**Core Concept**
Acute pyelonephritis is a type of kidney infection characterized by inflammation of the renal parenchyma, usually caused by a bacterial infection. The kidneys are the primary organs involved in filtering waste and excess fluids from the blood, and an infection in this area can lead to severe complications if not treated promptly.

**Why the Correct Answer is Right**
The commonest organism producing acute pyelonephritis is **Escherichia coli (E. coli)**, a gram-negative rod-shaped bacterium. E. coli is a normal inhabitant of the gastrointestinal tract but can cause infections when it enters the urinary tract through the urethra, often due to urethral catheterization, instrumentation, or other predisposing factors. Once in the urinary tract, E. coli can adhere to the bladder and renal epithelial cells, invade the renal parenchyma, and produce virulence factors that contribute to the development of pyelonephritis.

**Why Each Wrong Option is Incorrect**
**Option A:** Klebsiella pneumoniae is another gram-negative bacterium that can cause urinary tract infections, but it is less common than E. coli in causing acute pyelonephritis.
**Option B:** Staphylococcus saprophyticus is a gram-positive bacterium that is more commonly associated with urinary tract infections in young women, but it is not the most common cause of acute pyelonephritis.
**Option C:** Pseudomonas aeruginosa is a gram-negative bacterium that can cause a variety of infections, including urinary tract infections, but it is not typically associated with acute pyelonephritis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that E. coli is the most common cause of acute pyelonephritis, and the risk factors for developing this condition include urinary tract abnormalities, diabetes mellitus, and immunocompromised states. A high index of suspicion and prompt diagnosis are crucial in managing this condition effectively.

**Correct Answer:** C. E. coli