior mechanical properties compared to the native tendon. This is primarily due to:
. Excessive collagen Type II deposition
. Persistent inflammatory response
. Formation of disorganized Type III collagen
. Reduced vascularization
. Lack of neural innervation

Correct Answer & Explanation

. Formation of disorganized Type III collagen

Explanation

Tendon healing, particularly after a rupture, is a complex process that often leads to scar tissue formation. The reparative tissue initially consists of granulation tissue rich in fibroblasts, macrophages, and new capillaries. During the remodeling phase, there is an increase in collagen synthesis. However, a significant portion of the newly synthesized collagen is Type III collagen, which is mechanically weaker and has smaller, less organized fibrils compared to the predominantly Type I collagen found in healthy, mature tendons. Over time, there is some conversion to Type I collagen and better organization, but the healed tendon rarely retains the full strength and elasticity of the original tissue due to the persistent presence of more disorganized Type III collagen and altered collagen cross-linking.

Question 787

Topic: Biology, Genetics & Bone Healing

Endochondral ossification is the primary process by which long bones are formed. What is the initial template for bone formation in endochondral ossification?

. Fibrous connective tissue
. Cartilaginous model
. Dermal condensation
. Mesenchymal membrane
. Woven bone scaffold

Correct Answer & Explanation

. Cartilaginous model

Explanation

Endochondral ossification is one of the two main processes of bone formation (osteogenesis). In this process, bone develops from a pre-existing hyaline cartilage model. Mesenchymal cells first differentiate into chondrocytes, which form a cartilage template that largely mimics the future bone shape. This cartilage model then undergoes calcification, hypertrophy, and ultimately apoptosis, and is gradually replaced by bone tissue laid down by osteoblasts. This mechanism is responsible for the formation of most bones in the appendicular skeleton and many in the axial skeleton (e.g., vertebrae). Intramembranous ossification, in contrast, forms bone directly from mesenchymal tissue without a cartilage intermediate.

Question 788

Topic: Biology, Genetics & Bone Healing

Osteoprotegerin (OPG) plays a vital role in regulating bone resorption. What is its primary mechanism of action?

. Directly inhibiting osteoclast activity
. Stimulating osteoblast differentiation
. Sequestration of RANKL
. Promoting osteoclast apoptosis
. Enhancing calcium reabsorption in the kidney

Correct Answer & Explanation

. Sequestration of RANKL

Explanation

Osteoprotegerin (OPG) is a soluble decoy receptor for Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). RANKL is a transmembrane protein primarily expressed by osteoblasts and stromal cells that binds to its receptor, RANK, on osteoclast precursors and mature osteoclasts, leading to their differentiation, activation, and survival, thus promoting bone resorption. OPG competes with RANK for RANKL binding. By binding to and sequestering RANKL, OPG prevents RANKL from interacting with RANK, thereby inhibiting osteoclast formation and activity and effectively reducing bone resorption.

Question 789

Topic: Biology, Genetics & Bone Healing

Which of the following systemic factors is most detrimental to fracture healing by inhibiting angiogenesis and collagen synthesis, and promoting catabolism?

. Vitamin D deficiency
. Growth hormone excess
. Uncontrolled diabetes mellitus
. Estrogen deficiency
. Glucocorticoid excess

Correct Answer & Explanation

. Glucocorticoid excess

Explanation

Glucocorticoids, particularly in excessive or chronic doses, are well-known inhibitors of fracture healing. They exert multiple detrimental effects, including: suppressing osteoblast proliferation and differentiation, inhibiting collagen synthesis, decreasing local growth factor production, reducing vascularization (angiogenesis), and promoting osteoclastogenesis and bone resorption. This overall effect shifts the balance towards delayed healing, non-union, and increased risk of complications. While other factors like Vitamin D deficiency and uncontrolled diabetes can also impair healing, glucocorticoid excess is a particularly potent inhibitor of the entire repair process. Growth hormone is generally anabolic, and estrogen deficiency can lead to osteoporosis but its direct acute effect on fracture healing is less profound than glucocorticoid excess.

Question 790

Topic: Biology, Genetics & Bone Healing

Bisphosphonates are a cornerstone treatment for osteoporosis. Their primary mechanism of action involves:

. Stimulating osteoblast activity
. Increasing calcium absorption in the gut
. Inhibiting parathyroid hormone secretion
. Inducing osteoclast apoptosis and inhibiting their activity
. Enhancing calcitonin release

Correct Answer & Explanation

. Inducing osteoclast apoptosis and inhibiting their activity

Explanation

Bisphosphonates are synthetic analogues of pyrophosphate that have a high affinity for hydroxyapatite crystals in bone. Once incorporated into the bone matrix, they are released during osteoclastic bone resorption. When internalized by osteoclasts, bisphosphonates interfere with the mevalonate pathway, disrupting critical metabolic functions necessary for osteoclast survival and activity. This leads to reduced osteoclast numbers, impaired osteoclast function, and ultimately a decrease in bone resorption, thereby increasing bone mineral density and reducing fracture risk.

