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Biology, Genetics & Bone Healing MCQs

Practice Set 189 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 3761

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy presents with short trunk dwarfism, a barrel chest, and coxa vara. Radiographs reveal flattening of the vertebral bodies and delayed ossification of the femoral head and neck epiphyses. An abnormality in which of the following proteins is the underlying cause?

. Type I collagen
. Type II collagen
. Type X collagen
. COMP
. Aggrecan

Correct Answer & Explanation

. Type II collagen

Explanation

Spondyloepiphyseal dysplasia (SED) affects both the spine and epiphyses. It is caused by a mutation in the COL2A1 gene, resulting in defective Type II collagen.

Question 3762

Topic: Biology, Genetics & Bone Healing

A 6-year-old girl with osteogenesis imperfecta is treated with intravenous pamidronate.

What is the primary mechanism of action of this medication in this patient?

. Stimulation of osteoblast proliferation
. Induction of osteoclast apoptosis and inhibition of bone resorption
. Enhancement of Type I collagen cross-linking
. Direct stimulation of renal calcium reabsorption
. Inhibition of parathyroid hormone secretion

Correct Answer & Explanation

. Induction of osteoclast apoptosis and inhibition of bone resorption

Explanation

Bisphosphonates like pamidronate are internalized by osteoclasts and induce apoptosis by inhibiting the mevalonate pathway. This reduces bone resorption and increases bone mass in patients with OI.

Question 3763

Topic: Biology, Genetics & Bone Healing

An 18-month-old child presents with enlarged wrists, bowing of the legs, and frontal bossing. Laboratory studies demonstrate low serum 25-hydroxyvitamin D, low-normal calcium, low phosphorus, and significantly elevated parathyroid hormone. Which of the following is the most likely diagnosis?

. Nutritional rickets
. X-linked hypophosphatemic rickets
. Renal osteodystrophy
. Hypophosphatasia
. Vitamin D-dependent rickets type II

Correct Answer & Explanation

. Nutritional rickets

Explanation

Nutritional rickets is characterized by vitamin D deficiency leading to decreased calcium and phosphate absorption. This causes secondary hyperparathyroidism, which normalizes calcium at the expense of severe hypophosphatemia and bone demineralization.

Question 3764

Topic: Biology, Genetics & Bone Healing

A 55-year-old woman presents with a pathologic fracture of her proximal humerus. Radiographs show a well-defined lytic lesion. Laboratory values reveal hypercalcemia, hypophosphatemia, and markedly elevated alkaline phosphatase. Which of the following describes the histological appearance of this bone lesion?

. Sheets of atypical plasma cells with eccentric nuclei
. Woven bone trabeculae in a fibrous stroma without osteoblastic rimming
. Increased numbers of osteoclasts and giant cells in a hemorrhagic fibrovascular stroma
. Malignant spindle cells producing disorganized osteoid
. Nests of uniform round cells with fried egg appearance

Correct Answer & Explanation

. Increased numbers of osteoclasts and giant cells in a hemorrhagic fibrovascular stroma

Explanation

The clinical scenario is classic for primary hyperparathyroidism leading to a brown tumor (osteitis fibrosa cystica). Histologically, brown tumors consist of numerous multinucleated giant cells and osteoclasts in a highly vascular, hemorrhagic stroma.

Question 3765

Topic: Biology, Genetics & Bone Healing

An 8-year-old boy presents with the ability to bring his shoulders together in the midline. Radiographs reveal hypoplastic clavicles as shown.

Which of the following genes is mutated in this condition?

. COL1A1
. RUNX2
. FGFR3
. COMP
. SLC26A2

Correct Answer & Explanation

. RUNX2

Explanation

Cleidocranial dysplasia is caused by a mutation in the RUNX2 (CBFA1) gene, which is essential for osteoblast differentiation. It is characterized by delayed fontanelle closure, hypoplastic clavicles, and supernumerary teeth.

Question 3766

Topic: Biology, Genetics & Bone Healing

A 12-year-old boy with a history of multiple fractures presents with progressive loss of forearm rotation. Radiographs demonstrate calcification of the interosseous membrane and a dislocated radial head. Which of the following is characteristic of his specific condition?

