Question 1

All of the following have been used as viral vectors for gene delivery except:

Explanation

A retroviral vector derived from the Moloney murine leukemia retrovirus is among the best-developed viral vectors. Other viral vectors include adenovirus, adeno-associated virus, and herpes simplex virus. Novel vector systems based on lentivirus, a type of retrovirus, which includes human immunodeficiency virus (HIV), are being developed.

Question 2

The principle of homologous recombination in gene therapy is used to:

Explanation

Novel approaches to treating genetic diseases involve gene repair or replacement rather than gene supplementation. One such approach is based on homologous recombination (replacement of a defective gene by a normal gene).

Question 3

The virus associated with most immune reactions is:

Explanation

Adenoviral vectors can cause inflammatory reaction due to immune activation, an event linked to the first death related to gene therapy. This occurred in September 1999 at the University of Pennsylvania in a clinical trial in which an 18-year-old patient received infusion of over a trillion adenoviral vectors directed to his liver, which triggered a systemic inflammatory response that became uncontrollable, leading to organ failure and death. Newer generation gutted or gutlessâ adenovirus vectors are nonimmunogenic.

Question 4

Compared to ex vivo gene delivery system, the in vivo system is:

Explanation

Two basic strategies exist for gene delivery. Direct, or in vivo, gene therapy involves direct introduction of vectors into the body. Indirect, or ex vivo, gene therapy involves removal of target cells from the body, vector introduction by incubation of the cells in vitro, and reimplantation. The in vivo system is less invasive.

Question 5

The gene that has been studied in greatest detail for application in osteoarthritis is:

Explanation

Gene therapy has been suggested as a means of delivering sustained therapeutic levels of anti-arthritic gene products to diseased joints. Local gene delivery to the synovial tissue is preferred for osteoarthritis and other conditions affecting a few joints. It is less suited to address the extra- articular components of systemic conditions, such as rheumatoid arthritis. The gene that has been studied in greatest detail encodes the human IL-1 receptor antagonist (IL-Ra).

Question 6

The osteoinductive potential of LIM mineralization protein (LMP)-1 gene has been studied for clinical application in:

Explanation

Identification of LIM mineralization protein-1 (LMP-1), a novel intracellular protein, is a step forward in osteoinductive proteins. Unlike BMP, which is a secreted protein that binds to cell-surface receptor to initiate a response, LMP-1 is an intracellular signaling molecule. Boden transfected bone marrow cells from rats ex vivo with LMP-1 gene using a DNA plasmid vector and used them during posterior thoracic and lumbar spine fusion in rats.

Question 7

The gene studied for application in osteoporosis and wear-induced osteolysis is:

Explanation

Various cytokines and cytokine antagonists hold promise as new therapeutic agents for osteoporosis. Baltzer et al showed that intramedullary injection of Ad-IL-1Ra gene in a murine ovariectomy model strongly reduced the loss of bone mass. Using a similar model, Bolon et al studied the effect of adenovirus-mediated transfer of osteoprotegerin, which showed more bone volume with reduced osteoclast numbers in axial and appendicular bones after 4 weeks compared to sham-operated mice.

Question 8

Gene transfer to a cell using viral vectors is called:

Explanation

In vivo gene delivery involves the direct injection of vectors containing the genes into the body with the expectation that they will reach and transduce the target cell.

Question 9

Which of the following genes has been shown to stimulate proteoglycan synthesis for prevention of disk degeneration:

Explanation

Intervertebral disk degeneration has been associated with a progressive decrease in proteoglycan content of nucleus pulposus. The potential application of gene therapy for prevention of disk degeneration is to increase or maintain the proteoglycan content of nucleus pulposus. Thompson et al reported that addition of TGF-b1 to canine disk tissue in culture stimulated in vitro proteoglycan synthesis.

Question 10

This patient has numerous (> 6) cafÃ© au lait spots (Slide). The most likely diagnosis is:

Explanation

Neurofibromatosis is an autosomal dominant condition that occurs in a peripheral and central form. The peripheral type is more common and called neurofibromatosis 1 (NF1). The central type is characterized by bilateral acoustic neurofibromatosis (NF2). The clinical presentation is variable. Patients may either have a severe form of NF1 with hundreds of neurofibromas in the skin or they may have only cafa-au lait spots and deep neurofibromas that are easily apparent. Demographics of NF1 include: 1. Autosomal dominant with high degree of penetrance 2. C hromosomal abnormality - 17, NF1 gene (encodes neurofibromin) 3. Diagnostic criteria - two or more of the following: Orthopedic Prometric Exam Chapter 3 Image

Orthopedic Prometric Exam Chapter 3 Image

Six or more cafu au lait spots Two or more neurofibromas of any type or one plexiform neurofibroma Freckling in the axillary or inguinal area Optic glioma Two or more Lisch nodules Bone lesion First-degree relative with NF1

Question 11

This patient has numerous (> 6) cafa au lait spots (Slide). Which of the following describes the correct inheritance pattern:

Explanation

Neurofibromatosis is an autosomal dominant condition that occurs in a peripheral and central form. The peripheral type is more common and called neurofibromatosis 1 (NF1). The central type is characterized by bilateral acoustic neurofibromatosis (NF2). The clinical presentation is variable. Patients may either have a severe form of NF1 with hundreds of neurofibromas in the skin or they may have only cafa au lait spots and deep neurofibromas that are easily apparent. Demographics of NF1 include: 1. Autosomal dominant with high degree of penetrance 2. C hromosomal abnormality - 17, NF1 gene (encodes neurofibromin) 3. Diagnostic criteria - two or more of the following: Six or more cafa au lait spots Two or more neurofibromas of any type or one plexiform neurofibroma Freckling in the axillary or inguinal area Optic glioma Two or more Lisch nodules Bone lesion First-degree relative with NF1

Question 12

The bone graft used in the Grice procedure for the treatment of neurogenic valgus feet should be placed:

Explanation

In 1945, Grice developed the use of subtalar extra-articular arthrodesis as a treatment for paralytic valgus feet. Grice demonstrated that blocking the tarsal sinus opening prevents the calcaneus from rotating posterolaterally and thus blocks the heel of a foot from shifting into the valgus position. A bone block must be placed at such an angle in the tarsal sinus that when posterolateral migration of the calcaneus is attempted, the graft compresses. If a graft is placed too far forward, then the posterolateral movement of the calcaneus increases insertional distance between blocked areas, the graft collapses, and the foot goes into valgus.

Question 13

A patient with myelomeningocele has good quadriceps function, fair hamstring motion, and no active ankle movement. He has bilateral clubfoot. His motor level is:

Explanation

The hamstrings are innervated at a level just proximal to the anterior tibialis muscle (with the highest innervated ankle motor at L5). Therefore, the patientâ s motor level is best estimated at L4.

Question 14

In a type 1 Monteggiaâ s fracture, the position of reduction is:

Explanation

Type 1 Monteggis fracture is associated with an anterior dislocation of the radial head and an apex anterior fracture of the ulna. Type 1 Monteggi s fracture is best treated by flexion and supination. The same maneuvers that reduce the ulna will also stabilize the radial head.

Question 15

What is the approximate risk of growth disturbance of distal radial Salter II fractures:

Explanation

Although distal radial physeal fractures are common, growth disturbance is rare with a 1% occurence. Growth disturbance poses a challenge for follow- up, but physicians should at least mention the risk to patients.

Question 16

Which of the following fracture situations would be most appropriate for internal fixation vs. attempted closed reduction:

Explanation

Remodeling potential is significantly decreased at skeletal maturity. The first four scenarios represent skeletally immature patients. The 15-year-old girl should be treated like an adult to achieve the most acceptable result. All other patients deserve an attempt at closed reduction.

Question 17

All of the following may be signs and symptoms of a tethered cord except:

Explanation

A tethered cord is common in patients with myelomeningocele. A tethered cord occurs due to adhesion of the dysplastic distal cord to its surrounding structures. The onset of a tethered cord may be exacerbated by closure of the open cord. Signs and symptoms of a tethered cord may include back pain, leg pain, increasing motor weakness, and increasing contracture. Alternate explanations for some of these signs and symptoms exist as well. An adult with Meryon's sign may drop a child he or she is holding due to weakness of the pectorals and latissimus muscles. Meryon's sign is also common in patients with myelomeningocele who have a C hiari's malformation. In this case, the patient with myelomeningocele does not experience weakness of the pectorals and latissimus muscles due to a tethered cord. Meryon's sign also occurs in some myopathies.

Question 18

A 15-year-old patient with osteogenesis imperfecta (OI) is examined due to increasing knee flexion contractures and generalized muscle tightness of the lower extremities. The most likely explanation for these symptoms is:

Explanation

Patients with OI have a significant risk of basilar invagination. In patients with OI the skull settles around the foramen magnum and cervical spine, causing the brainstem to become indented. Signs and symptoms include increased spasticity (described here as muscle tightness), contractures, hyperreflexia, clonus, respiratory depression, and changes in voice or swallowing. Basilar invagination is most common in type IV OI. Although osteoarthritis is more common in patients with OI than in the general population at a given age, the 15-year-old patient in this case is too young to develop osteoarthritis. Spondylolysis would not produce this constellation of symptoms unless associated with severe listhesis.

Question 19

A 6-year-old boy with osteogenesis imperfecta has had three fractures of his proximal femur resulting in progressive bowing, which has now reached 70°. He has pain in his thigh and a sclerotic stress line at the apex of the bow. The best treatment is:

Explanation

As bowing becomes greater, the likelihood of a re-fracture increases. The presence of a lucent sclerotic line and pain increases the risk of re-fracture. Intramedullary fixation is preferred to plate fixation because plate fixation increases the risk of stress risers. Expandable rods that bridge the entire bone from one epiphysis to the other are the best treatment for excessive bowing. Examples include the Bailey Dubow or the Fassier rods.

