**Core Concept**
Cystitis is an inflammation of the bladder, often caused by infection. The most common pathogens responsible for cystitis are Gram-negative bacteria that can ascend from the lower urinary tract.

**Why the Correct Answer is Right**
E. coli is a common Gram-negative bacterium that is frequently found in the gastrointestinal tract. It can ascend to the urinary tract through the urethra and cause infection. E. coli produces virulence factors such as adhesins and toxins that enable it to colonize and infect the bladder mucosa. The pathogenesis of E. coli-induced cystitis involves the attachment of bacteria to the bladder epithelial cells, followed by the production of inflammatory mediators that lead to symptoms such as dysuria and frequency.

**Why Each Wrong Option is Incorrect**
**Option B:** Pseudomonas is a Gram-negative bacterium that is commonly found in water and soil. While it can cause urinary tract infections, it is less common than E. coli and typically affects patients with underlying conditions such as catheter-associated urinary tract infections.

**Option C:** Proteus mirabilis is a Gram-negative bacterium that can cause urinary tract infections, particularly in patients with underlying conditions such as urinary tract obstruction. However, it is less common than E. coli and typically produces symptoms such as pyuria and bacteriuria.

**Option D:** Neisseria gonorrhea is a Gram-negative bacterium that can cause gonococcal cystitis, particularly in sexually active young women. However, it is less common than E. coli and typically requires specific diagnostic testing for confirmation.

**Clinical Pearl / High-Yield Fact**
The most common symptoms of cystitis include dysuria, frequency, and urgency. A positive urine dipstick test for leukocyte esterase and nitrite is often used as a screening test for cystitis. However, a negative test does not rule out the diagnosis, and further diagnostic testing such as urine culture may be necessary.

**✓ Correct Answer: A. E. coli**