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Biology, Genetics & Bone Healing MCQs

Practice Set 188 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 3741

Topic: Biology, Genetics & Bone Healing

A 30-year-old female undergoes a biopsy for an eccentric, lytic epiphyseal lesion in her distal femur. The pathology report confirms a Giant Cell Tumor (GCT) of bone, describing a mix of mononuclear cells and multinucleated giant cells. Which of the following best describes the pathogenesis involving these mononuclear cells?

. They are non-neoplastic reactive cells attempting to clear tumor debris
. They are the true neoplastic cells that secrete RANKL to stimulate osteoclast differentiation
. They are derived strictly from the monocyte-macrophage hematopoietic lineage
. They secrete Denosumab to inhibit excessive local bone resorption
. They display characteristic reciprocal translocations t(11;22)

Correct Answer & Explanation

. They are the true neoplastic cells that secrete RANKL to stimulate osteoclast differentiation

Explanation

In Giant Cell Tumor (GCT) of bone, the spindle-shaped mononuclear cells are the actual neoplastic cells. They strongly express RANK ligand (RANKL), which recruits and activates the non-neoplastic, multinucleated giant cells (osteoclast-like cells) to destroy bone.

Question 3742

Topic: Biology, Genetics & Bone Healing

A 7-year-old boy presents with a waddling gait. His mother has similar features. Physical exam reveals an unusually large head, a delayed closure of the anterior fontanelle, and the ability to bring his shoulders together anteriorly.

Which of the following genes is most likely mutated in this condition?

. FGFR3
. COMP
. RUNX2
. COL1A1
. SLC26A2

Correct Answer & Explanation

. RUNX2

Explanation

The clinical presentation and image demonstrate absent or hypoplastic clavicles typical of cleidocranial dysplasia. This is an autosomal dominant condition caused by a mutation in the RUNX2 (CBFA1) gene, which is essential for osteoblast differentiation.

Question 3743

Topic: Biology, Genetics & Bone Healing

A 12-year-old boy presents with a history of recurrent fractures and newly diagnosed facial nerve palsy. Radiographs reveal generalized, extreme osteosclerosis and a classic "bone-in-bone" appearance of the vertebrae. What is the primary cellular defect responsible for this condition?

. Defective osteoblast differentiation
. Failure of osteoclast ruffled border formation
. Overactivity of osteoclasts
. Defective collagen type I cross-linking
. Impaired mineralization of osteoid

Correct Answer & Explanation

. Failure of osteoclast ruffled border formation

Explanation

Osteopetrosis is caused by defective osteoclast function, frequently due to mutations (e.g., carbonic anhydrase II or TCIRG1) preventing ruffled border formation and acid secretion. This results in incredibly dense, brittle bones and narrowed neural foramina leading to cranial nerve entrapment.

Question 3744

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy presents with severe genu varum and short stature. Laboratory evaluation reveals normal serum calcium, low serum phosphate, normal PTH, normal 25-hydroxyvitamin D levels, and highly elevated FGF-23. What is the most appropriate medical management?

. High-dose Vitamin D supplementation alone
. Calcium and Vitamin D supplementation
. Oral phosphate and calcitriol (1,25-dihydroxyvitamin D)
. Intravenous bisphosphonates
. Recombinant growth hormone injections

Correct Answer & Explanation

. Oral phosphate and calcitriol (1,25-dihydroxyvitamin D)

Explanation

X-linked hypophosphatemic rickets is caused by a PHEX gene mutation leading to excess FGF-23, which inappropriately wastes phosphate in the kidneys. Standard treatment includes frequent oral phosphate replacement combined with calcitriol to prevent secondary hyperparathyroidism.

Question 3745

Topic: Biology, Genetics & Bone Healing
A 2-year-old presents with bowing of the long bones, premature loss of fully rooted deciduous teeth, and a history of recurrent respiratory infections. Laboratory testing reveals strikingly low serum alkaline phosphatase levels and elevated urinary phosphoethanolamine. What is the primary deficiency in this condition?
. Vitamin D receptor
. Tissue-nonspecific alkaline phosphatase (TNSALP)
. Carbonic anhydrase II
. Fibroblast growth factor 23 (FGF-23)
. Type I collagen

Correct Answer & Explanation

. Tissue-nonspecific alkaline phosphatase (TNSALP)

Explanation

Hypophosphatasia is a rare metabolic bone disease caused by an ALPL gene mutation, leading to tissue-nonspecific alkaline phosphatase (TNSALP) deficiency. This results in defective bone mineralization, classic premature tooth loss, and characteristically low serum alkaline phosphatase.

