**Core Concept**
Chlamydia is a unique intracellular bacterium that exhibits a biphasic life cycle, consisting of an infectious elementary body (EB) and a non-infectious, replicative reticulate body (RB). The EB is the form that infects host cells, while the RB is responsible for replication within the host cell.

**Why the Correct Answer is Right**
The elementary body of Chlamydia is not metabolically active. In fact, it is the metabolically inactive form that can survive outside the host cell. Upon entry into the host cell, the EB undergoes a process called "differentiation" to become the reticulate body, which is highly metabolically active and responsible for replication. The reticulate body uses the host cell's metabolic machinery to replicate and produce new RBs. The RB then undergoes a process called "re-differentiation" to become a new EB, which can then be released from the host cell to infect new cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is true. Chlamydia exhibits a biphasic life cycle, which is a unique characteristic of this bacterium.
**Option C:** This statement is true. The reticulate body of Chlamydia undergoes binary fission to replicate, which is a characteristic of bacterial cell division.
**Option D:** This statement is true. Once Chlamydia invades the host cell, it can prevent phagolysosomal fusion, which allows it to survive and replicate within the host cell.

**Clinical Pearl / High-Yield Fact**
Chlamydia is a common cause of sexually transmitted infections (STIs) and can also be transmitted from mother to child during childbirth. It is essential to diagnose and treat Chlamydia infections promptly to prevent long-term complications, such as pelvic inflammatory disease (PID) in women and epididymitis in men.

**✓ Correct Answer: B. Elementary body is metabolically active**