Question 791

Topic: Biology, Genetics & Bone Healing

Osteoblasts produce a specific factor that promotes the differentiation and activity of osteoclasts. This factor is crucial in the RANK/RANKL/OPG system. Identify this factor.

. Calcitonin
. Parathyroid hormone (PTH)
. Osteoprotegerin (OPG)
. Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL)
. Bone morphogenetic protein (BMP)

Correct Answer & Explanation

. Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL)

Explanation

The RANK/RANKL/OPG system is a critical signaling pathway that regulates osteoclast formation, function, and survival, thus controlling bone resorption. Osteoblasts and stromal cells express Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL) on their cell surfaces. This ligand binds to its receptor, RANK, which is found on the surface of osteoclast precursors and mature osteoclasts. This binding initiates a signaling cascade within the osteoclast precursor, leading to its differentiation into a mature osteoclast and subsequent activation of bone resorptive activity. Osteoprotegerin (OPG) acts as a decoy receptor, inhibiting RANKL activity. Calcitonin and PTH are systemic hormones that influence this system. BMPs are involved in bone formation.

Question 792

Topic: Biology, Genetics & Bone Healing

Which vitamin is essential for the hydroxylation of proline and lysine residues during collagen synthesis, a crucial step for the stability of the collagen triple helix?

. Vitamin A
. Vitamin D
. Vitamin K
. Vitamin C
. Vitamin B12

Correct Answer & Explanation

. Vitamin C

Explanation

Vitamin C (ascorbic acid) is an essential cofactor for the enzymes prolyl hydroxylase and lysyl hydroxylase. These enzymes catalyze the hydroxylation of proline and lysine residues within the procollagen alpha chains. Hydroxyproline and hydroxylysine residues are critical for forming stable hydrogen bonds that stabilize the collagen triple helix and for subsequent cross-linking, which gives collagen its tensile strength. Deficiency of Vitamin C leads to scurvy, characterized by impaired collagen synthesis, resulting in fragile blood vessels, poor wound healing, and weakened connective tissues.

Question 793

Topic: Biology, Genetics & Bone Healing

Which property of bone graft material refers to its ability to stimulate host mesenchymal stem cells to differentiate into osteoblasts and form new bone?

. Osteoconduction
. Osteointegration
. Osteogenesis
. Osteoinduction
. Osteopromotion

Correct Answer & Explanation

. Osteoinduction

Explanation

Bone grafts facilitate bone healing through several mechanisms: Osteoconduction provides a scaffold or matrix for host bone cells to grow onto and into. Osteogenesis involves viable cells within the graft (e.g., autograft) that directly form new bone. Osteoinduction relies on growth factors (e.g., BMPs) within the graft or impregnated into it that recruit and differentiate host mesenchymal stem cells into osteoblasts. Osteointegration refers to the direct structural and functional connection between living bone and the surface of a load-carrying implant. Therefore, the ability to stimulate host stem cells to differentiate and form bone is termed osteoinduction.

Question 794

Topic: Biology, Genetics & Bone Healing

In the pathophysiology of rheumatoid arthritis, which cell type plays a critical role in the destruction of articular cartilage and subchondral bone by producing enzymes like matrix metalloproteinases (MMPs) and activating osteoclasts?

. Chondrocytes
. Osteocytes
. Synoviocytes
. T-lymphocytes
. B-lymphocytes

Correct Answer & Explanation

. Synoviocytes

Explanation

In rheumatoid arthritis (RA), the synovial membrane undergoes significant pathological changes, characterized by hyperplasia and inflammation. Fibroblast-like synoviocytes (FLS) become activated and highly proliferative, forming an aggressive pannus tissue that invades and destroys the adjacent articular cartilage and subchondral bone. These activated FLS, along with macrophages, produce a range of pro-inflammatory cytokines (e.g., TNF-alpha, IL-1, IL-6) and proteolytic enzymes, notably matrix metalloproteinases (MMPs), which directly degrade the cartilage matrix. They also contribute to bone erosion by promoting osteoclastogenesis. While T and B lymphocytes initiate and perpetuate the immune response, the FLS are the primary effector cells directly causing tissue destruction in the joint.

Question 795

Topic: Biology, Genetics & Bone Healing

The healing of an avascular necrosis (AVN) lesion in the femoral head primarily relies on which biological process?

