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Biology, Genetics & Bone Healing MCQs

Practice Set 119 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 2361

Topic: Biology, Genetics & Bone Healing

A 65-year-old female with severe osteoporosis is started on a nitrogen-containing bisphosphonate (alendronate). Which of the following best describes the molecular mechanism of action of this medication in preventing bone resorption?

. Inhibition of osteoclast ruffled border formation by blocking carbonic anhydrase II
. Inhibition of farnesyl pyrophosphate synthase in the mevalonate pathway
. Direct binding to RANKL, preventing osteoclast activation
. Formation of toxic ATP analogs that induce osteoclast apoptosis
. Inhibition of cathepsin K, preventing collagen matrix degradation

Correct Answer & Explanation

. Inhibition of farnesyl pyrophosphate synthase in the mevalonate pathway

Explanation

Nitrogen-containing bisphosphonates (e.g., alendronate, zoledronate) act by inhibiting farnesyl pyrophosphate synthase in the mevalonate pathway. This prevents the prenylation of small GTPase proteins (such as Ras, Rho, and Rac), which are essential for osteoclast function, ruffled border formation, and cell survival. Non-nitrogenous bisphosphonates (e.g., etidronate) work by forming toxic ATP analogs. Denosumab binds to RANKL. Odanacatib inhibits cathepsin K.

Question 2362

Topic: Biology, Genetics & Bone Healing

A 35-year-old male presents with a large lytic lesion in his distal femur extending into the epiphysis. Biopsy confirms a Giant Cell Tumor of bone. He is treated with Denosumab. What is the specific target of this monoclonal antibody?

. RANK receptor on osteoclasts
. Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL)
. Vascular Endothelial Growth Factor (VEGF)
. Colony Stimulating Factor 1 (CSF-1)
. Tumor Necrosis Factor alpha (TNF-alpha)

Correct Answer & Explanation

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

Explanation

Denosumab is a fully human monoclonal antibody that binds to RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand). By binding RANKL, it prevents it from interacting with the RANK receptor on the surface of osteoclasts and their precursors, thereby inhibiting osteoclastogenesis and function. It is a highly effective targeted therapy for Giant Cell Tumor of bone, which is driven by neoplastic stromal cells secreting excessive RANKL.

Question 2363

Topic: Biology, Genetics & Bone Healing

A patient with severe malnutrition presents with bleeding gums, petechiae, and poor wound healing. Radiographs show periosteal elevation and a dense zone of provisional calcification. This condition is caused by a deficiency in a cofactor required for which of the following steps in collagen synthesis?

. Gamma-carboxylation of glutamate residues
. Cross-linking of tropocollagen molecules by lysyl oxidase
. Cleavage of procollagen C-terminal propeptides
. Hydroxylation of proline and lysine residues in procollagen
. Glycosylation of hydroxylysine residues in the Golgi apparatus

Correct Answer & Explanation

. Hydroxylation of proline and lysine residues in procollagen

Explanation

The patient has scurvy (Vitamin C deficiency). Ascorbic acid is an essential cofactor for the enzymes prolyl hydroxylase and lysyl hydroxylase, which are responsible for the hydroxylation of proline and lysine residues on the preprocollagen chains in the rough endoplasmic reticulum. This step is critical for the stable triple-helix formation of collagen.

Question 2364

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy presents with bowing of the lower extremities, waddling gait, and short stature. Laboratory tests reveal normal calcium, very low serum phosphate, normal PTH, normal 25(OH) vitamin D, and appropriately normal but functionally low 1,25(OH)2 vitamin D. What is the most likely pathophysiological mechanism of his condition?

. Mutation in the PHEX gene leading to elevated FGF-23 levels
. Inactivating mutation in the 1-alpha-hydroxylase enzyme
. Defect in the intestinal Vitamin D receptor (VDR)
. Autoimmune destruction of the parathyroid glands
. Activating mutation of the calcium-sensing receptor (CaSR)

Correct Answer & Explanation

. Mutation in the PHEX gene leading to elevated FGF-23 levels

Explanation

The clinical and laboratory profile is classic for X-linked hypophosphatemic rickets (XLH). This is caused by a mutation in the PHEX gene, which leads to unregulated overproduction of Fibroblast Growth Factor 23 (FGF-23) by osteocytes. FGF-23 inhibits renal phosphate reabsorption in the proximal tubule and inhibits 1-alpha-hydroxylase, preventing the expected rise in 1,25(OH)2 vitamin D in response to hypophosphatemia.

Question 2365

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of bone healing, the mechanical environment dictates the type of tissue that forms in a fracture gap. What range of strain promotes secondary bone healing with callus formation?

