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1. General Principles & Basic Science MCQs

Practice Set 317 of 789

This practice set contains high-yield board review questions covering key concepts in 1. General Principles & Basic Science. Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 6321

Topic: Biology, Genetics & Bone Healing

A 68-year-old woman with metastatic breast cancer is receiving denosumab to prevent skeletal-related events. Denosumab functions by mimicking the action of which naturally occurring physiological molecule?

. Sclerostin
. Calcitonin
. Osteoprotegerin (OPG)
. Receptor activator of nuclear factor kappa-B (RANK)
. Transforming growth factor-beta (TGF-beta)

Correct Answer & Explanation

. Sclerostin

Explanation

Denosumab is a monoclonal antibody that binds directly to RANKL, preventing it from interacting with the RANK receptor on osteoclasts. This mimics the physiologic role of Osteoprotegerin (OPG), a soluble decoy receptor produced by osteoblasts that naturally inhibits RANKL.

Question 6322

Topic: Biology, Genetics & Bone Healing

A 65-year-old woman is prescribed Denosumab for the treatment of severe osteoporosis. What is the precise mechanism of action of this medication?

. Inhibition of farnesyl pyrophosphate synthase
. Activation of the Wnt/beta-catenin pathway
. Direct binding to RANKL, preventing its interaction with RANK
. Stimulation of intermittent parathyroid hormone receptors
. Inhibition of sclerostin

Correct Answer & Explanation

. Inhibition of farnesyl pyrophosphate synthase

Explanation

Denosumab is a monoclonal antibody that binds directly to RANKL, preventing it from binding to the RANK receptor on osteoclasts. This inhibits osteoclast maturation, function, and survival, thereby strongly reducing bone resorption.

Question 6323

Topic: 1. General Principles & Basic Science

Which of the following modifications to a cortical screw will most significantly increase its pull-out strength in bone?

. Decreasing the outer diameter
. Increasing the inner (root) diameter
. Increasing the outer diameter
. Increasing the thread pitch
. Decreasing the length of screw engagement

Correct Answer & Explanation

. Decreasing the outer diameter

Explanation

Screw pull-out strength is most significantly determined by the outer diameter of the screw. Other factors that increase pullout strength include decreased root diameter, decreased thread pitch, and increased length of engagement in the bone.

Question 6324

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory, what is the maximum interfragmentary strain environment that allows for primary bone healing (direct osteonal reconstruction)?

. Less than 2%
. 2% to 10%
. 10% to 20%
. 20% to 30%
. Greater than 30%

Correct Answer & Explanation

. Less than 2%

Explanation

Primary bone healing occurs under conditions of absolute stability, requiring an interfragmentary strain of less than 2%. Strains between 2% and 10% promote secondary bone healing via endochondral ossification and callus formation.

Question 6325

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) play a crucial role in osteoinduction. They initiate intracellular signaling primarily through which of the following pathways?

. Wnt/beta-catenin pathway
. MAPK cascade
. Smad 1/5/8 pathway
. RANK/RANKL pathway
. OPG/RANKL cascade

Correct Answer & Explanation

. Wnt/beta-catenin pathway

Explanation

BMPs are members of the TGF-beta superfamily and initiate signaling primarily by binding to serine/threonine kinase receptors that phosphorylate Smad 1/5/8. These phosphorylated Smads translocate to the nucleus to upregulate osteogenic genes like Runx2.

Question 6326

Topic: Biomechanics & Biomaterials

Which of the following orthopedic biomaterials has a modulus of elasticity that is closest to that of human cortical bone?

. Stainless steel
. Cobalt-chromium alloy
. Titanium alloy
. Tantalum
. Polymethylmethacrylate (PMMA)

Correct Answer & Explanation

. Stainless steel

Explanation

Titanium alloy has a modulus of elasticity (approx. 110 GPa) that is much closer to cortical bone (15-20 GPa) than stainless steel (200 GPa) or cobalt-chromium (210 GPa). This closer biomechanical match helps reduce stress shielding around implants.

Question 6327

Topic: Biomechanics & Biomaterials

A surgeon decides to use a stainless steel screw to secure a titanium plate during fracture fixation. Which of the following modes of corrosion is most likely to occur at the interface of these two metals?

. Fretting corrosion
. Crevice corrosion
. Galvanic corrosion
. Pitting corrosion
. Intergranular corrosion

Correct Answer & Explanation

. Fretting corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals with different anodic indices are placed in contact within an electrolytic solution like bodily fluids. This causes an electrochemical reaction that accelerates the corrosion of the less noble metal.

Question 6328

Topic: Biology, Genetics & Bone Healing

A 7-year-old child presents with diffuse bone pain, gingival bleeding, and metaphyseal widening on radiographs. The underlying nutritional deficiency primarily impairs which step in collagen synthesis?