. Primary bone healing
. Intramembranous ossification
. Creeping substitution
. Endochondral ossification
. Direct osteosynthesis

Correct Answer & Explanation

. Creeping substitution

Explanation

Avascular necrosis (AVN) involves the death of osteocytes and marrow elements due to interrupted blood supply. The natural healing process for AVN primarily involves "creeping substitution." In this process, new blood vessels invade the necrotic bone, bringing with them osteoclasts that resorb the dead bone, and osteoblasts that lay down new viable bone on the remnants of the necrotic trabeculae. This is a slow process, and if the mechanical loads on the femoral head exceed the strength of the weakened necrotic and repairing bone, collapse can occur before complete revascularization and repair. This process is distinct from primary bone healing (direct cortical union), intramembranous ossification, or endochondral ossification as primary mechanisms forfracturehealing.

Question 796

Topic: Biology, Genetics & Bone Healing

Which type of bone cell is terminally differentiated and embedded within the mineralized bone matrix, primarily functioning in mechanosensation and local regulation of bone remodeling?

. Osteoprogenitor cell
. Osteoblast
. Osteoclast
. Osteocyte
. Chondroblast

Correct Answer & Explanation

. Osteocyte

Explanation

Osteocytes are mature bone cells that originate from osteoblasts and become embedded within the mineralized bone matrix. They reside in lacunae and extend dendritic processes through canaliculi, forming an extensive network throughout the bone. Osteocytes are the primary mechanosensory cells of bone, detecting mechanical strain and initiating signals that regulate both osteoblast and osteoclast activity, thus playing a crucial role in bone remodeling and adaptation to mechanical loads. Osteoprogenitor cells are precursors, osteoblasts form bone, osteoclasts resorb bone, and chondroblasts form cartilage.

Question 797

Topic: Biology, Genetics & Bone Healing

Which of the following describes the change in bone density that results from the removal of mechanical stress, such as prolonged bed rest or microgravity?

. Osteosclerosis
. Hyperostosis
. Osteoporosis
. Osteopetrosis
. Paget's disease

Correct Answer & Explanation

. Osteoporosis

Explanation

Wolff's Law states that bone adapts its structure to the loads placed upon it. Conversely, the removal of mechanical stress leads to a decrease in bone density, a phenomenon known as disuse osteoporosis. Prolonged bed rest, immobilization, or exposure to microgravity (e.g., in space) reduces the mechanical stimuli on bone, leading to an imbalance between bone formation (decreased osteoblast activity) and bone resorption (increased osteoclast activity), resulting in net bone loss and reduced bone mineral density. Osteosclerosis and hyperostosis refer to increased bone density, while osteopetrosis is a genetic disorder of dense bone. Paget's disease is characterized by disorganized bone remodeling.

Question 798

Topic: Biology, Genetics & Bone Healing
Bone morphogenetic proteins (BMPs) are critical for inducing bone formation. Their mechanism of action primarily involves:
. Directly stimulating osteoclast activity
. Inhibiting chondrocyte proliferation
. Recruiting and differentiating mesenchymal stem cells into osteoblasts
. Enhancing collagen degradation
. Promoting angiogenesis only

Correct Answer & Explanation

. Recruiting and differentiating mesenchymal stem cells into osteoblasts

Explanation

Bone morphogenetic proteins (BMPs) are a family of growth factors belonging to the TGF-β superfamily, renowned for their osteoinductive properties. Their primary mechanism of action involves recruiting undifferentiated mesenchymal stem cells from the surrounding tissues and inducing their differentiation into osteoblasts and chondroblasts, thereby leading to the formation of new bone and cartilage. They also promote angiogenesis (formation of new blood vessels), which is essential for bone healing, but their defining characteristic is their ability to initiate the entire cascade of bone formation. They do not directly stimulate osteoclast activity or inhibit chondrocyte proliferation in the context of bone formation, nor do they enhance collagen degradation.

Question 799

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory, what is the maximum tissue strain that allows for the formation of lamellar bone during secondary fracture healing?

. Less than 2%
. 2% to 10%
. 10% to 30%
. 30% to 100%
. Greater than 100%

Correct Answer & Explanation

. Less than 2%

Explanation

Perren's theory dictates that lamellar bone can only form under strain conditions of less than 2%. Granulation tissue tolerates up to 100% strain, while cartilage tolerates up to 10% strain before tearing.

Question 800

Topic: Biology, Genetics & Bone Healing

Osteoclasts resorb bone by creating an acidic microenvironment at the ruffled border. Which enzyme is primarily responsible for generating the intracellular protons required for this acidification process?

. Cathepsin K
. Alkaline phosphatase
. Carbonic anhydrase II
. Tartrate-resistant acid phosphatase (TRAP)
. Matrix metalloproteinase-9 (MMP-9)

Correct Answer & Explanation

. Carbonic anhydrase II

Explanation

Carbonic anhydrase II catalyzes the conversion of carbon dioxide and water into carbonic acid, which dissociates into protons and bicarbonate. The protons are then pumped into the Howship's lacuna via a vacuolar-type H+-ATPase to dissolve bone mineral.

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