Final answer:
The baby is most likely suffering from Impetigo, which is characterized by large bullae filled with cloudy yellow fluid and crusts formed after rupture. It is a bacterial infection common in children.
Step-by-step explanation:
If a baby with large bullae filled with cloudy yellow fluid; some have ruptured and the bases are covered with a thin crust, the most likely diagnosis is Impetigo. This condition often presents with large fluid-filled blisters (bullae) around the nose and mouth that rupture and become encrusted with a yellowish crust.
Impetigo especially the bullous type, is common in children and is highly contagious. It is usually caused by bacterial infection with either Staphylococcus aureus, Streptococcus pyogenes or a combination of both.