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Biology, Genetics & Bone Healing MCQs

Practice Set 83 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 1641

Topic: Biology, Genetics & Bone Healing
During the process of secondary fracture healing, which type of collagen is predominantly synthesized during the soft callus phase, and which cells are primarily responsible for its production?
. Type I collagen; osteoblasts
. Type II collagen; chondrocytes
. Type III collagen; fibroblasts
. Type X collagen; hypertrophic chondrocytes
. Type I collagen; mesenchymal stem cells

Correct Answer & Explanation

. Type II collagen; chondrocytes

Explanation

Secondary fracture healing occurs via endochondral ossification. During the soft callus phase, the fracture gap is stabilized by cartilage formation in hypoxic conditions. Chondrocytes synthesize predominantly Type II collagen (along with aggrecan) to form this cartilaginous soft callus. Later, hypertrophic chondrocytes produce Type X collagen, the matrix mineralizes, and osteoblasts lay down Type I collagen to form the hard callus.

Question 1642

Topic: Biology, Genetics & Bone Healing

Monoclonal antibodies targeting sclerostin, such as romosozumab, are utilized in the management of osteoporosis. What is the primary molecular mechanism by which sclerostin normally influences bone remodeling?

. It activates the RANKL receptor on osteoclast precursors to promote osteoclastogenesis.
. It binds to LRP5/6 receptors on osteoblasts, inhibiting the Wnt/beta-catenin signaling pathway.
. It directly stimulates osteoblast differentiation by upregulating the Runx2 transcription factor.
. It inhibits the secretion of osteoprotegerin (OPG) by bone marrow stromal cells.
. It promotes chondrocyte hypertrophy during endochondral ossification.

Correct Answer & Explanation

. It binds to LRP5/6 receptors on osteoblasts, inhibiting the Wnt/beta-catenin signaling pathway.

Explanation

Sclerostin is a glycoprotein secreted primarily by mature osteocytes. It acts as a negative regulator of bone formation by binding to the LRP5/6 co-receptors on the surface of osteoblasts. This binding competitively blocks the Wnt/beta-catenin signaling pathway, leading to decreased osteoblastic activity and reduced bone formation. Inhibiting sclerostin results in a dual effect: increased bone formation and decreased bone resorption.

Question 1643

Topic: Biology, Genetics & Bone Healing

Bone Morphogenetic Proteins (BMPs) belong to the TGF-beta superfamily and play critical roles in osteoinduction. Which of the following recombinant human BMPs is FDA-approved for use with an absorbable collagen sponge in the treatment of acute, open tibial shaft fractures?

. rhBMP-2
. rhBMP-3
. rhBMP-4
. rhBMP-7
. rhBMP-9

Correct Answer & Explanation

. rhBMP-2

Explanation

Recombinant human BMP-2 (rhBMP-2) is FDA-approved for application via an absorbable collagen sponge for acute, open tibial shaft fractures, as well as for anterior lumbar interbody fusions (ALIF) with certain interbody devices. rhBMP-7 (also known as Osteogenic Protein-1 or OP-1) previously had a Humanitarian Device Exemption for recalcitrant nonunions but is no longer commercially available. BMP-3 actually acts as an antagonist to bone formation.

Question 1644

Topic: Biology, Genetics & Bone Healing

A 68-year-old female with severe postmenopausal osteoporosis is started on teriparatide to reduce her fracture risk. Which of the following best describes the primary mechanism of action of this pharmacological agent?

. Monoclonal antibody that binds to RANKL, preventing osteoclast activation
. Inhibition of the farnesyl pyrophosphate synthase enzyme in the mevalonate pathway
. Intermittent administration of a recombinant parathyroid hormone analog that stimulates osteoblast activity
. Direct binding to hydroxyapatite crystals to prevent osteoclastic dissolution
. Inhibition of sclerostin, promoting the Wnt signaling pathway

Correct Answer & Explanation

. Intermittent administration of a recombinant parathyroid hormone analog that stimulates osteoblast activity

Explanation

Teriparatide is a recombinant human parathyroid hormone (PTH 1-34) analog. When given via daily intermittent injections, it acts as an anabolic agent, preferentially stimulating osteoblast activity over osteoclast activity to build new bone. Denosumab binds RANKL. Bisphosphonates inhibit farnesyl pyrophosphate synthase and bind hydroxyapatite. Romosozumab inhibits sclerostin.