. COL1A1 mutation
. Hypertrophic callus formation
. Blue sclerae
. Dentinogenesis imperfecta
. Severe wormian bones

Correct Answer & Explanation

. Hypertrophic callus formation

Explanation

This presentation is classic for Osteogenesis Imperfecta Type V, caused by an IFITM5 mutation (not COL1A1/COL1A2). It is uniquely associated with hyperplastic callus formation and calcification of the forearm interosseous membrane.

Question 3767

Topic: Biology, Genetics & Bone Healing

A pediatric patient with osteogenesis imperfecta is started on intravenous pamidronate. Which of the following best describes the cellular mechanism of action of this medication?

. Stimulates osteoblast differentiation
. Inhibits farnesyl pyrophosphate synthase in osteoclasts
. Agonizes the Wnt/beta-catenin pathway
. Blocks the RANKL receptor
. Inhibits sclerostin

Correct Answer & Explanation

. Inhibits farnesyl pyrophosphate synthase in osteoclasts

Explanation

Nitrogen-containing bisphosphonates like pamidronate inhibit farnesyl pyrophosphate synthase in the mevalonate pathway. This leads to osteoclast apoptosis, thereby reducing bone resorption and increasing bone density.

Question 3768

Topic: Biology, Genetics & Bone Healing

A newborn presents with micromelic short stature, bilateral clubfeet, "hitchhiker" thumbs, and cystic swelling of the pinnae. A defect in which of the following cellular processes is responsible for this condition?

. Type I collagen synthesis
. Fibroblast growth factor signaling
. Transmembrane sulfate transport
. Cartilage oligomeric matrix protein secretion
. Osteoclast ruffled border formation

Correct Answer & Explanation

. Transmembrane sulfate transport

Explanation

Diastrophic dysplasia is characterized by hitchhiker thumbs, cauliflower ears, and severe clubfeet. It is caused by a mutation in the SLC26A2 gene, resulting in a defect in the diastrophic dysplasia sulfate transporter (DTDST).

Question 3769

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy presents with progressive bowing of the legs and short stature. Laboratory tests show normal serum calcium, low serum phosphate, normal PTH, and elevated alkaline phosphatase. Genetic testing reveals a PHEX mutation. Which of the following is the primary pathophysiologic mechanism?

. Inadequate dietary vitamin D intake
. Decreased sensitivity of the vitamin D receptor
. Excessive renal phosphate reabsorption
. Increased FGF23 leading to renal phosphate wasting
. Deficient 1-alpha-hydroxylase activity

Correct Answer & Explanation

. Increased FGF23 leading to renal phosphate wasting

Explanation

X-linked hypophosphatemic rickets is caused by a PHEX mutation, resulting in elevated levels of FGF23. Excessive FGF23 inhibits renal reabsorption of phosphate, leading to profound phosphaturia and rickets.

Question 3770

Topic: Biology, Genetics & Bone Healing

A 7-year-old child with X-linked hypophosphatemic rickets has been treated with oral phosphate and calcitriol but continues to have severe limb deformities and progressive nephrocalcinosis. What novel targeted therapy can be initiated to directly address the underlying pathophysiology?

. Bisphosphonates
. Denosumab
. Teriparatide
. Burosumab
. Asfotase alfa

Correct Answer & Explanation

. Burosumab

Explanation

Burosumab is a monoclonal antibody that binds and inhibits FGF23. It directly addresses the underlying cause of X-linked hypophosphatemia, improving phosphate homeostasis and reducing the need for oral supplements that cause nephrocalcinosis.

Question 3771

Topic: Biology, Genetics & Bone Healing

An infant presents with failure to thrive, hepatosplenomegaly, and cranial nerve palsies. Radiographs show generalized osteosclerosis with a "bone-in-bone" appearance. The most severe form of this disease is characterized by a defect in which of the following?

