Osteosarcoma is the most common primary malignancy of bone, excluding hematopoietic malignancies, making up approximately 20% of all primary bone cancers. 

Fibrosarcoma and Ewing's sarcoma are relatively rare conditions, and osteoid osteomas are is non-malignant.

Roughly 75% of osteosarcomas occur in the distal femur or around the knee. The next most common site is the upper arm. Spinal or pelvic involvement are rare. 

The majority of cases of osteosarcoma are in children and adolescents between the age of 10 and 25. It's the most common type of bone cancer in children and young adults, and the third most common cancer in teens.

Dystonia is a condition of sustained muscle contractions causing abnormal posture, twisting, restricted movement, and pain. Torticollis is a form of dystonia specific to the muscles of the neck. Ballismus is a condition of rapid, violent involuntary motions. Geniospasm is specific to the mentalis muscle, causing involuntary movements of the jaw.

Elevation of the limb above the heart may slow arterial perfusion and raise venous pressure, and worsen circulation. Ice may cause vasoconstriction and further lead to circulation compromise. Both of these interventions would not be appropriate for compartment syndrome. 

Know the six P's of compartment syndrome: pallor, pain on passive movement, paresthesia, paralysis, pulses, pressure: all of these may be present in compartment syndrome. Pulselessness is an ominous sign of impending permanent damage, the physician should be notified. Muscle damage can lead to myoglobinemia of the blood thereby damaging the kidneys leading to rhabdomyolysis and dark urine. A tight bandage could further increase the pressure in the compartment and should be loosened if this is so. 

The correct answer is "costochondritis." Costochondritis is an inflammation of the cartilage that bridges the sternum to the ribs. It is a very common cause of acute chest pain in young, healthy, physically active individuals without known cardiovascular disease. In this case, we have a very prototypical case: a young, physically active (especially with physical activity involving the chest like weight-lifting and rowing crew), otherwise healthy male with focal, reproducible chest pain in the parasternal area, and normal vital signs on presentation. When you see this history, this is a rather convincing case for costochondritis. There is no information given that would seem to indicate a personal or family history of cardiovascular disease, and thus, costochondritis would be the best answer choice. He can be treated conservatively with NSAID's for anti-inflammatory and pain control effects.

Acute coronary syndrome (e.g. myocardial infarction) is very unlikely in this patient as he is young and healthy, has reproducible, focal chest pain, is very physically active, and has stable vital signs. If there was a personal or family history of cardiovascular disease, or any report of recent drug use (particularly cocaine), then this could be higher on the differential.

Pulmonary embolism is very unlikely in this patient as there is no leg swelling reported, the chest pain is focal, the patient is healthy and active, and his vital signs are normal.

Peripheral vascular disease would not present with chest pain, and would be highly unlikely to be present in an otherwise healthy 19-year old.

There is nothing to suggest that this healthy, active 19-year old patient has diabetes mellitus from the information provided.

The correct answer is "ACL, MCL, and Medial meniscus." This is the only answer choice that correctly lists all three components of the "unhappy triad." The unhappy triad is frequently injured in athletes who participate in contact sports, such as football. The injury mechanism often involves a direct, forceful hit to a firmly planted leg. The hit is forceful enough to rupture two ligaments (ACL and MCL), as well as the medial meniscus. The treatment for such an injury involves surgical intervention and a lengthy recovery period, though for patients inclined to partake in the rehabilitation process, they can often return to their desired sports after completion of surgery and rehab.

The correct answer is "compartment syndrome." Compartment syndrome is a surgical emergency in which an injury results in hemorrhage or swelling in an enclosed space that causes dangerously high pressures to build within the enclosed space, resulting in compromised bloodflow to the muscles and nerves within and distal to the space. The symptoms include a cool, pale, pulseless extremity that may also experience paresthesias, and that is visibly swollen and tense. In this case, the patient's right arm exhibits all of these characteristics, and this presentation is highly consistent with compartment syndrome.

The other choices are incorrect. Vasculitis and lupus are both autoimmune conditions that may result in limb swelling, but would be unlikely causes of this patient's presentation, especially in the setting of a recent local trauma to the affected limb. Fat embolism would not account for the patient's constellation of symptoms.There is no evidence of limb necrosis at this stage of the patient's injury. Should the compartment syndrome not be addressed and vascular flow remain compromised by persistently high pressures, then limb ischemia and consequent necrosis could be a downstream effect.

Polymyalgia rheumatica is an inflammatory musculoskeletal condition that presents with pain or stiffness, usually in the neck, shoulders, and hips. Amyotrophic lateral sclerosis (ALS) is a genetic disorder of muscle wasting and neuronal death. Symptoms include progressive muscular weakness, leading to paralysis and death. Guillain–Barré syndrome is a rapid-onset condition of reversible muscle paralysis and weakness following infection, most commonly Campylobacter jejuni. Myositis is a general term for inflammation of the muscles, and can be due to various causes.

Duchenne's muscular dystrophy (DMD) is caused by dysfunctional production of dystrophin, a protein that is part of a complex of proteins that anchor the cytoskeleton of a muscle cell to the extracellular matrix, also known as the dystrophin-associated protein complex. Its absence disrupts multiple intracellular signaling pathways, leading to muscle necrosis. Dystroglycan is another protein in this complex, but it is unaffected in DMD. Actin and troponin are also unaffected in DMD.