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Biology, Genetics & Bone Healing MCQs

Practice Set 58 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 1141

Topic: Biology, Genetics & Bone Healing
A patient with a history of recurrent fractures and dense, brittle bones on radiographs is diagnosed with malignant infantile osteopetrosis. This condition is most commonly caused by a loss-of-function mutation affecting which of the following?
. Type I collagen (COL1A1)
. Fibroblast growth factor receptor 3 (FGFR3)
. Tissue nonspecific alkaline phosphatase (TNSALP)
. TCIRG1 (V-type proton ATPase subunit)
. Core binding factor alpha-1 (Cbfa1/Runx2)

Correct Answer & Explanation

. TCIRG1 (V-type proton ATPase subunit)

Explanation

Osteopetrosis represents a failure of osteoclast-mediated bone resorption. The most common mutation in the severe autosomal recessive form involves TCIRG1, which encodes a subunit of the V-type H+ ATPase required to acidify the resorption pit.

Question 1142

Topic: Biology, Genetics & Bone Healing

Nitrogen-containing bisphosphonates (e.g., alendronate, zoledronic acid) inhibit osteoclast function primarily by targeting which of the following enzymes in the mevalonate pathway?

. Carbonic anhydrase II
. Farnesyl pyrophosphate synthase
. HMG-CoA reductase
. Cathepsin K
. Cyclooxygenase-2

Correct Answer & Explanation

. Farnesyl pyrophosphate synthase

Explanation

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

Question 1143

Topic: Biology, Genetics & Bone Healing

Sclerostin is a glycoprotein secreted by osteocytes that serves as a potent negative regulator of bone formation. It achieves this primarily by binding to which of the following cellular targets?

. RANKL
. Osteoprotegerin (OPG)
. LRP5/6 receptors
. Bone morphogenetic protein receptor type II (BMPR-II)
. Transforming growth factor-beta (TGF-b) receptor

Correct Answer & Explanation

. LRP5/6 receptors

Explanation

Sclerostin binds to the LRP5/6 coreceptors on the surface of osteoblasts. This action blocks the canonical Wnt/beta-catenin signaling pathway, effectively inhibiting osteoblast differentiation and bone formation.

Question 1144

Topic: Biology, Genetics & Bone Healing

Endochondral ossification is the primary mechanism of bone formation in fracture callus. Which of the following transcription factors is recognized as the master regulator of chondrocyte differentiation in this process?

. Sox9
. Runx2
. Osterix
. Beta-catenin
. PPAR-gamma

Correct Answer & Explanation

. Sox9

Explanation

Sox9 is the crucial transcription factor for chondrocyte differentiation and cartilage matrix production during endochondral ossification. Runx2 (Cbfa1) is the master transcription factor for osteoblast differentiation.

Question 1145

Topic: Biology, Genetics & Bone Healing

Continuous, chronic elevation of parathyroid hormone (PTH) leads to net bone resorption. This resorptive effect is primarily mediated by PTH initially binding to receptors on which cell type?

. Osteoclasts
. Osteoblasts
. Osteocytes
. Macrophages
. Chondrocytes

Correct Answer & Explanation

. Osteoblasts

Explanation

Osteoclasts lack PTH receptors. PTH binds directly to receptors on osteoblasts, causing them to increase expression of RANKL and decrease expression of OPG, which secondarily stimulates osteoclast recruitment and bone resorption.

Question 1146

Topic: Biology, Genetics & Bone Healing

The primary mechanism of action of denosumab in treating osteoporosis is through binding to and inhibiting which of the following molecules?

. OPG
. RANK
. RANKL
. Osteocalcin
. Cathepsin K

Correct Answer & Explanation

. RANKL

Explanation

Denosumab is a human monoclonal antibody that binds to RANKL, preventing its interaction with RANK on osteoclasts. This effectively inhibits osteoclast formation, function, and survival.

Question 1147

Topic: Biology, Genetics & Bone Healing

During distraction osteogenesis (e.g., Ilizarov technique), new bone is formed primarily through which of the following processes?

