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Biology, Genetics & Bone Healing MCQs

Practice Set 111 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 2201

Topic: Biology, Genetics & Bone Healing

A biopsy is taken from a lytic, expansile lesion in the proximal femur of a 20-year-old patient. Histological examination reveals irregular, woven bone trabeculae lacking osteoblastic rimming, set within a bland fibrous stroma. The trabeculae are often described as having a 'Chinese character' appearance. Which of the following molecular abnormalities is the primary driver of this pathology?

. Defective synthesis of heparan sulfate
. Constitutive activation of the adenylate cyclase pathway
. Loss of function of the retinoblastoma (Rb) tumor suppressor protein
. Overexpression of fibroblast growth factor receptor 3 (FGFR3)
. Defective osteoclast-mediated bone resorption

Correct Answer & Explanation

. Constitutive activation of the adenylate cyclase pathway

Explanation

Correct Answer: Constitutive activation of the adenylate cyclase pathwayThe histology describes Fibrous Dysplasia (woven bone lacking osteoblastic rimming in a fibrous stroma, 'Chinese characters'). Fibrous dysplasia is caused by a somatic, post-zygotic activating mutation in the GNAS gene. This mutation affects the alpha subunit of the stimulatory G protein (Gs-alpha), leading to constitutive activation of adenylate cyclase and an overproduction of intracellular cyclic AMP (cAMP). This downstream signaling cascade results in the anarchic proliferation of undifferentiated mesenchymal cells that fail to mature into normal osteoblasts. Defective heparan sulfate synthesis is seen in Multiple Hereditary Exostoses.

Question 2202

Topic: Biology, Genetics & Bone Healing

A 12-year-old boy with Ollier disease has a 6 cm leg length discrepancy due to extensive involvement of his left femur. He undergoes a femoral lengthening procedure using an external fixator. Which of the following complications is most specifically associated with distraction osteogenesis in patients with this condition?

. Formation of cartilaginous tissue within the distraction gap leading to delayed or non-union.
. Premature consolidation of the regenerate bone.
. Uncontrollable pin tract infections due to underlying immunodeficiency.
. Rapid malignant transformation of the enchondromas during the distraction phase.
. Severe heterotopic ossification around the pin sites.

Correct Answer & Explanation

. Formation of cartilaginous tissue within the distraction gap leading to delayed or non-union.

Explanation

Correct Answer: Formation of cartilaginous tissue within the distraction gap leading to delayed or non-union.Ollier disease (multiple enchondromatosis) involves a defect in normal endochondral ossification, where chondrocytes fail to undergo normal hypertrophy and apoptosis, leaving islands of unmineralized cartilage in the metaphyses and diaphyses. When distraction osteogenesis is performed through or near these dysplastic segments, the regenerate tissue often forms abnormal cartilaginous tissue rather than normal woven bone. This leads to poor quality regenerate, delayed consolidation, and a high risk of non-union or fracture of the regenerate after frame removal. Slower distraction rates and prolonged frame time are often required.

Question 2203

Topic: Biology, Genetics & Bone Healing
A 2-year-old child with multiple café-au-lait spots presents with anterolateral bowing of the tibia. Radiographs demonstrate medullary sclerosis and a frank pseudarthrosis at the junction of the middle and distal thirds of the tibia (Crawford Type IV). What is the most critical biological factor contributing to the failure of bone healing in this specific condition?
. Hyperactive osteoclast function due to RANKL overexpression
. Thickened, highly cellular, and fibromatosis-like periosteum
. Complete absence of the nutrient artery to the tibia
. Defective type I collagen synthesis
. Overexpression of FGFR3 inhibiting chondrocyte proliferation

Correct Answer & Explanation

. Thickened, highly cellular, and fibromatosis-like periosteum

Explanation

Congenital pseudarthrosis of the tibia (CPT) is strongly associated with Neurofibromatosis type 1 (NF1). The primary biological barrier to healing in CPT is the presence of a thickened, hamartomatous, fibromatosis-like periosteum surrounding the pseudarthrosis site. This abnormal periosteum restricts blood supply, causes local bone resorption, and mechanically prevents callus formation. Successful surgical treatment requires complete excision of this diseased periosteum (hamartoma) down to healthy tissue.

