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Biology, Genetics & Bone Healing MCQs

Practice Set 175 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 3481

Topic: Biology, Genetics & Bone Healing

Demineralized bone matrix (DBM) is widely used in orthopedic surgery as a bone graft substitute. What are the primary biological properties of DBM?

. Osteoconductive only
. Osteogenic and osteoinductive
. Osteoconductive and osteoinductive
. Osteogenic only
. Osteogenic, osteoconductive, and osteoinductive

Correct Answer & Explanation

. Osteoconductive and osteoinductive

Explanation

DBM provides a collagenous scaffold for bone growth (osteoconductive) and contains viable bone morphogenetic proteins (BMPs) that stimulate bone formation (osteoinductive). It lacks living cells, so it is not osteogenic.

Question 3482

Topic: Biology, Genetics & Bone Healing

During bone remodeling, osteoclastogenesis is primarily driven by the binding of RANKL to RANK. Which of the following molecules acts as a soluble decoy receptor to inhibit this interaction?

. Osteocalcin
. Sclerostin
. Osteoprotegerin
. Fibroblast growth factor 23
. Macrophage colony-stimulating factor

Correct Answer & Explanation

. Osteoprotegerin

Explanation

Osteoprotegerin (OPG) is secreted by osteoblasts and binds to RANKL, preventing it from binding to the RANK receptor on osteoclast precursors. This prevents osteoclast differentiation and activation, thereby inhibiting bone resorption.

Question 3483

Topic: Biology, Genetics & Bone Healing

A 5-year-old child presents with bowing of the lower extremities and a waddling gait. Laboratory studies reveal profound hypophosphatemia, normal calcium, and normal parathyroid hormone levels. A mutation in the PHEX gene is suspected. What is the primary pathophysiological consequence of this mutation?

. Decreased intestinal calcium absorption
. Excessive production of Fibroblast Growth Factor 23 (FGF-23)
. Inability to convert 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D
. Defective collagen type I cross-linking
. Hyperactivity of tissue nonspecific alkaline phosphatase

Correct Answer & Explanation

. Excessive production of Fibroblast Growth Factor 23 (FGF-23)

Explanation

X-linked hypophosphatemic rickets is caused by a PHEX mutation leading to elevated levels of FGF-23. High FGF-23 causes renal phosphate wasting and inhibits 1-alpha-hydroxylase, preventing the formation of active Vitamin D.

Question 3484

Topic: Biology, Genetics & Bone Healing

Teriparatide is an anabolic agent used in the treatment of severe osteoporosis. What is its mechanism of action when administered as a daily subcutaneous injection?

. Inhibits osteoclast ruffled border formation
. Neutralizes RANKL in the extracellular fluid
. Directly binds to calcium crystals to prevent dissolution
. Stimulates intermittent parathyroid hormone receptor activation, favoring osteoblastogenesis
. Inhibits sclerostin, releasing the block on the Wnt signaling pathway

Correct Answer & Explanation

. Stimulates intermittent parathyroid hormone receptor activation, favoring osteoblastogenesis

Explanation

Teriparatide is a recombinant portion of human parathyroid hormone (PTH 1-34). When given intermittently, it preferentially stimulates osteoblast activity over osteoclast activity, leading to net bone formation.

Question 3485

Topic: Biology, Genetics & Bone Healing

Demineralized bone matrix (DBM) is commonly used as an adjunct in spinal fusion. Based on its biologic properties, which of the following elements of bone healing does DBM lack compared to autogenous iliac crest bone graft?

. Osteoconduction
. Osteoinduction
. Osteogenesis
. Bone Morphogenetic Proteins (BMPs)
. Type I collagen scaffold

Correct Answer & Explanation

. Osteogenesis

Explanation

DBM provides an osteoconductive scaffold (collagen matrix) and is osteoinductive (contains native BMPs). However, it lacks viable osteoblasts or osteoprogenitor cells, meaning it does not possess osteogenic properties.

Question 3486

Topic: Biology, Genetics & Bone Healing

A 65-year-old woman is prescribed denosumab for osteoporosis. Which of the following best describes the molecular mechanism of action of this medication?