. Endochondral ossification
. Intramembranous ossification
. Appositional ossification
. Creeping substitution
. Osteoclastic resorption

Correct Answer & Explanation

. Intramembranous ossification

Explanation

Distraction osteogenesis under stable fixation and an appropriate distraction rate typically proceeds via intramembranous ossification. This process directly forms bone without a cartilaginous intermediate stage.

Question 1148

Topic: Biology, Genetics & Bone Healing

A patient with severe malnutrition presents with bleeding gums, petechiae, and poor wound healing. A deficiency in which of the following vitamins is responsible for these symptoms by impairing the hydroxylation of proline and lysine in collagen synthesis?

. Vitamin A
. Vitamin B12
. Vitamin C
. Vitamin D
. Vitamin K

Correct Answer & Explanation

. Vitamin C

Explanation

Vitamin C (ascorbic acid) is an essential cofactor for prolyl and lysyl hydroxylase in collagen synthesis. Deficiency causes scurvy, characterized by defective collagen cross-linking and subsequent tissue fragility.

Question 1149

Topic: Biology, Genetics & Bone Healing

Sclerostin is a protein that regulates bone mass. It functions primarily by antagonizing which of the following cellular signaling pathways?

. RANKL/OPG pathway
. Wnt/beta-catenin pathway
. BMP/Smad pathway
. Notch signaling pathway
. FGF pathway

Correct Answer & Explanation

. Wnt/beta-catenin pathway

Explanation

Sclerostin, produced primarily by osteocytes, binds to LRP5/6 receptors on osteoblasts. This action inhibits the canonical Wnt/beta-catenin signaling pathway, leading to decreased bone formation.

Question 1150

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) initiate intracellular signaling through transmembrane serine/threonine kinase receptors. Which of the following downstream intracellular mediators are primarily activated by BMPs?

. Smad 1, 5, and 8
. Smad 2 and 3
. Smad 4
. Beta-catenin
. NF-kappa B

Correct Answer & Explanation

. Smad 1, 5, and 8

Explanation

BMPs signal primarily through the phosphorylation of Smad 1, 5, and 8. These phosphorylated Smads then form a complex with Smad 4 to translocate to the nucleus and regulate gene transcription for osteoblastogenesis.

Question 1151

Topic: Biology, Genetics & Bone Healing

Which of the following cells produces osteoprotegerin (OPG), acting as a decoy receptor to prevent bone resorption?

. Osteoclasts
. Osteoblasts
. Macrophages
. T-lymphocytes
. Chondrocytes

Correct Answer & Explanation

. Osteoblasts

Explanation

Osteoblasts and osteocytes secrete OPG, which binds to RANKL and prevents it from interacting with RANK on osteoclast precursors. This inhibits osteoclast differentiation and activation, thereby decreasing bone resorption.

Question 1152

Topic: Biology, Genetics & Bone Healing

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

. Inhibition of osteoclast ruffled border formation
. Stimulation of intermittent parathyroid hormone receptors to increase osteoblast activity
. Binding to hydroxyapatite to inhibit farnesyl pyrophosphate synthase
. Neutralization of sclerostin
. Antagonism of the RANK ligand

Correct Answer & Explanation

. Stimulation of intermittent parathyroid hormone receptors to increase osteoblast activity

Explanation

Teriparatide is a recombinant human parathyroid hormone (PTH 1-34) analog. When administered intermittently, it strongly stimulates osteoblastic bone formation, unlike continuous PTH exposure which promotes bone resorption.

Question 1153

Topic: Biology, Genetics & Bone Healing
During secondary bone healing, which type of collagen is predominantly synthesized during the soft callus phase?
. Type I
. Type II
. Type III
. Type IV
. Type X

Correct Answer & Explanation

. Type II

Explanation

The soft callus phase of endochondral ossification is characterized by the production of fibrocartilage and hyaline cartilage, which are predominantly composed of Type II collagen. This is later replaced by Type I collagen during the hard callus phase.

Question 1154

Topic: Biology, Genetics & Bone Healing

A patient presents with hypocalcemia following a total thyroidectomy. How does the body naturally attempt to correct this via parathyroid hormone (PTH) release?

