Vitamin K deficiency: Given the patient's history of multiple episodes of C. difficile colitis and treatment with antibiotics, there is a high likelihood of vitamin K deficiency, which can lead to coagulopathy and spontaneous bruising. The patient's drop in hemoglobin and slight decrease in platelet count also support this diagnosis.

Thrombocytopenia: Although the patient's platelet count is currently within the normal range, the slight decrease from baseline levels could contribute to bruising, especially in the context of other potential coagulopathies.
Anemia of chronic disease: The patient's drop in hemoglobin over the past 6 months could be related to chronic inflammation or disease, which may also contribute to bruising.
Medication side effects: Although the patient has not started any new medications, long-term use of anti-rejection medications can have various side effects, including effects on blood cell production or coagulation.

Heparin-induced thrombocytopenia (HIT): Although less likely, HIT is a potentially life-threatening condition that can occur in patients on peritoneal dialysis who may be exposed to heparin. It is crucial to consider this diagnosis due to its high mortality rate if missed.
Disseminated intravascular coagulation (DIC): This condition can occur in patients with severe infections, such as recurrent C. difficile colitis, and can lead to both thrombosis and bleeding. Early recognition is critical for effective management.
Bone marrow failure: Conditions such as aplastic anemia or myelodysplastic syndrome can cause pancytopenia, including thrombocytopenia, leading to bruising. These diagnoses are critical to consider due to their potential for severe morbidity and mortality.

Amyloidosis: This condition can cause bleeding due to amyloid deposition in blood vessels, leading to their fragility. Although rare, it should be considered in patients with unexplained bruising and other systemic symptoms.
Vasculitis: Certain types of vasculitis, such as Henoch-Schönlein purpura, can cause spontaneous bruising. These conditions are rare but can be severe and require prompt diagnosis and treatment.
Inherited coagulopathies: Conditions such as von Willebrand disease or factor deficiencies can cause bleeding and bruising. Although less likely in a patient presenting for the first time at age 54, they should be considered if other diagnoses are ruled out.