Question 3746

Topic: Biology, Genetics & Bone Healing

A 10-year-old girl is evaluated for short stature and delayed dental eruption. On physical examination, she is able to bring her shoulders together anteriorly in the midline.

What is the genetic mutation responsible for this condition?

. FGFR3
. COMP
. RUNX2 (CBFA1)
. COL2A1
. SOX9

Correct Answer & Explanation

. RUNX2 (CBFA1)

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by mutations in the RUNX2 (CBFA1) gene. This gene is a critical transcription factor essential for osteoblast differentiation and bone formation.

Question 3747

Topic: Biology, Genetics & Bone Healing

In children with severe osteogenesis imperfecta, cyclical intravenous bisphosphonates are frequently utilized to decrease fracture rates. What is the primary mechanism of action of this medication class at the cellular level?

. Stimulates osteoblast proliferation and differentiation
. Induces osteoclast apoptosis and inhibits bone resorption
. Enhances renal calcium and phosphate reabsorption
. Increases intestinal calcium absorption via vitamin D pathways
. Promotes type 1 collagen cross-linking in the extracellular matrix

Correct Answer & Explanation

. Induces osteoclast apoptosis and inhibits bone resorption

Explanation

Bisphosphonates, such as pamidronate, accumulate in bone and are ingested by osteoclasts during bone resorption. They inhibit the mevalonate pathway, leading to osteoclast apoptosis and a significant reduction in bone resorption.

Question 3748

Topic: Biology, Genetics & Bone Healing

A 6-year-old child presents with progressive genu varum, widening of the wrists, and a rachitic rosary. Laboratory studies reveal a normal serum calcium, low phosphorus, and significantly elevated alkaline phosphatase. Which of the following is the most likely initial step in the pathophysiology of this metabolic bone disease?

. Primary renal tubular defect in phosphorus reabsorption
. Defective intestinal absorption of calcium due to low 1,25-dihydroxyvitamin D
. Autonomous secretion of parathyroid hormone
. Overproduction of Fibroblast Growth Factor 23 (FGF-23)
. Mutation in the PHEX gene leading to phosphate wasting

Correct Answer & Explanation

. Defective intestinal absorption of calcium due to low 1,25-dihydroxyvitamin D

Explanation

Nutritional rickets typically begins with Vitamin D deficiency, leading to decreased intestinal calcium absorption. The resulting secondary hyperparathyroidism normalizes serum calcium at the expense of phosphorus via renal wasting.

Question 3749

Topic: Biology, Genetics & Bone Healing

A 5-year-old boy presents with progressive bowing of the lower extremities. Laboratory tests show normal calcium, extremely low serum phosphorus, and normal PTH levels. He is diagnosed with X-linked hypophosphatemic rickets. What is the primary mediator of renal phosphate wasting in this disease?

. PTH-related peptide (PTHrP)
. Sclerostin
. Fibroblast growth factor 23 (FGF-23)
. Osteocalcin
. Calcitonin

Correct Answer & Explanation

. Fibroblast growth factor 23 (FGF-23)

Explanation

X-linked hypophosphatemic rickets is caused by a mutation in the PHEX gene, which leads to elevated circulating levels of FGF-23. FGF-23 potently decreases renal phosphate reabsorption in the proximal tubule, causing profound hypophosphatemia.

Question 3750

Topic: Biology, Genetics & Bone Healing

An infant presents with severe anemia, cranial nerve palsies, and diffuse symmetric osteosclerosis on radiographs. A bone marrow transplant is being considered. What is the primary cellular defect causing this condition?