Question 20

A 10-year-old boy steps on a nail. The nail is promptly removed and inspected in the emergency department. No evidence of foreign material is found in the patientâ s foot. One week later, the boy develops increased pain and swelling in his foot with inability to bear weight. Plain films show no abnormality. The appropriate next step of treatment would be:

Explanation

Penetrating injuries to the sole of the foot commonly inoculate bacteria to the deep spaces of the foot. Pseudomonas, other gramnegative organisms, and Staphylococcus are examples of bacteria that commonly can become inoculated. An MRI would help to localize the site and stage of the infection. The MRI determines whether the surgeon should aspirate the infection and treat it with antibiotics or operatively debride the infection. Pseudomonas often requires debridement.

Question 21

Lentiviral vectors are increasingly used in orthopedic gene therapy research. Which of the following represents the primary advantage of lentiviral vectors over standard retroviral vectors?

Explanation

Lentiviruses are a subclass of retroviruses unique in their ability to transduce both dividing and non-dividing cells. This is highly advantageous for targeting mature, slow-dividing orthopedic tissues like articular cartilage.

Question 22

Non-viral gene delivery using naked plasmid DNA is being evaluated for fracture healing. What is the main limitation of this technique compared to viral vectors?

Explanation

Naked plasmid DNA delivery has the advantages of safety and low immunogenicity but suffers from very low transfection efficiency and transient gene expression. It typically requires physical methods like electroporation to improve cellular uptake.

Question 23

Adeno-associated virus (AAV) vectors are highly favored in orthopedic gene therapy for sustained expression. Which of the following is a key characteristic of AAV?

Explanation

AAV is a dependent parvovirus that requires a helper virus (like adenovirus or herpesvirus) to replicate. It is non-pathogenic, elicits minimal immune response, and typically remains episomal rather than integrating.

Question 24

In designing a gene therapy construct to enhance bone formation, researchers include a specific DNA sequence located upstream of the target gene to regulate its transcription. What is this sequence called?

Explanation

A promoter is a regulatory DNA sequence located upstream of a gene that binds RNA polymerase and transcription factors to initiate transcription. Tissue-specific promoters can restrict gene expression to target cells, such as osteoblasts.

Question 25

BMP-2 is delivered via an adenoviral vector to stimulate spinal fusion in an animal model. Through which intracellular signaling pathway does BMP-2 primarily exert its osteoinductive effects?

Explanation

Bone morphogenetic proteins (BMPs) bind to serine/threonine kinase receptors, leading to the phosphorylation of receptor-regulated Smads (Smad 1, 5, and 8). These complex with Smad 4 to enter the nucleus and regulate transcription of osteogenic genes.

Question 26

A novel therapy utilizes small interfering RNA (siRNA) to treat an autosomal dominant orthopedic condition. What is the mechanism of action of siRNA?

Explanation

siRNA mediates post-transcriptional gene silencing by binding to complementary messenger RNA (mRNA) sequences. This leads to mRNA degradation, effectively preventing translation into the disease-causing protein.

Question 27

Ex vivo gene therapy is utilized to deliver therapeutic proteins for articular cartilage repair. Which of the following is a characteristic advantage of the ex vivo approach compared to in vivo gene delivery?

Explanation

Ex vivo gene therapy involves harvesting cells, transducing them in vitro, and implanting them back into the patient. This allows for the selection and expansion of successfully genetically modified cells, ensuring high therapeutic efficacy.

Question 28

CRISPR/Cas9 technology is being investigated for correcting mutations in osteogenesis imperfecta. What is the primary function of the Cas9 enzyme in this system?

Explanation

In the CRISPR system, Cas9 acts as an endonuclease that creates site-specific double-strand breaks in DNA, guided by a specific RNA sequence. The cell's repair mechanisms are then exploited to correct the mutation.

Question 29

A patient with Osteogenesis Imperfecta Type I has a mutation causing a premature stop codon in the COL1A1 gene. This primarily results in which of the following?

Explanation

OI Type I is typically caused by a null allele mutation resulting in haploinsufficiency, meaning a decreased amount of structurally normal collagen is produced. Missense mutations usually cause more severe types via dominant-negative effects.

Question 30

Retroviral vectors require the conversion of their RNA genome into DNA before integration into the host cell. Which enzyme is responsible for this critical step?

Explanation

Retroviruses utilize the enzyme reverse transcriptase to synthesize a complementary DNA (cDNA) strand from their RNA genome. This DNA is then integrated into the host genome via an integrase enzyme.

Question 31

When utilizing adenoviral vectors for gene therapy in bone healing models, researchers must account for the specific characteristics of this vector. Which of the following is true regarding adenoviral vectors?

Explanation

Adenoviral vectors deliver their genetic payload to the nucleus where it remains episomal (non-integrating), resulting in high-level but transient gene expression. They are also highly immunogenic.

