• Ammonia Neurotoxicity and Cerebral Defense in Teleosts:
• Ammonia ($NH_3 / NH_4^+$) crosses the blood-brain barrier and can cause astrocyte swelling and cerebral edema.
• Because most teleost fishes lack a complete functional hepatic Ornithine-Urea Cycle (O-UC) to convert ammonia into urea:
• The brain relies primarily on Glutamine Synthetase (GS) located in astrocytes:

$$\text{Glutamate} + NH_4^+ + \text{ATP} \xrightarrow{\text{Glutamine Synthetase}} \mathbf{\text{Glutamine}} + \text{ADP} + P_i$$

• Detoxifying excess ammonia into neutral, non-toxic glutamine is the primary cerebral pathway preventing ammonia neurotoxicity.
• nalysis:
• (Glutamine): Correct.

Correct option: (3) Glutamine

• Ureotelism Among Teleost Fishes:
• Virtually all teleost fishes are strictly ammonotelic. However, a few rare teleosts retain and express a complete, functional set of hepatic Ornithine-Urea Cycle (O-UC) enzymes (CPSase III, OTC, ASS, ASL, arginase):
• Gulf toadfish (Opsanus beta) and Oyster toadfish (Opsanus tau): When subjected to confinement, crowding, or air exposure, toadfish downregulate gill ammonia excretion and activate the O-UC, becoming ureotelic (excreting $\gt 90\%$ of nitrogen as urea across the gills via the ut-A urea transporter).
• Lake Magadi tilapia (Alcolapia grahami): Obligate ureotelic teleost adapted to hyper-alkaline waters ($pH \approx 10$).
• nalysis:
• (Opsanus beta): Correct ureotelic teleost.

Correct option: (2) Opsanus beta