A patient presenting with swelling in the left big toe and a WBC count of 2543/mL suggests:

The correct choice emphasizes that the presentation of swelling in the left big toe, combined with a low white blood cell (WBC) count of 2543/mL, aligns with gout. Gout is a type of inflammatory arthritis characterized by sudden and severe attacks of pain, swelling, and redness, often affecting the big toe. This condition is caused by the accumulation of uric acid crystals in the joints.

In cases of an acute gout flare, it is common for patients to exhibit inflammation in a single joint, like the big toe. While elevated WBC counts can occur in response to inflammation, they are not necessary for the diagnosis of gout, and a low WBC count does not rule it out. Instead, the presentation of localized swelling and pain points to an inflammatory process, consistent with gout.

The other potential options do not fit the clinical picture as appropriately. Chronic infection would typically present with elevated WBC counts due to a sustained inflammatory response. Acute inflammation usually results in an increase in WBC counts as well. Osteoporosis, starting with its impact on bone density and strength, does not present with symptoms such as joint swelling. Thus, gout is the most fitting diagnosis in this context.