Question 32

Fibroblast Growth Factor Receptor 3 (FGFR3) mutations are the cause of achondroplasia. In a normal growth plate, what is the physiologic effect of FGFR3 activation?

Explanation

FGFR3 is a negative regulator of bone growth. In a normal growth plate, its activation inhibits chondrocyte proliferation and hypertrophy. In achondroplasia, a gain-of-function mutation causes constitutive activation.

Question 33

A family presents with multiple male members affected by Duchenne muscular dystrophy (DMD). The gene responsible for DMD is located on the X chromosome. What is the chance that an affected male will pass the mutated gene to his son?

Explanation

DMD is an X-linked recessive disorder. A male passes his Y chromosome to his sons and his X chromosome to his daughters. Therefore, an affected male will pass the mutation to all daughters (carriers) but no sons (0%).

Question 34

Polymerase chain reaction (PCR) is used to amplify specific DNA sequences for diagnosing orthopedic infections. What is the sequence of temperature phases in a standard PCR cycle?

Explanation

A standard PCR cycle consists of denaturation (heating to separate strands), annealing (cooling to allow primers to bind), and extension (optimal temperature for DNA polymerase to synthesize the new strand).

Question 35

A researcher is evaluating the expression level of a newly discovered cartilage degrading enzyme in synovial fluid. Which of the following laboratory techniques is most appropriate to detect and quantify this specific protein?

Explanation

Western blotting uses gel electrophoresis to separate proteins by size, followed by transfer to a membrane and probing with specific antibodies. Southern blots detect DNA, and Northern blots detect RNA.

Question 36

To understand the global genetic changes occurring during fracture nonunion, researchers isolate mRNA from the fracture site to compare expression levels of thousands of genes simultaneously. Which technology is best suited for this analysis?

Explanation

DNA microarrays allow for the simultaneous measurement of the expression levels of thousands of genes (transcriptomics). This provides a global view of gene expression changes in tissues like a fracture callus.

Question 37

During endochondral ossification, hypertrophic chondrocytes undergo programmed cell death to allow vascular invasion and bone formation. Which of the following is the key executioner enzyme group in this apoptotic process?

Explanation

Caspases (cysteine-aspartic proteases) are the central executioner enzymes in apoptosis. Their activation leads to the organized cleavage of cellular proteins, resulting in programmed cell death without eliciting an inflammatory response.

Question 38

When designing a plasmid vector for gene delivery to mesenchymal stem cells, an antibiotic resistance gene is typically included. What is the primary purpose of this specific gene?

Explanation

Antibiotic resistance genes in plasmids act as selectable markers. When the cell population is exposed to the antibiotic, only the cells that successfully took up the plasmid survive, allowing for their isolation.

Question 39

The image represents basic viral integration. The occurrence of leukemia in some early gene therapy trials using retroviral vectors was primarily attributed to which of the following mechanisms?

Explanation

Retroviruses integrate semi-randomly into the host genome. In early trials, this integration occurred near proto-oncogenes (insertional mutagenesis), inadvertently activating them and leading to clonal expansion and leukemia.

Question 40

This image illustrates a physical method of introducing DNA into cells using short, high-voltage electrical pulses. This technique is known as:

Explanation

Electroporation uses brief, high-voltage electrical pulses to create transient pores in the cell membrane, allowing naked DNA to enter the cell. It is an effective non-viral, physical method of gene transfer.

Question 41

Which viral vector integrates its genetic material into the host cell genome, carrying a risk of insertional mutagenesis?

Explanation

Retroviruses integrate into the host genome, providing stable long-term expression but with a risk of insertional mutagenesis. Adenoviruses remain episomal.

Question 42

In orthopedic gene therapy, what is the primary advantage of ex vivo gene transfer compared to in vivo transfer?

Explanation

Ex vivo gene therapy involves harvesting cells, transducing them in vitro, and implanting them back. This reduces the systemic immune response to the viral vector compared to direct in vivo injection.

Question 43

Adeno-associated viruses (AAV) are frequently used in orthopedic research for gene delivery. Which of the following is a primary characteristic of AAV?

Explanation

AAV is a parvovirus that requires a helper virus (like adenovirus) to replicate. It has low immunogenicity and a relatively small packaging capacity (~4.7 kb).

Question 44

Which non-viral method of gene transfer utilizes lipid bilayers to encapsulate plasmid DNA for delivery into cells?

Explanation

Lipofection uses liposomes (lipid vesicles) to encapsulate DNA, allowing it to merge with the cell membrane and deliver genetic material. This method typically has very low immunogenicity compared to viral vectors.

Question 45

A researcher is evaluating the expression of BMP-2 protein levels in a harvested osteoblast culture following gene transduction. Which laboratory technique is most appropriate for quantifying specific protein levels?

Explanation

Western blotting is used to detect and quantify specific proteins in a sample using antibodies. Southern blotting detects DNA, while Northern blotting detects RNA.

