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Biology, Genetics & Bone Healing MCQs

Practice Set 142 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 2821

Topic: Biology, Genetics & Bone Healing
Which type of collagen is the predominant structural component of the initial soft callus formed during secondary fracture healing?
. Type I collagen
. Type II collagen
. Type III collagen
. Type IX collagen
. Type X collagen

Correct Answer & Explanation

. Type II collagen

Explanation

During secondary fracture healing, the soft callus is primarily composed of cartilage, making Type II collagen the predominant structural protein. This is later replaced by Type I collagen as the hard callus forms and woven bone remodels into lamellar bone.

Question 2822

Topic: Biology, Genetics & Bone Healing

During the remodeling phase of tendon healing, the cellularity decreases and collagen fibers align along the direction of stress. Which cell type is most responsible for this ongoing matrix synthesis and remodeling?

. Neutrophils
. Macrophages
. Tenoblasts
. Myofibroblasts
. Osteoblasts

Correct Answer & Explanation

. Neutrophils

Explanation

Tenoblasts (and mature tenocytes) are the specialized fibroblasts of tendons responsible for synthesizing collagen and extracellular matrix. During the remodeling phase, they align parallel to the lines of tension and organize the collagen network.

Question 2823

Topic: Biology, Genetics & Bone Healing

A 6-year-old child presents with bowed legs and a waddling gait. Radiographs show widening and cupping of the physes. Laboratory studies reveal low serum calcium, low phosphorus, and elevated parathyroid hormone (PTH). The primary defect in the most common heritable form of this disease involves:

. Inability to 1-alpha-hydroxylate 25-hydroxyvitamin D
. Defect in the PHEX gene leading to renal phosphate wasting
. Defect in the calcium-sensing receptor
. Vitamin C deficiency affecting collagen cross-linking
. Autoantibodies against the parathyroid gland

Correct Answer & Explanation

. Inability to 1-alpha-hydroxylate 25-hydroxyvitamin D

Explanation

The presentation describes rickets. The most common heritable form is X-linked hypophosphatemic rickets, caused by a mutation in the PHEX gene leading to excess FGF23 and subsequent renal phosphate wasting.

Question 2824

Topic: Biology, Genetics & Bone Healing

Denosumab is a monoclonal antibody used in the treatment of osteoporosis and giant cell tumor of bone. Which of the following is its specific molecular target?

. Cathepsin K
. RANK receptor
. RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand)
. Osteoprotegerin (OPG)
. Sclerostin

Correct Answer & Explanation

. Cathepsin K

Explanation

Denosumab specifically binds to and inhibits RANKL, preventing it from interacting with the RANK receptor on osteoclasts and their precursors. This directly inhibits osteoclast formation, function, and survival.

Question 2825

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of fracture healing, primary bone healing (direct Haversian remodeling) can only occur if the interfragmentary strain is below which of the following thresholds?

. 2%
. 10%
. 20%
. 50%
. 100%

Correct Answer & Explanation

. 2%

Explanation

Perren's strain theory states that the type of tissue that forms in a fracture gap is dictated by the interfragmentary strain. Tissues cannot form if the strain exceeds their tolerance. Woven bone can form at strains up to 2% (allowing for primary bone healing if stable). Fibrocartilage can tolerate up to 10% strain (secondary healing), and granulation tissue can tolerate up to 100% strain.

Question 2826

Topic: Biology, Genetics & Bone Healing

Bone Morphogenetic Proteins (BMPs) play a critical role in osteoinduction during fracture healing. BMPs bind to serine/threonine kinase receptors on the cell membrane. Which intracellular signaling molecules are directly phosphorylated to transduce this osteogenic signal to the nucleus?

. JAK/STAT
. Smad 1, 5, and 8
. Smad 2 and 3
. Beta-catenin
. MAPK/ERK

Correct Answer & Explanation

. JAK/STAT

Explanation

BMPs signal primarily through the canonical Smad pathway. Binding to the receptor leads to phosphorylation of receptor-regulated Smads (R-Smads) 1, 5, and 8. These form a complex with the common-partner Smad (Co-Smad 4) and translocate to the nucleus to regulate target genes like RUNX2. Smad 2 and 3 are primarily activated by the TGF-beta pathway.

