**Core Concept**
The 'thimble' bladder appearance is a radiological finding that indicates a significant reduction in bladder volume, often associated with bladder outlet obstruction or neurogenic bladder. This term is used to describe the characteristic shape of the bladder, which resembles a thimble due to its small size and irregular contours.

**Why the Correct Answer is Right**
The 'thimble' bladder appearance is typically seen in conditions that cause increased resistance to urine flow, such as benign prostatic hyperplasia (BPH), bladder neck contracture, or bladder outlet obstruction. In these cases, the bladder muscle becomes hypertrophied, and the bladder capacity is reduced. As a result, the bladder contracts to a smaller size, leading to the characteristic 'thimble' appearance on imaging studies. The detrusor muscle in the bladder wall becomes thickened due to the increased pressure.

**Why Each Wrong Option is Incorrect**
* **Option A:** Neurogenic bladder is a correct association with the 'thimble' bladder appearance, but it is not the most common cause.
* **Option B:** Bladder cancer can cause a variety of radiological appearances, but the 'thimble' bladder is not a characteristic finding.
* **Option C:** Spina bifida is a condition that can cause neurogenic bladder, but it is not the most common cause of the 'thimble' bladder appearance.

**Clinical Pearl / High-Yield Fact**
The 'thimble' bladder appearance is a classic radiological finding that should raise suspicion for bladder outlet obstruction or neurogenic bladder. It is essential to consider these conditions in the differential diagnosis when interpreting imaging studies of the bladder.

**Correct Answer: C. Neurogenic bladder**