**Core Concept**
Pyelonephritis in pregnancy is a urinary tract infection (UTI) affecting the kidneys, with a higher risk of complications due to hormonal and physiological changes during pregnancy. The most common cause of pyelonephritis in pregnant women is bacterial invasion of the urinary tract.

**Why the Correct Answer is Right**
The correct answer is Escherichia coli (E. coli), a gram-negative bacterium. E. coli is the leading cause of pyelonephritis in pregnant women due to its virulence factors, such as adhesins, which enable it to colonize and invade the bladder and kidneys. The hormonal changes during pregnancy, including increased estrogen levels, can enhance the adherence of E. coli to the uroepithelial cells, increasing the risk of infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** Klebsiella pneumoniae is another common cause of UTIs, but it is less common than E. coli in pregnant women.
* **Option B:** Staphylococcus aureus is a common cause of skin and soft tissue infections, but it is not typically associated with pyelonephritis in pregnancy.
* **Option C:** Pseudomonas aeruginosa is a gram-negative bacterium that can cause UTIs, but it is more commonly associated with hospital-acquired infections and is less common in pregnant women.

**Clinical Pearl / High-Yield Fact**
E. coli pyelonephritis in pregnancy is often associated with symptoms such as flank pain, fever, and urinary frequency. Prompt diagnosis and treatment are essential to prevent complications, such as preterm labor and fetal growth restriction.

**Correct Answer:** A. E. coli