In virology, identify the morphological class of bacteriophages described as the most complex type, with a hexagonal head, a rigid tail that has a contractile sheath, and distinct tail fibers.

Introduction / Context:This item tests recognition of classical bacteriophage morphology. Early electron microscopy led to a practical scheme (commonly attributed to Bradley) that groups tailed bacteriophages by tail structure. Correct classification is important in understanding attachment, genome injection mechanisms, and even phage therapy selection.

Given Data / Assumptions:

• Head: hexagonal outline (icosahedral symmetry).
• Tail: rigid tail equipped with a contractile sheath.
• Accessories: distinct tail fibers are present.
• We are choosing a morphological class label rather than a specific species.

Concept / Approach:In Bradley’s morphology-based scheme, three major tailed groups are highlighted: Type A (contractile tail), Type B (long, flexible, noncontractile tail), and Type C (short, noncontractile tail). The presence of a sheath that actively shortens to drive the tail tube through the bacterial envelope is the hallmark of Type A phages (typified by the family Myoviridae).

Step-by-Step Solution:Identify the diagnostic feature: a contractile sheath around the tail.Map to class: contractile tail corresponds to Bradley type A.Confirm supportive features: hexagonal head and tail fibers are consistent with tailed phages of type A.Select the option that explicitly states type A / contractile tail.

Verification / Alternative check:Review reference micrographs: Type A phages show extended and contracted tail states; the sheath visibly shortens during genome injection.

Why Other Options Are Wrong:Type B and C describe noncontractile tails; they lack a sheath. The ”no tail” option refers to entirely different, nontailed icosahedral viruses.

Common Pitfalls:Confusing “rigid” with “noncontractile”; assuming all tailed phages contract—only type A does.

Final Answer:Bradley type A (contractile tail; Myoviridae-like)