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Biology, Genetics & Bone Healing MCQs

Practice Set 103 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 2041

Topic: Biology, Genetics & Bone Healing

An infant presents with recurrent fractures, cranial nerve palsies, and diffuse uniform osteosclerosis on whole-body radiographs. Bone marrow aspiration is dry, and a diagnosis of infantile malignant osteopetrosis is suspected. What is the primary cellular defect in this disorder?

. Failure of osteoblasts to produce osteoid
. Defective osteoclast ruffled border formation and acidic microenvironment
. Hyperactivity of osteocytes
. Absence of mesenchymal stem cells in the marrow
. Defective primary mineralization of the organic matrix

Correct Answer & Explanation

. Defective osteoclast ruffled border formation and acidic microenvironment

Explanation

Osteopetrosis is characterized by the failure of osteoclasts to resorb bone due to defects in maintaining an acidic environment (e.g., CAII or TCIRG1 mutations) or defective ruffled border formation. This failure of bone turnover leads to overly dense, brittle bone and subsequent marrow obliteration.

Question 2042

Topic: Biology, Genetics & Bone Healing

A 70-year-old man presents with increasing hat size, sensorineural hearing loss, and a significant bowing deformity of his right femur. Laboratory evaluation shows an isolated marked elevation of serum alkaline phosphatase. The initial pathogenic phase of this disease process is driven primarily by which of the following?

. Intense osteoblastic woven bone formation
. Intense, localized osteoclastic bone resorption
. Replacement of the normal marrow space with dense fibrous stroma
. Failure of physiologic osteoid mineralization
. Ischemic necrosis of the diaphyseal marrow elements

Correct Answer & Explanation

. Intense, localized osteoclastic bone resorption

Explanation

Paget disease of bone begins with an initial, intensely osteoclastic (lytic) phase characterized by massive localized bone resorption. This is followed sequentially by a mixed osteoclastic/osteoblastic phase, and finally an inactive osteosclerotic phase.

Question 2043

Topic: Biology, Genetics & Bone Healing
An 8-year-old boy presents with severe, progressive thoracic scoliosis. Physical examination reveals 7 café-au-lait spots measuring >5 mm and axillary freckling. Radiographs demonstrate a sharp, short-segment angular kyphoscoliosis. What is the primary cellular function of the protein encoded by the mutated gene in this condition?
. Downregulates Ras signaling
. Acts as a Tyrosine kinase receptor
. Upregulates the Wnt pathway
. Functions as an osteoclast proton pump
. Acts as a BMP antagonist

Correct Answer & Explanation

. Downregulates Ras signaling

Explanation

Neurofibromatosis Type 1 is caused by a mutation in the NF1 gene, which produces neurofibromin. Neurofibromin is a GTPase-activating protein (GAP) that normally downregulates the Ras signaling pathway.

Question 2044

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with knee pain. Radiographs reveal an eccentric, lytic lesion in the proximal tibia that extends to the subchondral bone. Biopsy demonstrates sheets of mononuclear stromal cells and numerous multinucleated giant cells. Which of the following best describes the mechanism of the targeted medical therapy for this condition?

. Binds directly to the ruffled border of osteoclasts
. Binds RANK ligand (RANKL)
. Inhibits vascular endothelial growth factor (VEGF)
. Inhibits the osteoclast proton pump
. Causes direct cytotoxicity to the mononuclear spindle cells

Correct Answer & Explanation

. Binds RANK ligand (RANKL)

Explanation

Giant Cell Tumor of bone consists of neoplastic mononuclear stromal cells that express RANKL, which recruits reactive, bone-destroying multinucleated giant cells. Denosumab is a monoclonal antibody that binds RANKL, effectively halting osteoclastogenesis and tumor-associated osteolysis.

Question 2045

Topic: Biology, Genetics & Bone Healing

A 70-year-old man complains of progressive bowing of his femurs, increasing hat size, and new-onset hearing loss. Labs show a markedly elevated alkaline phosphatase but normal serum calcium and phosphate. A biopsy of the affected bone would most likely demonstrate which of the following?