. Less than 2%
. 2% to 10%
. 10% to 15%
. 15% to 30%
. Greater than 30%

Correct Answer & Explanation

. 2% to 10%

Explanation

Perren's strain theory posits that tissue cannot form if the strain (deformation) exceeds the yield strain of that tissue. Primary bone healing requires absolute stability with strain < 2%. Secondary bone healing (callus formation via endochondral ossification) occurs with relative stability, corresponding to a strain environment between 2% and 10%. Granulation tissue tolerates up to 100% strain, while cartilage tolerates about 10%.

Question 2366

Topic: Biology, Genetics & Bone Healing

Bone Morphogenetic Proteins (BMPs) are utilized clinically to enhance spinal fusion and fracture healing. Recombinant human BMP-2 acts on mesenchymal stem cells primarily through which of the following intracellular signaling pathways to promote osteoblastic differentiation?

. Wnt / beta-catenin pathway
. Smad 1/5/8 pathway
. JAK / STAT pathway
. MAP kinase pathway
. Notch signaling pathway

Correct Answer & Explanation

. Smad 1/5/8 pathway

Explanation

BMPs are members of the TGF-beta superfamily. Upon binding to their serine/threonine kinase cell surface receptors, BMPs (including BMP-2 and BMP-7) induce phosphorylation of the intracellular Smad proteins, specifically Smad 1, 5, and 8. These phosphorylated Smads then form a complex with Smad 4, translocate to the nucleus, and upregulate the transcription of osteogenic genes like Runx2.

Question 2367

Topic: Biology, Genetics & Bone Healing

A 12-year-old boy presents with multiple low-energy fractures and is found to have 'rugger jersey spine' and 'bone-in-bone' appearances on radiographs. A mutation affecting which of the following is most likely responsible for his condition?

. Type I collagen
. Fibroblast growth factor receptor 3 (FGFR3)
. Carbonic anhydrase II
. Cathepsin K
. Runx2

Correct Answer & Explanation

. Carbonic anhydrase II

Explanation

The patient has osteopetrosis, characterized by dense, brittle bones due to osteoclast dysfunction. A well-known cause of the autosomal recessive form (which can present with renal tubular acidosis and cerebral calcification) is a mutation in the carbonic anhydrase II gene. This enzyme is crucial for generating the acidic environment (via hydrogen ion secretion at the ruffled border) necessary to dissolve bone mineral.

Question 2368

Topic: Biology, Genetics & Bone Healing

Cancellous autograft is often considered the 'gold standard' for bone grafting due to possessing osteogenic, osteoinductive, and osteoconductive properties. Which of the following components of the graft provides its osteoinductive capacity?

. The three-dimensional trabecular bone matrix
. Surviving mesenchymal stem cells
. Bone morphogenetic proteins (BMPs) in the matrix
. Live osteoblasts transferred with the graft
. High vascularity of the graft tissue

Correct Answer & Explanation

. Bone morphogenetic proteins (BMPs) in the matrix

Explanation

Osteoinduction is the ability of a graft to stimulate host stem cells to differentiate into osteoblasts. This is primarily mediated by growth factors, such as Bone Morphogenetic Proteins (BMPs), contained within the bone matrix. Osteoconduction is provided by the physical 3D trabecular matrix, and osteogenesis is provided by the surviving cellular elements (MSCs and osteoblasts).

Question 2369

Topic: Biology, Genetics & Bone Healing

A 2-year-old child presents with bowing of the lower extremities, widened physes, and costochondral junction enlargements. Laboratory evaluation reveals normal serum calcium, significantly low serum phosphate, and elevated alkaline phosphatase. Which of the following is the most likely underlying mechanism of this condition?

. Deficiency of dietary vitamin D
. Mutation in the PHEX gene leading to excess FGF23
. Mutation in the COL1A1 gene
. Deficiency of 1-alpha-hydroxylase
. Resistance to parathyroid hormone

Correct Answer & Explanation

. Mutation in the PHEX gene leading to excess FGF23

Explanation

The patient has X-linked hypophosphatemic rickets (XLH), characterized by low phosphate, normal calcium, and elevated alkaline phosphatase, along with clinical signs of rickets. XLH is caused by a mutation in the PHEX gene, leading to overproduction of FGF23. FGF23 decreases renal tubular reabsorption of phosphate, causing profound hypophosphatemia.

