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Biology, Genetics & Bone Healing MCQs

Practice Set 69 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 1361

Topic: Biology, Genetics & Bone Healing

During the endochondral ossification phase of secondary fracture healing, hypertrophic chondrocytes secrete which of the following factors to stimulate angiogenesis?

. Bone morphogenetic protein-2 (BMP-2)
. Vascular endothelial growth factor (VEGF)
. Transforming growth factor-beta (TGF-beta)
. Platelet-derived growth factor (PDGF)
. Fibroblast growth factor (FGF)

Correct Answer & Explanation

. Bone morphogenetic protein-2 (BMP-2)

Explanation

Hypertrophic chondrocytes in the fracture callus secrete VEGF, which is essential for initiating angiogenesis. The invasion of blood vessels brings osteoprogenitor cells and osteoclasts to replace the cartilage model with woven bone.

Question 1362

Topic: Biology, Genetics & Bone Healing

Sclerostin, an inhibitor of bone formation, exerts its effect primarily by blocking which of the following signaling pathways in osteoblasts?

. RANK/RANKL
. Wnt/beta-catenin
. Notch
. Hedgehog
. BMP/Smad

Correct Answer & Explanation

. RANK/RANKL

Explanation

Sclerostin is produced by osteocytes and binds to LRP5/6 receptors on osteoblasts, inhibiting the canonical Wnt/beta-catenin signaling pathway. This leads to decreased osteoblast proliferation and bone formation.

Question 1363

Topic: Biology, Genetics & Bone Healing

Denosumab, used in the treatment of osteoporosis, acts by mimicking the physiologic action of which endogenous molecule?

. Parathyroid hormone
. Osteocalcin
. Sclerostin
. Osteoprotegerin (OPG)
. Calcitonin

Correct Answer & Explanation

. Parathyroid hormone

Explanation

Denosumab is a monoclonal antibody that targets and binds to RANKL, preventing it from activating RANK on osteoclasts. This mechanism of action directly mimics the role of endogenous osteoprotegerin (OPG).

Question 1364

Topic: Biology, Genetics & Bone Healing

Nitrogen-containing bisphosphonates (e.g., alendronate) inhibit osteoclast function primarily by interfering with which intracellular pathway?

. Mevalonate pathway
. ATP hydrolysis
. Wnt signaling
. Caspase-3 activation
. Cytochrome c release

Correct Answer & Explanation

. Mevalonate pathway

Explanation

Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate (FPP) synthase within the mevalonate pathway. This prevents the prenylation of small GTPases (like Ras and Rho), leading to osteoclast apoptosis and decreased bone resorption.

Question 1365

Topic: Biology, Genetics & Bone Healing

Osteoprotegerin (OPG) regulates bone resorption by:

. Binding to RANKL, preventing it from interacting with RANK on osteoclasts
. Directly stimulating osteoblast differentiation
. Binding to RANK on osteoclasts, stimulating their apoptosis
. Inhibiting the secretion of parathyroid hormone (PTH)
. Promoting the differentiation of macrophage colony-stimulating factor (M-CSF)

Correct Answer & Explanation

. Binding to RANKL, preventing it from interacting with RANK on osteoclasts

Explanation

OPG is a decoy receptor produced by osteoblasts that binds to RANKL. This prevents RANKL from binding to RANK on osteoclast precursors, thereby inhibiting osteoclast differentiation and bone resorption.

Question 1366

Topic: Biology, Genetics & Bone Healing

Which of the following gene mutations is most commonly associated with the pathogenesis of malignant infantile osteopetrosis?

. COL1A1
. FGFR3
. TCIRG1
. CBFA1 (RUNX2)
. COMP

Correct Answer & Explanation

. COL1A1

Explanation

Mutations in TCIRG1 (encoding a vacuolar proton pump) are responsible for more than 50% of cases of malignant infantile osteopetrosis. This mutation leads to defective osteoclast acidification and failure of bone resorption.

Question 1367

Topic: Biology, Genetics & Bone Healing

Vitamin C deficiency impairs bone formation by disrupting which of the following cellular processes?

. Hydroxylation of proline and lysine in collagen synthesis
. Cleavage of procollagen extensions
. Cross-linking of collagen fibers by lysyl oxidase
. Mineralization of the osteoid matrix
. Glycosylation of hydroxylysine residues

Correct Answer & Explanation

. Hydroxylation of proline and lysine in collagen synthesis

Explanation

Vitamin C is an essential cofactor for prolyl and lysyl hydroxylases. Its deficiency impairs the hydroxylation of proline and lysine residues, weakening the collagen triple helix and leading to scurvy.