Question 2204

Topic: Biology, Genetics & Bone Healing

A 5-year-old boy presents with painless anterior bowing of the tibia. Radiographs show an intracortical, multi-loculated radiolucent lesion in the anterior diaphyseal cortex of the tibia with sclerotic margins. Biopsy shows fibrous tissue with trabeculae of woven bone rimmed by active osteoblasts. Which of the following is the most appropriate initial management?

. Immediate wide en bloc resection
. Curettage and bone grafting
. Observation and bracing
. Radiation therapy
. Intramedullary nailing

Correct Answer & Explanation

. Observation and bracing

Explanation

Correct Answer: Observation and bracingThe clinical and radiographic picture describes osteofibrous dysplasia (Campanacci disease), which almost exclusively affects the anterior cortex of the tibia in children under 10 years old. Histologically, it is distinguished from fibrous dysplasia by the presence of osteoblastic rimming around the bone trabeculae. The natural history of osteofibrous dysplasia is stabilization or regression after skeletal maturity. Early surgical intervention (curettage or excision) carries an extremely high recurrence rate. Therefore, observation and bracing (to prevent fracture) is the recommended initial management, delaying surgery until skeletal maturity unless there is an impending fracture or severe progressive deformity.

Question 2205

Topic: Biology, Genetics & Bone Healing

A 30-year-old male with polyostotic fibrous dysplasia complains of severe, persistent bone pain in his lower extremities that is refractory to NSAIDs. Radiographs show stable ground-glass lesions without impending fractures or significant deformity. Which of the following pharmacological interventions is most appropriate to manage his bone pain?

. Intravenous bisphosphonates
. Oral corticosteroids
. Methotrexate
. Teriparatide (PTH analog)
. Denosumab

Correct Answer & Explanation

. Intravenous bisphosphonates

Explanation

Correct Answer: Intravenous bisphosphonatesIn fibrous dysplasia, the anarchic fibrous tissue produces factors that stimulate osteoclastic bone resorption, leading to bone pain and structural weakness. Intravenous bisphosphonates (such as pamidronate or zoledronic acid) are the mainstay of medical therapy for symptomatic fibrous dysplasia. They inhibit osteoclast activity, significantly reducing bone pain and potentially improving the radiographic appearance of the lesions. Teriparatide is contraindicated as it stimulates bone turnover, and denosumab is generally reserved for giant cell tumors or refractory cases under strict protocols due to rebound hypercalcemia risks.

Question 2206

Topic: Biology, Genetics & Bone Healing

A neonate is evaluated for short limbs, bilateral severe rigid clubfeet, hitchhiker thumbs, and cystic swelling of the ear pinnae. Genetic testing reveals a mutation in the SLC26A2 gene. This defect primarily impairs which of the following cellular processes?

. Type II collagen synthesis
. Cartilage oligomeric matrix protein secretion
. Fibroblast growth factor receptor signaling
. Sulfate transport into chondrocytes
. Transcription of Runx2

Correct Answer & Explanation

. Sulfate transport into chondrocytes

Explanation

Diastrophic dysplasia is caused by a mutation in SLC26A2 (DTDST), leading to defective intracellular sulfate transport and underminosulfation of cartilage proteoglycans. Clinical hallmarks include hitchhiker thumbs, cauliflower ears, and severe clubfeet.

Question 2207

Topic: Biology, Genetics & Bone Healing

A 5-year-old presents with frequent fractures, hepatosplenomegaly, and cranial nerve palsies. Radiographs show diffusely dense bones with loss of medullary canals and an "Erlenmeyer flask" deformity of the distal femurs. The defective cellular mechanism primarily involves which of the following?

