Yellow nail syndrome
C. Order a thoracic CT scan
The key to the correct diagnosis in this patient is the triad of yellow thickened nails, sinopulmonary manifestations, and lymphedema, which point to a diagnosis of yellow nail syndrome. Onychomycosis (option A) would not explain the other manifestations described in this case. Although dystrophic nails can be seen in psoriasis, looking for evidence of psoriatic arthritis (option B) in the absence of joint symptoms would not be justified. Additionally, the other manifestations described in this case would not be explained by psoriasis. In the of setting of yellow thickened nails, sinopulmonary manifestations, and lymphedema, exertional shortness of breath is likely related to serositis, bronchiectasis, or both, and therefore the next step to evaluate these pulmonary manifestations of yellow nail syndrome would be to obtain a thoracic CT scan (option C). Severe iron deficiency (option D) can be associated with koilonychia (spooning of nails, in which nails lose their convexity and become flat or even concave in shape) but not with yellow dystrophic nails. It would also be unlikely in the absence of anemia. Although ANA testing was positive and C3 complement level was low, in the absence of antibodies to extractable nuclear antigens and double-stranded DNA, these laboratory findings are nonspecific and not diagnostic for systemic lupus erythematosus.