**Core Concept**
Streptomycin is an aminoglycoside antibiotic that can cause ototoxicity and nephrotoxicity. Its use during pregnancy is contraindicated due to the risk of fetal ototoxicity and potential effects on fetal renal function.

**Why the Correct Answer is Right**
Streptomycin crosses the placental barrier and can cause congenital deafness in the fetus due to its ototoxic effects. Aminoglycosides like streptomycin inhibit protein synthesis by binding to the 30S subunit of the bacterial ribosome, leading to cell death. However, this mechanism also disrupts the normal development of the fetal inner ear, resulting in permanent hearing loss. Additionally, streptomycin can cause fetal renal damage due to its nephrotoxic effects, which can lead to oligohydramnios and other complications.

**Why Each Wrong Option is Incorrect**
**Option B:** Isoniazid is a first-line antitubercular agent used during pregnancy, but it requires pyridoxine supplementation to prevent peripheral neuropathy. While it has potential teratogenic effects, it is not strictly contraindicated in pregnancy.
**Option C:** Cephalosporins are a class of beta-lactam antibiotics that are generally safe to use during pregnancy. They are not associated with fetal ototoxicity or nephrotoxicity.
**Option D:** Penicillin is a broad-spectrum beta-lactam antibiotic that is commonly used during pregnancy. It is not contraindicated in pregnancy and is often used to treat bacterial infections in pregnant women.

**Clinical Pearl / High-Yield Fact**
Aminoglycoside antibiotics like streptomycin can cause "fetal asphyxia" due to the inhibition of fetal renal function, leading to decreased fetal urine production and oligohydramnios. This can result in a variety of complications, including pulmonary hypoplasia and limb contractures.

**✓ Correct Answer: A. Streptomycin**