Final answer:
Based on the described symptoms and low platelet count, the child is likely suffering from thrombocytopenia, a condition leading to excessive bleeding due to an insufficient number of blood clotting cells.
Step-by-step explanation:
The child with a platelet count of 18,000/mm³, easy bruising, nosebleeds, black, tarry stools, and blood in the urine is likely suffering from thrombocytopenia. Thrombocytopenia is a condition characterized by an abnormally low number of platelets, which are critical for blood clotting. The symptoms described, including the low platelet count, suggest an impairment in the blood's ability to clot, which can lead to excessive bleeding even from minor wounds or in the absence of apparent trauma.