Question 2827

Topic: Biology, Genetics & Bone Healing

The normal direct insertion of a tendon into bone (enthesis) demonstrates a gradual transition of tissue types to minimize stress risers and dissipate load. From the tendon substance advancing toward the bone, what is the correct sequence of these histological zones?

. Tendon, mineralized fibrocartilage, unmineralized fibrocartilage, bone
. Tendon, unmineralized fibrocartilage, mineralized fibrocartilage, bone
. Tendon, loose connective tissue, mineralized fibrocartilage, bone
. Tendon, unmineralized fibrocartilage, tidemark, woven bone
. Tendon, Sharpey's fibers, hyaline cartilage, bone

Correct Answer & Explanation

. Tendon, mineralized fibrocartilage, unmineralized fibrocartilage, bone

Explanation

A direct enthesis consists of four distinct transitional zones: 1) Tendon proper, 2) Unmineralized fibrocartilage, 3) Mineralized fibrocartilage, and 4) Bone. A basophilic line called the tidemark separates the unmineralized and mineralized fibrocartilage zones.

Question 2828

Topic: Biology, Genetics & Bone Healing

During cortical bone remodeling, basic multicellular units (BMUs) form a characteristic 'cutting cone'. Which of the following best describes the spatial cellular arrangement and function within this advancing BMU?

. Osteoblasts at the leading edge resorbing bone, followed by osteoclasts forming new lamellar bone
. Osteoclasts at the leading edge resorbing bone, followed by osteoblasts forming new lamellar bone
. Osteocytes releasing enzymes to resorb bone, followed by mesenchymal stem cells forming woven bone
. Macrophages removing dead bone, followed by chondrocytes forming a cartilage template
. Osteoblasts at the leading edge, followed directly by mineralization via matrix vesicles

Correct Answer & Explanation

. Osteoblasts at the leading edge resorbing bone, followed by osteoclasts forming new lamellar bone

Explanation

In cortical bone remodeling, the basic multicellular unit (BMU) travels longitudinally through the bone. Osteoclasts are situated at the leading edge (the 'cutting cone'), resorbing bone to create a tunnel. They are trailed by a capillary loop and osteoblasts in the 'closing cone', which lay down new osteoid in concentric layers to form a new Haversian system (osteon).

Question 2829

Topic: Biology, Genetics & Bone Healing

A 4-year-old child presents with gingival bleeding, petechiae, and metaphyseal changes on radiographs. The underlying biochemical defect involves impaired hydroxylation of proline and lysine residues in collagen. The deficient essential cofactor in this condition is required for the function of which enzyme?

. Lysyl oxidase
. Procollagen peptidase
. Prolyl hydroxylase
. Collagenase
. Elastase

Correct Answer & Explanation

. Lysyl oxidase

Explanation

Scurvy is caused by a deficiency in Vitamin C (ascorbic acid), which acts as an essential electron donor (cofactor) for prolyl hydroxylase and lysyl hydroxylase. These enzymes are required for the intracellular hydroxylation of proline and lysine, a critical step that allows the formation of stable collagen triple helices. Lysyl oxidase (which cross-links extracellular collagen) requires copper as a cofactor.

Question 2830

Topic: Biology, Genetics & Bone Healing

The Wnt/beta-catenin signaling pathway is a major regulator of bone mass. The medication Romosozumab targets this pathway to treat osteoporosis. What is the precise mechanism by which Romosozumab increases bone mineral density?

. It binds directly to RANKL, preventing activation of osteoclasts.
. It inhibits sclerostin, which unblocks the Wnt/beta-catenin pathway and promotes osteoblast activity.
. It directly activates the parathyroid hormone receptor to stimulate bone formation.
. It inhibits cathepsin K, preventing the breakdown of the collagen matrix.
. It acts as a decoy receptor for osteoprotegerin (OPG).

Correct Answer & Explanation

. It binds directly to RANKL, preventing activation of osteoclasts.