. Carbonic anhydrase II
. Fibroblast growth factor receptor 3
. Osteoclast proton pump (TCIRG1)
. Type 1 collagen
. Cathepsin K

Correct Answer & Explanation

. Osteoclast proton pump (TCIRG1)

Explanation

Infantile malignant osteopetrosis is commonly caused by a mutation in the TCIRG1 gene, which encodes a subunit of the osteoclast vacuolar proton pump. This results in the failure of the ruffled border to acidify the resorption pit.

Question 3772

Topic: Biology, Genetics & Bone Healing

A 65-year-old man presents with increasing head size, unilateral hearing loss, and progressive anterior bowing of the tibia. A biopsy of the affected bone would most likely demonstrate which of the following histologic findings?

. Empty lacunae and necrotic trabeculae
. Mosaic pattern of lamellar bone with irregular cement lines
. Massive infiltration of plasma cells
. Diffuse proliferation of atypical chondrocytes
. Deficient osteoid mineralization with widened seams

Correct Answer & Explanation

. Mosaic pattern of lamellar bone with irregular cement lines

Explanation

Paget disease of bone is characterized by chaotic bone remodeling. The classic histologic finding is a "mosaic" or "jigsaw puzzle" pattern of lamellar bone demarcated by irregular, prominent cement lines.

Question 3773

Topic: Biology, Genetics & Bone Healing

A 55-year-old patient with end-stage renal disease presents with diffuse bone pain. Labs reveal elevated PTH, low calcium, and high phosphate. Radiographs show subperiosteal resorption of the radial aspect of the middle phalanges. What is the primary sequence of events causing this patient's elevated PTH?

. Excessive 1,25-dihydroxyvitamin D production causing calcium wasting
. Chronic hypocalcemia and hyperphosphatemia due to renal failure
. Autonomous hypersecretion from a parathyroid adenoma
. Deficient calcitonin secretion from the thyroid gland
. Inadequate dietary phosphorus absorption

Correct Answer & Explanation

. Chronic hypocalcemia and hyperphosphatemia due to renal failure

Explanation

Renal osteodystrophy is driven by secondary hyperparathyroidism. Failing kidneys cannot excrete phosphate or synthesize active Vitamin D (calcitriol), leading to hyperphosphatemia and hypocalcemia, which persistently stimulate the parathyroid glands.

Question 3774

Topic: Biology, Genetics & Bone Healing

A 6-month-old infant presents with craniosynostosis, failure to thrive, and severe rickets-like skeletal deformities. Laboratory findings are notable for significantly decreased serum alkaline phosphatase and elevated urinary phosphoethanolamine. What is the definitive treatment for this condition?

. High-dose Vitamin D supplementation
. Intravenous bisphosphonates
. Growth hormone therapy
. Bone marrow transplantation
. Asfotase alfa

Correct Answer & Explanation

. Asfotase alfa

Explanation

The patient has hypophosphatasia, an inborn error of metabolism caused by a mutation in the ALPL gene (tissue-nonspecific alkaline phosphatase). The definitive medical therapy is enzyme replacement with asfotase alfa.

Question 3775

Topic: Biology, Genetics & Bone Healing

A newborn is evaluated in the NICU. The infant has severely shortened limbs, bilateral rigid clubfeet, "hitchhiker" thumbs, and prominent cystic swelling of the external ears. The genetic mutation responsible for this condition primarily disrupts which of the following physiological processes?

. Fibroblast growth factor receptor inhibition
. Type 1 collagen synthesis
. Intracellular sulfate transport
. Cartilage oligomeric matrix protein secretion
. Vitamin D hydroxylation

Correct Answer & Explanation

. Intracellular sulfate transport

Explanation

The clinical picture describes diastrophic dysplasia, an autosomal recessive condition caused by mutations in the SLC26A2 gene. This gene encodes a sulfate transporter, and its defect impairs the sulfation of proteoglycans in cartilage matrix.

Question 3776

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy presents with progressive bowing of his lower extremities. Laboratory testing reveals normal serum calcium, low serum phosphorus, normal PTH, normal 25-OH vitamin D, and normal 1,25-OH vitamin D levels. Alkaline phosphatase is elevated. What is the primary pathophysiological mechanism underlying his condition?