. Decreasing renal reabsorption of calcium
. Increasing renal excretion of phosphate
. Decreasing 1-alpha-hydroxylase activity
. Inhibiting osteoclast activity
. Decreasing intestinal calcium absorption

Correct Answer & Explanation

. Increasing renal excretion of phosphate

Explanation

PTH responds to hypocalcemia by increasing bone resorption, increasing renal reabsorption of calcium, and increasing renal excretion of phosphate. It also stimulates 1-alpha-hydroxylase in the kidney to boost active vitamin D synthesis.

Question 1155

Topic: Biology, Genetics & Bone Healing

Denosumab is a targeted biological therapy used to treat severe osteoporosis. What is its specific molecular target?

. RANK
. RANKL
. Sclerostin
. Cathepsin K
. Osteoprotegerin

Correct Answer & Explanation

. RANKL

Explanation

Denosumab is a monoclonal antibody that binds directly to RANKL. By neutralizing RANKL, it prevents interaction with the RANK receptor on osteoclasts, severely inhibiting osteoclast formation, function, and survival.

Question 1156

Topic: Biology, Genetics & Bone Healing

Compared to mature lamellar bone, woven bone is characterized by which of the following properties?

. Higher mineral content
. Highly organized collagen alignment
. Slower rate of deposition
. Greater number of osteocytes per unit volume
. Increased biomechanical stiffness

Correct Answer & Explanation

. Greater number of osteocytes per unit volume

Explanation

Woven bone is immature, rapidly formed bone with a random, disorganized collagen fiber orientation. It contains a greater number of osteocytes per unit volume, higher water content, and lower mineral content compared to mature lamellar bone.

Question 1157

Topic: Biology, Genetics & Bone Healing

Sclerostin, a glycoprotein produced primarily by osteocytes, regulates bone formation by directly inhibiting which of the following intracellular signaling pathways?

. Wnt/beta-catenin
. RANK/RANKL
. TGF-beta/Smad
. Notch signaling
. Hedgehog signaling

Correct Answer & Explanation

. Wnt/beta-catenin

Explanation

Sclerostin binds to LRP5/6 receptors on osteoblasts, effectively antagonizing the canonical Wnt/beta-catenin signaling pathway. This inhibition halts osteoblast differentiation and fundamentally decreases bone formation.

Question 1158

Topic: Biology, Genetics & Bone Healing

During normal bone remodeling, RANKL binds to RANK on the surface of osteoclast precursors to stimulate differentiation. Which of the following molecules acts as a decoy receptor to inhibit this specific interaction?

. Sclerostin
. Osteoprotegerin (OPG)
. Osteocalcin
. Cathepsin K
. Macrophage colony-stimulating factor (M-CSF)

Correct Answer & Explanation

. Osteoprotegerin (OPG)

Explanation

Osteoprotegerin (OPG) is secreted by osteoblasts and binds to RANKL, preventing it from binding to RANK. This inhibition effectively downregulates osteoclastogenesis and reduces bone resorption.

Question 1159

Topic: Biology, Genetics & Bone Healing

Intermittent low-dose administration of teriparatide promotes an overall anabolic effect on bone. What is the primary cellular mechanism responsible for this bone formation?

. Inhibition of osteoclast apoptosis
. Direct binding to the RANK receptor
. Stimulation of osteoblast survival and proliferation
. Upregulation of sclerostin production
. Irreversible inhibition of cathepsin K

Correct Answer & Explanation

. Stimulation of osteoblast survival and proliferation

Explanation

Intermittent administration of parathyroid hormone (PTH) analogues like teriparatide strongly stimulates bone formation by increasing osteoblast proliferation, differentiation, and lifespan. This anabolic effect overrides the typical resorptive effects seen with continuous high PTH levels.

Question 1160

Topic: Biology, Genetics & Bone Healing

Sclerostin is a glycoprotein secreted by osteocytes that serves as a key negative regulator of bone mass. It achieves this primarily by binding to LRP5/6 receptors and inhibiting which intracellular signaling pathway?

. Wnt/beta-catenin
. Transforming growth factor-beta (TGF-beta)
. Notch
. Hedgehog
. Bone morphogenetic protein (BMP)/Smad

Correct Answer & Explanation

. Wnt/beta-catenin

Explanation

Sclerostin antagonizes the canonical Wnt/beta-catenin signaling pathway by binding to LRP5/6. Inhibition of this pathway decreases osteoblastogenesis and bone formation.

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