. Lack of osteoblast differentiation
. Failure of osteoclasts to resorb bone due to a defective ruffled border
. Excessive production of osteoprotegerin (OPG) by stromal cells
. Defective mineralization of hypertrophic chondrocytes
. Mutation in type I collagen synthesis pathway

Correct Answer & Explanation

. Failure of osteoclasts to resorb bone due to a defective ruffled border

Explanation

Osteopetrosis is caused by defective osteoclast function. Mutations in carbonic anhydrase II or the TCIRG1 gene prevent the formation of a functional ruffled border and the acidic microenvironment required for bone resorption.

Question 3751

Topic: Biology, Genetics & Bone Healing

An 80-year-old man complains of increasing hat size and dull, aching pain in his right thigh. Radiographs of the femur demonstrate cortical thickening and coarse trabeculae. Which of the following best describes the histologic appearance of his femur during the mixed phase of the disease?

. Woven bone with hypervascularity
. Mosaic pattern of lamellar bone with prominent cement lines
. Accumulation of unmineralized osteoid seams
. Extensive cutting cones with empty lacunae
. Palisading granulomas with multinucleated giant cells

Correct Answer & Explanation

. Mosaic pattern of lamellar bone with prominent cement lines

Explanation

Paget's disease in its mixed phase is characterized by a pathognomonic mosaic or jigsaw pattern of lamellar bone. This is due to chaotic osteoclastic resorption followed by rapid, disorganized osteoblastic bone formation leaving prominent cement lines.

Question 3752

Topic: Biology, Genetics & Bone Healing

A 25-year-old patient of Ashkenazi Jewish descent presents with severe hip pain. Radiographs demonstrate bilateral avascular necrosis of the femoral heads and an Erlenmeyer flask deformity of the distal femora. Laboratory tests show pancytopenia. What is the most appropriate specific medical therapy for this underlying condition?

. High-dose intravenous bisphosphonates
. Recombinant enzyme replacement therapy with imiglucerase
. Chronic phlebotomy
. Bone marrow transplant only
. Splenectomy alone

Correct Answer & Explanation

. Recombinant enzyme replacement therapy with imiglucerase

Explanation

Gaucher disease is a lysosomal storage disorder caused by a deficiency of glucocerebrosidase. Recombinant enzyme replacement therapy (e.g., imiglucerase) is the standard of care to reduce hepatosplenomegaly, improve cytopenias, and stabilize skeletal disease.

Question 3753

Topic: Biology, Genetics & Bone Healing

A 7-year-old girl is evaluated for short stature and dental anomalies, including supernumerary teeth. On physical examination, she is able to bring her shoulders together anteriorly in the midline.

A mutation in which of the following genes is responsible for her presentation?

. RUNX2 (CBFA1)
. FGFR3
. COL2A1
. SQSTM1
. SLC26A2

Correct Answer & Explanation

. RUNX2 (CBFA1)

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by a mutation in the RUNX2 (CBFA1) gene. This gene is an essential transcription factor for osteoblast differentiation.

Question 3754

Topic: Biology, Genetics & Bone Healing

A 5-year-old boy presents with severe genu varum. Laboratory studies show normal serum calcium, critically low phosphorus, normal parathyroid hormone, and elevated alkaline phosphatase. Which of the following is the most appropriate definitive medical management?

. Vitamin D supplementation alone
. Oral calcium and Vitamin D
. Oral phosphorus and calcitriol
. Intravenous bisphosphonates
. Recombinant growth hormone

Correct Answer & Explanation

. Oral phosphorus and calcitriol

Explanation

The patient has X-linked hypophosphatemic rickets (PHEX mutation). Standard treatment requires combined oral phosphate and calcitriol (1,25-dihydroxyvitamin D) to correct the deficiency and prevent secondary hyperparathyroidism.

Question 3755

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy presents with severe anemia, recurrent infections, and hepatosplenomegaly. Skeletal survey reveals generalized, profound osteosclerosis with a "bone-within-a-bone" appearance in the long bones. What is the primary cellular defect responsible for this condition?

. Failure of osteoblast differentiation
. Congenital absence of osteoclasts
. Defect in the osteoclast ruffled border
. Overactivity of osteoblasts
. Defect in osteocyte mechanotransduction

Correct Answer & Explanation

. Defect in the osteoclast ruffled border

Explanation

Osteopetrosis is caused by defective osteoclastic bone resorption, most commonly due to a failure to form the osteoclast ruffled border (often via TCIRG1 or CA2 mutations). Osteoclasts are present in normal or increased numbers but are functionally incompetent.