. Cleavage of the procollagen C-terminal
. Hydroxylation of proline and lysine residues
. Glycosylation of hydroxylysine
. Cross-linking by lysyl oxidase
. Assembly of the triple helix

Correct Answer & Explanation

. Cleavage of the procollagen C-terminal

Explanation

Scurvy is caused by Vitamin C deficiency, which acts as an essential cofactor for prolyl and lysyl hydroxylases. Without hydroxylation of proline and lysine, collagen triple helices cannot form stable hydrogen bonds, resulting in defective tissue.

Question 6329

Topic: Biomechanics & Biomaterials

When a compressive load is applied to cortical bone at a very high strain rate compared to a low strain rate, how do the mechanical properties of the bone change?

. It becomes less stiff and fails at a lower load
. It becomes more stiff and absorbs more energy before failure
. The stiffness remains unchanged, but it fails at a lower load
. It undergoes significantly more plastic deformation before failure
. It exhibits purely elastic behavior independent of the strain rate

Correct Answer & Explanation

. It becomes less stiff and fails at a lower load

Explanation

Bone is a viscoelastic material, meaning its mechanical behavior depends on the rate of loading. At higher strain rates, bone becomes stiffer, requires a higher load to fail, and absorbs more energy prior to fracture.

Question 6330

Topic: 1. General Principles & Basic Science

In normal articular cartilage, which zone is characterized by the highest concentration of water and collagen fibrils oriented parallel to the joint surface?

. Superficial (tangential) zone
. Middle (transitional) zone
. Deep (radial) zone
. Calcified cartilage zone
. Subchondral bone

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

The superficial (tangential) zone of articular cartilage contains the highest water content and the lowest proteoglycan concentration. Its collagen fibers are densely packed and oriented parallel to the joint surface to effectively resist shear forces.

Question 6331

Topic: Biology, Genetics & Bone Healing

A 70-year-old man takes alendronate for Paget's disease of bone. By which intracellular mechanism does this nitrogen-containing bisphosphonate inhibit osteoclast function?

. Inhibition of the Wnt signaling pathway
. Direct binding to the RANK receptor
. Inhibition of farnesyl pyrophosphate (FPP) synthase
. Generation of a cytotoxic ATP analog
. Stimulation of osteoprotegerin (OPG) release

Correct Answer & Explanation

. Inhibition of the Wnt signaling pathway

Explanation

Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate (FPP) synthase in the mevalonate pathway. This prevents the prenylation of small GTPases (like Rho and Rac) that are essential for osteoclast ruffled border formation and survival.

Question 6332

Topic: 1. General Principles & Basic Science

A 35-year-old patient with a tibial shaft fracture is taking daily high-dose ibuprofen for pain. This medication may impair fracture healing primarily by inhibiting the synthesis of which of the following?

. Bone morphogenetic proteins
. Prostaglandins
. Leukotrienes
. Interleukin-1
. Transforming growth factor-beta

Correct Answer & Explanation

. Bone morphogenetic proteins

Explanation

NSAIDs inhibit cyclooxygenase (COX) enzymes, which are responsible for the synthesis of prostaglandins. Prostaglandins, particularly PGE2, are essential for the early inflammatory phase of fracture healing and the subsequent recruitment of osteoprogenitor cells.

Question 6333

Topic: Biology, Genetics & Bone Healing

Osteoclasts are specialized, multinucleated bone-resorbing cells. They are directly derived from which of the following cellular lineages?

. Mesenchymal stem cells
. Neural crest cells
. Endodermal progenitors
. Monocyte/macrophage hematopoietic lineage
. Fibroblastic lineage

Correct Answer & Explanation

. Mesenchymal stem cells

Explanation

Osteoclasts are derived from the monocyte/macrophage hematopoietic stem cell lineage. Their differentiation and multinucleation are primarily driven by Macrophage Colony-Stimulating Factor (M-CSF) and RANKL.

Question 6334

Topic: 1. General Principles & Basic Science

Which enzyme is responsible for the final conversion of 25-hydroxyvitamin D to its most biologically active form, and in which organ is it primarily located?

. 25-hydroxylase in the liver
. 1-alpha-hydroxylase in the kidney
. 24-hydroxylase in the kidney
. 1-alpha-hydroxylase in the skin
. 7-dehydrocholesterol reductase in the liver

Correct Answer & Explanation

. 25-hydroxylase in the liver

Explanation

The conversion of 25-hydroxyvitamin D to the biologically active 1,25-dihydroxyvitamin D is catalyzed by the enzyme 1-alpha-hydroxylase. This enzyme is located primarily in the proximal tubules of the kidney and its activity is upregulated by PTH.

Question 6335

Topic: Biology, Genetics & Bone Healing

Teriparatide is utilized for the treatment of severe osteoporosis to decrease fracture risk. What is its fundamental mechanism of action when administered via daily subcutaneous injections?

