What lab would be most appropriate for a patient presenting with petechiae on the feet?

In evaluating a patient presenting with petechiae, particularly on the feet, a complete blood count (CBC) is the most appropriate lab test. Petechiae can be indicative of several underlying conditions, including thrombocytopenia (low platelet count), which can be identified through a CBC. This test provides critical information regarding the different components of blood, including red blood cells, white blood cells, and platelets.

When assessing the cause of petechiae, the platelet count is especially important, as low platelets can lead to bleeding manifestations such as petechiae. Additionally, the CBC can also help evaluate for other potential issues, such as anemia or leukopenia, that may provide further context to the patient's overall health status and symptoms.

Other laboratory tests, while important in certain clinical scenarios, do not directly assess the potential hematologic causes of petechiae in the same comprehensive manner that a CBC does. Serum iron levels, vitamin B12, folate levels, and PT/INR tests provide information on specific aspects of blood components and coagulation but would not offer the initial insight into the platelet status or possible hematologic abnormalities that a CBC would reveal.