. A mosaic pattern of lamellar bone with prominent cement lines
. Monotonous sheets of plasma cells
. Woven bone with prominent osteoblastic rimming
. Atypical chondrocytes in enlarged lacunae
. Dead trabeculae with empty lacunae and marrow fibrosis

Correct Answer & Explanation

. A mosaic pattern of lamellar bone with prominent cement lines

Explanation

Paget's disease of bone is characterized by excessive, uncoordinated osteoclastic and osteoblastic activity. This disordered remodeling results in thickened, mechanically weak bone with a pathognomonic 'mosaic' or 'jigsaw puzzle' pattern of cement lines on histology.

Question 2046

Topic: Biology, Genetics & Bone Healing
A 4-year-old boy has a history of multiple fractures, hepatosplenomegaly, and severe anemia. Radiographs demonstrate generalized, extreme bone density with a 'bone-within-bone' appearance. What is the fundamental cellular defect in this condition?
. Defective type I collagen synthesis
. Defective osteoclast resorption due to an absent ruffled border
. Constitutive overactivation of osteoblasts
. Impaired mineralization of osteoid matrix
. Excessive, uncontrolled cartilage proliferation

Correct Answer & Explanation

. Defective osteoclast resorption due to an absent ruffled border

Explanation

Osteopetrosis (Albers-Schönberg disease) results from defective osteoclastic bone resorption, frequently due to mutations in TCIRG1 or CLCN7. The osteoclasts lack the ruffled border necessary to secrete protons and acidify the resorption pit, leading to dense but extremely brittle bones.

Question 2047

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with progressive knee pain. Radiographs reveal an eccentric, expansile, lytic lesion in the epiphysis of the distal femur that extends to the subchondral bone without a sclerotic margin. A core needle biopsy reveals a proliferation of uniform mononuclear cells interspersed with numerous multinucleated giant cells. Which of the following best describes the underlying cellular pathogenesis and the precise mechanism of targeted medical therapy for this condition?

. The multinucleated giant cells are neoplastic and express RANKL; denosumab binds to these cells to induce apoptosis.
. The mononuclear stromal cells are neoplastic and express RANK; denosumab acts as a RANK antagonist to prevent osteolysis.
. The mononuclear stromal cells are neoplastic and express RANKL; denosumab binds RANKL to prevent activation of osteoclast-like giant cells.
. The multinucleated giant cells are neoplastic and express RANK; bisphosphonates inhibit their farnesyl pyrophosphate synthase activity.
. The lesion is driven by a USP6 gene rearrangement in both cell populations; denosumab inhibits the resulting inflammatory cascade.

Correct Answer & Explanation

. The mononuclear stromal cells are neoplastic and express RANKL; denosumab binds RANKL to prevent activation of osteoclast-like giant cells.

Explanation

In giant cell tumor (GCT) of bone, the mononuclear spindle-like stromal cells are the true neoplastic component and they heavily express RANKL. This RANKL recruits and activates benign, osteoclast-like multinucleated giant cells (which express RANK) to cause bone resorption; denosumab is a monoclonal antibody that binds RANKL to halt this destructive process.

Question 2048

Topic: Biology, Genetics & Bone Healing

A neonate born with a short trunk, prominent joints, and a cleft palate undergoes a radiographic skeletal survey. The films show characteristic coronal clefts of the vertebral bodies and dumbbell-shaped femora. Which specific gene mutation is responsible for this skeletal dysplasia?

. FGFR3
. SLC26A2
. COL2A1
. SOX9
. RUNX2

Correct Answer & Explanation

. COL2A1

Explanation

The clinical presentation and radiographic findings (coronal clefts of vertebrae, dumbbell-shaped femora) are pathognomonic for Kniest dysplasia. This is a type II collagenopathy caused by a mutation in the COL2A1 gene.

Question 2049

Topic: Biology, Genetics & Bone Healing

A newborn presents with severe anterior bowing of the tibiae, skin dimples over the tibial apex, and immediate, severe respiratory distress. Radiographs show hypoplastic cervical vertebral bodies and absent pedicles. What is the underlying genetic mutation for this condition?

. SOX9
. FGFR3
. COMP
. SLC26A2
. DLL3

Correct Answer & Explanation

. SOX9

Explanation

This presentation is highly characteristic of camptomelic dysplasia, an autosomal dominant disorder caused by mutations in the SOX9 gene. Cervical spine instability due to absent pedicles and fatal respiratory distress from tracheobronchomalacia are hallmarks.