Explanation

Romosozumab is a humanized monoclonal antibody that binds to and inhibits sclerostin. Sclerostin is a glycoprotein secreted by osteocytes that normally inhibits the Wnt/beta-catenin pathway. By blocking sclerostin, Romosozumab "takes the brakes off" Wnt signaling, leading to robust osteoblast differentiation and marked bone formation, alongside a mild decrease in bone resorption.

Question 2831

Topic: Biology, Genetics & Bone Healing

In normal adult articular cartilage, which zone is characterized by the highest concentration of water and collagen fibers oriented parallel to the articular surface?

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

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

The superficial (tangential) zone of articular cartilage contains the highest water content (approximately 80%), the lowest proteoglycan concentration, and collagen type II fibers that are oriented parallel to the joint surface to resist shear forces. The deep zone contains collagen fibers oriented perpendicular to the joint surface and has the highest concentration of proteoglycans with the lowest water content.

Question 2832

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) play a crucial role in osteogenesis by binding to cell surface receptors. Following binding, which intracellular proteins are phosphorylated to translocate to the nucleus and induce transcription of osteogenic genes?

. Beta-catenin
. Smad 1/5/8
. JAK2
. STAT3
. NF-kappa B

Correct Answer & Explanation

. Beta-catenin

Explanation

BMPs are members of the TGF-beta superfamily. They bind to serine/threonine kinase cell surface receptors. Once activated, these receptors phosphorylate intracellular receptor-regulated Smads (R-Smads), specifically Smad 1, 5, and 8. These phosphorylated Smads then form a complex with the common-partner Smad (Smad 4) and translocate to the nucleus to regulate target gene transcription. Beta-catenin is involved in the Wnt signaling pathway.

Question 2833

Topic: Biology, Genetics & Bone Healing

Which of the following molecules acts as an antagonist to the Wnt/beta-catenin signaling pathway, thereby inhibiting osteoblastogenesis and reducing bone formation?

. Sclerostin
. Osteoprotegerin (OPG)
. Runx2
. Osterix
. Transforming growth factor-beta (TGF-beta)

Correct Answer & Explanation

. Sclerostin

Explanation

Sclerostin (produced primarily by osteocytes) binds to the LRP5/6 receptors on osteoblasts, competitively inhibiting the Wnt signaling pathway. This prevents the accumulation of beta-catenin, leading to decreased osteoblast differentiation and reduced bone formation. Monoclonal antibodies targeting sclerostin (e.g., romosozumab) are used therapeutically to increase bone mass.

Question 2834

Topic: Biology, Genetics & Bone Healing

Nitrogen-containing bisphosphonates (such as alendronate and zoledronic acid) inhibit bone resorption primarily by inhibiting which of the following enzymes in the osteoclast?

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

Correct Answer & Explanation

. Cathepsin K

Explanation

Nitrogen-containing bisphosphonates work by inhibiting farnesyl pyrophosphate synthase, an enzyme in the mevalonate pathway. This prevents the prenylation of small GTPase proteins (like Ras, Rho, and Rac) that are essential for osteoclast function, ruffled border formation, and survival, ultimately leading to osteoclast apoptosis.

Question 2835

Topic: Biology, Genetics & Bone Healing

Osteoprotegerin (OPG) is a critical regulator of bone remodeling that acts to inhibit osteoclast differentiation and activation. What is the precise molecular mechanism by which OPG exerts this effect?

. It directly degrades RANKL in the extracellular matrix via enzymatic cleavage
. It acts as a soluble decoy receptor, binding directly to RANKL
. It competitively binds to the RANK receptor on the osteoclast surface
. It phosphorylates intracellular Smad proteins within osteoclasts
. It inhibits farnesyl pyrophosphate synthase in mature osteoclasts

Correct Answer & Explanation

. It directly degrades RANKL in the extracellular matrix via enzymatic cleavage

Explanation

Osteoprotegerin (OPG) is secreted by osteoblasts and acts as a soluble decoy receptor for Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). By directly binding RANKL in the extracellular space, OPG prevents RANKL from interacting with its true receptor (RANK) on the surface of osteoclast precursors, thereby halting osteoclastogenesis and bone resorption.

