A 42-year-old female has been fatigued for the past few months. The onset of her illness was poorly defined, and aside from a lack of energy she has no other complaints. She reported becoming short of breath when climbing the stairs or walking for a prolonged distance. She told her doctor that she had recently experienced some weight loss following the death of her mother. What diseases or conditions should be considered in this patient (debate why or why not some disease might fit with the description)? What additional information do you need to make a correct diagnosis? What tests might you run?