Final answer:
In the case where a patient's output is less than intake with a weight gain of 1 kg, the nurse should assess the patient's lungs for signs of fluid overload before considering further interventions.
Step-by-step explanation:
The clinical scenario of a patient having markedly lower output compared to intake and experiencing a 1 kg weight increase suggests the possibility of fluid overload. Therefore, among the given options, the most appropriate immediate action would be to assess the patient's lungs. This assessment could reveal signs of fluid overload such as pulmonary edema, which may manifest as crackles on auscultation or increased work of breathing. Fluid restriction or further interventions might be necessary based on the assessment findings, but an initial lung assessment is crucial to determine the next steps in management.