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Biology, Genetics & Bone Healing MCQs

Practice Set 70 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 1381

Topic: Biology, Genetics & Bone Healing

Osteopetrosis is a genetic condition characterized by overly dense, disorganized, and brittle bones. What is the primary underlying cellular and molecular defect in this disease?

. Defective type I collagen synthesis by osteoblasts
. Hyperactive osteoblasts leading to excessive osteoid production
. Impaired acidification of the Howship lacunae by osteoclasts
. Inability of osteoblasts to synthesize tissue-nonspecific alkaline phosphatase
. Excessive production of parathyroid hormone related peptide (PTHrP)

Correct Answer & Explanation

. Defective type I collagen synthesis by osteoblasts

Explanation

Osteopetrosis results from impaired osteoclast function, most commonly due to mutations affecting carbonic anhydrase II or the proton pump. This prevents the acidification necessary to dissolve bone mineral, leading to dense but mechanically inferior bone.

Question 1382

Topic: Biology, Genetics & Bone Healing

During bone remodeling under mechanical loading, the convex side of a bowed long bone experiences tension, while the concave side experiences compression. According to the piezoelectric effect, what is the electrical charge and corresponding cellular activity on the compressive side?

. Electronegative, stimulating osteoblastic bone formation
. Electropositive, stimulating osteoblastic bone formation
. Electronegative, stimulating osteoclastic bone resorption
. Electropositive, stimulating osteoclastic bone resorption
. Neutral, with balanced remodeling

Correct Answer & Explanation

. Electronegative, stimulating osteoblastic bone formation

Explanation

The piezoelectric effect dictates that mechanical compression of bone generates an electronegative charge on the surface. This electronegative environment attracts and stimulates osteoblasts, leading to adaptive bone formation on the concave (compressive) side, illustrating Wolff's law.

Question 1383

Topic: Biology, Genetics & Bone Healing

A pediatric patient presents with bleeding gums, petechiae, and metaphyseal bone pain. Radiographs reveal a densely sclerotic 'white line of Frankel' at the metaphyses. The underlying defect is a failure of which biochemical process?

. Mineralization of the osteoid matrix
. Hydroxylation of proline and lysine residues in collagen
. Cleavage of procollagen to tropocollagen
. Cross-linking of collagen molecules via lysyl oxidase
. Secretion of osteocalcin by active osteoblasts

Correct Answer & Explanation

. Mineralization of the osteoid matrix

Explanation

This presentation is classic for scurvy, caused by a severe deficiency of Vitamin C. Vitamin C is a vital cofactor for prolyl and lysyl hydroxylases; without it, the hydroxylation of proline and lysine fails, preventing the formation of stable type I collagen triple helices.

Question 1384

Topic: Biology, Genetics & Bone Healing

Nitrogen-containing bisphosphonates, such as alendronate, inhibit osteoclast function and induce cellular apoptosis by directly disrupting which of the following intracellular pathways?

. Mevalonate pathway via inhibition of farnesyl pyrophosphate synthase
. Glycolytic pathway via inhibition of hexokinase
. RANKL signaling via competitive binding to the RANK receptor
. Calcineurin pathway via inhibition of NFATc1
. Apoptotic pathway via direct activation of caspase-3

Correct Answer & Explanation

. Mevalonate pathway via inhibition of farnesyl pyrophosphate synthase

Explanation

Nitrogen-containing bisphosphonates selectively inhibit the enzyme farnesyl pyrophosphate synthase within the mevalonate pathway. This blocks the prenylation of essential small GTP-binding proteins (like Rho and Rab), leading to the loss of the osteoclast ruffled border and eventual apoptosis.

Question 1385

Topic: Biology, Genetics & Bone Healing

Which of the following osseous structures develops primarily through the process of intramembranous ossification, bypassing a cartilaginous anlage intermediate stage?

. Proximal femur
. Distal radius
. Clavicle
. Tibial diaphysis
. Scaphoid

Correct Answer & Explanation

. Proximal femur

Explanation

The clavicle, along with the flat bones of the skull and portions of the mandible, develops via intramembranous ossification. In this process, undifferentiated mesenchymal cells condense and differentiate directly into osteoblasts to form bone, without a preceding cartilage model.

Question 1386

Topic: Biology, Genetics & Bone Healing

Sclerostin is a protein that plays a key role in bone remodeling by inhibiting bone formation. Which of the following cells primarily secretes sclerostin?