Question 46

What is the primary function of RNA interference (RNAi) using small interfering RNA (siRNA) in orthopedic molecular biology?

Explanation

siRNA binds to specific messenger RNA (mRNA) molecules, leading to their degradation. This effectively silences the expression of the target gene post-transcriptionally.

Question 47

When designing an in vivo gene therapy strategy for bone healing using a plasmid vector, what is the most significant limitation compared to viral vectors?

Explanation

Plasmid (non-viral) vectors are safer and less immunogenic than viral vectors but suffer from significantly lower transfection efficiency. Their gene expression is also typically transient.

Question 48

In the context of cartilage repair gene therapy, transferring the gene for Interleukin-1 Receptor Antagonist (IL-1Ra) aims to:

Explanation

IL-1 is a potent catabolic cytokine in osteoarthritis that upregulates degrading enzymes. Overexpressing IL-1Ra competitively inhibits IL-1, reducing cartilage degradation and MMP production.

Question 49

Which viral vector is characterized by remaining episomal within the host nucleus, having a high transient transduction efficiency, but carrying a high risk of evoking an immune response?

Explanation

Adenoviruses do not integrate into the genome, thus remaining episomal, and provide high but transient gene expression. However, they are highly immunogenic, which limits their repeated use in vivo.

Question 50

A laboratory utilizes a technique that separates DNA fragments by size using an electrical field. This technique is known as:

Explanation

Gel electrophoresis uses an electrical field to push negatively charged DNA molecules through a gel matrix. Smaller DNA fragments travel further and faster than larger ones, separating them by size.

Question 51

Lentiviruses are a subclass of retroviruses increasingly used in orthopedic gene therapy. What unique advantage do they offer over traditional retroviruses?

Explanation

Unlike standard retroviruses which require cells to be actively dividing to enter the nucleus, lentiviruses can transport their genome into the nucleus of non-dividing cells. This broadens their applicability in tissues like cartilage.

Question 52

In the CRISPR-Cas9 gene editing system, what component provides the targeting specificity to direct the Cas9 nuclease to the exact genomic location?

Explanation

The guide RNA (gRNA) is designed to be complementary to the specific target DNA sequence. It guides the Cas9 enzyme to the correct locus to create a double-strand break.

Question 53

Gene therapy utilizing the delivery of the BMP-2 gene to muscle-derived stem cells has been shown to induce heterotopic ossification. What type of cellular process primarily characterizes this new bone formation?

Explanation

Muscle-derived stem cells transduced with BMP-2 typically undergo a cartilage intermediate phase followed by osteogenesis. This process mimics endochondral ossification to form ectopic bone.

Question 54

A researcher aims to amplify a specific sequence of DNA obtained from a giant cell tumor. Which of the following represents the correct sequence of thermal steps in a Polymerase Chain Reaction (PCR)?

Explanation

PCR cycles begin with denaturation of double-stranded DNA at high temperature (~94-98°C). This is followed by annealing of primers at a lower temperature, and finally extension by Taq polymerase.

Question 55

Plasmids are frequently used as non-viral vectors in basic science orthopedic research. What best describes the structure of a typical plasmid?

Explanation

Plasmids are extrachromosomal, circular, double-stranded DNA molecules found naturally in bacteria. They can replicate independently and are easily engineered to carry therapeutic genes of interest.

Question 56

When utilizing ex vivo gene therapy for delayed fracture healing, what is the most common and practical target cell harvested, genetically modified, and re-implanted?

Explanation

Mesenchymal stem cells (MSCs) are ideal vehicles for ex vivo gene therapy in orthopedics. They are easily harvested from bone marrow, expandable in culture, and possess osteogenic potential.

Question 57

Which of the following growth factors, commonly targeted in fracture healing gene therapy, functions as a powerful factor primarily stimulating the formation of new blood vessels?

Explanation

Vascular Endothelial Growth Factor (VEGF) is a primary driver of angiogenesis. This is a critical step in the early phases of fracture healing and endochondral ossification.

Question 58

The "bystander effect" in gene therapy, particularly relevant in orthopedic oncology and suicide gene strategies, refers to:

Explanation

The bystander effect occurs when the therapeutic product (e.g., a toxic metabolite activated by a suicide gene) is transferred from transduced cells to neighboring untransduced tumor cells. This amplifies the tumoricidal effect.

Question 59

Microarrays are powerful tools increasingly utilized in orthopedic molecular biology. What are they primarily used for?

Explanation

DNA microarrays allow researchers to analyze the global gene expression profile by measuring thousands of mRNA transcripts simultaneously. They are often used to compare normal versus diseased tissue, such as OA cartilage.

Question 60

Which specific vector-related complication led to significant setbacks in early human gene therapy trials due to the activation of an oncogene?

Explanation

Early human trials utilizing retroviral vectors for severe combined immunodeficiency (X-SCID) resulted in insertional mutagenesis near oncogenes like LMO2. This led to the development of leukemia in some patients, highlighting integration risks.

