⟨sthm⟩ — silent TH in the asthma family
In asthma and asthmatic, the ⟨th⟩ is silent — the cluster ⟨sthm⟩ is pronounced just /zm/.
Greek origin: "Asthma" comes from Greek ἆσθμα (asthma, "panting"). The ⟨th⟩ was always part of the Greek spelling, but English speakers drop it to ease the cluster.