. Continuous activation of PTH receptors leading to bone resorption
. Intermittent activation of PTH receptors stimulating osteoblast activity
. Inhibition of sclerostin leading to increased bone formation
. Direct inhibition of osteoclast ruffled border formation
. Upregulation of OPG production by T-cells

Correct Answer & Explanation

. Continuous activation of PTH receptors leading to bone resorption

Explanation

Teriparatide is a recombinant human parathyroid hormone fragment (PTH 1-34). While continuous PTH elevation causes net bone resorption, intermittent administration (such as daily injections) preferentially stimulates osteoblast activity, resulting in net bone formation.

Question 6336

Topic: Biology, Genetics & Bone Healing

During the incorporation of a massive cortical structural allograft, the process of "creeping substitution" occurs. This process is best characterized by:

. Simultaneous rapid resorption of the entire graft followed by replacement with woven bone
. Osteoclastic cutting cones advancing through the graft followed immediately by osteoblasts laying down new bone
. Complete retention and survival of donor osteocytes
. Rejection of the graft by the host's cellular immune system
. Immediate revascularization of the Haversian canals without any prior resorption

Correct Answer & Explanation

. Simultaneous rapid resorption of the entire graft followed by replacement with woven bone

Explanation

Creeping substitution in cortical bone grafts involves the invasion of osteoclastic cutting cones that resorb the dead allograft bone, followed closely by osteoblasts laying down viable host bone. This process temporarily weakens the graft mechanically before full incorporation.

Question 6337

Topic: Biology, Genetics & Bone Healing

A 45-year-old woman presents with diffuse skeletal pain and proximal muscle weakness. Laboratory studies reveal normal serum calcium, decreased phosphate, elevated alkaline phosphatase, and severely low 25-hydroxyvitamin D. A bone biopsy would most likely show:

. Increased woven bone with prominent irregular cement lines
. Thinning of trabeculae with structurally normal mineralization
. Accumulation of thick, unmineralized osteoid seams
. Complete absence of functioning osteoclasts
. Replacement of the marrow space with highly vascularized fibrous tissue

Correct Answer & Explanation

. Increased woven bone with prominent irregular cement lines

Explanation

The patient's clinical and laboratory findings are highly consistent with osteomalacia due to severe Vitamin D deficiency. Histologically, osteomalacia is defined by a profound defect in bone mineralization, leading to the accumulation of unmineralized osteoid.

Question 6338

Topic: Biology, Genetics & Bone Healing

Romosozumab is a newer monoclonal antibody utilized for osteoporosis treatment that functions by inhibiting sclerostin. What is the normal physiological function of sclerostin in bone metabolism?

. It binds to RANKL to inhibit osteoclastogenesis
. It promotes the differentiation of hematopoietic stem cells into macrophages
. It inhibits the Wnt/beta-catenin signaling pathway to decrease osteoblast activity
. It cleaves procollagen to form mature Type I collagen fibers
. It stimulates the conversion of 25-hydroxyvitamin D to its active form

Correct Answer & Explanation

. It binds to RANKL to inhibit osteoclastogenesis

Explanation

Sclerostin is a glycoprotein secreted primarily by mature osteocytes that negatively regulates bone formation. It functions by binding to LRP5/6 receptors on osteoblasts, which directly inhibits the osteogenic Wnt/beta-catenin signaling pathway.

Question 6339

Topic: Biomechanics & Biomaterials

Cortical bone exhibits viscoelastic properties. As the strain rate increases, such as during a high-energy traumatic impact, how does the mechanical behavior of the bone change?

. It becomes stiffer and stronger
. It becomes less stiff and weaker
. It becomes more ductile and weaker
. It becomes stiffer but fails at a lower load
. Its Young's modulus remains completely unchanged

Correct Answer & Explanation

. It becomes stiffer and stronger

Explanation

Cortical bone is viscoelastic, meaning its mechanical properties are strain-rate dependent. At higher strain rates, cortical bone becomes both stiffer (higher modulus) and stronger (can absorb more energy before failure).

Question 6340

Topic: Biology, Genetics & Bone Healing

A 65-year-old woman with severe osteoporosis is started on romosozumab. This medication increases bone mass primarily by inhibiting which of the following targets?

. Receptor activator of nuclear factor kappa-B ligand (RANKL)
. Sclerostin
. Cathepsin K
. Farnesyl pyrophosphate synthase
. Parathyroid hormone (PTH) receptor

Correct Answer & Explanation

. Receptor activator of nuclear factor kappa-B ligand (RANKL)

Explanation

Romosozumab is a monoclonal antibody that binds and inhibits sclerostin. Sclerostin normally inhibits the Wnt/beta-catenin signaling pathway, so its inhibition leads to increased osteoblastic bone formation.

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