Question 61

Which of the following viral vectors used in gene therapy integrates its genetic material into the host genome but is restricted to transducing only actively dividing cells?

Explanation

Retroviruses integrate their DNA into the host genome, allowing for stable, long-term expression, but they require the target cells to be actively dividing to gain access to the nucleus. Lentiviruses, a subclass of retroviruses, can infect both dividing and non-dividing cells.

Question 62

What is the primary advantage of utilizing an adeno-associated virus (AAV) over a standard adenovirus for gene delivery in orthopedic tissue engineering?

Explanation

AAV vectors provoke a very mild immune response compared to standard adenoviruses, allowing for more sustained gene expression in vivo. However, they have a limited packaging capacity (about 4.5 kb) and typically require a helper virus for replication.

Question 63

The CRISPR-Cas9 system has emerged as a powerful tool for genome editing in skeletal dysplasias. This system relies on which specific component to identify and bind the target DNA sequence?

Explanation

The CRISPR-Cas9 system uses a synthetic guide RNA (gRNA) that is complementary to the target DNA sequence. This gRNA directs the Cas9 nuclease to the specific genomic location to induce a double-strand break.

Question 64

A research team is attempting to detect the presence of a specific DNA mutation in a patient with Osteogenesis Imperfecta. Which of the following laboratory techniques is primarily used to detect specific DNA sequences?

Explanation

Southern blotting is used to detect specific DNA sequences. The mnemonic SNoW DRoP helps differentiate the techniques: Southern=DNA, Northern=RNA, Western=Protein.

Question 65

To evaluate the efficacy of a novel therapeutic for osteoarthritis, a researcher quantifies the mRNA expression levels of Aggrecan in articular chondrocytes. Which technique is most appropriate for this purpose?

Explanation

RT-qPCR is the most sensitive and quantitative method for measuring specific mRNA expression levels in cells. It first converts RNA to complementary DNA (cDNA) using reverse transcriptase, then amplifies and quantifies it in real-time.

Question 66

Achondroplasia is the most common form of short-limb dwarfism. It is caused by a mutation in the FGFR3 gene. Which best describes the molecular consequence of this specific mutation?

Explanation

Achondroplasia results from a gain-of-function mutation in the FGFR3 gene. This constitutively activates the receptor, which normally functions to inhibit chondrocyte proliferation, thus prematurely arresting endochondral ossification.

Question 67

Which of the following transcription factors is considered the master regulator essential for the differentiation of mesenchymal stem cells into the osteoblast lineage?

Explanation

Runx2 (also known as Cbfa1) is the master transcription factor required for osteoblast differentiation. Mice lacking Runx2 completely lack bone due to a maturation arrest of osteoblasts. Sox9 is the equivalent master regulator for chondrocytes.

Question 68

In the context of gene therapy for bone repair, which of the following accurately describes an 'ex vivo' gene delivery strategy?

Explanation

Ex vivo gene therapy involves isolating cells from the patient, modifying them genetically in a laboratory, and then implanting the transduced cells back into the patient. This allows for selection and quality control of the modified cells prior to implantation.

Question 69

The Wnt/β-catenin signaling pathway is a critical regulator of bone mass. Sclerostin (encoded by the SOST gene) inhibits bone formation by binding to and blocking which specific receptor complex?

Explanation

Sclerostin is a secreted glycoprotein that antagonizes Wnt signaling by binding to the LRP5/6 coreceptor on osteoblasts. This prevents the Wnt ligand from binding, leading to the degradation of β-catenin and a decrease in bone formation.

Question 70

What is the primary mechanism of action of small interfering RNA (siRNA) when used as a therapeutic tool in molecular orthopedics?

Explanation

siRNA mediates RNA interference (RNAi), a process that leads to specific post-transcriptional gene silencing. The siRNA binds to a complementary target mRNA sequence, leading to its cleavage and degradation by the RNA-induced silencing complex (RISC).

Question 71

A laboratory develops a viral vector that possesses an RNA genome, integrates stably into the host cell DNA, and is uniquely capable of infecting non-dividing cells such as mature neurons or resting osteocytes. Which vector fits this description?

Explanation

Lentiviruses are a subclass of retroviruses with an RNA genome that reverse transcribes into DNA and integrates into the host genome. Unlike simple retroviruses, lentiviruses have the machinery to actively transport their pre-integration complex through the nuclear membrane of intact, non-dividing cells.

Question 72

Marfan syndrome is classically caused by a mutation in the FBN1 gene coding for fibrillin-1. The pathophysiology of this disorder is primarily related to the abnormal regulation and excessive signaling of which molecule?

Explanation

Fibrillin-1 normally binds to and sequesters latent TGF-β in the extracellular matrix. Mutations in FBN1 lead to failed sequestration, resulting in excessive TGF-β signaling, which drives many of the phenotypic features of Marfan syndrome.