Question 2370

Topic: Biology, Genetics & Bone Healing
Osteoclasts attach to the bone surface to form a sealed zone, allowing for localized bone resorption in an acidic environment. Which specific integrin is primarily responsible for mediating the attachment of osteoclasts to bone matrix proteins such as osteopontin?
. Alpha-5 beta-1
. Alpha-v beta-3
. Alpha-2 beta-1
. Alpha-M beta-2
. Alpha-L beta-2

Correct Answer & Explanation

. Alpha-v beta-3

Explanation

Osteoclasts utilize the Alpha-v beta-3 (αvβ3) integrin to bind to specific amino acid sequences (RGD - arginine-glycine-aspartic acid motifs) found on bone matrix proteins like osteopontin and bone sialoprotein. This integrin-mediated binding is critical for forming the sealing zone required for effective bone resorption.

Question 2371

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) such as BMP-2 and BMP-7 induce osteoblastic differentiation and bone formation primarily through activation of which of the following intracellular signaling pathways?

. Wnt/beta-catenin
. JAK/STAT
. Smad 1/5/8
. Notch/Hes
. MAPK/ERK

Correct Answer & Explanation

. Smad 1/5/8

Explanation

BMPs bind to serine/threonine kinase receptors on the cell surface, which then phosphorylate intracellular Smad proteins, specifically Smad 1, 5, and 8. These complex with Smad 4, translocate to the nucleus, and upregulate the transcription of osteogenic genes like Runx2. The Wnt pathway uses beta-catenin, which is also osteoinductive but distinct from the direct BMP pathway.

Question 2372

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy is evaluated for recurrent fractures and vision loss. Radiographs reveal generalized dense, 'marble-like' bones with loss of the medullary canal. A mutation affecting which of the following enzymes is most likely responsible for this condition?

. Alkaline phosphatase
. Carbonic anhydrase II
. Type I collagen
. Fibroblast growth factor receptor 3
. Cathepsin K

Correct Answer & Explanation

. Carbonic anhydrase II

Explanation

This presentation is typical of osteopetrosis, characterized by defective osteoclast function leading to dense but brittle bones and cranial nerve entrapment (vision loss). A classic mutation responsible for an autosomal recessive form of osteopetrosis affects the carbonic anhydrase II enzyme, which is required by the osteoclast to generate protons for acidifying the resorption pit at the ruffled border. Cathepsin K mutation causes pycnodysostosis.

Question 2373

Topic: Biology, Genetics & Bone Healing
Aseptic loosening and osteolysis following total joint arthroplasty are primarily mediated by the biological response to ultra-high molecular weight polyethylene (UHMWPE) wear debris. Which cell type is the primary initiator of this cascade by phagocytosing particles sized 0.1 to 1.0 micrometers?
. Osteoblast
. Osteoclast
. Macrophage
. T-lymphocyte
. Polymorphonuclear leukocyte

Correct Answer & Explanation

. Macrophage

Explanation

The biological response to particulate wear debris is macrophage-mediated. Macrophages phagocytose the small submicron polyethylene particles and subsequently release pro-inflammatory cytokines such as IL-1, IL-6, and TNF-alpha. These cytokines then stimulate osteoclasts via the RANKL pathway, culminating in periprosthetic osteolysis.

Question 2374

Topic: Biology, Genetics & Bone Healing

A 12-year-old boy suffers a transverse fracture of the femoral shaft after a minor fall. Radiographs demonstrate diffusely dense, 'chalk-like' bones with loss of the medullary canal, and a 'rugger-jersey' appearance of the vertebral bodies. A genetic mutation affecting which of the following proteins is most commonly implicated in the subset of this disease associated with renal tubular acidosis?

. Type I collagen
. Fibroblast growth factor receptor 3 (FGFR3)
. Carbonic anhydrase II
. Cartilage oligomeric matrix protein (COMP)
. Core-binding factor alpha-1 (CBFA1)

Correct Answer & Explanation

. Carbonic anhydrase II

Explanation

The patient has osteopetrosis, characterized by defective osteoclast function. Mutations in carbonic anhydrase II (CAII) cause the form associated with renal tubular acidosis and cerebral calcifications. The lack of CAII prevents osteoclasts from generating the protons needed to acidify the resorption pit.

Question 2375

Topic: Biology, Genetics & Bone Healing

A 7-year-old girl is evaluated for short stature and delayed tooth eruption. On physical exam, she has a prominent forehead, wide-set eyes, and is able to approximate her shoulders anteriorly to the midline. A mutation in which of the following genes is responsible for her condition?

. RUNX2 (CBFA1)
. COL1A1
. FGFR3
. SHOX
. FBN1

Correct Answer & Explanation

. RUNX2 (CBFA1)

Explanation

The clinical presentation is classic for cleidocranial dysplasia (absent or hypoplastic clavicles, delayed fontanelle closure, dental anomalies). It is caused by an autosomal dominant mutation in the RUNX2 (CBFA1) gene, an essential transcription factor for osteoblast differentiation and membranous ossification.

