An organism suspected of causing a disease is described as being unicellular, having a cell wall and lacking a nucleus - HSC - SSCE Biology - Question 8 - 2018 - Paper 1

To classify this organism, we need to analyze the given characteristics:

1. Unicellular: This indicates that the organism is made up of a single cell.
2. Having a cell wall: This characteristic is crucial in determining the class of the organism since not all unicellular organisms have cell walls.
3. Lacking a nucleus: This detail is significant as it specifies that the organism is prokaryotic rather than eukaryotic.

Based on these characteristics:

• A bacterium (A) is unicellular, has a cell wall, and lacks a nucleus (cells are prokaryotic).
• A fungus (B) typically has a cell wall, but most fungi are multicellular (with some exceptions).
• A protozoan (C) is unicellular and eukaryotic (therefore has a nucleus).
• A virus (D) is not classified as a cellular organism and does not have a cell wall or a nucleus.

Given that the organism is unicellular, has a cell wall, and lacks a nucleus, it is classified as A. A bacterium.