Iron-deficiency anemia can cause fatigue (tiredness), shortness of breath, chest pain, and other symptoms. Severe iron-deficiency anemia can lead to heart problems, infections, problems with growth and development in children, and other complications.
Semantics is the study of meaning. It is a wide subject within the general study of language. An understanding of semantics is essential to the study of language acquisition (how language users acquire a sense of meaning, as speakers and writers, listeners and readers) and of language change (how meanings alter over time).
Topic/Issue. A topic is what the essay or research paper is about. It provides a focus for the writing. Of course, the major topic can be broken down into its components or smaller pieces (e.g., the major topic of nuclear waste disposal may be broken down into medical, economic, and environmental concerns).