Question 2050

Topic: Biology, Genetics & Bone Healing

When planning a deformity correction using an external fixator for distraction osteogenesis, a latency period of 7 to 10 days is typically observed. What is the primary biological purpose of this phase?

. To allow rigid consolidation of the osteotomy site
. To permit the initial inflammatory phase and soft callus formation
. To completely eliminate the risk of pin tract infections
. To allow the periosteum to completely detach from the cortex
. To promote immediate cortical bridging prior to distraction

Correct Answer & Explanation

. To permit the initial inflammatory phase and soft callus formation

Explanation

The 7-10 day latency period allows for the resolution of acute inflammation and the initiation of soft callus formation with mesenchymal stem cell aggregation. Immediate distraction disrupts this vital early phase, leading to poor regenerate bone formation.

Question 2051

Topic: Biology, Genetics & Bone Healing

During gradual distraction osteogenesis for lower limb lengthening, what is the primary mode of bone formation observed within the distraction gap?

. Endochondral ossification
. Intramembranous ossification
. Appositional ossification
. Creeping substitution
. Enchondral sequence

Correct Answer & Explanation

. Intramembranous ossification

Explanation

Bone formation during classic distraction osteogenesis (Ilizarov technique) occurs primarily via intramembranous ossification. This process requires stable fixation, proper latency, and an appropriate distraction rhythm.

Question 2052

Topic: Biology, Genetics & Bone Healing

A 25-year-old male undergoing tibial distraction osteogenesis at a rate of 1 mm/day is noted on 4-week follow-up radiographs to have premature consolidation of the regenerate. What is the most appropriate next step in management?

. Increase the distraction rate to 2 mm/day
. Administer systemic bisphosphonates to inhibit osteoblasts
. Return to the operating room for re-osteotomy
. Decrease the distraction rate to 0.25 mm/day
. Apply localized extracorporeal shockwave therapy

Correct Answer & Explanation

. Return to the operating room for re-osteotomy

Explanation

Premature consolidation occurs when the rate of bone formation outpaces the distraction rate. Once consolidation bridges the gap, closed distraction is impossible, and surgical re-osteotomy is required.

Question 2053

Topic: Biology, Genetics & Bone Healing

When performing a diaphyseal tibial corticotomy for Ilizarov distraction osteogenesis in an adult, what is the standard recommended latency period before initiating distraction?

. 0 to 1 days
. 2 to 3 days
. 7 to 10 days
. 14 to 21 days
. 28 days

Correct Answer & Explanation

. 7 to 10 days

Explanation

A latency period of 7 to 10 days is optimal for diaphyseal distraction in adults. This allows for initial fracture hematoma organization and the beginning of the soft callus phase prior to applying tension.

Question 2054

Topic: Biology, Genetics & Bone Healing

During distraction osteogenesis of the tibia using a circular fixator, a patient develops premature consolidation of the regenerate bone. What is the most appropriate initial management?

. Abandon the procedure and perform open bone grafting
. Increase the rate of distraction to 2 mm/day temporarily
. Perform a closed osteoclasis under anesthesia
. Remove the frame and apply a long leg cast
. Administer systemic bisphosphonates

Correct Answer & Explanation

. Perform a closed osteoclasis under anesthesia

Explanation

Premature consolidation occurs when the bone healing outpaces the distraction rate. The initial treatment of choice is closed osteoclasis (breaking the regenerate manually under anesthesia) to resume distraction.

Question 2055

Topic: Biology, Genetics & Bone Healing

A 35-year-old patient is undergoing tibial lengthening via distraction osteogenesis (Ilizarov technique) at a standard rate of 1 mm/day. Radiographs at 6 weeks reveal a central radiolucent fibrous interzone of 9 mm with poor regenerate bone formation. What is the most appropriate next step in management?

. Increase the distraction rate to 1.5 mm/day to stimulate the periosteum.
. Proceed immediately with autologous iliac crest bone grafting.
. Perform an accordion maneuver (sequential compression and distraction).
. Remove the external fixator and transition to a patellar tendon-bearing cast.
. Administrate low-dose bisphosphonates to prevent regenerate resorption.