. Osteoblast terminal differentiation
. Chondrocyte proliferation in the resting zone
. Carbonic anhydrase II deficiency inhibiting lacunar acidification
. Cleavage of type I procollagen C-propeptides
. Type X collagen synthesis in the hypertrophic zone

Correct Answer & Explanation

. Carbonic anhydrase II deficiency inhibiting lacunar acidification

Explanation

Osteopetrosis is caused by defective osteoclast function, frequently due to mutations affecting carbonic anhydrase II or TCIRG1, preventing the acidification of the resorption pit. This leads to dense, brittle bones, marrow obliteration, and extramedullary hematopoiesis.

Question 2208

Topic: Biology, Genetics & Bone Healing

A patient presents with delayed closure of cranial sutures, supernumerary teeth, and the ability to appose their shoulders in the midline. Radiographs reveal absent clavicles and delayed ossification of the pubic symphysis. This condition is caused by a defect in a transcription factor essential for which of the following processes?

. Osteoclast bone resorption
. Chondrocyte hypertrophy in endochondral ossification
. Osteoblast differentiation and intramembranous ossification
. Type I collagen post-translational cross-linking
. Sulfate transport in the cartilage matrix

Correct Answer & Explanation

. Osteoblast differentiation and intramembranous ossification

Explanation

Cleidocranial dysplasia is caused by a mutation in RUNX2 (CBFA1), a master transcription factor critical for osteoblast differentiation and intramembranous ossification. The defective ossification leads to absent clavicles, wide cranial sutures, and dental anomalies.

Question 2209

Topic: Biology, Genetics & Bone Healing

An infant presents with severe hypercalcemia, hypophosphatemia, and markedly irregular, cupped metaphyses on radiographs. Serum parathyroid hormone (PTH) and PTH-related peptide (PTHrP) levels are undetectable. What is the underlying pathophysiology?

. Vitamin D receptor mutation
. Ligand-independent activation of the PTH/PTHrP receptor
. Inactivating mutation of the calcium-sensing receptor
. Defect in renal 1-alpha-hydroxylase
. Hyperplasia of the parathyroid glands

Correct Answer & Explanation

. Ligand-independent activation of the PTH/PTHrP receptor

Explanation

Jansen metaphyseal chondrodysplasia is caused by an activating mutation in the PTH1R gene, leading to ligand-independent activation of the PTH/PTHrP receptor. This mimics hyperparathyroidism biologically, causing severe hypercalcemia and extensive metaphyseal abnormalities.

Question 2210

Topic: Biology, Genetics & Bone Healing

An infant is diagnosed with malignant infantile osteopetrosis. Radiographs reveal diffuse, uniform bone sclerosis ('bone within bone' appearance). Genetic testing reveals a mutation in the TCIRG1 gene. At the cellular level, what is the primary consequence of this mutation?

. Excessive proliferation of osteoblasts
. Failure of osteoclasts to undergo apoptosis
. Failure of osteoclasts to form a functioning ruffled border
. Defective cleavage of type I collagen propeptides
. Inability of osteocytes to signal mechanotransduction

Correct Answer & Explanation

. Failure of osteoclasts to form a functioning ruffled border

Explanation

TCIRG1 mutations cause a defect in the vacuolar proton pump required for acidification of the resorption pit. This prevents osteoclasts from forming a functioning ruffled border, halting normal bone resorption and leading to osteopetrosis.

Question 2211

Topic: Biology, Genetics & Bone Healing
A female newborn presents with asymmetric limb shortening, cataracts, and a scaling skin rash (ichthyosis). Radiographs show prominent stippled epiphyses in the proximal femurs and humeri. Genetic analysis indicates an X-linked dominant inheritance pattern. This disorder is characterized by a defect in which of the following metabolic pathways?
. Glycosaminoglycan degradation
. Type I collagen synthesis
. Lysosomal trafficking
. Cholesterol biosynthesis
. Wnt-beta catenin signaling

Correct Answer & Explanation

. Cholesterol biosynthesis

Explanation

This describes Conradi-Hünermann-Happle syndrome, an X-linked dominant form of chondrodysplasia punctata. It is caused by mutations in the EBP gene, leading to defective cholesterol biosynthesis and accumulation of sterol precursors.

