Step 1: Understanding the Concept
Platelets (thrombocytes) start blood clotting by forming a plug and releasing clotting factors.
Step 2: Link with the condition
In thrombocytopenia the platelet count is abnormally low. Even small vessel injuries keep bleeding, giving internal haemorrhage and bruising.
Step 3: Why the other options fail
Polycythemia is an excess of RBCs, erythropoiesis is RBC formation and leucopenia is a low WBC count. None of them causes bleeding directly.
Final Answer:
Internal bleeding can result from thrombocytopenia.
\[ \boxed{\text{(C) }\text{thrombocytopenia}} \]