. Defective intestinal calcium absorption
. Excessive production of FGF23
. Deficient 1-alpha hydroxylase activity
. Autoimmune destruction of the parathyroid glands
. Mutation in tissue-nonspecific alkaline phosphatase

Correct Answer & Explanation

. Excessive production of FGF23

Explanation

This presentation is characteristic of X-linked hypophosphatemic rickets, caused by a PHEX gene mutation that leads to excessive levels of FGF23. Elevated FGF23 inhibits renal phosphate reabsorption and 1-alpha hydroxylase activity, resulting in renal phosphate wasting with normal calcium levels.

Question 3777

Topic: Biology, Genetics & Bone Healing

A 2-year-old girl with recurrent fragility fractures, osteopenia, and blue sclerae

is diagnosed with Osteogenesis Imperfecta and is initiated on cyclic intravenous pamidronate. What is the primary mechanism of action of this pharmacological therapy?

. Stimulation of osteoblast proliferation and differentiation
. Inhibition of farnesyl pyrophosphate synthase in osteoclasts
. Cross-linking of defective Type I collagen fibers
. Activation of the calcium-sensing receptor
. Direct neutralization of RANKL in the bone microenvironment

Correct Answer & Explanation

. Inhibition of farnesyl pyrophosphate synthase in osteoclasts

Explanation

Pamidronate is a nitrogen-containing bisphosphonate used to increase bone mineral density in children with Osteogenesis Imperfecta. It acts by inhibiting the enzyme farnesyl pyrophosphate synthase (FPPS) in the mevalonate pathway, leading to osteoclast apoptosis and decreased bone resorption.

Question 3778

Topic: Biology, Genetics & Bone Healing

A 12-year-old child with a RUNX2 gene mutation presents with excessively mobile shoulders and delayed closure of cranial sutures. Radiographic evaluation of the spine in this specific condition is most likely to reveal which of the following?

. Extensive syringomyelia
. Central vertebral body beaking
. Spina bifida occulta and neural arch defects
. Decreased interpedicular distance
. Basilar invagination

Correct Answer & Explanation

. Spina bifida occulta and neural arch defects

Explanation

Cleidocranial dysplasia (RUNX2 mutation) is a defect in intramembranous ossification. Common spinal manifestations include delayed ossification of the neural arches, presenting as spina bifida occulta, particularly in the cervical and thoracic regions.

Question 3779

Topic: Biology, Genetics & Bone Healing

A 7-year-old boy presents to the orthopedic clinic with a waddling gait. Examination reveals hypermobility of his shoulders, allowing them to be approximated anteriorly in the midline. Pelvic radiographs show bilateral coxa vara and widened pubic symphysis. What is the genetic mutation associated with this syndrome?

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

Correct Answer & Explanation

. RUNX2 (CBFA1)

Explanation

Cleidocranial dysplasia is characterized by absent or hypoplastic clavicles, delayed skull suture closure, supernumerary teeth, and coxa vara. It is caused by a mutation in the RUNX2 (CBFA1) gene, essential for osteoblast differentiation.

Question 3780

Topic: Biology, Genetics & Bone Healing

An 18-year-old male presents with bilateral lower extremity aching pain and fatigue with walking. Radiographs demonstrate marked symmetric cortical thickening of the diaphyseal regions of the femur and tibia, with obliteration of the medullary canal.

Which of the following is true regarding his condition?

. Inherited in an autosomal recessive pattern
. Caused by a defect in osteoclast function
. Associated with elevated serum alkaline phosphatase
. Associated with a mutation in the TGFB1 gene
. Curative treatment involves bisphosphonate administration

Correct Answer & Explanation

. Associated with a mutation in the TGFB1 gene

Explanation

Progressive diaphyseal dysplasia (Camurati-Engelmann disease) is an autosomal dominant disorder characterized by thickened diaphyses and muscle weakness. It is associated with a mutation in the TGFB1 gene.

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