Aminoglycosides are usually used as:

Aminoglycosides are a class of medications that are primarily known for their antibacterial properties. They work by inhibiting protein synthesis in bacteria, which prevents the bacteria from growing and multiplying. This makes them effective against a variety of bacterial infections, especially those caused by Gram-negative bacteria.

Now, let's examine the given options:

Option A: Antibiotic - This refers to a substance that kills or inhibits the growth of bacteria.

Option B: Analgesic - This refers to a drug that relieves pain.

Option C: Hypnotic - This refers to a drug that induces sleep.

Option D: Antifertility - This refers to a drug that prevents conception or fertility.

Since aminoglycosides specifically target bacteria and are used to treat bacterial infections, they fall under the category of antibiotics. Common examples include gentamicin, tobramycin, and amikacin, which are prescribed for serious infections like sepsis, urinary tract infections, and certain types of pneumonia.

Therefore, aminoglycosides are classified as antibiotics.

Hence, the correct answer is Option A.