Biosafety level 4 (BSL-4) pathogen is:

 # Biosafety level 4 (BSL-4) pathogen is:

 A. XDR strain of Mycobacterium tuberculosis

B. SARS CoV-2

C. Ebola

D. Measles

The correct answer is C. Ebola.

Biological Safety Levels (BSL) are a series of protections relegated to the activities that take place in particular biological labs. They are individual safeguards designed to protect laboratory personnel, as well as the surrounding environment and community.

These levels, which are ranked from one to four, are selected based on the agents or organisms that are being researched or worked on in any given laboratory setting. For example, a basic lab setting specializing in the research of nonlethal agents that pose a minimal potential threat to lab workers and the environment are generally considered BSL-1—the lowest biosafety lab level. A specialized research laboratory that deals with potentially deadly infectious agents like Ebola would be designated as BSL-4—the highest and most stringent level.

The Centers for Disease Control and Prevention (CDC) sets BSL lab levels as a way of exhibiting specific controls for the containment of microbes and biological agents. Each BSL lab level builds upon on the previous level—thereby creating layer upon layer of constraints and barriers. These lab levels are determined by the following:

• Risks related to containment
• Severity of infection
• Transmissibility
• Nature of the work conducted
• Origin of the microbe
• Agent in question
• Route of exposure

 Summary of Biological Agents and BSL levels

Reference: The University of Texas Rio Grande Valley

Once bone is formed, it grows by:

 # Once bone is formed, it grows by:

 A. Interstitial growth only

B. Appositional growth only

C. Both appositional and interstitial growth

D. Degenerative changes into bony structures

The correct answer is B. Appositional growth only.

Which is growth by the addition of new layers on those previously formed. Bone formation begins in the embryo where mesenchymal cells differentiate into either fibrous membrane or cartilage. 

This leads to two paths of bone development:

1. Intramembranous ossification is so called because it takes place in membranes of connective tissue. Osteoprogenitor cells in the membrane differentiate into osteoblasts: a collagen matrix is formed which undergoes ossification. Note: The maxilla and mandible as well as the cranial vault are forrmed this way.

2. Endochondral ossification is how the remainder of the skeleton forms and takes place within a hyaline cartilage model. Cartilage cells are replaced by bone cells (osteocytes replace chondrocytes), organic matrix is laid down and calcium and phosphate are deposited. This type of ossification is principally responsible for the formation of short and long bones. Note: The ethmoid, sphenoid, and occipital bones (bones of the cranial base) form this way.

The incisor overjet is increased for a class II case that is undergoing treatment. The reason is:

 # The incisor overjet is increased for a  class II case that is undergoing treatment. The reason is:

 A. Loss of anchorage

B. Arch collapse

C. Crowding in the lower arch

D. None of the above

The correct answer is A. Loss of anchorage.

Following are the features of anchorage loss in class II cases:

- Increase in overjet

- Molar relation  becoming more class II

- Normal canine relation without any change

Which of the following are typical and acceptable preventive and therapeutic measures for dealing with the periodontal problems during fixed appliance therapy?

 # Which of the following are typical and acceptable preventive and therapeutic measures for dealing with the periodontal problems during fixed appliance therapy?

 A. Elimination of gingivitis prior to placing orthodontic appliances

B. Home care instructions regarding the use of toothbrush and water pik during orthodontic treatment

C. Megavitamin therapy

D. Scaling and curettage immediately after appliance removal

The correct answer is A. Elimination of gingivitis prior to placing orthodontic appliances.

The mean value of visible plaque and visible inflammation showed significant increases during orthodontic treatment. Therefore, prior to orthodontic treatment, patients should have a high level of periodontal health and it should be maintained during the treatment period. Considering the relationship between orthodontic treatment and gingival health, patients, orthodontists and periodontists should cooperate during orthodontic treatment.

Reference: Boke F, Gazioglu C, Akkaya S, Akkaya M. Relationship between orthodontic treatment and gingival health: A retrospective study. Eur J Dent. 2014;8(3):373-380. doi:10.4103/1305-7456.137651