A patient with an acute viral infection undergoes electron microscopy of the pathogen, revealing genetic material enclosed within a protective protein structure, with some virions also displaying an outer lipid layer derived from the host cell membrane. Which statement best describes the basic structure of a virus?

Viral genomes consist of either DNA or RNA, but never both at the same time. This is a fundamental principle in virology and is used to classify viruses into DNA viruses and RNA viruses. The genome may be single-stranded or double-stranded and can exist in linear, circular, or segmented forms, depending on the specific virus. Unlike human and other living cells, which contain both DNA (as genetic material) and RNA (for protein synthesis), viruses rely on only one type of nucleic acid for replication. This distinction is important because the type of genome determines the virus’s replication strategy, mutation rate, and response to antiviral therapies.

A. Flagella: Flagella are whip-like appendages that provide motility to bacteria, allowing them to move toward nutrients or away from harmful environments. While motility can contribute to colonization, flagella do not provide a protective barrier against phagocytosis or immune system attack.

B. Capsule: The capsule is an external polysaccharide or protein layer that surrounds the bacterial cell wall. It prevents recognition and ingestion by phagocytes, inhibits complement activation, and contributes to increased virulence. Encapsulated bacteria, such as Streptococcus pneumoniae, are particularly problematic in patients with weakened immune systems, like those with chronic alcoholism, because the capsule impedes effective immune clearance.

C. Fimbriae: Fimbriae are short, hair-like projections on the bacterial surface that facilitate attachment to host cells and surfaces. They play a role in colonization and biofilm formation but do not directly prevent phagocytosis or enhance survival against immune defenses.

D. Lipopolysaccharide: Lipopolysaccharide (LPS) is a component of the outer membrane of Gram-negative bacteria that contributes to endotoxin activity, triggering inflammation and septic shock. While LPS can affect host immune responses, it does not provide a direct physical barrier to phagocytosis.

E. Peptidoglycan: Peptidoglycan forms the rigid structural layer of the bacterial cell wall, providing shape and protection against osmotic pressure. Although essential for bacterial integrity, peptidoglycan does not prevent immune recognition or phagocytosis in the way that a capsule does.