. Inhibits osteoclast formation by binding to RANKL
. Binds directly to RANK receptor on osteoclasts to induce apoptosis
. Inhibits farnesyl pyrophosphate synthase in the mevalonate pathway
. Acts as a competitive antagonist at the PTH receptor
. Stimulates Wnt signaling pathway via sclerostin inhibition

Correct Answer & Explanation

. Inhibits osteoclast formation by binding to RANKL

Explanation

Denosumab is a monoclonal antibody that binds to RANKL, preventing it from interacting with the RANK receptor on osteoclasts and their precursors, thereby inhibiting osteoclastogenesis and reducing bone resorption. Bisphosphonates inhibit farnesyl pyrophosphate synthase. Romosozumab binds to sclerostin.

Question 3487

Topic: Biology, Genetics & Bone Healing

During the incorporation of a cortical bone allograft, the process of 'creeping substitution' is best described as which of the following?

. Simultaneous osteoclastic resorption of the graft and osteoblastic bone formation
. Direct transformation of fibrous tissue into bone without a cartilage intermediate
. Recruitment of mesenchymal stem cells by BMPs to form new bone
. Replacement of the allograft through endochondral ossification
. Initial formation of woven bone followed by remodeling into lamellar bone

Correct Answer & Explanation

. Simultaneous osteoclastic resorption of the graft and osteoblastic bone formation

Explanation

Creeping substitution is the process by which a bone graft (especially cortical allograft) is incorporated. It involves a coordinated process where osteoclasts resorb the necrotic bone of the graft while osteoblasts simultaneously lay down new viable bone in its place. Cortical grafts incorporate slowly and are initially weaker due to porosity from osteoclastic resorption.

Question 3488

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory, what is the maximum amount of interfragmentary strain that will still permit primary bone healing (absolute stability)?

. 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

According to Perren's strain theory, primary bone healing (which occurs without callus formation) requires absolute stability, defined as interfragmentary strain of less than 2%. Secondary bone healing occurs with strains between 2% and 10%. Strain above 10% promotes fibrous nonunion or granulation tissue formation.

Question 3489

Topic: Biology, Genetics & Bone Healing

A 72-year-old man presents with progressive bowing of his right tibia and dull, aching bone pain. Laboratory studies reveal an isolated elevation of serum alkaline phosphatase. Radiographs show cortical thickening and coarse trabeculae. Which of the following viral inclusions is historically implicated in the pathophysiology of this disease?

. Cytomegalovirus
. Paramyxovirus
. Epstein-Barr virus
. Herpes simplex virus
. Human papillomavirus

Correct Answer & Explanation

. Paramyxovirus

Explanation

The patient has Paget's disease of bone. Pathophysiologically, it is characterized by increased and disorganized bone remodeling. Historically and microscopically, paramyxovirus (such as measles or respiratory syncytial virus) nucleocapsid-like inclusion bodies have been found in the osteoclasts of patients with Paget's disease.

Question 3490

Topic: Biology, Genetics & Bone Healing
A 4-year-old child presents with multiple fractures, cranial nerve palsies, and pancytopenia. Radiographs demonstrate a 'bone-within-a-bone' appearance and generalized osteosclerosis. A defect in which of the following enzymes is most likely responsible for this condition?
. Alkaline phosphatase
. Tissue nonspecific alkaline phosphatase (TNSALP)
. Carbonic anhydrase II
. Lysyl oxidase
. Type I collagenase

Correct Answer & Explanation

. Carbonic anhydrase II

Explanation

The patient has osteopetrosis (marble bone disease), characterized by defective osteoclast function resulting in dense but brittle bones. The autosomal recessive (malignant) form is most commonly caused by a mutation in the gene encoding carbonic anhydrase II, which is essential for osteoclasts to generate the acidic environment needed for bone resorption.

Question 3491

Topic: Biology, Genetics & Bone Healing

A 12-year-old boy with blue sclerae, hearing loss, and a history of multiple low-energy fractures undergoes a skin biopsy. Analysis of the extracellular matrix will most likely reveal a quantitative deficiency in which of the following?

