In a patient with positive nitrates and leukocytes along with flank pain, what condition is most likely?

The presence of positive nitrates and leukocytes, in conjunction with flank pain, strongly suggests a urinary tract infection (UTI) that has ascended to involve the kidneys, which is termed pyelonephritis. In this context, positive nitrates indicate the presence of nitrate-reducing bacteria, commonly associated with UTIs, while leukocytes (white blood cells) point to an inflammatory response often seen with infection.

Flank pain is specifically indicative of kidney involvement; it typically presents in renal conditions, including pyelonephritis, rather than lower urinary tract infections like cystitis, which usually causes suprapubic pain and dysuria but not flank pain. Therefore, the combination of these clinical signs and symptoms is characteristic of pyelonephritis, confirming it as the most likely diagnosis in this scenario.