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Biology, Genetics & Bone Healing MCQs

Practice Set 95 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 1881

Topic: Biology, Genetics & Bone Healing

A diaphyseal fracture is treated with rigid plate fixation and absolute stability, achieving a gap of less than 0.1 mm. Which of the following best describes the mechanism of bone healing expected in this scenario?

. Endochondral ossification
. Intramembranous ossification followed by endochondral ossification
. Direct bridging by cutting cones without callus formation
. Formation of a cartilaginous soft callus
. Periosteal bridging callus

Correct Answer & Explanation

. Direct bridging by cutting cones without callus formation

Explanation

Absolute stability with interfragmentary compression and a gap < 0.1 mm leads to primary (contact) bone healing. This occurs via direct remodeling by osteoclastic cutting cones followed by osteoblasts, without the formation of an intermediate fracture callus.

Question 1882

Topic: Biology, Genetics & Bone Healing

A 2-year-old child presents with bowing of the legs, alopecia, and hypocalcemia. Laboratory tests show elevated levels of 1,25-dihydroxyvitamin D and elevated parathyroid hormone. Which of the following is the most likely underlying defect?

. Deficiency of 1-alpha-hydroxylase in the kidney
. Defective Vitamin D receptor
. Deficiency of 25-hydroxylase in the liver
. Inactivating mutation of the calcium-sensing receptor
. FGF-23 overproduction

Correct Answer & Explanation

. Defective Vitamin D receptor

Explanation

Vitamin D-dependent rickets type II is caused by an end-organ resistance to active vitamin D due to a mutation in the Vitamin D receptor. This leads to high levels of 1,25-dihydroxyvitamin D, hypocalcemia, and frequently alopecia.

Question 1883

Topic: Biology, Genetics & Bone Healing

In the medical management of an unresectable or recurrent giant cell tumor of bone, denosumab is frequently utilized. Which of the following best describes its primary mechanism of action?

. Inhibits farnesyl pyrophosphate synthase
. Binds to and neutralizes RANKL
. Directly induces osteoblast apoptosis
. Blocks the calcitonin receptor on osteoclasts
. Inhibits matrix metalloproteinases

Correct Answer & Explanation

. Binds to and neutralizes RANKL

Explanation

Denosumab is a monoclonal antibody that binds to Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). This prevents RANKL from activating RANK on osteoclasts and their precursors, thereby profoundly inhibiting bone resorption.

Question 1884

Topic: Biology, Genetics & Bone Healing

A 4-year-old child presents with multiple fractures and is found to have diffuse osteosclerosis and 'bone-in-bone' appearance on radiographs. Genetic testing reveals a mutation in the carbonic anhydrase II gene. This condition is primarily caused by a defect in which cellular process?

. Osteoblast-mediated matrix mineralization
. Osteoclast-mediated acidification of the resorption pit
. Chondrocyte hypertrophy in the physis
. Osteocyte canalicular communication
. Fibroblast collagen synthesis

Correct Answer & Explanation

. Osteoclast-mediated acidification of the resorption pit

Explanation

Osteopetrosis caused by carbonic anhydrase II deficiency results in the inability of osteoclasts to acidify the clear zone (resorption pit). This leads to defective bone resorption and dense, brittle bones.

Question 1885

Topic: Biology, Genetics & Bone Healing

A 6-year-old boy presents with progressive bowing of his lower extremities. Laboratory evaluation demonstrates a normal serum calcium, extremely low phosphorus, and elevated alkaline phosphatase. Which of the following is the most likely diagnosis?

. Nutritional rickets
. X-linked hypophosphatemic rickets
. Osteogenesis imperfecta
. Achondroplasia
. Renal osteodystrophy

Correct Answer & Explanation

. X-linked hypophosphatemic rickets

Explanation

X-linked hypophosphatemic rickets is caused by a PHEX mutation leading to elevated FGF-23, which results in severe renal phosphate wasting. Labs characteristically show low phosphorus, normal calcium, and elevated alkaline phosphatase.

Question 1886

Topic: Biology, Genetics & Bone Healing

A patient with severe dietary restrictions presents with bleeding gums, petechiae, and joint pain. Radiographs show osteopenia and a dense provisional zone of calcification. The underlying cellular defect involves failure of which step in collagen synthesis?

. Cleavage of procollagen C-terminal propeptides
. Extracellular cross-linking by lysyl oxidase
. Intracellular hydroxylation of proline and lysine residues
. Transcription of alpha-1 collagen genes
. Assembly of the triple helix

Correct Answer & Explanation

. Intracellular hydroxylation of proline and lysine residues

Explanation

Scurvy is caused by Vitamin C deficiency, a necessary cofactor for prolyl and lysyl hydroxylases. Without proper intracellular hydroxylation of proline and lysine, stable collagen triple helices cannot form.