Question 73

Diastrophic dysplasia is an autosomal recessive disorder caused by a mutation in the SLC26A2 (DTDST) gene. What is the primary molecular consequence of this defect in cartilage tissue?

Explanation

The SLC26A2 gene encodes a sulfate transporter. Mutations cause defective intracellular transport of sulfate, leading to the undersulfation of proteoglycans in the cartilage matrix, which severely impairs cartilage structural integrity.

Question 74

During the polymerase chain reaction (PCR), what is the primary purpose of the initial heating phase, typically set between 94°C and 98°C?

Explanation

The high temperature (94-98°C) is used for the denaturation step in PCR. This breaks the hydrogen bonds holding the complementary DNA strands together, yielding single-stranded DNA required for primer binding in the subsequent annealing phase.

Question 75

A researcher conducts an assay to detect the presence of specific bone morphogenetic proteins (BMPs) in a tissue sample.

Based on standard molecular techniques, which method uses gel electrophoresis followed by antibody-based detection to confirm the presence and size of a specific protein?

Explanation

Western blotting is the standard technique for detecting specific proteins in a sample. It involves separating proteins by gel electrophoresis, transferring them to a membrane, and incubating with specific antibodies.

Question 76

The schematic illustrates a cellular process targeted in a novel orthopedic gene therapy.

When utilizing a non-viral plasmid vector for in vivo gene transfer as part of a bone healing strategy, what is the most characteristic outcome regarding gene expression?

Explanation

Non-viral plasmid vectors are typically safer than viral vectors because they do not elicit a strong immune response and do not integrate into the host genome. However, their major limitation is low transfection efficiency and purely transient gene expression.

Question 77

Which of the following mutations results in a classic 'dominant negative' effect, as frequently observed in severe, lethal forms of Osteogenesis Imperfecta (Type II)?

Explanation

In dominant negative mutations, the defective gene product structurally interferes with the function of the normal protein from the wild-type allele. A missense mutation substituting glycine prevents proper folding of the entire type I collagen triple helix, leading to severe qualitative defects (Type II, III, IV OI).

Question 78

Epigenetic regulation controls gene expression in bone and cartilage without altering the underlying DNA sequence. Which of the following modifications is most commonly associated with transcriptional silencing of a gene?

Explanation

DNA methylation, particularly at CpG islands within a gene's promoter region, generally leads to transcriptional repression or silencing. In contrast, histone acetylation is typically associated with open chromatin and active gene transcription.

Question 79

In the process of endochondral ossification, the sequential differentiation of mesenchymal cells into chondrocytes is heavily dependent on specific transcription factors. Which transcription factor is essential for this initial chondrogenic commitment?

Explanation

Sox9 is the critical master transcription factor that drives the differentiation of mesenchymal stem cells into chondrocytes. Without Sox9, mesenchymal condensation and cartilage formation during endochondral ossification cannot occur.

Question 80

Despite their high transduction efficiency and ability to produce high levels of therapeutic proteins like BMP-2, standard adenoviral vectors have a significant limitation that often precludes their systemic clinical use. What is this primary limitation?

Explanation

Adenoviral vectors do not integrate into the host genome (reducing oncogenic risk) and can infect non-dividing cells. Their main clinical limitation is their high immunogenicity, which can elicit severe, sometimes fatal, inflammatory responses.

Question 81

Southern blotting is a foundational laboratory technique used primarily in basic orthopedic science to detect a specific sequence of which of the following macromolecules?

Explanation

Southern blotting is used to detect specific DNA sequences in DNA samples. Northern blotting is used for RNA, and Western blotting is used to detect specific proteins.

Question 82

In molecular biology studies evaluating the upregulation of specific inflammatory markers in osteoarthritis, which technique is most appropriate for detecting specific mRNA sequences to quantify gene expression?

Explanation

Northern blotting detects specific RNA sequences (specifically mRNA) to study gene expression. It involves separating RNA fragments by size via electrophoresis and probing with a complementary sequence.

Question 83

An orthopedic researcher is evaluating a new bone graft substitute and wants to quantify the production of Osteocalcin protein by osteoblasts in vitro. Which laboratory test should be utilized?

Explanation

Western blotting is an analytical technique used to detect specific proteins in a sample of tissue homogenate or extract. It relies on the use of specific antibodies to identify the target protein.

Question 84

Which of the following viral vectors used in gene therapy is capable of permanently integrating into the host cell's genome but is historically limited by its ability to infect actively dividing cells only?

Explanation

Standard retroviruses require target cells to be actively dividing for genome integration to occur. In contrast, lentiviruses (a subclass of retroviruses) can integrate into the genomes of non-dividing cells.

Question 85

Adeno-associated viruses (AAV) are frequently preferred over adenoviruses for orthopedic gene therapy applications. Which of the following best describes the primary advantage of AAV in this context?

Explanation

AAVs are highly favored for in vivo gene therapy because they elicit a very low immune response and cause no known human diseases. Adenoviruses, conversely, can provoke severe and highly destructive cellular immune responses.

