If a swab taken from my paronychia shows pseudomonas, does it rule out sub ungual melanoma? the paronychia is not responding to prescription (4 months).

See a melanoma exper: Any inflammation that does not heal over such a long time raises the possibility of something unusual and not simply infection. I would advise that you see an epert in diagnosing melanoma(dermatologist or a surgeon) and discuss the need for a biopsy to rule out melanoma. Subungual melanoma usually starts at the base of the nail(top part under the skin), it is pigmented but it can be amelanotic too.

No: But probably then a chronic infection probably need nail removed, correct antibiotics, and possible biopsy.