Question 1887

Topic: Biology, Genetics & Bone Healing

Parathyroid hormone (PTH) regulates serum calcium levels by eventually stimulating bone resorption. Which of the following cells does PTH directly bind to in order to initiate this resorptive cascade?

. Osteoclasts
. Osteoblasts
. Osteocytes
. Chondrocytes
. Macrophages

Correct Answer & Explanation

. Osteoblasts

Explanation

Osteoclasts do not have PTH receptors; rather, PTH binds directly to receptors on osteoblasts. This stimulates osteoblasts to increase RANKL expression, which secondarily activates osteoclasts to resorb bone.

Question 1888

Topic: Biology, Genetics & Bone Healing

Romosozumab, an agent used for the treatment of severe osteoporosis, exerts its anabolic effect on bone by directly binding and inhibiting which of the following molecules?

. RANKL
. Sclerostin
. Cathepsin K
. Osteoprotegerin
. Dickkopf-1 (DKK1)

Correct Answer & Explanation

. Sclerostin

Explanation

Romosozumab is a monoclonal antibody that targets and inhibits sclerostin, a glycoprotein produced by osteocytes. Blocking sclerostin disinhibits the Wnt/beta-catenin signaling pathway in osteoblasts, robustly promoting bone formation.

Question 1889

Topic: Biology, Genetics & Bone Healing

A 65-year-old man presents with progressive bowing of his tibiae and an increasing hat size. Laboratory tests show markedly elevated serum alkaline phosphatase with normal calcium and phosphorus. A bone biopsy would most likely demonstrate which microscopic finding?

. Woven bone with a prominent mosaic pattern of cement lines
. Sheets of small round blue cells
. Prominent unmineralized osteoid seams
. Subperiosteal bone resorption with dissecting osteitis
. Hypocellular dense bone with absent Haversian systems

Correct Answer & Explanation

. Woven bone with a prominent mosaic pattern of cement lines

Explanation

Paget's disease of bone is characterized by chaotic and excessive bone remodeling. The hallmark histologic feature is a "mosaic pattern" of irregular cement lines reflecting haphazard episodes of bone resorption and formation.

Question 1890

Topic: Biology, Genetics & Bone Healing

During the incorporation of a cortical structural allograft, the process by which host osteoclasts resorb the graft and osteoblasts lay down new bone in its place is known as:

. Osteoinduction
. Osteoconduction
. Creeping substitution
. Endochondral ossification
. Intramembranous ossification

Correct Answer & Explanation

. Creeping substitution

Explanation

Creeping substitution is the biological process where a dead bone graft is gradually resorbed by host osteoclasts and simultaneously replaced by new bone deposited by host osteoblasts.

Question 1891

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of fracture healing, which type of tissue is capable of tolerating the highest amount of interfragmentary strain before rupturing?

. Woven bone
. Lamellar bone
. Cartilage
. Granulation tissue
. Fibrous tissue

Correct Answer & Explanation

. Granulation tissue

Explanation

Granulation tissue can tolerate up to 100% strain without rupturing, bridging the initial highly mobile fracture gap. As stability increases and strain decreases, the tissue differentiates into cartilage (tolerates ~10% strain) and eventually bone (tolerates ~2% strain).

Question 1892

Topic: Biology, Genetics & Bone Healing

Denosumab is used in the treatment of postmenopausal osteoporosis and giant cell tumor of bone. What is its precise mechanism of action?

. Binds directly to the ruffled border of active osteoclasts
. Inhibits farnesyl pyrophosphate synthase in the mevalonate pathway
. Acts as a human monoclonal antibody against RANKL
. Competitively blocks the RANK receptor on osteoblasts
. Stimulates endogenous osteoprotegerin (OPG) production

Correct Answer & Explanation

. Acts as a human monoclonal antibody against RANKL

Explanation

Denosumab is a human monoclonal antibody that specifically binds to RANKL. By doing so, it prevents RANKL from interacting with the RANK receptor on osteoclasts, profoundly inhibiting osteoclast formation, function, and survival.

Question 1893

Topic: Biology, Genetics & Bone Healing

During normal secondary fracture healing, which phase is distinctly characterized by the highest peak of cellular proliferation and the critical transition of soft cartilaginous callus to hard woven bone callus?

. Hematoma formation phase
. Acute inflammation phase
. Reparative phase
. Remodeling phase
. Direct ossification phase

Correct Answer & Explanation

. Reparative phase

Explanation

The reparative phase features intense cellular proliferation, initially forming a soft cartilaginous callus. This soft callus subsequently undergoes endochondral ossification to form a mechanically stable hard bony callus.

Question 1894

Topic: Biology, Genetics & Bone Healing

Which of the following signaling molecules, produced primarily by mature osteocytes, acts as a potent negative regulator of bone formation by inhibiting the Wnt/beta-catenin pathway in osteoblasts?