Question 1645

Topic: Biology, Genetics & Bone Healing

According to Perren’s strain theory of fracture healing, the type of tissue that forms in a fracture gap is determined by the local interfragmentary strain. Lamellar bone can only form when the interfragmentary strain is below which of the following thresholds?

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

Correct Answer & Explanation

. 2%

Explanation

Perren's strain theory dictates that a specific tissue can only form and survive in a fracture gap if the interfragmentary strain is lower than the tissue's ultimate failure strain. Granulation tissue can tolerate up to 100% strain, fibrocartilage roughly 10-15%, and woven bone about 5%. Lamellar bone requires an extremely stable environment and can only form when the strain is less than 2%.

Question 1646

Topic: Biology, Genetics & Bone Healing
Sclerostin is an important regulator of bone metabolism and the target of the osteoporosis medication romosozumab. Which of the following best describes the primary mechanism of action of sclerostin?
. Stimulates RANKL expression by osteoblasts to increase osteoclastogenesis
. Binds to LRP5/6 receptors to inhibit the canonical Wnt/β-catenin signaling pathway
. Acts as a soluble decoy receptor for RANKL to prevent osteoclast activation
. Promotes the differentiation of mesenchymal stem cells into the chondrocytic lineage
. Cleaves osteopontin to facilitate initial osteoclast attachment to the bone surface

Correct Answer & Explanation

. Binds to LRP5/6 receptors to inhibit the canonical Wnt/β-catenin signaling pathway

Explanation

Sclerostin is a glycoprotein secreted primarily by mature osteocytes that acts as a potent negative regulator of bone formation. It functions by binding to the LRP5/6 co-receptors on the surface of osteoblasts, which competitively inhibits the canonical Wnt/β-catenin signaling pathway. This pathway is essential for osteoblast differentiation, proliferation, and survival. Inhibition of sclerostin (e.g., via the monoclonal antibody romosozumab) allows Wnt signaling to proceed, leading to increased bone formation.

Question 1647

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory regarding secondary fracture healing, the differentiation of tissues within a fracture gap is dictated by the amount of local strain. What is the maximum local tissue strain that allows for the formation and persistence of lamellar (compact) bone without resulting in microfracture?

. Less than 2%
. Up to 10%
. Up to 15%
. Up to 30%
. Up to 100%

Correct Answer & Explanation

. Less than 2%

Explanation

Perren's strain theory dictates that a specific tissue can only form if the strain in the fracture gap is less than the tissue's tolerance before rupture. Granulation tissue can tolerate up to 100% strain; fibrous connective tissue and cartilage up to 10-15%; and woven bone up to 10%. Lamellar (compact) bone is highly brittle and requires a rigid mechanical environment with less than 2% strain to form and persist without microfracturing.

Question 1648

Topic: Biology, Genetics & Bone Healing

Osteopetrosis is a genetic disease characterized by dense, brittle bones due to defective osteoclastic bone resorption. A mutation in the gene encoding for which of the following enzymes is most commonly associated with the failure of osteoclasts to create the necessary acidic microenvironment?

. Alkaline phosphatase
. Cathepsin K
. Carbonic anhydrase II
. Matrix metalloproteinase 9
. Tartrate-resistant acid phosphatase

Correct Answer & Explanation

. Carbonic anhydrase II

Explanation

Carbonic anhydrase II (CA II) is essential for producing the protons needed to acidify the resorption lacuna beneath the osteoclast's ruffled border. Mutations in CA II cause a subtype of autosomal recessive osteopetrosis that is characteristically associated with renal tubular acidosis and cerebral calcification. Cathepsin K degrades the organic matrix (type I collagen) but does not acidify the environment.

Question 1649

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of fracture healing, the type of tissue that forms in a fracture gap is dictated by the amount of mechanical strain at the local cellular level. What is the maximum strain environment that permits the formation of primary lamellar bone?

