Question: In a patient with mild chronic obstructive pulmonary disease (COPD) in stable phase, polyglobulia and respiratory insufficiency are evident. The chest radiograph shows no noticeable alterations. Which of the following procedures would you consider to carry out in the first place to rule out the coexistence of other diseases that may justify the described findings ?: 
 Options:  <1> Respiratory polygraphy <2> Echocardiogram. <3> Thoracic computed tomography. <4> Pulmonary scintigraphy <5> Puncture of bone marrow.
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