, a bacteriophage is a virus that infects bacteria and is typically characterised by a specific structural morphology . As shown in the diagram provided in the sources (Figure 2.6b), the four primary parts are: the Head (the top geometric portion containing the genetic material), the Collar (the 'neck' region), the Sheath (the cylindrical body), and the Tail fibres (the leg-like structures at the base) . Based on previous medical entrance examination data (specifically AIPMT MAINS 2010), the labels in this specific diagram correspond to: A–Head, B–Sheath, C–Collar, and D–Tail fibres .