Question 2212

Topic: Biology, Genetics & Bone Healing

A neonate is born with severe respiratory distress, marked bowing of the tibiae with overlying skin dimples, and hypoplastic scapulae. Chromosomal analysis reveals a 46,XY karyotype, but the genitalia are phenotypically female (sex reversal). Which transcription factor is mutated in this frequently lethal dysplasia?

. RUNX2
. SOX9
. CBFA1
. HoxA13
. GLI3

Correct Answer & Explanation

. SOX9

Explanation

Campomelic dysplasia presents with severe limb bowing, respiratory compromise (due to tracheomalacia), and 46,XY sex reversal. It is caused by a mutation in the SOX9 gene, a crucial transcription factor for chondrogenesis and testis development.

Question 2213

Topic: Biology, Genetics & Bone Healing

A 10-year-old boy with a known skeletal dysplasia develops progressive, massive hyperplastic callus formation following a closed femur fracture. Radiographs also demonstrate calcification of the interosseous membranes of the forearms. What is the inheritance pattern and associated gene mutation for this specific dysplasia?

. Autosomal recessive, SERPINF1
. Autosomal dominant, COL1A1
. Autosomal recessive, CRTAP
. Autosomal dominant, IFITM5
. Autosomal dominant, COMP

Correct Answer & Explanation

. Autosomal dominant, IFITM5

Explanation

Osteogenesis Imperfecta Type V is characterized by a classic triad: hyperplastic callus formation, interosseous membrane calcification, and radiodense metaphyseal bands. It is inherited in an autosomal dominant pattern due to a specific mutation in the IFITM5 gene.

Question 2214

Topic: Biology, Genetics & Bone Healing

A neonate is evaluated in the NICU for micromelic short stature, severe rigid equinovarus foot deformities, and "hitchhiker" thumbs. Physical examination also reveals cystic swelling of the external ears. The genetic defect responsible for this condition affects which of the following physiological processes?

. Transmembrane receptor tyrosine kinase signaling
. Intracellular sulfate transport in cartilage matrix
. Type II collagen triple-helix formation
. Chondrocyte apoptosis regulation
. Runx2 transcription factor binding during intramembranous ossification

Correct Answer & Explanation

. Intracellular sulfate transport in cartilage matrix

Explanation

Diastrophic dysplasia is characterized by hitchhiker thumbs, severe clubfeet, and cauliflower ears. It is caused by an autosomal recessive mutation in the SLC26A2 (DTDST) gene, leading to defective intracellular sulfate transport and undersulfated proteoglycans in the cartilage matrix.

Question 2215

Topic: Biology, Genetics & Bone Healing

A 12-year-old girl is evaluated for a prominent forehead, hypertelorism, and delayed eruption of permanent teeth. On physical examination, she is able to approximate her shoulders anteriorly in the midline. Which of the following best describes the underlying molecular defect in this condition?

. Defective intramembranous ossification due to a RUNX2 mutation
. Defective endochondral ossification due to an FGFR3 mutation
. Defective osteoclast ruffled border formation due to a TCIRG1 mutation
. Impaired cartilage oligomeric matrix protein assembly
. Defective type I collagen cross-linking

Correct Answer & Explanation

. Defective intramembranous ossification due to a RUNX2 mutation

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by a mutation in the RUNX2 (CBFA1) gene, an essential transcription factor for osteoblast differentiation. This defect primarily impairs intramembranous ossification, leading to hypoplastic or absent clavicles, delayed cranial suture closure, and dental anomalies.

Question 2216

Topic: Biology, Genetics & Bone Healing

A 14-year-old boy sustains a transverse subtrochanteric femur fracture after a minor fall. Radiographs show diffusely dense, chalk-like bones with a loss of corticomedullary differentiation and a "bone-within-bone" appearance. He has a history of mild anemia and hearing loss. What is the primary cellular dysfunction resulting in this skeletal phenotype?