. Osteoblasts
. Osteoclasts
. Osteocytes
. Chondrocytes
. Mesenchymal stem cells

Correct Answer & Explanation

. Osteoblasts

Explanation

Sclerostin is primarily secreted by mature osteocytes embedded in the bone matrix. It inhibits bone formation by antagonizing the Wnt/beta-catenin signaling pathway in osteoblasts.

Question 1387

Topic: Biology, Genetics & Bone Healing

Denosumab is used in the treatment of osteoporosis and giant cell tumors of bone. It acts by directly binding to and inhibiting which of the following?

. RANK
. RANKL
. Osteoprotegerin (OPG)
. Macrophage colony-stimulating factor (M-CSF)
. Cathepsin K

Correct Answer & Explanation

. RANK

Explanation

Denosumab is a monoclonal antibody that mimics OPG by directly binding to RANKL. This prevents RANKL from binding to RANK on osteoclasts, thereby inhibiting osteoclast maturation and function.

Question 1388

Topic: Biology, Genetics & Bone Healing

Which of the following local factors is considered the most critical initiator of the fracture healing cascade immediately following injury and hematoma formation?

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

Correct Answer & Explanation

. Bone morphogenetic protein-2 (BMP-2)

Explanation

Following the initial hematoma formation, PDGF is released by degranulating platelets. It acts as a potent early chemoattractant for macrophages and mesenchymal stem cells, initiating the inflammatory phase.

Question 1389

Topic: Biology, Genetics & Bone Healing

Demineralized bone matrix (DBM) primarily provides which of the following properties to aid in bone healing?

. Osteogenesis and osteoinduction
. Osteoinduction and osteoconduction
. Osteoconduction only
. Osteogenesis only
. Osteogenesis and osteoconduction

Correct Answer & Explanation

. Osteogenesis and osteoinduction

Explanation

DBM is prepared by acid extraction of allograft bone, removing the mineral phase but retaining the collagen matrix and growth factors like BMPs. Thus, it provides both osteoinduction and osteoconduction, but lacks live cells.

Question 1390

Topic: Biology, Genetics & Bone Healing

In the classical pathway of vitamin D metabolism, which of the following enzymes is responsible for the conversion of 25-hydroxyvitamin D to its most active form, 1,25-dihydroxyvitamin D?

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

Correct Answer & Explanation

. 25-hydroxylase in the liver

Explanation

The final step in the synthesis of the active form of vitamin D occurs in the kidneys. The enzyme 1-alpha-hydroxylase converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D, a process stimulated by PTH.

Question 1391

Topic: Biology, Genetics & Bone Healing

Parathyroid hormone (PTH) maintains serum calcium levels through several mechanisms. Which of the following is a direct physiologic effect of PTH?

. Increased intestinal absorption of calcium
. Decreased renal reabsorption of calcium
. Increased osteoclast activity via direct binding to osteoclasts
. Increased renal reabsorption of calcium and excretion of phosphate
. Inhibition of 1-alpha-hydroxylase in the kidney

Correct Answer & Explanation

. Increased intestinal absorption of calcium

Explanation

PTH directly increases renal tubular reabsorption of calcium and decreases reabsorption of phosphate. It stimulates osteoclasts indirectly by binding to osteoblasts, and increases intestinal calcium absorption indirectly via Vitamin D activation.

Question 1392

Topic: Biology, Genetics & Bone Healing

The primary defect in Paget's disease of bone is characterized by which of the following cellular abnormalities?

. Decreased osteoblast proliferation
. Abnormal, large, multinucleated osteoclasts
. Defective mineralization of osteoid by osteoblasts
. Absence of osteocytes within lacunae
. Overproduction of poorly cross-linked collagen

Correct Answer & Explanation

. Decreased osteoblast proliferation

Explanation

The initial phase of Paget's disease is characterized by intense osteoclastic bone resorption mediated by abnormal, giant, multinucleated osteoclasts. This is later followed by a disorganized, excessive bone formation phase.

Question 1393

Topic: Biology, Genetics & Bone Healing

Which of the following is the hallmark histological finding in osteomalacia?

