Differential Diagnosis

The patient's presentation of small reddish-purple lesions on the extremities and a low platelet count suggests a bleeding disorder. Here's a differential diagnosis organized into categories:

• Single most likely diagnosis
  • B. petechiae: The patient's small (less than 3 mm diameter) reddish-purple lesions are characteristic of petechiae, which are often seen in patients with thrombocytopenia (low platelet count). The platelet count of 20 x 10^9/L supports this diagnosis, as petechiae are a common manifestation of platelet deficiency.
• Other Likely diagnoses
  • C. ecchymoses: While ecchymoses are larger than petechiae, patients with severe thrombocytopenia can present with both petechiae and ecchymoses. The low platelet count increases the likelihood of ecchymoses, especially if the patient has experienced trauma or injury.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Thrombotic thrombocytopenic purpura (TTP): Although not listed as an option, TTP is a life-threatening condition characterized by thrombocytopenia, microangiopathic hemolytic anemia, and organ dysfunction. The patient's low platelet count and presence of petechiae-like lesions warrant consideration of TTP, which requires prompt diagnosis and treatment.
  • Immune thrombocytopenic purpura (ITP): ITP is an autoimmune condition that can cause severe thrombocytopenia and bleeding manifestations, including petechiae and ecchymoses. While the patient's platelet count is low, ITP should be considered as a potential underlying cause.
• Rare diagnoses
  • Vasculitis: Certain types of vasculitis, such as Henoch-Schönlein purpura or leukocytoclastic vasculitis, can present with petechiae-like lesions. However, these conditions are less likely given the patient's isolated thrombocytopenia and lack of other systemic symptoms.
  • Scurvy: Vitamin C deficiency (scurvy) can cause petechiae, ecchymoses, and other bleeding manifestations. However, scurvy is relatively rare and would typically be accompanied by other symptoms such as fatigue, malaise, and joint pain.