Question 2836

Topic: Biology, Genetics & Bone Healing
Rigid internal fixation with an anatomically contoured dynamic compression plate results in absolute stability and minimal strain (< 2%) at the fracture site. This precise biomechanical environment forces the bone to heal via which primary mechanism?
. Primary intramembranous bone healing with robust external callus
. Secondary bone healing via endochondral ossification
. Primary bone healing via Haversian remodeling (cutting cones)
. Fibrocartilage intermediary healing
. Creeping substitution mediated by osteoprogenitor cells

Correct Answer & Explanation

. Primary bone healing via Haversian remodeling (cutting cones)

Explanation

Absolute stability (rigid fixation with interfragmentary compression, resulting in <2% strain) prevents the formation of a fibrocartilaginous callus. Instead, it leads to primary bone healing, which occurs via direct Haversian remodeling. Osteoclast 'cutting cones' traverse the fracture line, followed immediately by osteoblasts laying down new lamellar bone. Secondary healing (with a callus) occurs under conditions of relative stability (e.g., intramedullary nailing or cast immobilization).

Question 2837

Topic: Biology, Genetics & Bone Healing

Denosumab is utilized in the management of osteoporosis and giant cell tumors of bone. What is its exact cellular mechanism of action?

. Binds directly to the RANK receptor on osteoclasts
. Binds to RANKL, preventing its interaction with the RANK receptor
. Directly inhibits the proton pumps on the osteoclast ruffled border
. Stimulates Osteoprotegerin (OPG) production by mature osteoblasts
. Inhibits the Wnt/beta-catenin signaling pathway in osteoprogenitors

Correct Answer & Explanation

. Binds directly to the RANK receptor on osteoclasts

Explanation

Denosumab is a monoclonal antibody that acts as a decoy receptor, binding directly to RANKL. This prevents RANKL from binding to the RANK receptor on osteoclasts, thus inhibiting their activation and survival.

Question 2838

Topic: Biology, Genetics & Bone Healing

Recombinant human Bone Morphogenetic Protein-2 (rhBMP-2) is heavily utilized in anterior lumbar interbody fusions. BMPs belong to which larger superfamily of growth factors?

. Insulin-like growth factors (IGF)
. Transforming growth factor-beta (TGF-beta) superfamily
. Fibroblast growth factors (FGF)
. Platelet-derived growth factors (PDGF)
. Vascular endothelial growth factors (VEGF)

Correct Answer & Explanation

. Insulin-like growth factors (IGF)

Explanation

Bone Morphogenetic Proteins (BMPs) are part of the TGF-beta superfamily of signaling molecules. They induce osteoblastic differentiation from mesenchymal stem cells through the Smad intracellular signaling pathway.

Question 2839

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of fracture healing, which of the following tissues can tolerate the highest amount of interfragmentary strain before mechanical failure?

. Primary lamellar bone
. Woven bone
. Fibrocartilage
. Granulation tissue
. Hyaline cartilage

Correct Answer & Explanation

. Primary lamellar bone

Explanation

Granulation tissue can tolerate up to 100% strain before tearing. As the fracture stabilizes and strain decreases, the tissue transitions to fibrocartilage (tolerates 10-15% strain), woven bone (tolerates up to 2% strain), and finally lamellar bone.

Question 2840

Topic: Biology, Genetics & Bone Healing

In the microscopic anatomy of mature cortical bone, what is the primary structural function of the Volkmann canals?

. They contain the primary ossification center during development
. They connect adjacent Haversian systems perpendicularly to the long axis of the bone
. They house resting osteoprogenitor cells exclusively in the endosteum
. They provide the central longitudinal axis for the osteon
. They store large aggregations of unmineralized osteoid

Correct Answer & Explanation

. They contain the primary ossification center during development

Explanation

Volkmann canals transmit blood vessels laterally from the periosteum into the bone, connecting the longitudinally oriented Haversian canals. This creates an interconnected vascular network throughout the dense cortical bone.

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