Balantidium coli trophozoites can be distinguished microscopically by which feature?

• A. Internal spore formation
• B. Large macronucleus
• C. Blue-staining flagella
• D. Single ameboid nucleus

Answer: (B)

Balantidium coli trophozoites are distinguished by a very large macronucleus. In ciliates, the macronucleus governs everyday cell functions and is typically prominent, often sausage- or kidney-shaped, with a much smaller micronucleus nearby. This large macronucleus is a hallmark visible under light microscopy and helps separate Balantidium from other intestinal protozoa that may not have such a conspicuous nuclear feature. The other options don’t fit because Balantidium trophozoites are covered with cilia (not blue-staining flagella), they do have two nuclei (macronucleus and micronucleus) rather than a single ameboid nucleus, and they do not form spores.