. Woven bone interspersed with lamellar bone
. Thickened, unmineralized osteoid seams
. Empty osteocyte lacunae
. Howship's lacunae filled with osteoclasts
. Disorganized chondrocyte columns in the physis

Correct Answer & Explanation

. Woven bone interspersed with lamellar bone

Explanation

Osteomalacia is characterized by defective mineralization of newly formed osteoid. This leads to abnormally wide and thickened unmineralized osteoid seams on histological examination.

Question 1394

Topic: Biology, Genetics & Bone Healing

Which of the following cytokines is secreted by osteoblasts to directly inhibit osteoclastogenesis?

. RANKL
. Osteoprotegerin (OPG)
. Macrophage colony-stimulating factor (M-CSF)
. Interleukin-1 (IL-1)
. Tumor necrosis factor-alpha (TNF-a)

Correct Answer & Explanation

. RANKL

Explanation

Osteoprotegerin (OPG) is a decoy receptor produced by osteoblasts that binds to RANKL, preventing it from interacting with RANK on osteoclast precursors, thereby inhibiting osteoclastogenesis.

Question 1395

Topic: Biology, Genetics & Bone Healing

Sclerostin, an important regulator of bone mass, primarily functions by inhibiting which of the following signaling pathways?

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

Correct Answer & Explanation

. RANKL/RANK

Explanation

Sclerostin is secreted by osteocytes and inhibits bone formation by antagonizing the Wnt/beta-catenin signaling pathway in osteoblasts. Monoclonal antibodies targeting sclerostin are used to treat osteoporosis.

Question 1396

Topic: Biology, Genetics & Bone Healing

Which of the following bones undergoes formation primarily via intramembranous ossification?

. Femur
. Clavicle
. Vertebral body
. Phalanges
. Tibia

Correct Answer & Explanation

. Femur

Explanation

The clavicle, flat bones of the skull, and portions of the mandible form via intramembranous ossification. In this process, mesenchymal cells differentiate directly into osteoblasts without a cartilaginous model.

Question 1397

Topic: Biology, Genetics & Bone Healing

During the incorporation of a cortical bone allograft, the process of simultaneous bone resorption and formation by osteoclasts and osteoblasts cutting cones is termed:

. Osteoinduction
. Osteoconduction
. Creeping substitution
. Distraction osteogenesis
. Endochondral ossification

Correct Answer & Explanation

. Osteoinduction

Explanation

Creeping substitution is the process by which cortical bone grafts are slowly resorbed by osteoclasts and replaced by new bone from host osteoblasts. This progressive remodeling process can take years to complete.

Question 1398

Topic: Biology, Genetics & Bone Healing

In the metabolism of Vitamin D, the conversion of 25-hydroxyvitamin D to its most active form, 1,25-dihydroxyvitamin D, occurs primarily in which organ?

. Skin
. Liver
. Kidney
. Bone
. Intestine

Correct Answer & Explanation

. Skin

Explanation

The kidney enzyme 1-alpha-hydroxylase is responsible for converting 25-hydroxyvitamin D into the active 1,25-dihydroxyvitamin D. This step is strongly stimulated by parathyroid hormone (PTH).

Question 1399

Topic: Biology, Genetics & Bone Healing

Nitrogen-containing bisphosphonates (e.g., alendronate) inhibit bone resorption primarily by inhibiting which of the following enzymes?

. Cathepsin K
. Carbonic anhydrase II
. Farnesyl pyrophosphate synthase
. Alkaline phosphatase
. Tartrate-resistant acid phosphatase

Correct Answer & Explanation

. Cathepsin K

Explanation

Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate synthase in the mevalonate pathway. This prevents prenylation of small GTPases essential for osteoclast function and survival.

Question 1400

Topic: Biology, Genetics & Bone Healing

Continuous, high-dose administration of Parathyroid Hormone (PTH) leads to which of the following net effects on bone?

. Net bone formation via direct stimulation of osteoblasts
. Net bone resorption via direct stimulation of osteoclasts
. Net bone resorption via osteoblast-mediated stimulation of osteoclasts
. Net bone formation via inhibition of osteoclasts
. No net change in bone mass

Correct Answer & Explanation

. Net bone formation via direct stimulation of osteoblasts

Explanation

Continuous exposure to PTH causes net bone resorption by binding to osteoblasts, which then upregulate RANKL to stimulate osteoclast activity. Conversely, intermittent, low-dose PTH promotes net bone formation.

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