. Less than 2%
. Between 2% and 10%
. Between 10% and 30%
. Between 30% and 50%
. Greater than 50%

Correct Answer & Explanation

. Less than 2%

Explanation

Perren's strain theory postulates that a specific tissue can only form under conditions where the local mechanical strain does not exceed its failure strain. Lamellar bone is highly intolerant of strain and requires an environment with less than 2% strain to form (primary bone healing). Cartilage can tolerate up to 10% strain (allowing secondary callus formation), while granulation tissue can tolerate up to 100% strain.

Question 1650

Topic: Biology, Genetics & Bone Healing

Teriparatide is utilized in the management of severe osteoporosis. Which of the following best describes its mechanism of action when administered via daily subcutaneous injections?

. Inhibition of the mevalonate pathway in osteoclasts
. Binding to RANKL, preventing osteoclast activation
. Intermittent activation of parathyroid hormone (PTH) receptors, predominantly stimulating osteoblasts
. Continuous activation of parathyroid hormone (PTH) receptors, predominantly stimulating osteoclasts
. Direct inhibition of Cathepsin K

Correct Answer & Explanation

. Intermittent activation of parathyroid hormone (PTH) receptors, predominantly stimulating osteoblasts

Explanation

Teriparatide is a recombinant form of human parathyroid hormone (PTH 1-34). While continuous elevation of endogenous PTH (as seen in hyperparathyroidism) leads to increased osteoclast activity and bone resorption, the intermittent administration (daily subcutaneous injection) of Teriparatide has a net anabolic effect on bone by predominantly stimulating osteoblast proliferation, differentiation, and activity. Bisphosphonates inhibit the mevalonate pathway, and Denosumab binds to RANKL.

Question 1651

Topic: Biology, Genetics & Bone Healing

A novel pharmacological agent designed to accelerate fracture healing is found to act by directly inhibiting sclerostin. Which of the following best describes the downstream cellular effect of this medication?

. Increased osteoclast apoptosis
. Decreased RANKL expression by osteoblasts
. Increased beta-catenin translocation to the nucleus
. Inhibition of Runx2 transcription
. Direct activation of osteoprotegerin (OPG)

Correct Answer & Explanation

. Increased beta-catenin translocation to the nucleus

Explanation

Sclerostin is a glycoprotein secreted primarily by osteocytes that acts as an antagonist to the Wnt/beta-catenin signaling pathway. By binding to LRP5/6 receptors, it prevents Wnt signaling, thereby inhibiting bone formation. Medications that inhibit sclerostin (e.g., romosozumab) allow Wnt to bind to its receptors, leading to intracellular stabilization and accumulation of beta-catenin, which then translocates to the nucleus to upregulate target genes that drive osteoblastogenesis and bone formation.

Question 1652

Topic: Biology, Genetics & Bone Healing
A novel therapeutic agent is being studied for the treatment of Paget's disease. The drug targets the sealing zone of osteoclasts, preventing the creation of the isolated acidic microenvironment necessary for bone resorption. Which cell surface receptor is primarily targeted by this medication to disrupt osteoclast attachment to the bone matrix?
. Integrin alpha-5 beta-1 (α5β1)
. Integrin alpha-v beta-3 (αvβ3)
. Integrin alpha-2 beta-1 (α2β1)
. Cadherin-11
. CD44

Correct Answer & Explanation

. Integrin alpha-v beta-3 (αvβ3)

Explanation

Osteoclasts attach firmly to the bone matrix to form a 'sealing zone,' isolating the resorption pit (Howship's lacuna). This attachment is primarily mediated by the transmembrane receptor integrin alpha-v beta-3 (αvβ3), which recognizes and binds to RGD (Arginine-Glycine-Aspartic acid) amino acid sequences present on bone matrix proteins such as osteopontin and bone sialoprotein.

Question 1653

Topic: Biology, Genetics & Bone Healing

Which of the following accurately describes the mechanism of action of osteoprotegerin (OPG) in bone metabolism?