. Type I collagen
. Type II collagen
. Type X collagen
. Fibrillin-1
. Elastin

Correct Answer & Explanation

. Type I collagen

Explanation

The patient has Osteogenesis Imperfecta (OI) Type I, which is a mild form of OI characterized by a quantitative deficiency of structurally normal Type I collagen (due to mutations in COL1A1 or COL1A2). Type I collagen is the primary collagen in bone, sclerae, and ligaments.

Question 3492

Topic: Biology, Genetics & Bone Healing

BMP-2 (rhBMP-2) is utilized in spine fusion surgery. Which of the following intracellular signaling molecules is directly phosphorylated following BMP binding to its serine/threonine kinase receptor?

. JAK/STAT
. cAMP/PKA
. Smad 1/5/8
. Wnt/beta-catenin
. MAPK/ERK

Correct Answer & Explanation

. Smad 1/5/8

Explanation

Bone Morphogenetic Proteins (BMPs) belong to the TGF-beta superfamily. When a BMP binds to its cell surface serine/threonine kinase receptor, it induces the phosphorylation of specific R-Smads (Smad 1, 5, and 8). These then form a complex with the Co-Smad (Smad 4) and translocate to the nucleus to regulate gene transcription for osteoblast differentiation.

Question 3493

Topic: Biology, Genetics & Bone Healing

A 5-year-old child newly immigrated from a northern latitude presents with bowing of the legs and widening of the wrists. Laboratory evaluation reveals hypocalcemia and hypophosphatemia. Which of the following additional lab profiles is most likely present in nutritional rickets?

. Decreased PTH, decreased alkaline phosphatase
. Increased PTH, increased alkaline phosphatase
. Decreased PTH, increased alkaline phosphatase
. Increased PTH, decreased alkaline phosphatase
. Normal PTH, normal alkaline phosphatase

Correct Answer & Explanation

. Increased PTH, increased alkaline phosphatase

Explanation

In nutritional rickets (Vitamin D deficiency), reduced intestinal absorption of calcium leads to hypocalcemia. This triggers a secondary hyperparathyroidism (increased PTH), which causes phosphaturia (leading to hypophosphatemia). Increased osteoblastic activity in an attempt to form bone leads to highly elevated alkaline phosphatase.

Question 3494

Topic: Biology, Genetics & Bone Healing

Intermittent subcutaneous administration of Teriparatide (recombinant human PTH) is used to treat severe osteoporosis. What is the primary mechanism by which intermittent, low-dose PTH increases bone mass?

. It directly inhibits osteoclast ruffled border formation
. It stimulates osteoblast survival and bone formation via the Wnt/beta-catenin pathway
. It decreases the production of 1,25-dihydroxyvitamin D
. It binds to RANKL, preventing osteoclast activation
. It competitively inhibits sclerostin binding to LRP5/6

Correct Answer & Explanation

. It stimulates osteoblast survival and bone formation via the Wnt/beta-catenin pathway

Explanation

While continuous high levels of PTH cause bone resorption, intermittent low-dose administration of PTH (e.g., Teriparatide) has an anabolic effect on bone. It directly stimulates osteoblasts, increases their lifespan by inhibiting apoptosis, and promotes bone formation, largely through down-regulation of sclerostin and activation of the Wnt/beta-catenin signaling pathway.

Question 3495

Topic: Biology, Genetics & Bone Healing

A 72-year-old female is treated for osteoporosis with denosumab. Which of the following best describes the precise mechanism of action of this medication?

. Binds directly to the RANK receptor on osteoclasts
. Binds to RANK-L, preventing the activation of the RANK receptor
. Inhibits the farnesyl pyrophosphate synthase pathway
. Stimulates osteoblast bone formation via the Wnt/beta-catenin pathway
. Acts as a decoy receptor similar to naturally occurring osteoprotegerin (OPG)

Correct Answer & Explanation

. Binds to RANK-L, preventing the activation of the RANK receptor

Explanation

Denosumab is a human monoclonal antibody that binds directly to Receptor Activator of Nuclear factor Kappa-B Ligand (RANK-L). This prevents RANK-L from binding to the RANK receptor on osteoclast precursors, thereby inhibiting osteoclast differentiation, function, and survival. Bisphosphonates inhibit the farnesyl pyrophosphate synthase pathway.