Question 2376

Topic: Biology, Genetics & Bone Healing

A surgeon utilizes demineralized bone matrix (DBM) to augment a posterolateral lumbar spine fusion. Which of the following biological properties does DBM possess that distinguishes it from a processed structural cortical bone allograft?

. Osteogenesis
. Osteoconduction only
. Osteoinduction
. Both osteogenesis and osteoinduction
. Superior mechanical stability

Correct Answer & Explanation

. Osteoinduction

Explanation

Demineralized bone matrix (DBM) is created by acid-extracting the mineral component of bone, which exposes bone morphogenetic proteins (BMPs) and other growth factors. Thus, DBM is both osteoconductive and osteoinductive, whereas a standard processed structural cortical allograft is primarily osteoconductive. Neither possesses osteogenic properties (living osteoblasts/mesenchymal stem cells), which are unique to autograft or bone marrow aspirate.

Question 2377

Topic: Biology, Genetics & Bone Healing

Which of the following best describes the process of 'creeping substitution' during the incorporation of a cortical bone graft?

. Simultaneous osteoclastic resorption and osteoblastic deposition along existing haversian canals
. Rapid revascularization followed by enchondral ossification
. Initial formation of a cartilage model that is subsequently mineralized
. Direct differentiation of mesenchymal stem cells into osteocytes within the graft
. Encapsulation of the graft by fibrous tissue without remodeling

Correct Answer & Explanation

. Simultaneous osteoclastic resorption and osteoblastic deposition along existing haversian canals

Explanation

Creeping substitution is the process by which cortical bone grafts are incorporated. It involves osteoclasts cutting cones through the dense cortical allograft/autograft, closely followed by osteoblasts laying down new bone. This coordinated replacement of dead bone with new host bone occurs along existing haversian systems.

Question 2378

Topic: Biology, Genetics & Bone Healing

A 72-year-old woman with severe osteoporosis and a history of a vertebral compression fracture is started on teriparatide. Which of the following best describes the physiological basis for its efficacy in increasing bone mass?

. Continuous activation of osteoblasts
. Inhibition of the RANKL pathway
. Intermittent administration preferentially stimulates osteoblastic bone formation over osteoclastic resorption
. Irreversible binding to farnesyl pyrophosphate synthase in osteoclasts
. Direct binding to osteoid inhibiting osteoclastic attachment

Correct Answer & Explanation

. Intermittent administration preferentially stimulates osteoblastic bone formation over osteoclastic resorption

Explanation

Teriparatide is a recombinant parathyroid hormone (PTH 1-34) analog. While continuous high levels of PTH lead to net bone resorption (hyperparathyroidism), intermittent daily subcutaneous injection of PTH paradoxically stimulates osteoblastic bone formation more than bone resorption, resulting in a net increase in bone mass.

Question 2379

Topic: Biology, Genetics & Bone Healing

Intermittent administration of recombinant human parathyroid hormone (teriparatide) exerts an anabolic effect on bone mass. This effect is primarily mediated by the activation of which of the following intracellular signaling pathways in osteoblasts?

. Inhibition of the Wnt/beta-catenin pathway
. Direct inhibition of osteoclast ruffled border formation
. Activation of the adenylate cyclase/cAMP/PKA pathway
. Upregulation of the Smad 1/5/8 transcription factors
. Downregulation of the Runx2 transcription factor

Correct Answer & Explanation

. Activation of the adenylate cyclase/cAMP/PKA pathway

Explanation

Teriparatide binds to the PTH1 receptor, a G-protein coupled receptor on osteoblasts. This binding primarily activates the adenylate cyclase/cAMP/Protein Kinase A (PKA) pathway, promoting osteoblast survival and bone formation.

Question 2380

Topic: Biology, Genetics & Bone Healing

A 70-year-old female with severe postmenopausal osteoporosis is treated with denosumab. What is the precise molecular target of this pharmacological agent?

. Farnesyl pyrophosphate synthase
. Cathepsin K
. Osteoprotegerin (OPG)
. Receptor activator of nuclear factor kappa-B ligand (RANKL)
. Sclerostin

Correct Answer & Explanation

. Receptor activator of nuclear factor kappa-B ligand (RANKL)

Explanation

Denosumab is a fully human monoclonal antibody that specifically binds to and inhibits RANKL. This prevents RANKL from binding to its receptor (RANK) on osteoclasts, thereby inhibiting osteoclast maturation and survival.

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