Cyanide taken up by child. First one to be affected in Kreb’s cycle is?
**Core Concept**
The underlying principle being tested involves the **toxicity of cyanide** and its impact on cellular respiration, specifically the **Krebs cycle** (also known as the citric acid cycle or TCA cycle). Cyanide is a potent inhibitor of cellular respiration.
**Why the Correct Answer is Right**
Cyanide binds to **cytochrome c oxidase** in the electron transport chain, preventing the transfer of electrons and thereby inhibiting the production of ATP. However, in the context of the Krebs cycle, the first enzyme to be affected by the inhibition of the electron transport chain would be **Ξ±-ketoglutarate dehydrogenase** or more directly, the step involving the **succinate dehydrogenase (Complex II)**, as these are crucial for the continuation of the cycle and are directly linked to the electron transport chain. However, the most direct impact in the Krebs cycle due to cyanide poisoning would be on the **succinate dehydrogenase** step because it is both part of the Krebs cycle and the electron transport chain.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while it's part of the Krebs cycle, it's not the first to be affected by cyanide poisoning.
**Option B:** Incorrect as it is not directly related to the first impact of cyanide in the Krebs cycle.
**Option D:** Incorrect because, although important, it's not the first step in the Krebs cycle to be affected by cyanide.
**Clinical Pearl / High-Yield Fact**
Cyanide poisoning leads to cellular asphyxiation by inhibiting the electron transport chain, and understanding its impact on the Krebs cycle and cellular respiration is crucial for managing such poisonings. The key to remembering the effect of cyanide is its binding to **cytochrome c oxidase**.
**Correct Answer:** D. Succinate dehydrogenase