Question 1368

Topic: Biology, Genetics & Bone Healing

Intermittent administration of parathyroid hormone (PTH) analogues (e.g., teriparatide) primarily leads to:

. Increased osteoclastogenesis and net bone loss
. Enhanced Wnt/beta-catenin signaling and net bone formation
. Decreased intestinal calcium absorption
. Suppression of 1-alpha-hydroxylase activity
. Inhibition of osteoblast apoptosis via suppression of Runx2

Correct Answer & Explanation

. Increased osteoclastogenesis and net bone loss

Explanation

Intermittent PTH administration stimulates osteoblastic bone formation by downregulating sclerostin, which enhances Wnt/beta-catenin signaling. In contrast, continuous PTH elevation leads to net bone resorption.

Question 1369

Topic: Biology, Genetics & Bone Healing

The primary cellular defect in Paget disease of bone resides in the:

. Osteoblast
. Osteoclast
. Osteocyte
. Chondrocyte
. Macrophage

Correct Answer & Explanation

. Osteoblast

Explanation

Paget disease is primarily characterized by hyperactive, multinucleated osteoclasts that cause excessive and disorganized bone resorption. This is followed by a compensatory but structurally abnormal increase in osteoblastic bone formation.

Question 1370

Topic: Biology, Genetics & Bone Healing

Nutritional rickets is characterized by an initial defect in which of the following processes?

. Osteoid synthesis
. Collagen cross-linking
. Mineralization of the physis and osteoid
. Osteoclast differentiation
. Intramembranous ossification

Correct Answer & Explanation

. Osteoid synthesis

Explanation

Rickets (and osteomalacia in adults) is typically caused by a deficiency in Vitamin D or calcium. This results in a failure to mineralize the osteoid matrix and, in growing children, the cartilaginous growth plate (physis).

Question 1371

Topic: Biology, Genetics & Bone Healing

A patient presents with generalized bone pain and proximal muscle weakness. Laboratory tests reveal low serum calcium, low phosphorus, elevated alkaline phosphatase, and elevated parathyroid hormone. What is the most likely diagnosis?

. Osteoporosis
. Primary hyperparathyroidism
. Paget disease
. Osteomalacia
. Osteopetrosis

Correct Answer & Explanation

. Osteoporosis

Explanation

These laboratory findings represent the classic profile of osteomalacia, often due to severe Vitamin D deficiency. It triggers secondary hyperparathyroidism, leading to low/normal calcium, low phosphorus, and elevated alkaline phosphatase.

Question 1372

Topic: Biology, Genetics & Bone Healing

Denosumab is an effective pharmacologic treatment for severe osteoporosis because it functions as a monoclonal antibody that directly targets and inhibits:

. RANK
. RANKL
. OPG
. Sclerostin
. Cathepsin K

Correct Answer & Explanation

. RANK

Explanation

Denosumab is a human monoclonal antibody that specifically binds to RANKL, preventing it from activating RANK on the surface of osteoclasts. This mechanism mimics the endogenous action of osteoprotegerin (OPG), robustly inhibiting bone resorption.

Question 1373

Topic: Biology, Genetics & Bone Healing

Parathyroid hormone (PTH) regulates bone remodeling. Continuous administration of PTH leads to net bone resorption. Through which direct cellular mechanism does PTH stimulate osteoclastogenesis?

. Direct binding to PTH receptors on osteoclast precursors
. Inhibition of macrophage colony-stimulating factor (M-CSF)
. Stimulation of osteoblasts to secrete RANKL
. Direct inhibition of osteoprotegerin (OPG) secretion by osteocytes
. Activation of the Wnt/beta-catenin pathway

Correct Answer & Explanation

. Direct binding to PTH receptors on osteoclast precursors

Explanation

PTH binds to receptors on osteoblasts, stimulating them to express RANKL and M-CSF while inhibiting OPG. RANKL then binds to RANK on osteoclast precursors, promoting their differentiation and activation into mature osteoclasts. Osteoclasts themselves do not possess PTH receptors.

Question 1374

Topic: Biology, Genetics & Bone Healing

A patient with frequent fractures and a dense "rugger jersey" spine on radiographs is diagnosed with osteopetrosis. Genetic testing reveals a defect in carbonic anhydrase II. This primary defect impairs the function of which cell by what mechanism?

. Osteoblasts; failure to mineralize osteoid
. Osteoclasts; inability to acidify the clear zone/Howship's lacuna
. Osteocytes; defective mechanotransduction signaling
. Chondrocytes; failure of endochondral ossification
. Macrophages; excessive production of IL-1

Correct Answer & Explanation

. Osteoblasts; failure to mineralize osteoid

Explanation

Osteopetrosis is characterized by defective osteoclast-mediated bone resorption. Carbonic anhydrase II is required to generate the protons that osteoclasts secrete via vacuolar H+-ATPases to acidify the resorption pit (Howship's lacuna) and dissolve bone mineral.

