What is the appropriate specimen to diagnose a viral cause for hemorrhagic cystitis in an immunocompromised patient?

The appropriate specimen to diagnose a viral cause for hemorrhagic cystitis in an immunocompromised patient is urine for the BK virus. In immunocompromised individuals, particularly those who have undergone renal transplantation, the BK virus can cause significant urinary tract issues, including hemorrhagic cystitis. The presence of the virus in urine can be identified using specific viral assays, making urine a valuable specimen for this diagnosis.

In contrast, while other viruses listed may be associated with a range of systemic infections, they do not have a direct link to hemorrhagic cystitis in this patient population. Human papillomavirus primarily affects epithelial tissues, especially in the skin and mucous membranes, and would not typically be associated with a urinary tract condition. Hepatitis B virus, found in serum, relates more to liver disease than urinary complications. Epstein-Barr virus is linked to various lymphoproliferative disorders and infectious mononucleosis but is not a common cause of hemorrhagic cystitis. Therefore, urine is the most relevant specimen for diagnosing the BK virus in this context.