The correct answer is Chloramphenicol.

Gray baby syndrome is a serious and potentially fatal condition that occurs primarily in neonates treated with chloramphenicol. This is due to the neonates’ immature liver enzyme systems, specifically the UDP-glucuronyl transferase, which is responsible for the glucuronidation of chloramphenicol. Because neonates cannot effectively metabolize and eliminate chloramphenicol, the drug accumulates to toxic levels, causing symptoms such as:

- Grayish skin discoloration
- Hypotension
- Profound cyanosis
- Vomiting
- Cardiovascular collapse

In contrast, older children and adults efficiently metabolize chloramphenicol via hepatic glucuronidation, preventing such adverse effects.

Key Points:
- Chloramphenicol requires hepatic glucuronidation for elimination.
- Neonates have deficient UDP-glucuronyl transferase activity.
- Accumulation leads to toxic effects called gray baby syndrome, characterized by gray discoloration, cardiovascular collapse, and death if untreated.
- Other antibiotics listed (Gentamicin, Tetracycline, Rifampicin) do not cause gray baby syndrome.

Reference: *Harrison's Principles of Internal Medicine, 20th Edition, Volume 2, Chapter 130, Page 1499.*