Question 86

An experimental model utilizes a naked plasmid vector injected directly into an osteoporotic fracture site to induce localized bone formation. This represents which type of gene delivery system?

Explanation

Directly injecting a plasmid (which lacks viral components) into a living subject is a non-viral, in vivo method of gene delivery. While simpler and safer, it often suffers from lower transfection efficiency compared to viral vectors.

Question 87

Review the cell cycle model representing dividing osteoblasts:

During the standard eukaryotic cell cycle, in which phase does DNA replication primarily occur?

Explanation

The Synthesis (S) phase of the cell cycle is when DNA replication takes place, resulting in the duplication of the cell's genetic content prior to entering the G2 phase and eventually Mitosis (M phase).

Question 88

In an ex vivo gene therapy approach designed to accelerate the healing of critical-sized bone defects, which of the following best describes the core methodology?

Explanation

Ex vivo gene therapy involves isolating cells from a patient, cultivating and genetically modifying them in a laboratory setting, and then transplanting these optimized cells back into the patient.

Question 89

Polymerase chain reaction (PCR) involves repeated cycles of temperature changes to amplify DNA. What is the primary purpose of the initial high-temperature step (typically 94-98°C)?

Explanation

The high-temperature denaturation step separates the double-stranded DNA into single strands. This exposes the target sequences, allowing primers to bind during the subsequent annealing phase.

Question 90

An oncology researcher uses small interfering RNA (siRNA) to target mutant oncogenes in an osteosarcoma cell line. What is the expected mechanism of action of this targeted therapy?

Explanation

siRNA utilizes the RNA interference (RNAi) pathway to achieve post-transcriptional gene silencing. It binds to complementary mRNA transcripts, leading to their degradation and preventing protein translation.

Question 91

Achondroplasia is the most common form of disproportionate short stature and is caused by a specific mutation in the FGFR3 gene. At the molecular level, this mutation typically results in which of the following?

Explanation

Achondroplasia is caused by a gain-of-function mutation in FGFR3, leading to its constitutive activation. This abnormal continuous signaling powerfully inhibits chondrocyte proliferation in the growth plate.

Question 92

A researcher is evaluating reverse transcription polymerase chain reaction (RT-PCR) to study gene expression:

Which enzyme is uniquely essential for synthesizing complementary DNA (cDNA) from a messenger RNA (mRNA) template?

Explanation

Reverse transcriptase is an RNA-dependent DNA polymerase. It is responsible for creating a complementary DNA (cDNA) strand from an RNA template, a critical step in RT-PCR and retroviral replication.

Question 93

Osteogenesis imperfecta (OI) type II is typically lethal in the perinatal period. This extreme severity is most commonly explained by which genetic mechanism affecting the assembly of the type I collagen triple helix?

Explanation

Severe forms of OI (Types II, III, IV) are usually caused by missense mutations (often substituting glycine) that produce abnormal collagen chains. These mutant chains disrupt the entire triple helix, exhibiting a dominant negative effect.

Question 94

Which of the following viral vectors is characterized by its high transduction efficiency and ability to infect both dividing and non-dividing cells, but primarily remains episomal, leading to transient gene expression?

Explanation

Adenoviruses do not integrate into the host genome but instead persist as episomes within the nucleus. This results in robust but transient expression of the transgene, alongside a strong host immune response.

Question 95

In the process of gene cloning and recombinant DNA technology, what is the primary and specific function of restriction endonucleases?

Explanation

Restriction endonucleases act as molecular scissors, cutting double-stranded DNA at specific recognition sites. The resulting fragments can then be ligated into vectors during cloning.

Question 96

When designing a gene delivery protocol to target fully differentiated, non-dividing articular chondrocytes, what is the major advantage of using lentiviral vectors over classic retroviral vectors?

Explanation

Unlike standard retroviruses, which require host cell division (nuclear envelope breakdown) to access the genome, lentiviruses possess pre-integration complexes that can cross the intact nuclear membrane of non-dividing cells.

Question 97

The CRISPR/Cas9 system is a revolutionary tool for targeted genome editing in orthopedic genetic research. In this system, what specific component directs the Cas9 nuclease to the exact target DNA sequence?

Explanation

The CRISPR system relies on a guide RNA (gRNA) specifically designed to be complementary to the target DNA sequence. The gRNA binds to the Cas9 enzyme and guides it to the precise genomic location to create a double-strand break.

Question 98

Which of the following methods of non-viral gene delivery utilizes short, high-voltage electrical pulses to temporarily create pores in the cell membrane, allowing exogenous DNA to enter?

Explanation

Electroporation uses an electrical field to increase the permeability of the cell membrane, introducing pores through which plasmids or other macromolecules can pass. It is highly efficient for in vitro cell transfection.

Orthopedic Prometric MCQs - Chapter 3 Part 1

Orthopedic Prometric MCQs - Chapter 3 Part 9

Orthopedic Prometric MCQs - Chapter 3 Part 9

Chapter Index