Question 3756

Topic: Biology, Genetics & Bone Healing
An infant presents with failure to thrive, hypotonia, and severe bowing of all extremities. Radiographs reveal profound global demineralization and widened physes. Laboratory studies demonstrate hypercalcemia and a critically low serum alkaline phosphatase. Which gene is most likely mutated?
. PHEX
. ALPL
. FGF23
. COL1A1
. ENPP1

Correct Answer & Explanation

. ALPL

Explanation

Hypophosphatasia is a severe metabolic bone disease caused by a mutation in the ALPL gene. This leads to deficient tissue-nonspecific alkaline phosphatase (TNSALP) activity, impairing skeletal mineralization and mimicking rickets radiographically.

Question 3757

Topic: Biology, Genetics & Bone Healing

A 7-year-old girl is evaluated for short stature and an unusual shoulder appearance. Clinical examination reveals hypermobility of the shoulders, allowing them to touch in the midline.

Which of the following genes is mutated in this patient's condition?

. COL1A1
. FGFR3
. RUNX2 (CBFA1)
. COMP
. SLC26A2

Correct Answer & Explanation

. RUNX2 (CBFA1)

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by a mutation in the RUNX2 (CBFA1) transcription factor. It is characterized by hypoplastic or absent clavicles and delayed closure of cranial sutures.

Question 3758

Topic: Biology, Genetics & Bone Healing

A 65-year-old man presents with progressive bowing of his right tibia, increasing thigh pain, and increasing hat size. Radiographs reveal cortical thickening and coarsened trabeculae of the tibia. Which of the following best describes the initial pathogenesis of this disease?

. Abnormal mineralization of osteoid
. Intense focal osteoclastic resorption of bone
. Defective osteoclast ruffled border
. Excessive uncoupled osteoblastic bone formation
. Failure of endochondral ossification

Correct Answer & Explanation

. Intense focal osteoclastic resorption of bone

Explanation

Paget's disease begins with an initial osteolytic phase driven by intense focal osteoclastic resorption. This is followed by a mixed osteoblastic-osteoclastic phase, and finally a sclerotic phase.

Question 3759

Topic: Biology, Genetics & Bone Healing

A 3-year-old boy presents with progressive bowing of the legs and short stature. Laboratory evaluation shows low serum phosphate, normal calcium, normal PTH, and elevated alkaline phosphatase. Genetic testing reveals a mutation in the PHEX gene. Which of the following is the primary pathophysiologic mechanism?

. Decreased renal tubular reabsorption of phosphate due to elevated FGF23
. Decreased intestinal absorption of calcium due to low 1,25-dihydroxyvitamin D
. Excessive renal loss of calcium leading to secondary hyperparathyroidism
. Mutation in tissue-nonspecific alkaline phosphatase preventing mineralization
. Autoimmune destruction of parathyroid glands

Correct Answer & Explanation

. Decreased renal tubular reabsorption of phosphate due to elevated FGF23

Explanation

In X-linked hypophosphatemic rickets, a PHEX mutation leads to overproduction of FGF23. FGF23 decreases renal phosphate reabsorption, causing hypophosphatemia and rickets.

Question 3760

Topic: Biology, Genetics & Bone Healing

A 10-month-old infant presents with irritability, bleeding gums, and painful swollen lower extremities. Radiographs of the knees show a dense zone of provisional calcification, a radiolucent band beneath it, and a marginal spur (Pelkan spur). Which of the following steps in collagen synthesis is primarily impaired?

. Cleavage of procollagen N- and C-terminals
. Hydroxylation of proline and lysine residues
. Assembly of three alpha chains into a triple helix
. Glycosylation of hydroxylysine residues
. Cross-linking of tropocollagen molecules by lysyl oxidase

Correct Answer & Explanation

. Hydroxylation of proline and lysine residues

Explanation

The clinical presentation describes scurvy (Vitamin C deficiency). Vitamin C is an essential cofactor for prolyl and lysyl hydroxylase, and its absence prevents proper hydroxylation of proline and lysine.

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