"50 mL" does not seem to be supported by anything that I can find in AMBOSS/FA25/NBME/online.
AMBOSS says that "PVR < 100 mL is usually considered normal" however notes that "there is no consensus on the cutoff value for abnormal PVR."
This is confirmed by NIH's "Bladder Post Void Residual Volume" by Ballstaedt et al.:
"
- Less than 100 mL PVR is considered normal.
- Up to 200 mL PVR may be acceptable.
- Over 200 mL PVR indicates inadequate emptying.
- Over 300 mL is suggestive of urinary retention.
- Over 400 mL is considered urinary retention.
"
The one step 2 UW tag does not talk about mLs of PVR. The NBME Form tagged (Form 14, section 2) has a question very similar to the UW question, but again, has no discussion of PVR diagnostic criteria (however, in the vignette, the patient's PVR is 200 mLs).
Source: AMBOSS - https://next.amboss.com/us/article/c50aQg#Y77627812ceaa13f17f04144c23ffd928
conflicting sources