**Core Concept**
Tetracycline antibiotics are contraindicated in pregnancy due to their potential to inhibit bone growth and cause tooth discoloration in the fetus. This is because tetracyclines are broad-spectrum antibiotics that can bind to calcium ions and form insoluble calcium-tetracycline complexes.

**Why the Correct Answer is Right**
Tetracyclines bind to the bacterial ribosomes, inhibiting protein synthesis. However, in pregnant women, this effect is also seen in the developing fetus. As the fetus's bones are still in the process of developing, tetracyclines can interfere with calcium deposition, potentially leading to growth retardation. Furthermore, the incorporation of tetracyclines into developing teeth can cause permanent discoloration.

**Why Each Wrong Option is Incorrect**
**Option A:** Folic acid is actually beneficial in pregnancy, particularly to prevent neural tube defects. It should be given as a supplement to pregnant women.
**Option B:** Metronidazole is generally considered safe in pregnancy, although it should be used with caution due to potential side effects.
**Option C:** Erythromycin is a macrolide antibiotic that is often preferred over tetracyclines in pregnancy due to its lower risk of tooth discoloration and bone growth inhibition.

**Clinical Pearl / High-Yield Fact**
When prescribing antibiotics in pregnancy, always consider the gestational age and the type of infection. If possible, choose antibiotics with a proven safety profile, such as erythromycin or amoxicillin.

**Correct Answer: C. Erythromycin.**