Final answer:
The bladder is difficult to palpate due to being protected by the pelvic bones and its variable distension, which depends on the volume of urine it contains.
Step-by-step explanation:
The bladder can be difficult to palpate primarily because it is protected by the pelvic bones. This is a correct description of its anatomical location, which shields it from direct palpation until it is significantly distended. The distension occurs superiorly when the bladder fills, which in some individuals may still keep it tucked under the pelvic bones.
When the bladder is empty or not fully distended, it falls below the pelvic margin and hence, is even less palpable. Only when it's sufficiently full and expands above the pelvic bones can it sometimes be palpated through the lower abdominal wall. The bladder's ability to significantly change its volume due to its distensible nature and the presence of the detrusor muscle also means its palpability varies widely.
In general, it is the bladder's location beneath the pelvic bones and its variable distension that contributes to the difficulty in palpating it, particularly when it is not full.