. It binds directly to the RANK receptor on osteoclasts to stimulate bone resorption
. It acts as a soluble decoy receptor for RANKL, preventing it from binding to RANK
. It upregulates sclerostin production by osteocytes to inhibit Wnt signaling
. It binds directly to hydroxyapatite and inhibits osteoclast ruffled border formation
. It promotes the differentiation of mesenchymal stem cells into mature osteoblasts

Correct Answer & Explanation

. It acts as a soluble decoy receptor for RANKL, preventing it from binding to RANK

Explanation

Osteoprotegerin (OPG) is a soluble decoy receptor produced primarily by osteoblasts. By binding to Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL), OPG prevents RANKL from interacting with its physiologic receptor, RANK, located on the surface of osteoclast precursors and mature osteoclasts. This effectively inhibits osteoclast differentiation, activation, and survival, leading to decreased bone resorption.

Question 1654

Topic: Biology, Genetics & Bone Healing

Figure 3 demonstrates the molecular pathway targeted by a newer class of osteoporosis medications. A 70-year-old woman is prescribed romosozumab for severe osteoporosis after suffering a fragility fracture. This medication primarily exerts an anabolic effect on bone by binding to and inhibiting which of the following proteins?

. RANK Ligand (RANKL)
. Cathepsin K
. Sclerostin
. Parathyroid hormone (PTH) receptor
. Osteocalcin

Correct Answer & Explanation

. Sclerostin

Explanation

Romosozumab is a monoclonal antibody that binds to and inhibits sclerostin. Sclerostin is a glycoprotein secreted predominantly by osteocytes that serves as a negative regulator of bone formation by inhibiting the Wnt/beta-catenin signaling pathway in osteoblasts. By blocking sclerostin, romosozumab disinhibits Wnt signaling, leading to a dual effect of promoting osteoblastic bone formation and concurrently decreasing bone resorption.

Question 1655

Topic: Biology, Genetics & Bone Healing

Which of the following intracellular signaling molecules is directly phosphorylated following the binding of Bone Morphogenetic Protein-2 (BMP-2) to its serine/threonine kinase receptor on osteoprogenitor cells?

. Beta-catenin
. Smad 1/5/8
. JAK/STAT
. NF-kappaB
. MAP kinase

Correct Answer & Explanation

. Smad 1/5/8

Explanation

BMP-2 binds to transmembrane serine/threonine kinase receptors, leading to the phosphorylation of receptor-regulated Smads (Smad 1, 5, and 8). These then form a complex with the co-Smad (Smad 4) and translocate to the nucleus to regulate the transcription of osteogenic genes, including RUNX2. Beta-catenin is the primary intracellular effector of the canonical Wnt signaling pathway. NF-kappaB is utilized by the RANKL pathway in osteoclastogenesis.

Question 1656

Topic: Biology, Genetics & Bone Healing

A 65-year-old female with severe osteoporosis is treated with romosozumab, a monoclonal antibody that promotes bone formation and decreases bone resorption. The target of this specific drug naturally exerts its biological effect by inhibiting which of the following signaling pathways?

. RANK/RANKL
. Osteoprotegerin (OPG)
. BMP/Smad
. Wnt/beta-catenin
. Notch signaling

Correct Answer & Explanation

. Wnt/beta-catenin

Explanation

Romosozumab is a monoclonal antibody that binds to and inhibits sclerostin. Sclerostin is a glycoprotein secreted primarily by osteocytes that naturally inhibits the Wnt/beta-catenin signaling pathway. The Wnt pathway is crucial for osteoblast differentiation and survival; therefore, blocking sclerostin leads to an upregulation of the Wnt pathway, resulting in increased bone formation.

Question 1657

Topic: Biology, Genetics & Bone Healing
Recombinant human bone morphogenetic protein-2 (rhBMP-2) is utilized off-label in various nonunion and spine fusion applications. Following the binding of BMP to its target cell surface receptor, which of the following best describes the immediate intracellular signaling event?
. Activation of a G-protein coupled receptor
. Cleavage of the Notch intracellular domain
. Activation of the JAK/STAT pathway
. Inhibition of GSK-3 beta
. Phosphorylation of Smad proteins by a serine/threonine kinase

Correct Answer & Explanation

. Phosphorylation of Smad proteins by a serine/threonine kinase

Explanation

Bone morphogenetic proteins (BMPs) belong to the transforming growth factor-beta (TGF-β) superfamily. They bind to heterodimeric transmembrane serine/threonine kinase receptors. Upon binding, the type II receptor phosphorylates the type I receptor, which subsequently phosphorylates intracellular R-Smad proteins (typically Smads 1, 5, and 8). These activated Smads form a complex with the co-Smad (Smad 4) and translocate to the nucleus to regulate the transcription of osteogenic genes.