Question 3496

Topic: Biology, Genetics & Bone Healing

Distraction osteogenesis (e.g., via an Ilizarov frame) relies on the application of gradual, controlled tension across a corticotomy site. What is the primary histologic mechanism of bone formation in the distraction gap?

. Intramembranous ossification
. Enchondral ossification
. Appositional ossification
. Creeping substitution
. Chondrolysis

Correct Answer & Explanation

. Intramembranous ossification

Explanation

Distraction osteogenesis primarily produces new bone through intramembranous ossification. Under conditions of absolute stability and gradual distraction, osteoblasts directly lay down woven bone parallel to the axis of distraction without a cartilaginous intermediate.

Question 3497

Topic: Biology, Genetics & Bone Healing

Denosumab has revolutionized the medical management of unresectable Giant Cell Tumors of Bone (GCTB). Which specific cell type within the tumor mass expresses the RANK-Ligand (RANK-L) that is targeted by this therapy?

. Multinucleated giant cells
. Mononuclear neoplastic stromal cells
. Reactive osteoblasts
. Tumor-associated macrophages
. Chondroid matrix cells

Correct Answer & Explanation

. Mononuclear neoplastic stromal cells

Explanation

In Giant Cell Tumor of bone, the multinucleated giant cells themselves are reactive (osteoclast-like) and express the RANK receptor. The true neoplastic cells are the mononuclear spindle (stromal) cells, which strongly express RANK-Ligand (RANK-L). Denosumab binds to this over-expressed RANK-L, preventing the recruitment and activation of the destructive giant cells.

Question 3498

Topic: Biology, Genetics & Bone Healing

A 5-year-old boy presents with progressive bowing of the lower extremities. Laboratory studies reveal a normal serum calcium, critically low serum phosphate, and markedly elevated alkaline phosphatase. Genetic testing confirms an inactivating mutation in the PHEX gene. Which of the following best describes the pathophysiology of this condition?

. Deficiency of 1-alpha-hydroxylase in the kidney
. Defective calcium sensing receptors in the parathyroid gland
. Mutations in the ALPL gene decreasing tissue-nonspecific alkaline phosphatase
. Elevated levels of Fibroblast Growth Factor 23 (FGF23) causing renal phosphate wasting
. Autoimmune destruction of osteoclasts leading to impaired bone remodeling

Correct Answer & Explanation

. Elevated levels of Fibroblast Growth Factor 23 (FGF23) causing renal phosphate wasting

Explanation

The patient has X-linked hypophosphatemic rickets (XLHR), caused by a mutation in the PHEX gene. This mutation leads to an overproduction and lack of degradation of FGF23. Elevated FGF23 causes profound renal phosphate wasting (downregulating Na/Pi cotransporters) and inhibits 1-alpha-hydroxylase, leading to defective bone mineralization (rickets).

Question 3499

Topic: Biology, Genetics & Bone Healing

A 70-year-old man is treated with alendronate for severe osteoporosis. What is the primary molecular target of this nitrogen-containing bisphosphonate?

. Cathepsin K
. Farnesyl pyrophosphate synthase
. Carbonic anhydrase II
. RANK ligand (RANKL)
. Osteoprotegerin (OPG)

Correct Answer & Explanation

. Farnesyl pyrophosphate synthase

Explanation

Nitrogen-containing bisphosphonates (like alendronate) inhibit farnesyl pyrophosphate synthase in the mevalonate pathway. This impairs osteoclast function and promotes apoptosis by preventing the prenylation of small GTPase proteins.

Question 3500

Topic: Biology, Genetics & Bone Healing

Mesenchymal stem cells differentiate into mature osteoblasts under the regulation of specific transcription factors. Which of the following is considered the master transcription factor for osteoblastic differentiation?

. Sox9
. Runx2 (Cbfa1)
. PPAR-gamma
. MyoD
. NFATc1

Correct Answer & Explanation

. Runx2 (Cbfa1)

Explanation

Runx2 (also known as Cbfa1) is the core binding factor critical for the commitment of mesenchymal stem cells to the osteoblast lineage. Mutations in Runx2 result in cleidocranial dysplasia.

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