. Osteocalcin
. Osteopontin
. Sclerostin
. Bone morphogenetic protein-2 (BMP-2)
. Receptor activator of nuclear factor kappa-B ligand (RANKL)

Correct Answer & Explanation

. Sclerostin

Explanation

Sclerostin is a glycoprotein selectively secreted by osteocytes that binds to LRP5/6 receptors on osteoblasts. This binding directly inhibits the Wnt/beta-catenin signaling pathway, thereby reducing bone formation.

Question 1895

Topic: Biology, Genetics & Bone Healing

A novel therapeutic agent targets the RANK receptor to inhibit osteoclastogenesis. Which of the following cells primarily expresses the RANK receptor?

. Osteoblasts
. Osteocytes
. Osteoclast precursors
. Mesenchymal stem cells
. Chondrocytes

Correct Answer & Explanation

. Osteoclast precursors

Explanation

The RANK receptor is expressed on the surface of osteoclast precursors and mature osteoclasts. Binding of RANKL (produced by osteoblasts) to RANK stimulates osteoclast differentiation and activation.

Question 1896

Topic: Biology, Genetics & Bone Healing

A 45-year-old woman with chronic kidney disease presents with diffuse bone pain. Laboratory studies reveal hypocalcemia, hyperphosphatemia, and elevated parathyroid hormone (PTH). What is the primary underlying pathophysiology of her bone disease?

. Deficiency of 25-hydroxyvitamin D
. Inability to convert 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D
. Primary hyperparathyroidism
. Defective Type I collagen synthesis
. Overproduction of osteoprotegerin (OPG)

Correct Answer & Explanation

. Inability to convert 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D

Explanation

Renal osteodystrophy is driven by the failing kidneys' inability to synthesize 1-alpha-hydroxylase, preventing the conversion of 25(OH)D to active 1,25(OH)2D. This leads to hypocalcemia, triggering secondary hyperparathyroidism.

Question 1897

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic protein-2 (BMP-2) promotes osteoinduction primarily by signaling through which of the following intracellular pathways?

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

Correct Answer & Explanation

. Smad 1/5/8

Explanation

BMPs bind to serine/threonine kinase receptors on the cell membrane, which subsequently phosphorylate intracellular Smad proteins (primarily Smad 1, 5, and 8). These translocate to the nucleus to regulate osteogenic gene expression.

Question 1898

Topic: Biology, Genetics & Bone Healing

A 35-year-old pregnant woman presents with an expansile, lytic lesion in the distal femur.

Histology reveals multinucleated giant cells distributed uniformly among mononuclear stromal cells. Denosumab therapy is considered. What is the mechanism of action of denosumab in this disease?

. Inhibition of matrix metalloproteinases
. Direct binding and induction of apoptosis in giant cells
. Binding to RANKL to prevent osteoclast-like giant cell activation
. Inhibition of vascular endothelial growth factor (VEGF)
. Stimulation of osteoprotegerin (OPG) production

Correct Answer & Explanation

. Binding to RANKL to prevent osteoclast-like giant cell activation

Explanation

The diagnosis is Giant Cell Tumor (GCT) of bone. Denosumab is a monoclonal antibody that binds to RANKL, preventing it from interacting with RANK on the multinucleated giant cells, thereby inhibiting their bone-resorbing activity.

Question 1899

Topic: Biology, Genetics & Bone Healing

A diaphyseal radius fracture is treated with rigid internal fixation using a dynamic compression plate, achieving absolute stability and anatomic reduction. This construct primarily dictates which pathway of bone healing?

. Endochondral ossification
. Intramembranous ossification
. Primary bone healing via cutting cones
. Secondary bone healing with exuberant callus formation
. Fibrocartilaginous bridging

Correct Answer & Explanation

. Primary bone healing via cutting cones

Explanation

Rigid internal fixation with absolute stability minimizes interfragmentary strain, bypassing callus formation. It induces primary (direct) bone healing, characterized by osteoclast-led cutting cones crossing the fracture site followed by osteoblast bone deposition.

Question 1900

Topic: Biology, Genetics & Bone Healing

A 30-year-old female presents with knee pain. Radiographs reveal an eccentric, lytic, epiphyseal lesion in the distal femur that extends to the subchondral bone. Histopathologic analysis shows multinucleated giant cells in a background of mononuclear cells. The targeted biologic therapy denosumab acts by inhibiting which of the following factors expressed by the neoplastic cells in this lesion?

. RANK receptor
. RANK ligand (RANKL)
. Osteoprotegerin (OPG)
. Macrophage colony-stimulating factor (M-CSF)
. Vascular endothelial growth factor (VEGF)

Correct Answer & Explanation

. RANK ligand (RANKL)

Explanation

In a giant cell tumor of bone, the true neoplastic cells are the mononuclear stromal cells, which highly express RANKL. Denosumab is a monoclonal antibody that binds and inhibits RANKL, preventing the recruitment and activation of the destructive multinucleated giant cells.

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