Question 1375

Topic: Biology, Genetics & Bone Healing

A 65-year-old male presents with increasing hat size, bowing of tibiae, and hearing loss.

Laboratory values demonstrate normal calcium, normal phosphate, and markedly elevated alkaline phosphatase. Which of the following is the primary cellular abnormality in this disease?

. Defective mineralization of osteoid by osteoblasts
. Excessive bone resorption by large, multinucleated osteoclasts
. Overproduction of parathyroid hormone
. Inadequate production of 1,25-dihydroxyvitamin D
. Monoclonal proliferation of plasma cells

Correct Answer & Explanation

. Defective mineralization of osteoid by osteoblasts

Explanation

The clinical picture is classic for Paget's disease of bone. The primary pathologic event is excessive and haphazard bone resorption driven by hyperactive, highly multinucleated osteoclasts, which is followed by a disorganized osteoblastic response (woven bone).

Question 1376

Topic: Biology, Genetics & Bone Healing

A 45-year-old female with a history of a Roux-en-Y gastric bypass presents with diffuse bone pain and proximal muscle weakness. Laboratory evaluation reveals low serum calcium, low phosphorus, elevated alkaline phosphatase, and elevated parathyroid hormone. What is the most likely diagnosis?

. Osteoporosis
. Osteopetrosis
. Osteomalacia
. Primary hyperparathyroidism
. Renal osteodystrophy

Correct Answer & Explanation

. Osteoporosis

Explanation

The patient's malabsorptive history and laboratory profile indicate severe Vitamin D deficiency leading to osteomalacia. Impaired mineralization of osteoid presents with low calcium and phosphorus, which triggers compensatory secondary hyperparathyroidism.

Question 1377

Topic: Biology, Genetics & Bone Healing

Which of the following molecules acts as a decoy receptor to inhibit osteoclast differentiation and activity in the bone remodeling cycle?

. Receptor Activator of Nuclear factor Kappa-B (RANK)
. Receptor Activator of Nuclear factorB Ligand (RANKL)
. Osteoprotegerin (OPG)
. Macrophage Colony-Stimulating Factor (M-CSF)
. Osteocalcin

Correct Answer & Explanation

. Receptor Activator of Nuclear factor Kappa-B (RANK)

Explanation

Osteoprotegerin (OPG) is secreted by osteoblasts and binds to RANKL, preventing it from interacting with RANK on osteoclast precursors. This competitive inhibition prevents osteoclastogenesis and halts bone resorption.

Question 1378

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory, absolute stability at a fracture site, defined as strain less than 2%, promotes which type of bone healing?

. Primary bone healing via cutting cones
. Secondary bone healing via a cartilaginous callus
. Endochondral ossification
. Intramembranous ossification with robust callus formation
. Fibrous nonunion

Correct Answer & Explanation

. Primary bone healing via cutting cones

Explanation

Absolute stability reduces the mechanical strain at the fracture gap to less than 2%, which is required to prevent the disruption of regenerating blood vessels and cellular networks. This low-strain environment prevents callus formation and allows primary (direct) bone healing via osteoclast cutting cones.

Question 1379

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) primarily exert their profound osteoinductive effects by initiating intracellular signaling through which of the following pathways?

. Wnt/beta-catenin pathway
. JAK/STAT pathway
. MAP kinase pathway
. Smad 1/5/8 pathway
. Notch signaling pathway

Correct Answer & Explanation

. Wnt/beta-catenin pathway

Explanation

BMPs bind to serine/threonine kinase cell surface receptors, triggering the phosphorylation of intracellular Smad 1, 5, and 8. These complex with Smad 4 and translocate to the nucleus to upregulate osteogenic transcription factors like Runx2.

Question 1380

Topic: Biology, Genetics & Bone Healing

A 45-year-old patient with end-stage renal disease presents with metabolic bone disease (renal osteodystrophy). Which critical step of Vitamin D metabolism is most likely impaired in this patient?

. Conversion of 7-dehydrocholesterol to cholecalciferol in the skin
. 25-hydroxylation of cholecalciferol in the liver
. 1-alpha-hydroxylation of 25-hydroxyvitamin D in the kidney
. 24-hydroxylation of 25-hydroxyvitamin D in the liver
. Absorption of ergocalciferol in the gastrointestinal tract

Correct Answer & Explanation

. Conversion of 7-dehydrocholesterol to cholecalciferol in the skin

Explanation

The kidney utilizes the enzyme 1-alpha-hydroxylase to convert 25-hydroxyvitamin D into 1,25-dihydroxyvitamin D, the biologically active form. In chronic kidney disease, this enzyme's function is severely impaired, leading to hypocalcemia and secondary hyperparathyroidism.

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