Correct Answer & Explanation

. Perform an accordion maneuver (sequential compression and distraction).

Explanation

Delayed consolidation or poor regenerate during distraction osteogenesis is effectively managed with the accordion maneuver. This process of alternating compression and distraction stimulates local angiogenesis and enhances osteogenesis.

Question 2056

Topic: Biology, Genetics & Bone Healing

Which of the following is the primary mechanism of action for the class of medications most commonly used to reduce fracture risk in pediatric patients with Osteogenesis Imperfecta?

. Inhibition of osteoblast apoptosis
. Stimulation of Type I collagen synthesis
. Inhibition of osteoclast-mediated bone resorption
. Activation of the Wnt/beta-catenin pathway
. Enhancement of calcium intestinal absorption

Correct Answer & Explanation

. Inhibition of osteoclast-mediated bone resorption

Explanation

Bisphosphonates are the standard pharmacological treatment for Osteogenesis Imperfecta. They function by inhibiting osteoclast-mediated bone resorption, which increases bone density and decreases fracture incidence.

Question 2057

Topic: Biology, Genetics & Bone Healing

A newborn presents with delayed closure of the cranial sutures, wormian bones, and absent clavicles. What gene mutation is associated with this syndrome?

. FGFR3
. COL1A1
. RUNX2 (CBFA1)
. SOX9
. COMP

Correct Answer & Explanation

. RUNX2 (CBFA1)

Explanation

Cleidocranial dysplasia is an autosomal dominant condition characterized by absent or hypoplastic clavicles and delayed cranial suture closure. It is caused by a mutation in the RUNX2 (CBFA1) transcription factor, essential for osteoblast differentiation.

Question 2058

Topic: Biology, Genetics & Bone Healing

A newborn presents with short-limbed dwarfism, bilateral rigid clubfeet, "hitchhiker" thumbs, and cystic swelling of the pinnae. What is the primary pathophysiologic defect in this condition?

. Impaired cartilage sulfation due to a sulfate transporter defect
. Abnormal type II collagen formation
. Decreased degradation of dermatan sulfate
. Abnormal intramembranous ossification
. Defective osteoclast ruffled border formation

Correct Answer & Explanation

. Impaired cartilage sulfation due to a sulfate transporter defect

Explanation

Diastrophic dysplasia is caused by an autosomal recessive mutation in the SLC26A2 gene, resulting in a defect in the diastrophic dysplasia sulfate transporter (DTDST). This leads to profoundly impaired sulfation of cartilage proteoglycans.

Question 2059

Topic: Biology, Genetics & Bone Healing

A 7-year-old girl is evaluated for delayed dental eruption and abnormal shoulder mobility, allowing her to touch her shoulders together anteriorly. Radiographs show absent clavicles and delayed closure of cranial sutures. What is the mutated gene responsible for this condition?

. COL1A1
. COMP
. RUNX2 (CBFA1)
. SOX9
. FGFR3

Correct Answer & Explanation

. RUNX2 (CBFA1)

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by mutations in the RUNX2 (formerly CBFA1) gene, which is essential for osteoblast differentiation. It is characterized by absent or hypoplastic clavicles, delayed suture closure, and multiple dental anomalies.

Question 2060

Topic: Biology, Genetics & Bone Healing

A 4-year-old child with recurrent fractures, blue sclerae, and dentinogenesis imperfecta is treated with intravenous pamidronate. What is the primary mechanism of action of this medication in treating this condition?

. Stimulates osteoblast differentiation
. Inhibits osteoclast-mediated bone resorption by causing osteoclast apoptosis
. Increases the synthesis of type I collagen
. Enhances calcium absorption in the gut
. Promotes cross-linking of collagen fibrils

Correct Answer & Explanation

. Inhibits osteoclast-mediated bone resorption by causing osteoclast apoptosis

Explanation

Bisphosphonates like pamidronate are used in Osteogenesis Imperfecta to increase bone mineral density by inhibiting osteoclast-mediated bone resorption. They are internalized by osteoclasts, leading to disruption of the cytoskeleton and apoptosis.

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