Which chemotherapy agent is known to be toxic to nearly every organ except the heart?

Methotrexate is recognized for its wide-ranging toxicity across multiple organ systems, impacting the liver, kidneys, lungs, and bone marrow. It is an antimetabolite that interferes with DNA replication and cellular functioning, leading to its diverse side effects. In contrast, while the other options listed can cause significant organ toxicity, they are not known for the same broad impact across almost every organ.

Cyclophosphamide can be damaging to several organs, especially the bladder, which can lead to hemorrhagic cystitis. Cisplatin is notorious for causing renal toxicity but is also associated with significant ototoxicity and peripheral neuropathy. Vincristine tends to affect the nervous system primarily, leading to neuropathies, and does not produce the same extensive organ toxicity profile.

Thus, the characteristic of Methotrexate as being toxic to nearly every organ except the heart highlights its extensive side effect profile, which is critical for healthcare professionals to monitor and manage during treatment.