Question 1658

Topic: Biology, Genetics & Bone Healing

A rigid compression plate is applied to a transverse radial shaft fracture, achieving absolute stability and anatomic reduction with a gap of less than 0.1 mm. By which of the following biological mechanisms will this fracture primarily heal?

. Endochondral ossification with a cartilaginous intermediate
. Intramembranous ossification from the periosteum
. Primary bone healing via cutting cones
. Callus formation mediated by micromotion
. Appositional bone growth from the endosteum

Correct Answer & Explanation

. Primary bone healing via cutting cones

Explanation

When absolute stability and anatomic reduction are achieved (e.g., using a dynamic compression plate), interfragmentary strain is virtually eliminated (<2%). This environment dictates primary (direct) bone healing, which occurs without the formation of a fracture callus. It relies on Haversian remodeling, where 'cutting cones' composed of leading osteoclasts cross the fracture site, followed closely by osteoblasts laying down new osteons to bridge the gap.

Question 1659

Topic: Biology, Genetics & Bone Healing

Recombinant human bone morphogenetic protein-2 (rhBMP-2) is heavily utilized in anterior lumbar interbody fusions. Upon binding to its serine/threonine kinase receptor on the cell membrane of an osteoprogenitor cell, which intracellular signaling molecules are directly phosphorylated to translocate to the nucleus and initiate transcription of osteogenic genes?

. Wnt and beta-catenin
. Smad 1, 5, and 8
. RANKL and osteoprotegerin
. MAPK and ERK
. JAK and STAT

Correct Answer & Explanation

. Smad 1, 5, and 8

Explanation

Bone morphogenetic proteins (BMPs) signal primarily through the canonical Smad pathway. Binding of the BMP ligand to the receptor complex causes phosphorylation of receptor-regulated Smads (R-Smads 1, 5, and 8). These phosphorylated Smads form a complex with the common-partner Smad (co-Smad 4), which then translocates to the nucleus to regulate the transcription of osteoblast-specific genes like Runx2.

Question 1660

Topic: Biology, Genetics & Bone Healing
A 2-year-old boy presents with multiple fractures after minimal trauma, blue sclerae, and dentinogenesis imperfecta. Genetic testing reveals a point mutation resulting in the substitution of glycine by a bulkier amino acid within the triple helix domain of type I collagen. Which of the following best describes this specific genetic mechanism and its associated disease severity?
. Quantitative defect (null allele) resulting in mild Osteogenesis Imperfecta (Type I)
. Qualitative defect (dominant negative) resulting in severe Osteogenesis Imperfecta (Type III)
. Defect in core binding factor alpha-1 (Cbfa1/Runx2) resulting in cleidocranial dysplasia
. Mutation in the COMP gene resulting in multiple epiphyseal dysplasia
. Defect in the RANKL gene resulting in osteopetrosis

Correct Answer & Explanation

. Qualitative defect (dominant negative) resulting in severe Osteogenesis Imperfecta (Type III)

Explanation

Osteogenesis imperfecta (OI) is caused by mutations in the COL1A1 or COL1A2 genes. The substitution of glycine (the smallest amino acid, strictly required at every third position of the alpha chain to allow triple helix coiling) with a bulkier amino acid disrupts the triple helix formation. This causes a 'dominant negative' (qualitative) defect, meaning abnormal collagen is incorporated into the matrix, which typically results in more severe, structurally compromised forms of OI (e.g., Types II, III, IV). Mild OI (Type I) is generally caused by a premature stop codon (null allele), resulting in a decreased amount of structurally normal collagen (quantitative defect).

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