**Core Concept**
Cystitis in pregnancy requires careful management due to potential teratogenic effects of certain medications. The goal is to choose a drug that is effective in treating the infection while minimizing risks to the fetus.

**Why the Correct Answer is Right**
Nitrofurantoin is a safe option for treating cystitis in pregnancy, especially during the second and third trimesters. It is a urinary anti-infective agent that works by inhibiting bacterial DNA synthesis and function through the reduction of folic acid. This mechanism of action is specific to bacteria, minimizing the risk of teratogenic effects on the fetus. Nitrofurantoin is a preferred choice in pregnancy due to its efficacy and safety profile.

**Why Each Wrong Option is Incorrect**
**Option A:** Sulfonamides are contraindicated in pregnancy due to the risk of kernicterus in the newborn and potential inhibition of folic acid synthesis.
**Option B:** Tetracyclines are contraindicated in pregnancy due to the risk of inhibiting bone growth and causing tooth discoloration in the fetus.
**Option C:** Amoxicillin is generally safe in pregnancy, but it is not the first-line choice for treating cystitis. In addition, resistance patterns may vary, making nitrofurantoin a more reliable option.

**Clinical Pearl / High-Yield Fact**
When treating urinary tract infections in pregnancy, always consider the gestational age and potential risks to the fetus. Nitrofurantoin is a safe choice for treating cystitis in the second and third trimesters.

**Correct Answer: C. Nitrofurantoin**