Final answer:
The most likely adverse effect of Prednisone in a patient with rheumatoid arthritis from the provided options is a moon-shaped face, known as a Cushingoid appearance.
Step-by-step explanation:
If a patient with rheumatoid arthritis is prescribed Prednisone, which is a steroid medication, and is experiencing an adverse effect, the most likely symptom from the options provided would be a moon-shaped face. This is known as 'Cushingoid appearance' and can occur as a side effect of long-term steroid use, such as Prednisone. Increased joint mobility would likely be a therapeutic effect of steroid use in RA, not an adverse effect. A decreased white blood cell count might occur with immunosuppressive drugs, but this is not the most characteristic side effect of steroids. Weight loss is generally not associated with steroid use; instead, patients may experience weight gain due to increased appetite. Therefore, a moon-shaped face would be an indicator of Prednisone's adverse effects.