. Hyperactive osteoblast matrix deposition
. Failure of osteoclasts to form a ruffled border
. Excessive production of unmineralized osteoid
. Premature apoptosis of hypertrophic chondrocytes
. Defective osteocyte canalicular networks

Correct Answer & Explanation

. Failure of osteoclasts to form a ruffled border

Explanation

Osteopetrosis is caused by defective osteoclast-mediated bone resorption, most commonly due to mutations affecting the proton pump (TCIRG1) or chloride channels (CLCN7). This prevents the formation of a functional ruffled border and the acidic microenvironment required to dissolve bone mineral.

Question 2217

Topic: Biology, Genetics & Bone Healing

A neonate is born with severe respiratory distress, shortened and anterolaterally bowed tibiae with skin dimpling over the apex of the bow, and ambiguous genitalia. Chromosomal analysis reveals a 46,XY karyotype despite phenotypic female external genitalia. The mutated gene in this syndrome is crucial for which of the following processes?

. Osteoclast-mediated bone resorption
. Sulfate transport in articular cartilage
. Sex determination and chondrogenesis
. Type I collagen triple-helix folding
. Endochondral ossification of the appendicular skeleton only

Correct Answer & Explanation

. Sex determination and chondrogenesis

Explanation

Campomelic dysplasia is caused by a mutation in the SOX9 gene, a transcription factor that is essential for both chondrogenesis and normal testes development. This dual role explains the severe bowing of long bones, tracheobronchomalacia leading to respiratory failure, and XY sex reversal.

Question 2218

Topic: Biology, Genetics & Bone Healing

In a patient diagnosed with chondroblastoma of the proximal humerus, what specific histological feature is considered pathognomonic for this lesion?

. Woven bone with prominent osteoblastic rimming
. Herringbone pattern of spindle cells
. "Chicken wire" pericellular calcifications
. Homer-Wright rosettes
. Abundant myxoid stroma with lipoblasts

Correct Answer & Explanation

. "Chicken wire" pericellular calcifications

Explanation

Chondroblastoma typically features mononuclear chondroblasts, osteoclast-like giant cells, and islands of eosinophilic chondroid matrix. The presence of fine, lace-like pericellular calcifications (the "chicken wire" pattern) is pathognomonic.

Question 2219

Topic: Biology, Genetics & Bone Healing

A 6-year-old child presents with bowing of the legs and a waddling gait. Radiographs show widened physes and cupped, frayed metaphyses. Laboratory investigations reveal normal serum calcium, phosphorus, alkaline phosphatase, and Vitamin D levels. Which of the following is the most appropriate next step in management?

. High-dose Vitamin D supplementation
. Genetic testing for COL10A1 mutation
. Initiation of bisphosphonate therapy
. Parathyroidectomy
. Genetic testing for PHEX mutation

Correct Answer & Explanation

. Genetic testing for COL10A1 mutation

Explanation

Correct Answer: Genetic testing for COL10A1 mutationThe clinical and radiographic picture strongly resembles rickets, but the completely normal laboratory profile (normal Ca, PO4, ALP, Vit D) rules out nutritional and hypophosphatemic rickets. This presentation is classic for Schmid metaphyseal chondrodysplasia, which is caused by a COL10A1 mutation. Genetic testing confirms the diagnosis.

Question 2220

Topic: Biology, Genetics & Bone Healing
A 2-year-old child presents with delayed walking, bowing of the long bones, and premature loss of deciduous teeth. Radiographs show irregular, radiolucent metaphyses resembling severe rickets. Laboratory tests reveal hypercalcemia and significantly decreased serum alkaline phosphatase. Which of the following is the underlying defect?
. Defective type I collagen synthesis
. Mutation in the ALPL gene encoding tissue-nonspecific alkaline phosphatase
. Mutation in the PHEX gene
. Vitamin D receptor mutation
. Defective osteoclast function

Correct Answer & Explanation

. Mutation in the ALPL gene encoding tissue-nonspecific alkaline phosphatase

Explanation

Hypophosphatasia is a rare metabolic bone disease caused by mutations in the ALPL gene, leading to deficient tissue-nonspecific alkaline phosphatase (TNSALP). This causes defective bone mineralization (resembling rickets radiographically with metaphyseal irregularities) but is uniquely characterized by low alkaline phosphatase levels, hypercalcemia, and premature loss of deciduous teeth.

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