**Core Concept**
Cystitis is a common urinary tract infection (UTI) during pregnancy that can be caused by bacteria such as Escherichia coli. Antibiotics are often necessary to treat UTIs in pregnancy to prevent complications like pyelonephritis and preterm labor. However, the choice of antibiotic should take into account the potential risks to the fetus and the safety of the medication during pregnancy.

**Why the Correct Answer is Right**
Nitrofurantoin (Option A) is commonly used to treat uncomplicated cystitis in pregnancy, but it is contraindicated in the third trimester (after 37 weeks) due to the risk of hemolytic anemia in the neonate. Cephalexin (Option B) and trimethoprim-sulfamethoxazole (Option C) are considered safe alternatives for treating cystitis in pregnancy, especially in the third trimester.

**Why Each Wrong Option is Incorrect**
**Option A:** Nitrofurantoin is a good choice for treating cystitis in pregnancy, but it's not the correct answer because the question asks for the antibiotic that cannot be used to treat cystitis in pregnancy, and the other options are safer alternatives.
**Option B:** Cephalexin is a safe and effective antibiotic for treating cystitis in pregnancy, making it an incorrect answer.
**Option C:** Trimethoprim-sulfamethoxazole is also a safe alternative for treating cystitis in pregnancy, especially in the third trimester, making it an incorrect answer.

**Clinical Pearl / High-Yield Fact**
When choosing an antibiotic for cystitis in pregnancy, consider the trimester of pregnancy and the potential risks to the fetus. Nitrofurantoin is contraindicated in the third trimester, while cephalexin and trimethoprim-sulfamethoxazole are safer alternatives.

**Correct Answer:** A. Nitrofurantoin.